Dolby Digital Professional Encoding Guidelines Issue 1

Dolby Digital
Professional Encoding
Guidelines
Issue 1
S00/12972
Dolby Digital Professional Encoding Guidelines
Dolby Laboratories Inc
Corporate Headquarters
Dolby Laboratories Inc
100 Potrero Avenue
San Francisco, CA 94103-4813
Telephone 415-558-0200
Facsimile 415-863-1373
www.dolby.com
European Licensing Liaison Office
Dolby Laboratories
Wootton Bassett
Wiltshire, SN4 8QJ, England
Telephone (44) 1793-842100
Facsimile (44) 1793-842101
Far East
Dolby Laboratories
International Services, Inc.
Japan Branch
Fuji Chuo Building 6F
2-1-7, Shintomi, Chuo-ku
Tokyo 104-0041 Japan
Telephone (81) 3-5542-6160
Facsimile (81) 3-5542-6158
Dolby Laboratories
Representative Office
7/Fl., Hai Xing Plaza, Unit H
1 Rui Jin Road (S)
Shanghai 200023 China
Telephone (86) 21-6418-1015
Facsimile (86) 21-6418-1013
Dolby, Pro Logic, Surround EX, AC-3, and the double-D symbol are trademarks of Dolby Laboratories.
 2000 Dolby Laboratories Inc; all rights reserved.
S00/12972 Issue 1
Dolby Digital Professional Encoding Guidelines
Table of Contents
List of Figures
ix
List of Tables
x
Chapter 1
Introduction
1.1
Purpose and Scope
1.2
General Information
1.2.1 More than a Coding Technology
1.2.2 Features of Dolby Digital
1-1
Chapter 2
Production Environment
2.1
System Configuration
2.2
Monitoring Through a Decoder
2.3
Room Layout, Monitoring, and Calibration
2-1
2-1
2-7
2-8
Chapter 3
Consumer Decoder Products
3.1
Categories
3.1.1 Source Products
3.1.2 Decoder Products
3.1.3 Channel Output Categories
3.2
Features
3.3
Supported Data Rates
3.4
Compatibility
3.5
LFE and Bass Management
1-2
1-3
1-4
3-1
3-1
3-1
3-2
3-2
3-3
3-4
3-5
3-10
Dolby Digital Professional Encoding Guidelines
3.6
3.7
Chapter 4
Operational Modes
3.6.1 Line Mode
3.6.2 RF Mode
3.6.3 Using Operational Modes in Products
Laser Disc
Encoding
4.1
General Information
4.1.1 Preparing the Source Delivery Master
4.1.2 System Operation
4.2
Features
4.2.1 Dialog Normalization (dialnorm)
4.2.2 Dynamic Range Control (DRC)
4.3
Metering
4.3.1 Input Level Meter
4.3.2 Line Mode Meter
4.3.3 RF Mode Meter
4.3.4 Calibration of Dialog Normalization (dialnorm)
4.4
Parameter Default Values
4.5
Audio Service
4.5.1 Data Rate
4.5.2 Audio Coding Mode
4.5.3 LFE Channel
4.5.4 Bitstream Mode
4.5.5 Dialog Normalization (dialnorm)
iv
3-12
3-12
3-14
3-16
3-21
4-1
4-1
4-1
4-5
4-7
4-8
4-12
4-19
4-20
4-20
4-20
4-21
4-22
4-24
4-25
4-26
4-27
4-28
4-29
Dolby Digital Professional Encoding Guidelines
4.6
4.7
4.8
4.9
Bitstream Information
4.6.1 Center Downmix Level
4.6.2 Surround Downmix Level
4.6.3 Dolby Surround Mode
4.6.4 Language Code
4.6.5 Audio Production Information Exists
4.6.6 Mixing Level
4.6.7 Room Type
4.6.8 Copyright Bit
4.6.9 Original Bitstream
Preprocessing Options
4.7.1 Digital De-emphasis
4.7.2 DC Highpass Filter
4.7.3 Channel Bandwidth Lowpass Filter
4.7.4 LFE Lowpass Filter
4.7.5 Surround Channel 90-Degree Phase-Shift
4.7.6 Surround Channel 3 dB Attenuation
4.7.7 Dynamic Range Compression Profile
4.7.8 RF Overmodulation Protection (RF Pre-emphasis Filter)
Automatic Parameters
4.8.1 Audio Bandwidth
4.8.2 Coupling
Input/Output Control
4.9.1 Sampling Rate
4.9.2 Input Channels
4.9.3 Output Format
v
4-29
4-29
4-30
4-31
4-32
4-32
4-32
4-34
4-34
4-35
4-35
4-35
4-36
4-36
4-37
4-37
4-37
4-38
4-39
4-39
4-39
4-40
4-40
4-40
4-41
4-41
Dolby Digital Professional Encoding Guidelines
4.10
4.11
Chapter 5
Processing State Control
4.10.1 Start/Stop Encoding
4.10.2 Configuration Presets
4.10.3 Time Code Control
4.10.4 Record/Play Bitstream
Using the Dolby Model DP562 Professional Reference Decoder
4.11.1 Downmixing
4.11.2 Dynamic Range Control (DRC)
4.11.3 Bass Management
4.11.4 LFE Monitor Mode
Applications and Formats
5.1
DVD-Video
5.1.1 DVD-Video Specification
5.1.2 Supported Data Rates
5.1.3 Bit Resolution
5.1.4 Audio/Video Synchronization
5.1.5 Dolby Digital Encoding for DVD-Video
5.1.6 Music on DVD-Video
5.1.7 Karaoke DVD
5.1.8 Miscellaneous Issues
5.2
DVD-Audio
5.3
DVD-ROM
5.4
Digital Television (DTV)
5.4.1 ATSC DTV Constraints
5.4.2 Implementation
5.4.3 Main, Associated, and Multilingual Services
vi
4-41
4-42
4-42
4-42
4-43
4-43
4-44
4-49
4-49
4-50
5-1
5-1
5-2
5-2
5-3
5-3
5-4
5-5
5-6
5-7
5-7
5-7
5-9
5-9
5-10
5-11
Dolby Digital Professional Encoding Guidelines
5.4.4 Detailed Description of Service Types
5.4.5 Splicing Bitstreams
Laser Disc
5.5.1 Track Layout
5.5.2 Audio/Video Synchronization
5.5.3 Important Considerations
5-15
5-20
5-22
5-22
5-23
Chapter 6
Professional Encoders and Decoders
6.1
Dolby Digital Professional Encoders
6.1.1 Software vs. Hardware
6.1.2 Licensed Dolby Digital Encoders and Quality
6.1.3 Dolby Laboratories Encoders
6.1.4 Dolby Laboratories Licensed Encoders
6.1.5 Software Updates
6.2
Dolby Digital Professional Decoders
6.2.1 Dolby Laboratories Decoder
6.2.2 Licensed Dolby Digital Professional Decoders
6-1
6-1
6-1
6-2
6-2
6-3
6-3
6-4
6-4
6-5
Chapter 7
Miscellaneous Information
7.1
Technical Assistance
7.2
Contacting Dolby Laboratories
7.3
Trademark Usage
7-1
7-1
7-2
7-4
5.5
Appendix A The Dolby Digital Algorithm - Theory of Operations
A.1
Introduction
A.2
Perceptual Coding Principles
vii
A-6
A-6
A-7
Dolby Digital Professional Encoding Guidelines
Appendix B Bitstream Format
B.1
Output Mode
B.2
Audio/Non-Audio Bit
B-1
B-1
Appendix C Dynamic Range Control (DRC)
C.1
Background
C.2
Dynamic Range Control (DRC) Algorithm Overview
C.3
Compression Characteristic
C.4
Dynamic Range Compression Profiles
C-1
C-1
C-3
C-5
C-7
Appendix D Dolby Digital Time-Domain Filters
D.1
90-Degree Phase-Shift Filter
D.2
Digital Deemphasis Filter
D.3
DC Highpass Filter
D.4
Channel Bandwidth Lowpass Filter
D.5
LFE Lowpass Filter
D-1
D-1
D-2
D-4
D-4
D-5
Appendix E Mix and Mastering Data Sheets
E-7
Appendix F Glossary
F-1
Appendix G Bibliography
G-1
viii
Dolby Digital Professional Encoding Guidelines
List of Figures
2-1
2-2
2-3
2-4
2-5
3-1
3-2
3-3
3-4
3-5
3-6
3-7
4-1
4-2
5-1
A-1
A–2
A-3
B-1
C-1
C-2
Generic Dolby Digital Encoding System
Dolby Digital Recorded to a Computer
Dolby Digital Recorded to a Computer with DAT-Link+
Dolby Digital Encoding Using a Licensed PCI Card
Dolby Digital Encoding Using Licensed Computer Software
Audio Reproduction Hierarchy
Decoder Product Bass Management Configuration One
Decoder Product Bass Management Configuration Two
Signal Relationships in Line Mode
Signal Relationships in RF Mode
RF Modulator Signal Levels
Basic Laser Disc Player Structure
Dolby Surround Compatible Lt/Rt Downmix
Stereo Compatible Lo/Ro Downmix
Typical Dolby Digital Laser Disc Encoding Setup
Hearing Threshold
Effect of Masking
Coding (Quantization) Noise Below the Masking Curve
Screen Capture of Dolby Digital Data with Zero Padding
Overview of Dynamic Range Control Algorithm
Dynamic Range Compression Core
ix
2-2
2-4
2-5
2-6
2-7
3-6
3-11
3-11
3-13
3-15
3-16
3-22
4-46
4-47
5-24
A-8
A-9
A-10
B-3
C-4
C-6
Dolby Digital Professional Encoding Guidelines
List of Tables
3-1
3-2
3-4
3-5
3-6
3-7
4-1
4-2
4-3
4-4
4-5
4-6
4-7
4-8
4-9
5-1
5-2
5-3
7-1
C-1
Consumer Decoder Product Features
Dolby Digital Bitstreams Available from Existing Formats
DRC for a Typical Source Product
DRC for a Source Product with RF Modulated Output
DRC for a Typical Decoder Product
Alternative DRC for a Decoder Product
Channel-to-Track Layout Example
Recommended Parameter Default Values
Supported and Suggested Data Rates According to Audio Coding Mode
Audio Coding Mode
Bitstream Mode/Audio Service Type
Center Downmix Level
Surround Downmix Level
Dolby Surround Mode Indications
Room Type
ATSC DTV Audio Constraints
Audio Services
Typical D2 Track Layout
Dolby Email Contact Addresses
Dynamic Range Compression Profile Parameter Values
x
3-4
3-5
3-17
3-19
3-20
3-21
4-2
4-23
4-25
4-27
4-28
4-30
4-31
4-31
4-34
5-10
5-13
5-23
7-2
C-8
Dolby Digital Professional Encoding Guidelines
Chapter 1
Introduction
1.1
Purpose and Scope
This manual is intended to serve as a guide for performing Dolby Digital (formerly
AC-3) professional audio encoding in applications other than film production. It
contains information on the features of the Dolby Digital system, professional
encoders and decoders, consumer decoders, distribution formats, and Dolby
trademark use. The key features of the Dolby Digital system, including Dialog
Normalization (also referred to as volume normalization), Dynamic Range Control
(DRC), and downmixing (multiple channels through fewer outputs) are described and
guidelines for their use are presented.
For detailed descriptions of the Dolby Digital algorithm, refer to the Advanced
Television Systems Committee (ATSC) documents A/52, Digital Audio Compression
Standard (AC-3), and A/54, Guide to the Use of the ATSC Digital Television
Standard, which can be found on the ATSC web site at www.atsc.org. For a list of
additional technical papers on Dolby Digital, refer to Appendix G, Bibliography.
Dolby Digital Professional Encoding Guidelines
1.2
Introduction
General Information
Dolby Digital is a perceptual audio coding system that is based on the fundamental
principles of human hearing. It was first developed in 1992 as a means to allow 35
mm theatrical film prints to carry multichannel digital audio directly on the film
without sacrificing the standard analog optical soundtrack. Since its introduction the
system has been adopted for use with laser disc, ATSC high definition and standard
definition digital television, digital cable television, digital satellite broadcast, DVDVideo, DVD-ROM, DVD-Audio, and Internet audio distribution. It is intended for
use as an emissions coder that encodes audio for distribution to the consumer, not as a
multigenerational coder that is used to encode and decode audio multiple times. For
applications that require multigenerational coding, refer to Section 7.2, Contacting
Dolby Laboratories, to acquire information on Dolby E technology.
Dolby Digital divides the audio spectrum into narrow frequency bands using
mathematical models derived from the characteristics of the ear and analyzes each
band to determine the audibility of those signals. To maximize data efficiency, a
greater number of bits represent more audible signals; fewer bits represent less
audible signals. In determining the audibility of signals, one phenomenon that the
system makes use of is known as masking. Masking refers to the fact that the ear is
less sensitive to low-level sounds when there are higher-level sounds at nearby
frequencies. When this occurs, the high-level sound masks the low-level one,
rendering it either less audible or inaudible. By taking advantage of this phenomenon,
audio can be encoded much more efficiently than in other digital coding systems with
comparable audio quality, such as linear PCM. This makes Dolby Digital an excellent
Dolby Digital Professional Encoding Guidelines
Introduction
choice for systems where high audio quality is desired, but bandwidth or storage
space is restricted. This is especially true for multichannel audio, where the compact
Dolby Digital bitstream allows full 5.1-channel audio to occupy less space than a
single channel of linear PCM audio.
The Dolby Digital system is designed to allow the encoder to continue evolving and
improving. As more research is conducted, the encoding algorithm can be modified
for improved accuracy. The Dolby Digital system is also designed to pass encoding
improvements along to the decoder providing improved audio quality for all listeners.
1.2.1
More than a Coding Technology
Dolby Digital is more than just an audio coding technology. It is also a sophisticated
audio delivery and reproduction system that allows both the program producer and
the end listener to affect how the audio program will ultimately be heard. For the first
time in the consumer audio industry, a program producer can deliver multichannel
audio along with control parameters. These parameters can determine the relative
playback level, using Dialog Normalization, the preferred dynamic range
compression setting, and how the audio program will sound to consumers listening to
a stereo downmix of it. These capabilities not only provide the program producer
useful new tools that can enhance the listening experience, but they also create the
possibility of undesirable results if encoding parameters are not set correctly.
When Dolby Digital encoding it is important to understand the options typical
Dolby Digital decoders present to the consumer and how those options can affect
Dolby Digital Professional Encoding Guidelines
Introduction
the audio. Dolby Digital consumer decoders are used in a variety of listening
situations with high-end products offering the listener a wealth of options not found
in more basic implementations. Consumers can listen to multichannel audio in a
number of ways including:
• Full dynamic range 5.1-channel state
• Reduced dynamic range 5.1-channel state (for apartment dwellers or late night
listeners, etc.)
• Two-channel Dolby Surround compatible downmix (which can then be
Dolby Surround Pro Logic decoded)
• Normal two-channel stereo downmix
• Mono downmix
It is important to understand the various options presented by both Dolby Digital
professional encoders and consumer decoders, to be aware of how they interact with and
affect the audio, and to ensure that encoder options are used properly for best results.
1.2.2
Features of Dolby Digital
• Delivers from one (mono) to 5.1 discrete channels of audio in a variety of
configurations
Dolby Digital Professional Encoding Guidelines
Introduction
• Efficiently reduces audio data with typical savings of 12:1 (5.1-channel Dolby
Digital at 384 kb/s compared to a 16-bit, 48 kHz linear PCM source)
• Provides the capability for all main channels to deliver a frequency bandwidth
from 3 Hz (DC Highpass Filter enabled) to 20.7 kHz
• Provides an optional Low-Frequency Effects (LFE) channel for additional bass
with a frequency bandwidth from 3 Hz (DC Highpass Filter enabled) to 120 Hz
•
•
•
•
•
•
•
Supports a wide range of data rates from 32 kb/s to 640 kb/s
Accepts input sampling rates of 32, 44.1, and 48 kHz
Fully accepts input word lengths of 16, 18, 20, or 24 bits
Provides for uniform, level-matched playback for all sources
Allows producer and/or user to control dynamic range during playback
Provides for multichannel programs to be downmixed all the way to mono
Allows the creation of a Dolby Surround compatible downmix for listeners with
Dolby Surround Pro Logic systems
• Permits the producer to optimize Center and Surround channel levels for use in
stereo downmix mode
• Provides a Karaoke Mode
• Permits the transmission and disk storage of SMPTE time code stamps along with
the Dolby Digital data
Dolby Digital Professional Encoding Guidelines
Introduction
• Enables the producer to indicate:
!
!
!
The reference Sound Pressure Level (SPL) used when mixing the audio
program; allows for calibrated playback levels
The type of room (large or small) used to mix the audio program
That the two-channel audio program is Dolby Surround (Lt/Rt) encoded
• Provides an indicator for original bitstreams, to distinguish from a bitstream copy
• Provides a copyright control bit
• Provides a Bitstream Mode indicator (Main and Associated Services for
broadcast)
• Produces improved sound quality for all listeners through refinements in the
encoding algorithm
Dolby Digital Professional Encoding Guidelines
Chapter 2
Production Environment
2.1
System Configuration
There are many ways to configure a production environment for Dolby Digital
professional encoding. The type of Dolby Digital encoder and the associated
hardware determines how to connect the components.
As shown in Figure 2-1, the essential components for Dolby Digital professional
encoding are an audio source (mono to 5.1-channel), a Dolby Digital encoder, a
capture and storage device for the encoded output, and a professional reference
decoder. Not shown, although just as important, is a properly calibrated audio
reproduction system for monitoring the decoded output. Considering the wide variety
and availability of these components, general guidelines are given on the setup of
these systems. It is impossible to cover every configuration in this manual, therefore
the reader is encouraged to contact the appropriate equipment manufacturer for
details on each product.
2-1
Dolby Digital Professional Encoding Guidelines
Production Environment
Production
or
Broadcast
Digital Media
(Linear PCM)
L
R
C
LFE
LS
RS
Dolby
Digital
Encoder
Dolby
Digital
.ac3 files
DP562
Dolby
Digital
Decoder
SMPTE Time Code
5.1-Channel
Analog or
Digital Output
Monitoring
Figure 2-1 Generic Dolby Digital Encoding System
Following is a brief description of the components in Figure 2-1.
• The linear PCM source can be any digital audio storage device (e.g., Modular
Digital Multitrack (MDM), DAT, CD, etc.). Many types of media and products
are capable of storing digital audio content or files. Choosing the right medium
and product depends on whether the project requires real-time or non-real time
encoding. In addition, the number of channels to be encoded affects the choice of
medium.
2-2
Dolby Digital Professional Encoding Guidelines
Production Environment
• The Dolby Digital encoder can be any professional encoder manufactured or
licensed and approved by Dolby Laboratories and bearing the Dolby Digital logo.
These encoders are available either from Dolby Laboratories or Dolby
professional encoder licensees.
• The capture device for the Dolby Digital bitstream can be any computer with a
digital audio interface or equivalent.
• It is important that the monitoring environment for Dolby Digital professional
encoding includes a Dolby Laboratories Model DP562 professional reference
decoder. For more information, refer to Section 2.2, Monitoring Through a
Decoder, Section 4.11, Using the Dolby Model DP562 Professional Reference
Decoder, and Section 6.2, Dolby Digital Professional Decoders.
Figure 2-2 depicts a configuration in which a real-time Dolby Digital encoder
receives multichannel linear PCM audio and SMPTE time code from an MDM. The
encoder delivers a Dolby Digital bitstream and SMPTE time code data to the digital
audio input of a computer that is equipped with the appropriate hardware and
software. The Dolby Digital Recorder program for Windows 95 and Windows NT,
available from Dolby Laboratories, can capture and store the encoder output to a hard
drive. The recorded .ac3 file is compatible with all DVD authoring systems. A DP562
professional reference decoder is used for real-time monitoring of the Dolby Digital
bitstream.
2-3
Dolby Digital Professional Encoding Guidelines
Production Environment
Computer
with Digital Audio I/O
and REC/PLAY Capability
AES/EBU
Linear PCM
Modular
Digital
Multitrack
(MDM)
Dolby
Digital
Encoder
Dolby Digital
Bitstream
DP562
Dolby
Digital
Decoder
SMPTE Time Code
5.1-Channel
Analog or
Digital Output
Monitoring
Figure 2-2 Dolby Digital Recorded to a Computer
The configuration in Figure 2-3 is similar to the one in Figure 2-2 except that the
computer is not equipped with a digital audio input. In this case a DAT-Link+
(AES3/IEC 958 to SCSI) adapter converts the Dolby Digital bitstream to a SCSI
interface and delivers it to the SCSI bus of the computer. The rest of the configuration
is identical to that in Figure 2-2.
2-4
Dolby Digital Professional Encoding Guidelines
Production Environment
AES/EBU
Linear PCM
Modular
Digital
Multitrack
(MDM)
Computer
Dolby
Digital
Encoder
Dolby Digital
DAT-Link+
Bitstream
(AES3/IEC 958
to SCSI
Converter)
DP562
Dolby
Digital
Decoder
SMPTE Time Code
5.1-Channel
Analog or
Digital Output
Monitoring
Figure 2-3 Dolby Digital Recorded to a Computer with DAT-Link+
The configuration in Figure 2-4 depicts a real-time PCI card Dolby Digital encoder
installed in a computer. The PCI card (two- or 5.1-channel) can accept SMPTE time
code as well as AES/EBU or S/PDIF linear PCM audio input. In addition to being
stored to the hard drive of the computer for DVD authoring, the encoded output from
the card can be monitored by the decoder in real-time. The stored .ac3 file is
compatible with all DVD authoring systems. Dolby Digital professional encoder
cards are available from Dolby professional encoder licensees.
2-5
Dolby Digital Professional Encoding Guidelines
AES/EBU
Linear PCM
Modular
Digital
Multitrack
(MDM)
Production Environment
Computer with
Internal PCI
Real-time Encoder
Dolby Digital
Bitstream
DP562
Dolby
Digital
Decoder
5.1-Channel
Analog or
Digital Output
Monitoring
SMPTE Time Code
Figure 2-4 Dolby Digital Encoding Using a Licensed PCI Card
Figure 2-5 shows Dolby Digital professional encoder software running on a
computer. Like the PCI card, this solution is available from Dolby Laboratories
professional encoder licensees. Usually, the audio source material exists on digital
media as mono or stereo files. The computer encodes the linear PCM files and
outputs .ac3 files compatible with all DVD authoring systems. The software may be
able to control digital devices such as an MDM to capture the linear PCM files to a
hard drive for later encoding. Monitoring of the .ac3 files is done after the encoding
session is complete. The .ac3 files played from the digital audio card in the computer
are monitored using a DP562 decoder.
2-6
Dolby Digital Professional Encoding Guidelines
AES/EBU
Linear PCM
Production Environment
Computer with Audio
Card and Internal
Software Encoder
Dolby Digital
Bitstream
Modular
Digital
Multitrack
(MDM)
DP562
Dolby
Digital
Decoder
5.1-Channel
Analog or
Digital Output
Monitoring
Linear
PCM
Files
Figure 2-5 Dolby Digital Encoding Using Licensed Computer Software
2.2
Monitoring Through a Decoder
Dolby Digital decoders are divided into two fundamental categories: consumer and
professional. It is important to monitor the Dolby Digital encoding process using a
professional reference decoder such as the Dolby Model DP562, which also includes
Dolby Surround Pro Logic decoding. Unlike consumer decoders, a professional
reference decoder affords the user greater flexibility as well as the critical diagnostic
capabilities essential in a production environment. One such feature is the ability to
obtain real-time information on the effects of various parameter settings such as
2-7
Dolby Digital Professional Encoding Guidelines
Production Environment
Dialog Normalization and Dynamic Range Control (DRC). Another is the capability
to emulate any type of consumer decoder on the market whether it is a DVD player,
an A/V receiver, an HDTV, or a set-top box (STB). Since most consumer decoders
have some Dolby Digital features (Dialog Normalization, Dynamic Range Control,
downmixing, etc.) preset at the factory, it is critical that the encoding engineer use a
professional reference decoder to allow monitoring of all possible decoding options.
A professional reference decoder also offers a rack-mount style chassis, professional
electrical connections (XLR, AES/EBU, etc.), and comprehensive bass management
controls to accommodate various monitoring configurations. For further information,
refer to Section 4.11, Using the Dolby Model DP562 Professional Reference Decoder.
2.3
Room Layout, Monitoring, and Calibration
There are many standards and accepted practices as well as different opinions on
room layout, monitoring, and calibration in multichannel production and encoding
environments. Designs and implementations, therefore, can vary depending on
application, speaker selection, and personal preference. Refer to the 5.1-Channel
Production Guidelines available on the Dolby web site at www.dolby.com for
additional information.
2-8
Dolby Digital Professional Encoding Guidelines
Chapter 3
Consumer Decoder Products
It is important to note that parameter values set during the Dolby Digital professional
encoding process have a direct correlation to decoder behavior. Simply put, encoding
is essentially performed for the decoder. This chapter will help the encoding engineer
to better understand this relationship.
3.1
Categories
Consumer products incorporating Dolby Digital decoders are classified in two
fundamental categories: Source and Decoder. Requirements for a particular feature
can differ depending on whether the product is a Source product or Decoder product.
3.1.1
Source Products
Products designed with the primary purpose of decoding signals from one delivery
format, usually the particular medium and format supported by the product, are
classified as Source products. Source products receive and decode only specific Dolby
3-1
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Digital bitstream sources and have the source built into the product. Source products
must support all bitstream parameters allowed by the particular delivery format.
3.1.2
Decoder Products
Products designed with the primary purpose of decoding bitstreams from external
sources are classified as Decoder products. Because these products must accept
bitstreams from many different sources, Decoder products are generally required to
accept the full range of Dolby Digital bitstream parameters.
3.1.3
Channel Output Categories
Products are further categorized by the number of output channels provided: twochannel products and multichannel products.
Two-Channel Products
This category includes two-channel stereo DVD players, DTV sets, or set-top boxes
for satellite, cable, or DTV conversion.
All two-channel decoders use Dialog Normalization and require Line Mode Dynamic
Range Control (DRC) capability. Source products with RF modulation offer RF
Mode processing. Other modes are optional to the product designer. All two-channel
products offer Lt/Rt downmix mode; Lo/Ro downmix mode is optional.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Multichannel Decoders
This category includes multichannel A/V amplifiers, receivers, control centers, and
preamplifiers. All multichannel decoders include basic bass management capabilities.
Multichannel Adapters
This is a simplified type of decoder for adding Dolby Digital capability to an existing
Dolby Surround Pro Logic system. The end result meets all the same basic
requirements as those for a complete multichannel system.
3.2
Features
Table 3-1 provides a summary of consumer decoder product features.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Table 3-1 Consumer Decoder Product Features
Two-Channel
Decoder
Multichannel
Decoder
Multichannel
Adapter
Multichannel
DVD Player
Lt/Rt Downmix
"
"
"
"
"
"
"
"
"
"
"
"
Lo/Ro Downmix
optional
Optional
optional
optional
Bass Management
"
"
"*
Dolby Surround
Pro Logic
"
optional
optional
Feature
Line Mode
Dialog Normalization
3.3
Comments
* Simplified
design option
Supported Data Rates
Consumer sources of Dolby Digital bitstreams include NTSC laser disc (LD); digital
cable and satellite; digital television (DTV), encompassing standard definition
television (SDTV) and high definition television (HDTV); and digital versatile disc
(DVD). The maximum data rates for these formats are shown in Table 3-2.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Table 3-2 Dolby Digital Bitstreams Available from Existing Formats
Format
Sample Rate
Data Rate (max)
Laser Disc
48 kHz
384 kb/s
DTV
48 kHz
384 kb/s*
Digital Cable
48 kHz
448 kb/s
Digital Satellite
48 kHz
448 kb/s
DVD-Video
48 kHz
448 kb/s
* A proposal to change the ATSC maximum rate to 448 kb/s has been made.
DVB systems can employ any sample rate with a maximum data rate of 640 kb/s.
Decoders built into any of these source formats are only required to support the
sample rate and maximum data rate of that format. Decoders with an IEC 61937
(S/PDIF) input for Dolby Digital bitstreams must be able to accept data rates up to
640 kb/s, and sample rates of 48, 44.1, and 32 kHz, to allow for the possibility of new
delivery formats. This requirement does not apply to ATSC-compliant DTV sets,
which only need support data rates through 448 kb/s at the 48 kHz sample rate.
3.4
Compatibility
The same encoded multichannel content must play successfully on all decoders in the
different product categories. Refer to Figure 3-1.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Example
Source Product
Digital
Pass
Through
Disc
Internet
Broadcast
Digital
Home Theater
5.1-Channel
Discrete
Analog
Home Theater
A single bitstream
delivered to many receivers
Dolby
Digital
Encoder
Example
Decoder Product
Lt/Rt
Downmix
Dolby
Surround
Pro Logic
Hi-Fi VCR
Stereo
Downmix
Stereo,
Headphones
Mono
Downmix
TV RF
(antenna)
input
Figure 3-1 Audio Reproduction Hierarchy
Notes to Figure 3-1
(a) Discrete Multichannel: The encoded bitstream can be passed through to an A/V system with a 5.1-channel Dolby Digital
decoder. It is also possible to find DVD players that provide full 5.1-channel decoding capability.
(b)
Surround Downmix (Lt/Rt): The bitstream can be downmixed to a two-channel Dolby Surround Lt/Rt compatible format
using a preset downmix formula. This downmix can be played over a stereo system, decoded by a Dolby Surround Pro
Logic decoder, or recorded onto a VCR for later use.
(c)
Stereo Downmix (Lo/Ro): The bitstream can be downmixed to a two-channel stereo format using a defined downmix
formula with Center and Surround mixing level options. This downmix can be played over a stereo system or headphones.
(d)
Mono Downmix: The bitstream can be downmixed to a mono format. This downmix can be output from, for example, an
RF remodulator in a set-top box. It is equivalent to the Lo/Ro downmix summed to mono.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
While all decoders must accept bitstreams made in any Audio Coding Mode (also
referred to as Channel Mode) from mono through 5.1, there are options for how these
audio programs are processed. In some cases the signals are presented exactly as created,
in some cases the signals are downmixed, and in some cases the signals are upmixed by a
matrix decoder such as in Dolby Surround Pro Logic or Dolby 3 Stereo. Table 3-3 shows
how the various audio program configurations are processed with typical decoding
modes. The letter designations correspond with the notes for Figure 3-1.
Table 3-3 Output Signals from Various Program Content and Decoding Modes
Final Reproduction Mode
(b) Dolby
Dolby
Surround
(c) Stereo
3 Stereo
Pro Logic
$ 3/1 %
$ 3/0 %
$ 2/0
Delivered
Program
Content
(a) Discrete
Multichannel
3/2
# 3/2
3/1
# 3/1
$ 3/1 %
$ 3/0 %
$ 2/0
$ 1/0
3/0
# 3/0
$ 3/1 %
$ 3/0 %
$ 2/0
$ 1/0
2/2
# 2/2
$ 3/1 %
$ 3/0 %
$ 2/0
$ 1/0
2/1
# 2/1
$ 3/1 %
$ 3/0 %
$ 2/0
$ 1/0
2/0
# 2/0 or 3/1 %
3/1 %
3/0 %
# 2/0
$ 1/0
1/0
# 1/0
1/0 %
1/0 %
m 2/0
# 1/0
#
$
m
%
(d) Mono
$ 1/0
Denotes a delivered signal passed directly to the output channels.
Denotes a delivered signal downmixed before reproduction.
Denotes a mono program reproduced by two channels.
Denotes a matrix decoder upmixed from a two-channel signal to derive the output channels.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
The Discrete Multichannel column assumes that the Dolby Digital bitstream is being
passed through to a multichannel Decoder product. Because a multichannel Decoder
product also contains Dolby Surround Pro Logic, a 2/0 program can either be
reproduced directly as 2/0 or matrix decoded to produce a 3/1 output.
The Dolby Surround Pro Logic and Dolby 3 Stereo columns assume that the Lt/Rt
downmix from a two-channel Source product is being passed to a separate Dolby
Surround Pro Logic or Dolby 3 Stereo decoder. No consumer product is required to
downmix multichannel programs to Lt/Rt and apply Pro Logic decoding at the same
time. In the case of a 1/0 program, the two-channel Source product upmixes the 1/0
program to a 2/0 output. When this 2/0 output is matrix decoded, the result is again a
1/0 output.
To distinguish stereo signals from Dolby Surround Lt/Rt signals, the Dolby Digital
bitstream can carry a flag to indicate the format of 2/0 encoded programs. Decoders can
use this to drive a Dolby Surround indicator, or it can be used to automatically control the
Dolby Surround Pro Logic decoder. Decoders may ignore and/or override this flag, so the
user has final control over the stereo or Dolby Surround listening modes.
Decoders output 1/0 mode (mono) signals to either the Center channel or Left and
Right channels when the Center channel is not available. It is unnecessary and
therefore not recommended to encode mono signals in 2/0 mode.
High-end home theater systems often reproduce the program with as much of the
original quality as possible, and use neither downmixing nor Dynamic Range Control.
Many listeners use a form of downmixing whenever the number of delivered channels
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
exceeds the number of channels in the Dolby Digital decoder. When downmixing,
stereo products operate in Line Mode. Refer to Section 3.6, Operational Modes, for
more information.
It is crucial to set Dialog Normalization correctly to avoid the unwarranted
application of Dynamic Range Control for peak overload protection. When selecting
a Dynamic Range Compression profile (also referred to as a preset) during the
encoding process, e.g., film, music, etc., it is important that it matches the program
style and provides the intended result. Setting these parameters carefully ensures the
average reproduction loudness is consistent, and that the Dynamic Range Control
process operates with the correct program thresholds. Refer to Section 4.2, Features,
and Appendix C, Dynamic Range Control (DRC), for more information.
When downmixing multichannel programs to mono or Lo/Ro stereo, the relative
balance between the source channels is sometimes affected. This can be adjusted to
some degree with the Center Mix Level and Surround Mix Level encoder settings.
When downmixing multichannel programs to Dolby Surround compatible format, any
Surround signals that are correlated with the front signals, as in a front-back pan,
must be phase shifted to ensure the Lt/Rt downmix will decode properly. A ninetydegree phase shift is provided in the encoder. In the vast majority of cases, this filter
is inaudible with discrete reproduction.
Whenever downmixing takes place, the LFE signal is discarded. Essential LFE
program content must be included in the main Left and Right channels to ensure that
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
it will be heard by all listeners. The LFE channel is never required in a program, and
is not an option in mono or stereo modes.
3.5
LFE and Bass Management
Multichannel Dolby Digital decoders offer bass management systems with many
options including the ability to redirect bass from any channel where the speaker is
unable to reproduce it to the subwoofer. The Low-Frequency Effects (LFE) signal is
included in the total signal feeding the subwoofer. Many decoders offer great
flexibility in the setting of the bass management options and crossover filters. This
ensures that full-frequency content of all the channels in the audio program can be
heard. Professional decoders include the same bass management features so it is
possible to hear the signals reproduced in the same manner as the home theater
listener does. There are two bass management configurations required for consumer
products in the Decoder category. Figure 3-2 and Figure 3-3 illustrate these
mandatory configurations.
Two-channel products can accept any valid Dolby Digital bitstream, including
multichannel programs. Programs with more than two channels are automatically
downmixed as described in Section 3.4. The LFE channel, if present in the encoded
audio program, is always omitted during playback from a two-channel product.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
L
L
C
C
R
R
LS
LS
DOLBY DIGITAL
ONLY
RS
RS
-15 dB x 5
LFE
-5 dB
+
ANALOG
GAIN
SUB
+15 dB MAX
Figure 3-2 Decoder Product Bass Management Configuration One
-12 dB
L
+
L
-1.5 dB
LEVEL
ADJ
C
C
-12 dB
R
+
R
-1.5 dB
LS
LEVEL
ADJ
DOLBY DIGITAL
ONLY
LEVEL
ADJ
RS
LS
+
RS
-15 dB x 3
LFE
-5 dB
+
SUB
(OPTIONAL)
Figure 3-3 Decoder Product Bass Management Configuration Two
Dolby Digital Professional Encoding Guidelines g
3.6
Consumer Decoder Products
Operational Modes
To ease the design of consumer and professional decoder products, Dolby Digital
integrated circuits (ICs) offer standard Operational Modes (also referred to as
Dynamic Compression Modes) called Line Mode and RF Mode. This greatly
simplifies the implementation of Dialog Normalization, Dynamic Range Control, and
downmixing functions, all of which are necessary in Dolby Digital products.
Custom Modes are also available although rarely implemented. Custom Modes 1 and
0 can be used in addition to or instead of Line Mode to offer additional or enhanced
functionality. Settings equivalent to the required settings must still be provided.
3.6.1
Line Mode
Summary of Line Mode features:
• Dialog Normalization enabled
• Dialog reproduced at a constant level of -31 dBFS LAeq (3 dB lower in each
channel when downmixed to two-channel or mono); refer to Section 4.2.1, Dialog
Normalization (dialnorm), for more information.
• dynrng compression variable used for Dynamic Range Control
• High-level cut compression scaling allowed when not downmixing
• Low-level boost compression scaling allowed
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Line Mode is applicable in the widest range of products due to its flexibility and ease
of use. All line level or power amplified outputs from two-channel set-top decoders,
two-channel televisions, 5.1-channel televisions, A/V Surround decoders, and
outboard Dolby Digital adapters should be derived from this mode.
Figure 3-4 shows the signal relationships of Line Mode under different conditions.
Note that whether or not downmixing or Dynamic Range Control is active, the
average program loudness remains constant.
5.1-CH
MOVIE WITH
DIALNORM
5.1-CH WITH
VARIABLE
COMPRESSION
5.1 TO TWO-CH
DOWNMIX
0 dBFS
-10 dBFS
MIN
HIGH-LEVEL
COMPRESSION
MAX
-20 dBFS
-30 dBFS
DIALOG
DIALOG
-40 dBFS
-50 dBFS
MAX
-60 dBFS
LOW-LEVEL
COMPRESSION
dBFS
MIN
Figure 3-4 Signal Relationships in Line Mode
Dolby Digital Professional Encoding Guidelines g
3.6.2
Consumer Decoder Products
RF Mode
Summary of RF Mode features:
• Dialog Normalization enabled
• Dialog reproduced at a constant level of -20 dBFS LAeq (3 dB lower in each
channel when downmixed to two-channel or mono); refer to Section 4.2.1, Dialog
Normalization (dialnorm), for more information.
• compr compression variable used for Dynamic Range Control (dynrng used if
compr does not exist)
• High- and low-level compression scaling not allowed (always fully compressed)
• +11 dB gain shift to raise overall program level
RF Mode is optimized for products (i.e., set-top boxes) that generate a downmixed
signal for transmission to the RF (antenna) input of a television set. The overall
program level is raised 11 dB, while the peaks are limited to prevent signal overload
in the D/A converter. By limiting headroom to a maximum of 20 dB (3 dB greater in
each channel when downmixed to two-channel or mono) above average dialog level,
severe overmodulation of television receivers is prevented while providing a dialog
RF modulation level that compares well with quality television broadcasts and
premium movie channels. Figure 3-5 shows the signal relationships of RF Mode
under different conditions. Note that whether or not downmixing is active, the
average program loudness remains constant. Dynamic Range Control remains fully
on at all times.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
5.1-CH
MOVIE WITH
DIALNORM
0 dBFS
-10 dBFS
5.1-CH TO MONO
DOWNMIX
VALID
PEAK
OUTPUT
RANGE
-20 dBFS
DIALOG
-30 dBFS
+11dB
GAIN
SHIFT
-40 dBFS
-50 dBFS
MAX
LOW-LEVEL
COMPRESSION
-60 dBFS
dBFS
Figure 3-5 Signal Relationships in RF Mode
Refer to Figure 3-6, which shows an example of how a signal generated with RF and
Line Modes can relate to the modulation index of a typical RF modulator circuit.
While Line Mode can be used for this purpose, the improvement in program
dynamics comes with a lower average loudness than other television signal sources.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
RF MODE
5.1-CH TO MONO
DOWNMIX
0 dBFS
200%
+6 dBFS
100%
-12 dBFS
50%
-20 dBFS
20%
-26 dBFS
10%
-31 dBFS
6%
-36 dBFS
3%
LINE MODE
5.1-CH TO MONO
DOWNMIX
DIALOG
DIALOG
RF MOD
MAX
LOW-LEVEL
COMPRESSION
MAX
LOW-LEVEL
COMPRESSION
MIN
Figure 3-6 RF Modulator Signal Levels
3.6.3
Using Operational Modes in Products
More than one Operational Mode can be used in a product, and they can be used in
different ways depending on the type of product. Line Mode is intended for products
providing line level or power amplified outputs and is used in the majority of
products. RF Mode is intended primarily for products generating a downmixed signal
delivered to the RF (antenna) input of a television set, but can also be used in
conjunction with Line Mode to provide additional listening options.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Dynamic Range Control (DRC)
Dynamic Range Control (DRC) is required in all Dolby Digital products, but the
degree of user control differs depending on the product type. Some products require
Dynamic Range Control to suit the particular listening situation for which it was
designed. Other products support a variety of listening situations and offer various
dynamic range compression settings with the ability to set and store preferences.
Dynamic Range Control (DRC) for a Source Product
Table 3-4 shows the Dynamic Range Control (DRC) requirements, recommendations,
and options for a typical Source product.
Table 3-4 DRC for a Typical Source Product
Setting
Optional
Required
Recommended
Maximum dynamic range
Standard dynamic range
Minimum dynamic range
Operational
Mode
Scale Factors
(high/low)
Line
Line
RF
0.0/0.0
1.0/1.0
no scaling allowed
Gain
Correction
Required
None
None
None
A Source product, such as a cable or satellite receiver, or DVD player, needs the
ability to conform its audio output to traditional signal references, such as VHS Hi-Fi
tapes or broadcast television signals. The standard dynamic range setting is required
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
in all Source products and results in audio signals with good dynamic range, much
like a prerecorded VHS Hi-Fi movie. The majority of users who connect the audio
outputs to a Dolby Surround Pro Logic system do so with this setting. For example, it
may be the mode of choice for people who connect the audio to a Dolby Surround Pro
Logic system.
Other users may feel that the average loudness of the signal is too low when
switching between Dolby Digital programs and regular television programs, or that
the loud portions of the program are too loud compared with the average dialog level.
In these cases, the minimum dynamic range setting is recommended as it will raise
the average loudness of the dialog and restrict the program peaks, much in the style of
conventional television audio.
The optional maximum dynamic range setting is meant for use in multichannel
Source products to provide the widest dynamic range when the product is not
downmixing. Products that must downmix to two channels will not be able to
reproduce the full dynamic range in Line Mode because there are restrictions to
prevent digital overload.
For Source products that offer an RF modulated output in addition to line outputs, the
minimum dynamic range setting based on RF Mode becomes a requirement instead of
a recommendation, as shown in Table 3-5.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Table 3-5 DRC for a Source Product with RF Modulated Output
Optional
Maximum dynamic range
Line
0.0/0.0
Gain
Correction
Required
None
Required
Standard dynamic range
Minimum dynamic range
Line
RF
1.0/1.0
no scaling allowed
None
None
Setting
Operational
Mode
Scale Factors
(high/low)
Source products that offer only an RF modulated output with no other outputs can
reduce the DRC requirement to just the minimum dynamic range setting based on RF
Mode.
Dynamic Range Control (DRC) in Decoder Products
Table 3-6 shows the Dynamic Range Control (DRC) requirements and recommendations
for a typical Decoder product.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Table 3-6 DRC for a Typical Decoder Product
Setting
Required
Recommended
Maximum dynamic range
Standard dynamic range
Minimum dynamic range
Operational
Mode
Scale Factors
(high/low)
Line
Line
RF
0.0/0.0
1.0/1.0
no scaling allowed
Gain
Correction
Required
None
None
-11 dB
A Decoder product used in a home theater setting must be able to adapt to different
listening situations and user preferences. Both the standard and maximum dynamic
range settings are required so that the user has the ability to reproduce the audio
program with either the full or limited dynamic range intended by the producer.
The minimum dynamic range setting is also recommended for situations such as late
night viewing at reduced volume levels wherein low-levels signals must be brought up
to be heard, but peak level must be brought down so as to not disturb others. A -11 dB
gain correction is required when using RF Mode in a Decoder product so that dialog is
reproduced at a level consistent with the Line Mode output.
If a manufacturer wants to offer more flexible control for adjusting the amount of
dynamic range compression that is applied, a linearly variable control that adjusts the
scale factors from 0.0 to 1.0 can be used, as shown in Table 3-7.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
Table 3-7 Alternative DRC for a Decoder Product
Setting
Required
Recommended
Variable compression
Minimum dynamic range
Operational
Mode
Scale Factors
(high/low)
Line
RF
(0.0-1.0)/(0.0-1.0)
no scaling allowed
Gain
Correction
Required
None
-11 dB
A single control can adjust both high- and low-level scaling in tandem, or there can
be separate controls for each. A step size of no less than 0.1 is recommended. For
example, with a step size of 0.2, a six-position control is possible. With a step size of
0.5, a three-position control is possible. This type of control must include both
maximum dynamic range (0.0/0.0) and standard dynamic range (1.0/1.0).
3.7
Laser Disc
Laser disc is unique in that it is an established format already capable of delivering
two-channel audio from the original analog FM tracks (AFM), and also from the twochannel 16-bit linear PCM digital audio tracks. Dolby Digital compatible laser disc
players are able to provide both conventional stereo audio and Dolby Digital
bitstreams without an internal Dolby Digital decoder. This is because both linear
PCM tracks remain available and the Dolby Digital bitstream will usually represent
the discrete multitrack version of the same audio program found in those tracks.
Dolby Digital Professional Encoding Guidelines g
Consumer Decoder Products
To add Dolby Digital bitstreams in a way that preserves as much of the existing
format as possible, the Right channel AFM area carries the Dolby Digital RF signal.
It is a QPSK modulated carrier at 2.88 MHz. The data is Reed-Solomon coded for
error correction. This RF signal has a separate output from the player for external
decoding, and is demodulated into a standard Dolby Digital bitstream in the S/PDIF
format for connection to Dolby Digital A/V decoders, as shown in Figure 3-7.
PCM
Audio
Data
2-CH
D/A
L
R
L*
FM 1 RF
AFM
Demod
FM 2 RF
Dolby Digital
RF Out
R**
Dolby Digital
RF In
PCM
or
Analog
Audio
Source
Selector
Dolby Digital
RF Demod
L
Analog
Audio
Outputs
R
IEC 61937
(S/PDIF)
Formatter
Outboard Demodulator
Dolby
Digital
Output
*The Left channel of the AFM Demod ouput contains a mono mix delivered by FM 1 RF
**The Right channel of the AFM Demod contains a Dolby Digital bitstream delivered by FM 2 RF
Figure 3-7 Basic Laser Disc Player Structure
Dolby Digital Professional Encoding Guidelines
Chapter 4
Encoding
4.1
General Information
4.1.1
Preparing the Source Delivery Master
When preparing the source delivery master, adhere to accepted standards and
practices to ensure proper Dolby Digital encoding.
One of the most common source delivery formats for Dolby Digital encoding is the
Hi-8 mm tape used in many popular Modular Digital Multitracks (MDMs). Digital
Audio Workstations (DAWs), open reel digital multitracks, and other formats are also
used for this application, although less frequently.
Channel-to-Track Allocation
Dolby encourages adopting the channel-to-track allocation described in the ITU-R
recommendation, Parameters for Multichannel Sound Recording, and in SMPTE
standard 320M. Track layouts depend on channel complement although tracks 1, 2,
and 3 are always channels Left (L), Right (R), and Center (C) respectively. Table 4-1
4-1
Dolby Digital Professional Encoding Guidelines
Encoding
shows one possible configuration. Since inclusion of the LFE channel is optional and
the listener determines its reproduction from a decoder, essential low-frequency
information should not be mixed exclusively to the LFE channel. Alternative
practices exist within various industries so it is important to check the source and
accompanying documentation.
Table 4-1 Channel-to-Track Layout Example
Channel
L
R
C
LFE
LS
RS
Lt
Rt
Track
1
2
3
4
5
6
7
8
Channel Levels
The following text is in accordance with the ITU-R recommendation and SMPTE
standard referred to in the above section.
For consumer and DVD production studios, relative channel levels assume each
speaker delivers identical acoustic sound pressure levels to the listener. This excludes
the LFE channel, which is intended for reproduction at +10 dB SPL (with respect to
the main channels within the same 3 Hz to 120 Hz passband). Assuming that a
Surround (S) signal is delivered to a single speaker and two Surround signals (LS,
RS) are each delivered to individual speakers, Surround levels should be identical to
those for the front channels.
4-2
Dolby Digital Professional Encoding Guidelines
Encoding
In film sound practice, stereo Surround channel levels are typically recorded +3 dB
relative to the front channels. This is done to compensate for the -3 dB Surround
levels (relative to the front channel levels) encountered in cinema audio monitoring
systems. Calibrating cinemas in this manner allows for compatibility with other
soundtrack formats. Soundtracks mixed in film rooms require selecting the “3 dB
Attenuation” option for the LS and RS channels in the Dolby Digital encoder to
compensate for the difference in calibration.
When the Surround channel is mono, allocate it to both tracks 5 and 6 with 3 dB of
attenuation applied to each signal. Use the following formula:
Track 5 = Track 6 = 0.707 * S
Follow this recommendation even when track 4 also contains the S signal, which
should always be at normal level on this track. Label the tape clearly to indicate
tracks 5 and 6 each contain the S signal at -3 dB relative to their normal levels.
Reference Levels
The standard reference level is -20 dBFS for digital recorders (0 VU for analog
recorders). This level is typically +4 dBm from professional consoles and -10 dBV
from semi-professional consoles. When transferring from analog 35 mm magnetic
film, attenuation and/or peak limiting may be needed to avoid digital overload. These
processes require selecting a different Dialog Normalization value in the encoder and
necessitate complementary gain recovery in the reproduction chain.
4-3
Dolby Digital Professional Encoding Guidelines
Encoding
A 30-second, 1 kHz alignment signal at -20 dBFS should appear on all channels at the
beginning of the source delivery master prior to program start. The finished master
should contain at least 30 seconds of "digital black" after the alignment signal and
before each subsequent program. Each title should begin with at least two seconds of
encoded digital black.
Documentation
Complete, clear, and accurate documentation should always accompany the source
delivery master used for Dolby Digital encoding. This information is important not
only when the master is in use but also for reference once it is archived. Dolby has
created Mix Data and Mastering Information sheets to facilitate proper documentation
or to use as a guide for creating similar documents. The two documents correspond
with the stages at which they are used in production. These sheets are included in
Appendix E, Mix and Mastering Data Sheets, and are available on the Dolby web site
under the Technical Information heading at www.dolby.com.
The Mix Data sheet provides concise information about the source media to all the
engineers on a project. Typically, it includes information on sampling frequency, bit
resolution, time code, track assignment, titles, and program start and stop times. In
addition to being documented on the form, all mix data information should be
duplicated and attached to the source delivery master.
The Mastering Information sheet provides documentation relevant to the mastering
engineer or authoring facility on source media, timing, and encoder settings as well as
4-4
Dolby Digital Professional Encoding Guidelines
Encoding
general notes. This sheet can be used as a setup guideline for proper Dolby Digital
encoding.
These documents do not guarantee success, but are a starting point for customizing a
document(s) specific to the task(s) you perform. If you have questions about these
documents, refer to Section 7.2, Contacting Dolby Laboratories.
4.1.2
System Operation
There are many issues to consider before beginning Dolby Digital encoding. When
setting parameters it is important to take into account the type of content and the
distribution media. For example, DVDs and laser discs require different encoding
parameter settings. Following is a brief description of some of the issues that production
and authoring engineers face when generating Dolby Digital encoded content.
Whether an engineer uses a real-time encoder or a non-real-time encoder significantly
affects the production process. Real-time encoders, although generally more
expensive, offer the ability to check and monitor the Dolby Digital bitstream as it is
being encoded, saving the engineer a significant amount of verification time. A nonreal-time encoder generally offers the capability of batch encoding so the engineer
can run multiple sessions overnight and automate the encoding process.
When encoding for non-real-time applications, the process results in a Dolby Digital
file referred to as an .ac3 file. An .ac3 file adheres to the standard file format defined
by Dolby Laboratories for Dolby Digital files. All DVD authoring systems and the
4-5
Dolby Digital Professional Encoding Guidelines
Encoding
majority of Dolby Digital encoding products on the market today are capable of
processing the .ac3 file format.
Audio/video synchronization is a concern that engineers need to address with every
production. Although the use of SMPTE time code can help synchronize the audio
and video components of a production, occasionally slight timing adjustments need to
be made after the material has been assembled. Although an engineer may author a
DVD with an exact match in time code, differences in decoder latencies can produce
a noticeable discrepancy in audio and video synchronization when playing the final
production disc. Most authoring tools today offer a way to emulate a DVD before
production in order to adjust for such problems.
Latency is an issue when encoding simultaneous audio and video in real-time for
broadcast or DVD authoring. Since every real-time encoder has an inherent latency, it
is important to analyze the latencies of both the audio and the video encoders and
make appropriate delay adjustments to match the elements. When encoding in realtime for the purpose of storing a file to disk, latency is irrelevant.
Most real-time encoders allow the user to start and stop an encode session at
predetermined time code points. For this to occur, the source material must carry
SMPTE time code in addition to the audio tracks. Punching in and out using time code
is advantageous during encoding since it eliminates the need to edit the .ac3 files later.
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4.2
Encoding
Features
An important aspect of Dolby Digital is that it caters both to the critical and to the
casual listener. The former may wish to hear precisely what the mixer heard in the
studio; the latter may want a processed form of audio resembling current broadcast
practice. Neither would, however, voluntarily choose a system requiring continual
adjustment of the volume control, as is demanded by the present mixed formats (CD,
cassette, FM, AM, TV, DVD, etc.).
Dialog Normalization, Dynamic Range Control (DRC), and downmixing are
interdependent features and thus during encoding they cannot be treated separately.
Since Dolby Digital appears as a sound format in many different media and the
listener will want to switch between these media without dramatic volume changes, it
is necessary to consider all types of programming. In some applications, e.g., set-top
boxes for cable and/or satellite distribution, these Dolby Digital features also permit
matching loudness with present analog broadcast sources.
The effects of Dialog Normalization, Dynamic Range Control, and downmixing
should be assessed during program origination, preferably by monitoring through a
Dolby Digital encoder/decoder, so that the mixer can simulate worst-case as well as
best-case listening conditions.
Dynamic Range Control cannot be properly applied or assessed without correctly
setting Dialog Normalization, since some of the Dynamic Range Control parameters
depend upon this value.
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A note about terminology: Encoding parameters affect the presentation of the audio
program. During encoding, parameter values are embedded into the bitstream that
contains the coded audio. SMPTE and other technical organizations have adopted a
special term for data that is packaged or transmitted with program material, metadata.
The term loosely means “data about the data” and is intended to distinguish program
material, referred to as the audio or video essence, from the data that controls or
describes it. In recent technical papers and presentations, Dolby Laboratories has
begun referring to these encoding parameters (Dialog Normalization, Dynamic Range
Control, etc.) using the general term metadata.
4.2.1
Dialog Normalization (dialnorm)
For the purpose of Dialog Normalization (also referred to as volume normalization)
loudness is currently quantified using the equivalent loudness method LAeq, the longterm average of A-weighted sound pressure. The LAeq measurement correlates more
closely with subjective loudness but yields figures lower than VU meter readings.
The most useful measure for dialog level is the ratio of the LAeq measurement to
digital full-scale. Readings taken in this manner are noted as dBFS LAeq.
The following examples assume that program material entering the Dolby Digital
encoder has the dynamic range it received at its source: It is as the producer intended
and has not been further processed. A further assumption is that Line Mode is
employed with no downmixing.
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Consider a listener switching between a news bulletin, a wide-range movie, rock
music, and a symphony orchestra. These may be different TV channels or recordings,
or successive items from one source. In order of magnitude, these items will have
loudness values of about -14, -28 (the dialog), -8, and -25 dBFS LAeq. Thus if a
listener sets playback level (using a volume control) to the news bulletin and then
switches to the movie, its dialog will be about 14 (28–14) dB quieter than the voice of
the newsreader, and probably unintelligible. Conversely if the listener sets a playback
level appropriate for the movie, the rock music will be reproduced 20 (28–8) dB
higher than the dialog, probably intolerably loud. In fact with these items the typical
listener will need to adjust the playback level over at least a 15 dB range.
Note that the quietest source is the movie dialog. For many years, movie mixers have
used a standardized acoustic level for dialog; with digital formats this is equivalent to
between -25 and -31 dBFS LAeq. Since movies constitute an important part of the
material to be conveyed by Dolby Digital, this standardization is retained, and all
dialog should emerge from a Dolby Digital decoder at about -31 dBFS LAeq. The
average level of programming other than dialog should be adjusted appropriately
relative to dialog.
The Dolby Digital encoder sends a control word called dialnorm to command the
decoder to adjust the playback level. In other words, dialnorm acts as an automatic
volume control. In the example above, dialnorm needs to command about 17 dB of
attenuation on the news bulletin, only 3 dB on the movie, perhaps 15 dB on the rock
music (so that it is a little louder than speech), and about 6 dB on the symphony
orchestra.
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For speech, the dialnorm figure is the equivalent loudness level with respect to fullscale, and the attenuation introduced in the decoder is (31 + dialnorm) dB with
dialnorm being negative. Thus speech with an LAeq of -31 dBFS should have -31
entered; this commands 0 dB of attenuation in the decoder. Similarly, the news
bulletin with speech at -14 dBFS LAeq requires a dialnorm setting of -14, giving 17
dB of attenuation so that speech from the newsreader comes out of the decoder at -31
dBFS, matching the movie dialog.
If the source material is recorded at a lower level resulting in peaks that do not
approach digital full-scale, less attenuation is needed. Thus if the news bulletin used
in the example above had a loudness of -20 dBFS LAeq rather than -14 at its source, a
dialnorm setting of -20 would yield the standard level (-31 dBFS) at the decoder
output and a match other speech.
The desired attenuation (volume normalization) is performed at the decoder under the
control of dialnorm sent from the encoder. The actual audio data is not modified, and
therefore the original program level is available at the decoder. Most if not all Dolby
Digital consumer decoders automatically normalize the playback volume using the
Dialog Normalization feature.
In general there can be no default setting for dialnorm; the value depends on the
nature of the program, and in the context of mixed programming it is essential for the
setting to change from item to item. For a channel with uniform material, a fixed (but
appropriate) setting may be acceptable. Generally a setting for dialnorm of -31 is
unusual, required only for a few unprocessed wide-range movie soundtracks. For
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typical broadcast material (speech and popular music), the setting lies more often in
the range of -15 to -20.
In the future, Dolby Laboratories expects that the program material arriving at the
Dolby Digital encoder used for broadcast transmission will be accompanied by
metadata containing appropriate values for dialnorm (dynrng and compr as well), so
that no manual intervention at the emission stage will be required. Currently,
parameter values such as dialnorm must be set at the Dolby Digital encoder.
For programs including speech, the correct figure for dialnorm is the dBFS LAeq
value of that speech. Thus an objective setting involves measuring LAeq and entering
the result at the encoder. For non-speech items, it may initially be desirable to ask a
number of people to listen to speech at the standard level and then to adjust the music
volume to their taste; the amount of the adjustment can then be used as an offset to
the speech setting.
The above examples apply to the Line Mode output of the decoder. Dolby Digital
decoders can also operate in RF Mode, delivering an output intended primarily to
feed TV sets or VCRs via an RF modulator, typically mono. It is desirable that Dolby
Digital sources reproduced in this manner match analog sources (broadcast and cable
channels and VCR recordings), and the potential wide dynamic range of Dolby
Digital is usually inappropriate. In RF Mode the decoder switches in an output boost
of 11 dB relative to line output modes, and applies Dynamic Range Control. For more
information, refer to Dynamic Range Compression for RF Outputs (compr), in the
next section. The standard gain setting for the RF modulator (digital full-scale =
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200% of nominal maximum broadcast modulation) and correct selection of dialnorm
typically provide such a match.
4.2.2
Dynamic Range Control (DRC)
An important feature of Dolby Digital is that it conveys audio unaltered in dynamics.
Unlike almost any previous broadcast medium, it therefore gives the listener the
option to hear the program as the mixer intended, even if that means that it goes from
scarcely audible to extremely loud.
Present analog broadcast processors force the program level towards full modulation
of the transmitter for a substantial portion of the time, eliminating most of the
dynamic range; an incidental benefit being an approximate normalization of listening
level. In other words, the same device that reduces the dynamic range determines the
average volume. With Dolby Digital there are no technical pressures to reduce
dynamic range, and mean or average volume is addressed by dialnorm. Thus in
Dolby Digital, the need for dynamic range compression can be considered
independently of average listening levels.
After discrepancies in absolute or average volume have been reduced by the
application of dialnorm, many program items require no further processing for nonideal listening conditions. Some program items, however, have too great a dynamic
range for some listeners. An obvious example is the movie soundtrack; if the volume
is set for satisfactory intelligibility of dialog, sounds such as explosions may be
unacceptably loud when reproduced in the home. Another example is symphonic
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music; if the volume is set for comfort in loud passages, very quiet ones may be lost
in the background noise. In contrast, news bulletins or rock music have little inherent
dynamic range, and provided their absolute levels have been set appropriately there is
no reason to apply dynamic range compression.
Dynamic Range Control incorporates both selectable dynamic range compression
(refer to Profiles later in this section) and automatic overload protection limiting.
Dolby Digital encoders generate control words, dynrng and compr, which can be used
in the decoder to compress and limit the dynamic range of a program. The Dynamic
Range Compression profile algorithm is based on a simple audio loudness
measurement. In contrast, overload protection limiting is based on the peak levels.
Dynamic Range Compression for Line Outputs (dynrng)
Dolby Digital encoders generate Dynamic Range Compression profile information in
accordance with one of a number of selectable algorithms. In Line Mode, this
information (along with overload protection limiting) is contained in control words
called dynrng, accompanying the full dynamic range audio and used in the decoder to
apply dynamic range compression at the listener’s option. Some consumer decoders
also offer the option of using partial dynamic range compression.
As mentioned above, some types of programming have little inherent dynamic range
and therefore remain close to their average most of the time. The Dynamic Range
Compression profile algorithms provided in Dolby Digital encoders have a null band,
an intermediate range of input levels where no gain or loss is applied. Thus
programming with narrow dynamic range is substantially unchanged. Above the null
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band, the algorithms command attenuation in the decoder; below, they command
amplification. The width of the null band, the degrees of high- and low-level
compression and the time constants depend on the selected algorithm.
Average program levels should lie within the null band. Dialnorm represents the
average loudness of the input signals. Hence the thresholds of the various Dynamic
Range Compression profile algorithms (for example, the bottom and top of the null
band) are set relative to the dialnorm reference playback level. Refer to Appendix C,
Dynamic Range Control (DRC), for more information.
Dynamic Range Compression for RF Outputs (compr)
RF Mode employs a different dynamic range compression control word, compr. This
operates similarly to dynrng, apart from the overload protection. Refer to Appendix C,
Dynamic Range Control (DRC), for more information.
Profiles
There are several Dynamic Range Compression profiles (also referred to as presets), each
with a name denoting the most suitable application. They all share the property of a null
band, a region in the middle of the dynamic range where the gain is fixed at unity, no
boost or cut. The ends of this null band are referred to the value of dialnorm so that
dialog or average program lies within the null band and is not subject to gain variation.
Unless the feature is disabled at the decoder, sounds quieter or louder than the average
and outside the null band are boosted or cut in accordance with the profile selected at the
encoder. This provides a reduction in the dynamic range of the reproduced audio. The
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width of the null band determines the proportion of time that program level lies
outside it, and therefore the actual amount of dynamic range compression.
One result of implementing dynamic range compression in this way is that program
material with an already restricted dynamic range, whether inherent or because of
prior processing, lies primarily within the null band and hence is not subjected to
further compression. Another outcome is that dialog does not modulate background
noises, at least between syllables. The resultant processing reduces dynamic range
without the audible side effects often associated with broadcast processors, gain
pumping and transient distortion.
Irrespective of the selected Dynamic Range Compression profile, the encoder assesses
the possibility of peak overload in the decoder. If necessary, the encoder overrides and
increases the high-level compression gain word, preventing output overload. Refer to the
section on Downmixing and Overload Protection for more information.
Currently, there are five Dynamic Range Compression profiles in addition to “None.”
These profiles are divided into three groups that are described below. For more
detailed information refer to Appendix C, Dynamic Range Control (DRC).
1. Film
Movie soundtracks contain dialog at a standardized level with respect to digital fullscale. Sounds are rarely much quieter than dialog (they would not be audible beneath
the typical background noise of a movie theatre), so film soundtracks only call for
modest degrees of low-level boost. In addition, raising low-level sounds excessively
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would sometimes reveal unwanted background disturbances, such as camera and
traffic noise, which the film mixer did not intend to be audible in the theatre. Sounds
are, however, frequently much louder than dialog, by 20 dB or more, so large
amounts of gain reduction may be required.
There are two film profiles, standard and light, with null bands 10 and 20 dB wide
respectively, straddling the dialnorm setting. In both cases, low-level boost is applied
using a 2:1 compression ratio, with a maximum boost of 6 dB. Above the null band,
the compression adopts a characteristic (20:1 ratio) close to limiting, except that it is
based on RMS, not peak.
2. Music
The dynamic range of music varies according to type. Most popular music has an
inherently limited dynamic range, and requires little or no compression. It does,
however, demand an appropriate setting for dialnorm to ensure that its absolute
loudness is not out of line with that of other programming. The dialnorm setting also
determines whether the music will lie within the null band of the Dynamic Range
Compression profiles.
As with film, there are two music profiles, standard and light, with null bands 10 and
20 dB wide respectively, straddling the dialnorm setting. Below the null band, both
profiles offer up to 12 dB of boost with a 2:1 compression ratio. Above the null band,
a standard profile gives 20:1 compression and light 2:1.
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3. Speech
While any one source of speech usually has a limited dynamic range and can be
easily accommodated inside a null band, some speech programming may include
moments that are abnormally loud or soft, such as shouts or whispers. The speech
profile uses a 10 dB null band for average speech. The correct setting of dialnorm
usually ensures that average speech lies within the null band; outside this region fairly
heavy compression is applied: 5:1 ratio up to 15 dB boost for low levels and 20:1
(near limiting) for high levels. This technique retains the impression of quieter or
louder speech while ensuring that non-average voices remain intelligible and do not
get excessively loud.
The speech profile may be inappropriate when large amounts of background audio
accompanies the speech, as gaps in the speech would boost the background by 15 dB.
In these circumstances, one of the film profiles may be more suitable.
Downmixing and Overload Protection (dynrng or compr)
When multiple sources are mixed together in a studio, the engineer adjusts the
relative gain of each source for the desired subjective balance and the overall gain for
the desired total output level.
In contrast, when a multichannel program is downmixed inside a Dolby Digital decoder,
the mixing coefficients are in general fixed. The decoder performs downmixing in the
digital domain (except from two-channel to mono), so there is the possibility that the
downmix will overload the output digital-to-analog converters (DACs).
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If the coefficients in the decoder were chosen to ensure that downmixes could never
overload the DACs, many downmixes would sound somewhat quieter than the same
program reproduced in a multichannel mode or a mono or stereo program that did not
require downmixing.
The actual mixing coefficients are therefore chosen to give a more satisfactory match
in output volume between downmixed and non-downmixed sources. As a result
downmixes could lead to output overload on the comparatively rare occasions that a
multichannel source approached digital full-scale on all channels simultaneously.
As described earlier, most programming demands volume attenuation within the decoder
via dialnorm. This attenuation is applied in the digital domain prior to downmixing, and
therefore reduces the probability of overload. In RF Mode, though, the 11 dB of boost
provided by the decoder does increase the probability of downmix overload.
Dolby Digital prevents overload by invoking overload protection limiting during the
encoding process. The encoder generates several possible downmixes, estimates the
worst-case peak level at a decoder output, taking into account the setting of dialnorm,
and separately calculates the gain reduction required in the decoder to prevent
overload in Line Mode and RF Mode. Optionally, the gain reduction for RF Mode
can take into account the pre-emphasis employed in RF modulators. Whenever one of
these values of gain reduction exceeds the gain reduction demanded by a selected
Dynamic Range Compression profile, if any, it is substituted for the compression
value in the dynrng or compr control word.
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To put overload protection in perspective, consider the extraordinarily improbable
case of a five-channel source that reached full-scale simultaneously in all channels;
this represents a sound roughly 30 dB higher than standard dialog level.
In Line Mode, due to the choice of fixed mixing coefficients, overload prevention
would require 11 dB of gain reduction from dialnorm and dynrng combined. Such a
source would obviously be very loud and would probably already have demanded
volume reduction via dialnorm and dynamic range compression via dynrng. If such
reduction was 11 dB or more, no protection limiting would be required. In practice,
overload protection limiting operates rarely, if at all, on real programs.
In RF Mode, this source could demand 22 dB of total gain reduction (dialnorm plus
compr). Considering that in analog broadcasting average speech is typically within a
few dB of full modulation, it is not surprising that a sound 30 dB louder demands
large degrees of dynamic range compression and overload protection limiting.
4.3
Metering
Metering is not available on all Dolby Digital encoders and is generally implemented
in software. There can also be meters other than those described in this section.
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4.3.1
Encoding
Input Level Meter
The meter marked Input Level shows the input signal level that the compressor is
using to determine the amount of boost or cut. The displayed signal level is derived
from the largest of the individual channel signal levels (i.e., the input level meters)
and incorporates the attenuation applied by the dialnorm value.
4.3.2
Line Mode Meter
The meter marked Line Mode shows the value of dynrng being sent in the bitstream.
This is the amount of Dynamic Range Control that the decoder applies if it is
configured in Line Mode, and no dynamic range compression scaling is applied.
Green means boost, red means cut, and the meter shows a range of -24 dB to +24 dB.
4.3.3
RF Mode Meter
The meter marked RF Mode shows the value of compr being sent in the bitstream.
This is the amount of Dynamic Range Control that the decoder applies if it is
configured in RF Mode. Green means boost, red means cut, and the meter shows a
range of -24 dB to +24 dB (although compr can take on values as large as +/- 48 dB).
Frequently the RF Mode meter reads about the same as the Line Mode meter,
indicating that dynrng and compr are being set to comparable values. If signal
conditions arise that would cause digital overload in the decoder, the protection
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circuits in the encoder activate and cause dynrng and compr to be further limited. In
these conditions, compr tends to be more limited than dynrng, and thus the RF Mode
meter shows a larger amount of cut than the Line Mode meter.
4.3.4
Calibration of Dialog Normalization (dialnorm)
The preferred method for determining the Dialog Normalization (dialnorm) value for
speech is to use an LAeq meter. When this is impractical or impossible, the Line and RF
Mode meters can be used as a starting point to calibrate dialnorm. With only speech
being sent through the encoder, and using the appropriate Dynamic Range Compression
profile (also referred to as a preset), an operator can adjust dialnorm until the meters read
0 dB or show reasonable amounts of boost and cut. The same procedure can also be used
for programs that do not contain speech, such as music. Subjective listening should then
be used to set the final value. It is hoped that in the future a more practical and effective
solution for metering will be available for this application.
Dolby Laboratories has created a Dolby Digital Dialog Normalization Disc with
examples that can be used as references to assist in subjectively determining the
appropriate dialnorm value. For more information on this disc, refer to Section 7.2,
Contacting Dolby Laboratories.
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4.4
Encoding
Parameter Default Values
Although Dolby Digital professional encoders typically have preset default values,
parameter settings nearly always require adjustment to appropriate values for
specific content and applications.
In addition to the Data Rate and Audio Coding Mode (also referred to as Channel
Mode), the encoding engineer should pay particular attention to the Dialog
Normalization, Dynamic Range Compression profile, Surround Channel 3 dB
Attenuation, and 90-Degree Phase-Shift parameter settings. In addition, Input/Output
Control should be verified to ensure the appropriate clock source, input channel
assignment mode, and output format selections.
Dolby Laboratories recommends the default values given in Table 4-2 for the
parameters that are required and therefore common to all licensed encoders. This is
not to say that the same default values appear in all encoders, even if they are from
the same manufacturer. For some parameters, two-channel encoder defaults differ
from those for a 5.1-channel encoder. Refer to Chapter 5, Applications and Formats,
for more information on appropriate parameter values for various applications.
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Table 4-2 Recommended Parameter Default Values
Audio Service
Data Rate — two-channel
192 kb/s
Data Rate — 5.1-channel
448 kb/s
Audio Coding Mode — LFE Channel, two-channel
2/0, LFE disabled or off
Audio Coding Mode — LFE Channel, 5.1-channel
3/2, LFE enabled or on
Bitstream Mode
Complete Main
Dialog Normalization
-27 dB
Bitstream Information
Center/Surround downmix levels
-3 dB
Dolby Surround Mode
Not indicated
Audio Production Information Exists
No or 0
Audio Production Information — Mixing level
25
Audio Production Information — Room type
Small room
Copyright Bit
Original Bitstream
Copyright protected or 1
Original or 1
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Table 4-3 Recommended Parameter Default Values - continued
Preprocessing
4.5
DC Highpass Filter
On
Channel Bandwidth Lowpass Filter
On
LFE Lowpass Filter
On
Surround Channel 90-Degree Phase-Shift — two-channel
N/A
Surround Channel 90-Degree Phase-Shift — 5.1-channel
On
Surround Channel 3 dB Attenuation — two-channel
N/A
Surround Channel 3 dB Attenuation — 5.1-channel
Off
Dynamic Range Compression profile
On or Film Standard
RF Overmodulation Protection (RF Pre-emphasis filter)
On
Audio Service
The parameters in the Audio Service specify the fundamental aspects of the Dolby
Digital encoded program. They include the Data Rate, Sampling Rate, Audio Coding
Mode, Bitstream Mode, and the Dialog Normalization value.
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4.5.1
Encoding
Data Rate
The total Dolby Digital bitstream data rate is set using the bitstream Data Rate
parameter. The bitstream Data Rate value determines which of the 19 pre-defined
Dolby Digital data rates will be used. In order to maintain high audio quality, data
rates that are supported by a Dolby Digital encoder depend on the selected Audio
Coding Mode parameter. In general, Audio Coding Modes that include fewer
channels in the bitstream have lower data rate limits. Table 4-4 shows examples of
the supported and suggested data rates as a function of the Audio Coding Mode. The
suggested data rate assumes normal high-quality audio. For speech, low-bandwidth
audio, or when constrained by standards, it may be appropriate to use lower data rates
than those suggested.
Table 4-4 Supported and Suggested Data Rates According to Audio Coding Mode
Audio Coding Mode
1/0
2/0 or 1+1
3/1 or 2/2
3/2
Supported Data Rates
Suggested Data Rate
56–640 kb/s
96–640 kb/s
192–640 kb/s
224–640 kb/s
96 kb/s
192 kb/s
384 kb/s
448 kb/s
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Encoding
Audio Coding Mode
The Audio Coding Mode (also referred to as Channel Mode) parameter defines the
number of main audio channels within the encoded bitstream and also indicates the
channel format. The Audio Coding Mode is designated as two numbers, m/n, with m
indicating the number of front channels, and n indicating the number of rear
(Surround) channels. If the mode is set to 1+1, then two completely independent
program channels (dual-mono), referenced as Ch1 and Ch2, are encoded into the
bitstream. Note that the 1+1encoding mode is not allowed in ATSC DTV format, nor
in the DVD-V format. If the program material is encoded as 1/0 (mono), decoders output
the signal to either the Center channel or both Left and Right channels depending on the
system configuration. Table 4-5 lists all eight modes and defines which input channel is
used for encoding based on the selected mode.
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Table 4-5 Audio Coding Mode
Audio Coding Mode
1+1
1/0
2/0
3/0
2/1
3/1
2/2
3/2
4.5.3
Channel Format
L/Ch1, R/Ch2
C
L, R
L, C, R
L, R, S
L, C, R, S
L, R, LS, RS
L, C, R, LS, RS
LFE Channel
The LFE Channel parameter enables or disables the Low-Frequency Effects (LFE)
channel. Use of the LFE channel is optional with multichannel programs, but is not
available for mono, stereo, or surround-encoded programs.
Two-channel Dolby Digital products, or multichannel products operating in a twochannel downmix mode, omit the LFE signal. Therefore, low-frequency content
essential to the program should never be mixed exclusively to the LFE channel. Refer
to Section 3.5, LFE and Bass Management, for more information on decoder handling
of LFE signals.
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Encoding
Bitstream Mode
The Bitstream Mode parameter indicates the type of audio service that the bitstream
conveys. Complete Main (CM) is the normal mode of operation and contains a
complete audio program including dialog, music, and effects. The CM and ME Main
Services can be further enhanced by means of Associated Services. The Bitstream
Modes and audio service types are listed in Table 4-6.
Table 4-6 Bitstream Mode/Audio Service Type
Bitstream Mode/Audio Service Type
Main Service: Complete Main (CM)
Main Service: Music and Effects (ME)
Associated Service: Visually-Impaired (VI)
Associated Service: Hearing-Impaired (HI)
Associated Service: Dialog (D)
Associated Service: Commentary (C)
Associated Service: Emergency (E)
Associated Service: Voice Over (VO) / Karaoke
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Encoding
Dialog Normalization (dialnorm)
The Dialog Normalization (dialnorm) value indicates how far the average dialog level
of the encoded program is below digital 100% full scale (0 dBFS). Valid settings are 1 dB to -31 dB. This parameter determines the audio reproduction level and affects
other parameters and decoder operation. Refer to Section 4.2, Features, for detailed
information on this subject. A thorough definition of the dialnorm parameter can be
found in ATSC document A/52, Digital Audio Compression Standard (AC-3) as well.
4.6
Bitstream Information
The parameters in this group directly relate to the Dolby Digital Bitstream
Information (BSI) fields. Definitions for BSI parameters follow.
4.6.1
Center Downmix Level
The Center Downmix Level parameter indicates the nominal Lo/Ro downmix level of
the Center channel with respect to the Left and Right channels. This parameter setting
does not affect Lt/Rt downmixes. Table 4-7 lists the valid values for Center downmix
level. This parameter appears in the bitstream only when three front channels are in
use, i.e., only when the Audio Coding Mode is set to 3/0, 3/1, or 3/2.
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Table 4-7 Center Downmix Level
4.6.2
Center Downmix Level
Mix Coefficient
-3.0 dB
0.707
-4.5 dB
0.596
-6.0 dB
0.500
Surround Downmix Level
The Surround Downmix Level parameter indicates the nominal Lo/Ro downmix level
of the Surround channel(s) with respect to the Left and Right channels (consistent
with the ITU BR specification). This parameter setting does not affect Lt/Rt
downmixes. Table 4-8 lists the valid values for Surround downmix level.
This parameter appears in the bitstream only when a Surround channel is in use, i.e.,
only when the Audio Coding Mode is set to 2/1, 2/2, 3/1, or 3/2. It is recommended that
the parameter be user-adjustable only when one of these modes has been selected.
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Table 4-8 Surround Downmix Level
4.6.3
Surround Downmix Level
Mix Coefficient
-3.0 dB
-6.0 dB
-999 dB
0.707
0.500
0.000
Dolby Surround Mode
The Dolby Surround Mode parameter indicates whether or not a two-channel Dolby
Digital bitstream is conveying a Dolby Surround encoded program. This information
is not used by the Dolby Digital decoding algorithm, but can be used by other
portions of the audio reproduction equipment, such as a Dolby Surround Pro Logic
decoder. Table 4-9 lists the valid values for Dolby Surround Mode.
Table 4-9 Dolby Surround Mode Indications
Dolby Surround Mode Indications
Not indicated
Not Dolby Surround encoded
Dolby Surround encoded
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In some cases, an operator finds this parameter user-adjustable even when an Audio
Coding Mode other than 2/0 has been selected. The Dolby Surround Mode indicator,
however, appears in the bitstream only when operating in the two-channel mode, i.e.,
only when the Audio Coding Mode is set to 2/0.
4.6.4
Language Code
The Language Code was intended to represent the language of the Dolby Digital
audio service. Typically, there are MPEG system elements that are used for indicating
the service language (language descriptors, for example). At this time there is no
known use for this code and consequently there may be no reference to it on a Dolby
Digital encoder user interface.
4.6.5
Audio Production Information Exists
The Audio Production Information Exists flag indicates whether the Mixing Level
and Room Type parameters explained below exist within the Dolby Digital bitstream.
4.6.6
Mixing Level
The Mixing Level informational parameter indicates the absolute Sound Pressure Level
(SPL) of the audio program as heard by the original mixing engineer. This information
makes it possible to replay the program at exactly the same loudness, or at a known
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difference in loudness. By knowing how much lower a program is played at home, for
example, it is now possible to apply the correct degree of loudness compensation.
The value for Mixing Level represents the theoretical loudness of a full-scale
(0 dBFS) tone in one channel.
There are two kinds of encoders in use: The newer ones with the Windows interface
provide a range of adjustment from 80 to 111, whereas the older ones range from 0 to
31. The actual data encoded into the bitstream is exactly the same in either case. The
examples below are for the newer style encoders. If the older one is used, subtract 80
from the result.
Example A: Measure the SPL (C-weighted) of pink noise at -20 dBFS in the center
channel. The reading is 85 dB. Set the encoder Mixing Level value to (85 + 20) = 105.
(For the older type of encoder, the setting will be 105 – 80 = 25)
Example B: Measure the SPL (C-weighted) of pink noise at -30 dBFS in the left
channel. The reading is 72 dB. Set the encoder Mixing Level value to (72 + 30) = 102.
(For the older type of encoder, the setting will be 102 – 80 = 22)
A second value for this parameter type becomes active when the Audio Coding Mode
is 1+1 and Audio Production Information Exists is set to 1, or yes. This second item
applies to the second independent channel (Ch2) residing within the bitstream. This
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parameter appears in the bitstream only when the Audio Production Information
Exists parameter is set to 1, or yes.
4.6.7
Room Type
The Room Type informational parameter indicates the type and calibration of the
mixing room used for the final audio mixing session. The Room Type value is not
normally used within the Dolby Digital decoder but can be used by other elements in
the audio system. Table 4-10 lists the valid values for Room Type. This parameter
appears in the bitstream only when the Audio Production Information Exists
parameter is set to 1, or yes.
Table 4-10 Room Type
Room Type
Not indicated
Large room
Small room
4.6.8
Copyright Bit
The Copyright Bit informational parameter sets the value of a single bit within the
Dolby Digital bitstream. If this bit has a value of 1, the information in the bitstream is
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indicated as protected by copyright. If it has a value of 0, the information is not
copyright protected.
4.6.9
Original Bitstream
The Original Bitstream informational parameter sets the value of a single bit within
the Dolby Digital bitstream. This bit has a value of 1 if the bitstream is an original. If
it is a copy of an original bitstream, it has a value of 0.
4.7
Preprocessing Options
The Processing Options parameters listed in this group are used to precondition the
audio input signals before they are encoded. Refer to Appendix D, Dolby Digital
Time-Domain Filters, for further information.
4.7.1
Digital De-emphasis
Dolby Digital encoders can allow activation of digital de-emphasis applied to the
linear PCM input signals whenever it is detected that the input has been preemphasized. Detection is typically achieved by monitoring the pre-emphasis flags
within the channel status data of the incoming digital audio signal (e.g., AES/EBU or
S/PDIF). Since the value of this parameter depends on some other parameter(s) or
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condition(s), it does not require explicit user control and can be adjusted
automatically by the encoder. This parameter is not available on the Dolby Model
DP561 encoder. When using the Dolby Remote software and the DP561 the Digital
De-emphasis status will always be indicated as “Off.”
4.7.2
DC Highpass Filter
This parameter can be used to activate the DC Highpass filter for all input channels.
The DC Highpass filter should always be enabled unless the encoding engineer is
absolutely sure that there is no DC in the input audio.
4.7.3
Channel Bandwidth Lowpass Filter
The Channel Bandwidth Lowpass Filter parameter can be used to activate a low-pass
filter with a cut-off near the specified audio bandwidth that is applied to the main
input channels. If the digital signal fed to the main input channels does not contain
information above the specified audio bandwidth, this filter can be disabled. This
parameter is not available on the Dolby Model DP561. It has also been disabled in the
Dolby Remote software for the DP561.
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4.7.4
Encoding
LFE Lowpass Filter
The LFE Lowpass Filter parameter can be used to activate a 120 Hz low-pass filter
applied to the LFE input channel. If the digital signal fed to the LFE input does not
contain information above 120 Hz, this filter can be disabled. This parameter is useradjustable only when the LFE channel is enabled.
4.7.5
Surround Channel 90-Degree Phase-Shift
The Surround Channel 90-Degree Phase-Shift feature is useful for generating
multichannel Dolby Digital bitstreams that can be downmixed in an external twochannel decoder to create a true Dolby Surround compatible output. This parameter is
user-adjustable only when Surround channels are present in the bitstream, i.e., only
when Audio Coding Mode is set to 2/1, 2/2, 3/1, or 3/2.
The 90-Degree Phase-Shift parameter should always be left enabled except under
specific conditions. These include, but are not necessarily limited to, system
calibration, encoding of certain test signals, and in the extremely rare case when the
discrete playback of highly coherent program material may be compromised.
4.7.6
Surround Channel 3 dB Attenuation
The Surround Channel 3 dB Attenuation function is useful for applying a 3 dB
attenuation to the Surround channels of a multichannel soundtrack created in a room
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with film style calibration, when encoding it for consumer home theater playback.
Cinema soundtrack Surround channels are mixed +3 dB relative to the front channels
in order to account for cinema calibration standards. Home theater Surround channel
gains are calibrated differently, and so a -3 dB adjustment to the Surround tracks is
necessary. This parameter is user-adjustable only when Surround channels are present
in the bitstream, i.e., only when Audio Coding Mode is set to 2/1, 2/2, 3/1, or 3/2.
4.7.7
Dynamic Range Compression Profile
The Dynamic Range Compression profile (also referred to as a preset) determines the
characteristic curve of the dynamic range compression algorithm. Dynamic Range
Control generates dynrng and compr gain words during the encoding process. A
Dolby Digital decoder uses these gain words to reduce the dynamic range of the audio
program during playback. This feature can be disabled on the decoder (except when
downmixing) by the user who desires program reproduction with the original
dynamic range. Depending on encoder implementation, Dynamic Range Compression
can be set by merely enabling the parameter or by selecting one of several built-in
profiles. These profiles include Film Standard, Film Light, Music Standard, Music
Light, and Speech. Refer to Section 4.2 Features, and Appendix C, Dynamic Range
Control (DRC), for detailed information on this subject. A thorough definition of
Dynamic Range Control can be found in ATSC A/52, Digital Audio Compression
Standard (AC-3).
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4.7.8
Encoding
RF Overmodulation Protection (RF Pre-emphasis Filter)
The RF Overmodulation Protection parameter determines whether or not an RF preemphasis filter is used in the overload protection algorithm to prevent RF
overmodulation in set-top box decoders. It is primarily used for broadcast
applications. This parameter is not available on the Dolby Model DP561 Dolby
Digital encoder. It has been disabled in the Remote Control software for the DP561.
4.8
Automatic Parameters
The values for the Automatic Parameters are adjusted automatically by the Dolby Digital
professional encoder.
4.8.1
Audio Bandwidth
In general, audio bandwidth is adjusted automatically within the encoder. Under most
circumstances, the encoder maintains the full audio spectrum bandwidth. As the data
rate is decreased below a certain value, however, it becomes useful to decrease the
audio bandwidth in order to maintain high audio quality. The optimum choice for a
bandwidth value depends on the selected Data Rate and the Audio Coding Mode.
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4.8.2
Encoding
Coupling
In general, coupling is adjusted automatically within the encoder. It is used in the
cases where more than one audio channel is being encoded in order to increase coding
efficiency by sharing information across the channels. Dolby Digital encoders employ
coupling primarily at lower data rates and only when the audio signals meet the
proper criteria.
4.9
Input/Output Control
The parameters in the Input/Output Control group relate to physical and electrical
characteristics of the input and output connections for a Dolby Digital professional
encoder. Controls of this type are by nature dependent on the particular encoder
application because there are many possible ways to connect audio signals to and
from an encoder product. The controls listed here are suggestions; there may be other
input and output controls needed for a given Dolby Digital encoder.
4.9.1
Sampling Rate
Dolby Digital supports three standard sampling rates: 48 kHz, 44.1 kHz, and 32 kHz.
The linear PCM audio signals that are input to the Dolby Digital encoder must be
sampled at one of these rates. Since the value of this parameter depends on other
parameters or conditions, it does not require explicit user control and can be adjusted
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automatically by the encoder. It is important that the sampling rate conform to that of
the final release medium, which is typically 48 kHz.
4.9.2
Input Channels
Dolby Digital professional encoders typically implement a means to configure the
mapping of the input signals to the proper Dolby Digital encoded channel assignment.
The number of input channels that are actually encoded depends upon the Audio
Coding Mode and the LFE Channel setting.
4.9.3
Output Format
Some Dolby Digital encoders may include an Output Format control. These
parameters determine the output bitstream configuration and include format, audio
bit, and SMPTE time code selections. Refer to Appendix B, Bitstream Format, for
more information.
4.10 Processing State Control
The parameters in the Processing State Control group influence the processing state
of the Dolby Digital encoder. The simplest example is a control for starting and
stopping the encoder. Some of the controls explained here are only relevant for
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specific encoder applications. One such example is a user control for recording the
Dolby Digital bitstream to a disk file. This function can be useful for an application
such as DVD authoring, where the encoded data is to reside as a file on a computer
disk, but would not be useful in broadcast encoder applications.
4.10.1
Start/Stop Encoding
Many encoders have a control for starting and stopping the encoding process.
4.10.2
Configuration Presets
Some encoders may offer configuration presets (not to be confused with Dynamic
Range Compression profiles that are also sometimes referred to as presets) as a way
to save and recall parameter settings for encoder applications. Due to the large
number of possible parameter combinations, it can be useful to store a particular
combination that can be recalled later, thus saving setup time and reducing the
possibility of an erroneous setting.
4.10.3
Time Code Control
Some applications may require the use of SMPTE time code for controlling certain
aspects of the Dolby Digital encoder operation. An example would be DVD
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authoring, where it is necessary to resolve synchronization between the separate audio
and video elements before they are multiplexed into a single system bitstream.
Note that this example does not require the SMPTE time code values to be
transmitted as part of the elementary bitstream. Typically, there are MPEG system
elements that are used for audio/video synchronization at decode time (e.g.,
presentation time stamp, PTS). At this time there is no known use for the syntactic
time code elements, which are optional in the BSI portion of the elementary Dolby
Digital bitstream. Instead, the time stamp values that are associated with each Dolby
Digital synchronization frame can be outside of the elementary bitstream.
4.10.4
Record/Play Bitstream
Some applications may require the capability of recording the Dolby Digital bitstream
as a computer disk file. In these applications controls for destination file name, mode
selection and record/playback are useful.
4.11 Using the Dolby Model DP562 Professional
Reference Decoder
It is important to use a professional reference decoder to monitor content for Dolby
Digital encoding. Dolby Digital offers many features to maintain backward
compatibility as well as to allow consumers the ability to customize their listening
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environment. Features such as downmixing and Dynamic Range Control need to be
checked at various stages of content creation and delivery to ensure they meet the
intent of the content provider as well as the needs of the consumer. The Dolby Model
DP562 professional reference decoder provides monitoring capabilities for these
parameters in addition to being able to simulate almost any listening environment.
4.11.1
Downmixing
Downmixing has two, frequently interrelated applications: format compatibility and
channel redirection.
1. Format Compatibility
Dolby Surround-compatible, stereo, and mono mixes are often created when
multichannel material is downmixed to fewer channels. It is important to check a
number of aspects of the downmix to confirm that it translates as closely as possible
to the original intent of the mix.
Many consumers listen to Dolby Digital sources such as DVD or DTV without a full
5.1-channel Dolby Digital playback system. These consumers hear the two-channel
analog or linear PCM outputs of their DVD players or DTV set-top boxes through
stereo or Dolby Surround Pro Logic systems. All DVD video players and DTV settop boxes have the ability to create and deliver a Dolby Surround compatible, stereo,
or mono downmix from the two-channel analog or linear PCM outputs. The DP562
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Professional Reference Decoder can simulate what the consumer hears when listening
in these modes.
Example 1: Using a properly calibrated 5.1-channel monitoring system
(incorporating appropriate bass management), set the DP562 to “Dolby Digital” and
“Full.” In this configuration, a 5.1-channel bitstream reproduces all channels as a
consumer with a Dolby Digital 5.1-channel system hears them. Pressing “Pro Logic”
on the DP562 downmixes the five main channels (discarding the LFE channel) to a
Dolby Surround-compatible bitstream. The downmix is then Dolby Surround Pro
Logic decoded resulting in Left, Center, Right, and mono Surround channels at the
outputs. Monitoring in this mode simulates how a consumer hears the 5.1-channel
bitstream when downmixed and then reproduced through a Dolby Surround Pro
Logic system.
Example 2: With “Pro Logic” still engaged, select “Stereo” instead of “Full” in the
“Listening Mode” section. This mode allows monitoring the 5.1-channel bitstream as
a consumer hears it when downmixed and then reproduced through a two-channel
stereo system. Refer to Figure 4-1, Dolby Surround Compatible Lt/Rt Downmix.
Example 3: In addition to Dolby Surround (Lt/Rt) compatible downmixes, Lo/Ro
downmixes can be checked. Selecting “Stereo” mode without “Pro Logic” engaged
creates an Lo/Ro downmix at the outputs. Refer to Figure 4-2, Stereo Compatible
Lo/Ro Downmix.
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C
R
LS
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Encoding
+
+
RS
LFE
“Lt”
+
“Rt”
-3 dB
+
+
-3 dB
INSIDE
DECODER
SET IN
ENCODER
NOT USED
Figure 4-1 Dolby Surround Compatible Lt/Rt Downmix
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SET IN
ENCODER
L
C
+
Lo
+
CMIX
R
+
LS
SMIX
RS
SMIX
LFE
+
Ro
INSIDE
DECODER
NOT USED
Figure 4-2 Stereo Compatible Lo/Ro Downmix
2. Channel Redirection
The ability to redirect channel information provides a means to account for the design
of and number of speakers in the listening environment.
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Some consumers may not have or cannot use all 5.1 speakers with their Dolby Digital
decoder. Dolby Digital consumer decoders can redirect or downmix decoded
multichannel information such as a 5.1-channel audio program.
Example 1: Using a properly calibrated 5.1-channel monitoring system
(incorporating appropriate bass management), set the DP562 to “Dolby Digital” and
“Full.” In this configuration, a 5.1-channel bitstream reproduces all channels as a
consumer with a Dolby Digital 5.1-channel system hears them. Pressing any of the
other “Listening Modes” causes the DP562 to redirect audio to the outputs of the
selected speaker configuration.
Example 2: Select “3 Stereo” instead of “Full” in the “Listening Mode” section and
the Surround channel information is redirected to the Left and Right speakers to
simulate a monitoring system with no Surround speakers. Select “Phantom Center”
and the Center channel information is redirected to the Left and Right speakers to
simulate a monitoring system with no Center speaker.
Technical Note: Summing multiple channels of audio, as occurs in downmixing, has
the potential for overloading the channel outputs. The DP562 applies the necessary
level scaling to prevent overload in the DSP processing. This results in an 11dB
attenuation of the AES/EBU audio outputs in “Custom” and “None” Dynamic
Compression Modes (also referred to as Operational Modes) when downmixing. The
attenuation is recovered in the analog outputs.
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4.11.2
Encoding
Dynamic Range Control (DRC)
Dynamic Range Control (DRC) incorporates both selectable Dynamic Range
Compression profiles and automatic overload protection limiting. Many consumer
products allow the user to select reduced dynamic range when listening to a Dolby
Digital multichannel audio program. During downmixing, protection limiting can be
automatically applied to prevent overload. The DP562 has the capability to monitor
the DRC encoded into the Dolby Digital bitstream.
Example: Selecting “Line” from the “Dynamic Compression” section applies DRC
either fully or partially as determined by the configuration. Selecting “RF”
implements both Dynamic Range Control and Dialog Normalization as would be
applied for outputs that are connected to the RF (antenna) input of a television set.
“Custom” offers the same options for monitoring DRC as “Line” along with the
ability to defeat Dialog Normalization. “None” is strictly a professional mode that
defeats both DRC and Dialog Normalization. “Line” and “RF” always have Dialog
Normalization applied.
4.11.3
Bass Management
Bass management allows the user to redirect low-frequency information from any of
the five main speakers to the subwoofer or conversely, if there is no subwoofer the
LFE information can be redirected to the left and right speakers. This is important as
the vast majority of consumer home theater speaker systems require some degree of
bass management since typically none of the five main speakers are designed to
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reproduce frequencies below 80 Hz. The DP562 provides the same bass management
functions as a consumer Dolby Digital decoder. Even when monitoring with fullrange main speakers that require no bass management, this function is useful for
checking how low frequencies redirected from any of the main channels may interact
with the LFE channel information. Proper bass management is necessary to emulate a
consumer home theater system, as most consumers use some form of it. Two bass
management configurations are required for consumer products in the Decoder
category (refer to Section 3.5, LFE and Bass Management).
4.11.4
LFE Monitor Mode
The LFE Monitor Mode in the setup menu provides a means to monitor the use of the LFE
channel in a Dolby Digital downmix. The DP562 defaults to the “AUTO” mode, which
does not allow the LFE channel to be included in the downmix where it is not appropriate.
Example: Whenever Dolby Surround compatible downmixing occurs, the LFE
channel is not included in the downmix. With the LFE Monitor Mode set to “AUTO,”
the DP562 automatically mutes the LFE channel whenever “Pro Logic” is selected.
This is also true for a “Stereo” (Lo/Ro) or “Mono” Dolby Digital downmix without
“Pro Logic.” Conversely, in “AUTO” mode, the DP562 does not mute the LFE
channel when in “Dolby Digital” and either “Full,” “Phantom Center,” or “3 Stereo.”
Selecting LFE Monitor Mode “ON” allows the DP562 to pass LFE channel
information regardless of the downmix mode. LFE Monitor Mode “OFF” is
essentially a mute switch. “AUTO” mode should always be used unless there is a
specific requirement unrelated to checking downmixes.
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Chapter 5
Applications and Formats
5.1
DVD-Video
Dolby Digital is one of the standard audio formats for DVD-Video in all NTSC and
PAL countries. Refer to Section 5.1.1, DVD-Video Specification, for more
information. It is crucial for all engineers and technicians who author DVDs to be
familiar with the basic Dolby Digital audio requirements for DVD-Video before
starting the audio encoding process. This section covers the details of audio encoding
for DVD-Video. Any questions regarding DVD authoring or video encoding should
be addressed to the appropriate companies.
DVD players come in a few form factors: stand-alone home and portable units,
incorporated into personal computers, and convergence products. All these players
are capable of playing the same DVD-Video discs on the market today. In NTSC and
PAL countries all DVD players are equipped with Dolby Digital decoders that are
capable of playing DVD-Video discs with Dolby Digital tracks. The consumer
usually has the choice of listening to the DVD player decoded line outputs or
connecting the Dolby Digital data line to an external decoder (A/V receiver, etc.).
This gives the consumer more flexibility in the decoding process.
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Applications and Formats
The following guidelines apply when encoding audio material for the DVD-Video
format. Contact manufacturers of DVD authoring software and video encoding
equipment for more information in these areas.
5.1.1
DVD-Video Specification
The DVD Forum has issued a specification outlining the accepted audio formats for
DVD-Video in PAL countries. Any PAL DVD-Video disc can now have Dolby Digital
audio tracks only—without the need for linear PCM or MPEG audio to be present
anywhere on the disc. In NTSC countries, a DVD-Video disc conforms to the
specification if it carries either a Dolby Digital audio track or a linear PCM audio track.
5.1.2
Supported Data Rates
When encoding audio for DVD-Video one of the key parameters in the Dolby Digital
encoder is the Data Rate. The DVD-Video specification requires every DVD-Video
player to be able to decode Dolby Digital bitstreams up to and including 448 kb/s.
Through its rigorous licensing program, Dolby Laboratories guarantees that every DVDVideo player on the market is capable of decoding a Dolby Digital bitstream at 448 kb/s.
Dolby Laboratories recommends that all multichannel (more than two-channel) material
is encoded at 448 kb/s and that all two-channel stereo content is encoded at 192 kb/s.
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In some cases, video encoder inefficiencies may require a higher video data rate than
normal. In these cases the audio data rate for multichannel content could be lowered
to 384 kb/s. This lower data rate should only be used when it is clear that the video
encoder requires a higher data rate.
5.1.3
Bit Resolution
The Dolby Digital algorithm is capable of encoding up to 24-bit audio. In addition, all
DVD authoring systems can accept Dolby Digital (.ac3) files that were created from
24-bit audio sources. Dolby Digital decoders offer bit resolutions from 16 to 24 bits.
5.1.4
Audio/Video Synchronization
One of the issues facing DVD-Video authoring engineers is audio/video synchronization.
An important step to follow in the encoding stages of DVD-Video authoring is to include
SMPTE time code in both the video file and the Dolby Digital .ac3 file. Most DVD
authoring systems today accommodate audio/video synchronization using the imbedded
SMPTE time code in both the .ac3 file and in the MPEG-2 video file. Although using
SMPTE time code greatly reduces synchronization issues, fine-tuning may still be
required before the final production disc shows perfect synchronization. Experience has
shown that verifying correct synchronization prior to encoding can save time in the
DVD-Video production process.
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5.1.5
Applications and Formats
Dolby Digital Encoding for DVD-Video
When encoding audio content for DVD-Video authoring a few guidelines must be
kept in mind:
• Always have at least two seconds of digital black silence at the beginning of the
bitstream. This gives the large amount of digital circuitry in playback systems
time to lock and start decoding before the real material starts playing. It is not
necessary to leave digital black at the end of the file.
• Always encode at 448 kb/s data rate for multichannel material and at 192 kb/s for
two-channel stereo material.
• Make sure the source material has a sample rate of 48 kHz, otherwise the sample
rate of the material must be converted before the encoding begins. This is a
necessary step since the specifications for the DVD-Video format allow only a 48
kHz sample rate.
• Start with the best quality material possible. Dolby Digital encoders can
accommodate up to 24-bit audio.
• Select the most appropriate Dialog Normalization value and Dynamic Range
Compression profile for the material. Refer to Chapter 4, Encoding, for more
information. This is crucial since each type of material requires a different setting
of these parameters.
• Select the appropriate values for each of the parameters. If the original material is
Dolby Surround encoded, then enable the Dolby Surround flag. If mixing
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Applications and Formats
information exists fill-in the appropriate parameters. Many decoders react to these
parameters so the correct values must be set.
• Do not enable the LFE Channel unless there is dedicated Low-Frequency Effects
(LFE) material in the original audio source.
• Save the SMPTE time code, if present in the source material, with the Dolby
Digital output: It is crucial for synchronization with video.
• Always monitor the encoded output with a professional reference decoder.
Monitoring is the only way to check the integrity and accuracy of the encoded audio.
5.1.6
Music on DVD-Video
One exploding area in the DVD-Video format is the music disc. Since the
introduction of DVD-Video there have been many titles authored exclusively with
music content. The purpose of these discs is to provide the consumer with high
quality multichannel audio without the need to watch the video or maneuver with
software menus and buttons. The main goal is to have these discs behave as audio
compact discs from an authoring standpoint. The user can insert the disc in the player
without turning on the TV to listen to the content. There are many video options the
authoring engineer can choose from. Some discs have been authored to display one
video slide throughout an entire song, some display rolling credits and lyrics, and
some display a live or studio produced music video.
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5.1.7
Applications and Formats
Karaoke DVD
The key issues when creating a karaoke DVD-Video are the audio channel configuration,
the correct setting of the Dolby Digital bitstream, and the actual authoring technique.
Each karaoke audio element must be assigned to the correct channel of the Dolby Digital
encoder. The karaoke format is designed for two-channel reproduction, therefore route
the stereo music mix for a karaoke DVD-Video to the Left and Right channels of the
encoder. If a guide melody to assist the singer is available, assign it to the Center channel.
Allocate main and/or backing vocals to the Left and Right Surround channels. If there is
only one backing vocal, use the Left Surround channel.
It is important to set the Dolby Digital encoder Bitstream Mode to Karaoke. When
authoring, the engineer must be familiar with the karaoke capability of the software
tools since various settings determine how the disc behaves in the DVD-Video player.
Although a karaoke DVD-Video will play back on standard DVD-Video players, a
karaoke-capable player is required for the consumer to use all the features of this
mode. The karaoke capable player allows the user to mix his voice with the main
music as well as the guide and backing vocals, if present. Depending on the player
model, other features can include pan controls and “echo” or reverb units. For those
without karaoke capable players, a second audio track on the disc with either a
suitable two-channel or even five-channel mix can be useful. The Bitstream Mode for
the second audio track should be set to Complete Main.
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5.1.8
Applications and Formats
Miscellaneous Issues
There are areas of DVD authoring that could be covered in this manual but are best
answered by the DVD authoring companies. Issues such as seamless audio branching
and multiple language files are handled differently on various authoring systems.
Contact the respective DVD authoring company for more information.
5.2
DVD-Audio
Dolby Digital can be used as an alternative to linear PCM in the video zone of a
DVD-Audio disc. Dolby Laboratories recommends that all DVD-Audio discs include
a Dolby Digital bitstream with program content identical to the multichannel linear
PCM tracks. This ensures playback compatibility with all DVD-Video players.
5.3
DVD-ROM
Traditionally, when preparing material for multimedia, heavy peak limiting has been
used to maximize the volume of each sound element. When working with both .ac3
files and .wav files, examine the .wav files to determine the relative volume as
compared to the .ac3 files. If adjustment is required, either increase the level of the
.ac3 files to near 0 dBFS and encode them with a dialnorm setting of -31 dB, or
attenuate the level of the existing .wav files.
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Be advised that both program levels and Dolby Digital encoder settings are factors in
determining the need for peak overload protection. Dolby Digital encoders
automatically generate overload protection words (calculated by the worst-case
downmix) when the selected Dynamic Range Compression profile (also referred to as
a preset) is inadequate in preventing digital overload. This protection is applied to the
audio in the form of peak limiting when either downmixing or dynamic range
compression is active. Material with signals near 0 dBFS in multiple channels and a
dialnorm setting of -31 dB induces more overload protection than that of similar
material with a single channel near 0 dBFS and a higher dialnorm value. Dolby
Laboratories recommends monitoring Dolby Digital encoding using various decoding
modes during production to ensure that the audio will be reproduced as intended.
Currently, .ac3 files as elementary bitstreams can be played only through DirectShow
compatible decoders. Only a few DVD hardware decoder boards recognize the .ac3
file format when using MCI driver commands for a DVD-ROM title.
Many software decoders, however, now have the ability through DirectShow to
access the decoded linear PCM channels out of the Dolby Digital decoder and mix
them to L, R, LS, and RS and output them through a standard four-channel audio
card. This capability can be used to replace Redbook audio with 5.1-channel Dolby
Digital music or background tracks and retain the ability to use run-time sound effects
concurrently. Contact multimedia@dolby.com for more information.
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5.4
Applications and Formats
Digital Television (DTV)
Dolby Digital is widely used in digital television (DTV) broadcasting systems. Dolby
Digital was first standardized by the Advanced Television Systems Committee
(ATSC) in document A/52, Digital Audio Compression Standard (AC-3). The ATSC
DTV Standard, approved by the FCC for use in the United States, is ATSC document
A/53, ATSC Digital Television Standard. ATSC document A/54, Guide to the Use of
the ATSC Digital Television Standard is a useful tutorial. All of the ATSC documents
are available from the ATSC web page at www.atsc.org.
5.4.1
ATSC DTV Constraints
Certain Dolby Digital parameters are constrained in the ATSC DTV application. The
most significant constraints are:
• The sample rate is fixed at 48 kHz (which must be locked to the picture rate)
• The data rate for Main or Associated Services containing all necessary program
elements currently cannot exceed 384 kb/s.
• The 1+1 (or dual mono) mode is not allowed for emission. This mode is only for use
in professional production or distribution links where it is necessary to place two
completely independent programs into a single audio elementary bitstream.
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Other constraints apply to the maximum data rates for Associated Services (see
below). The constraints are summarized in Table 5-1, excerpted from ATSC
document A/53, ATSC Digital Television Standard, Annex B.
Table 5-1 ATSC DTV Audio Constraints
Dolby Digital
Syntactical Element
fscod
frmsizecod
frmsizecod
frmsizecod
frmsizecod
acmod
5.4.2
Comment
Allowed Value
Indicates sampling rate
Main or Associated Service containing
all necessary program elements
Single-channel Associated Service
containing a single program element
Two-channel Dialog Associated Service
Combined data rate of a Main and an
Associated Service intended to be
simultaneously decoded
‘00’ (indicates 48 kHz)
Indicates number of channels
≥ ‘001’
≤ ‘011100’ (indicates ≤ 384 kb/s)
≤ ‘010000’ (indicates ≤ 128 kb/s)
≤ ‘010100’ (indicates ≤ 192 kb/s)
(total ≤ 512 kb/s)
Implementation
Broadcasting typically employs hardware Dolby Digital professional encoders
working in real-time at the point of emission to the consumer. In two-channel stereo,
the encoder may be a Dolby Laboratories product such as the Model DP567, or may
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be an encoder that is embedded inside the video encoding system. In 5.1-channel
broadcasting, the encoder is a Dolby Laboratories product such as the Model DP569.
Dolby encoding products such as the DP567 and DP569, and some encoders built by
Dolby Laboratories licensees can pass through encoded bitstreams. Encoders that
encode linear PCM into Dolby Digital elementary bitstreams, when presented with an
encoded Dolby Digital bitstream at their input simply pass through the Dolby Digital
bitstream. In some broadcast systems it is thus possible for pre-encoded Dolby Digital
bitstreams to be transmitted.
5.4.3
Main, Associated, and Multilingual Services
The following information is excerpted from ATSC document A/54, Guide to the Use
of the ATSC Digital Television Standard.
A Dolby Digital elementary bitstream contains the encoded representation of a single
audio service. Multiple elementary bitstreams provide multiple audio services. Each
elementary bitstream is conveyed by the MPEG-2 transport multiplex with a unique PID.
There are a number of audio service types that can (individually) be coded into each
elementary bitstream. Each elementary bitstream is tagged as to its service type using the
bsmod bit field. There are two types of Main Services and six types of Associated
Services. Each Associated Service can be tagged (in the AC-3 audio descriptor in the
transport PSI data) as being associated with one or more main audio services.
Associated Services can contain complete program mixes, or can contain a single
program element. Associated Services that are complete mixes can be decoded and
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used without additional services. They are identified by the full svc bit in the AC-3
descriptor. Refer to ATSC document A/52, Digital Audio Compression Standard
(AC-3), Annex A. Typically, Associated Services that contain a single program
element are combined with the program elements from a Main Service.
In general, a complete audio program is presented to the listener over a set of
loudspeakers. This may consist of a Main Service, an Associated Service that is a
complete mix, or a Main Service combined with one Associated Service. The
capability to simultaneously decode one Main Service and one Associated Service is
required in order to form a complete audio program in certain service combinations
described in this section. This capability may not exist in some receivers.
Summary of Audio Service Types
The audio service types that correspond to each value of bsmod are defined in the
ATSC documents A/52, Digital Audio Compression Standard (AC-3) and A/53,
ATSC Digital Television Standard, Annex B. The information is reproduced in Table
5-2. The paragraphs that follow briefly describe the service types.
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Table 5-2 Audio Services
Audio Service Types
bsmod
000 (0)
Main Service: Complete Main (CM)
001 (1)
Main Service: Music and Effects (ME)
010 (2)
Associated Service: Visually-Impaired (VI)
011 (3)
Associated Service: Hearing-Impaired (HI)
100 (4)
Associated Service: Dialog (D)
101 (5)
Associated Service: Commentary (C)
110 (6)
Associated Service: Emergency (E)
111 (7)
Associated Service: Voice-Over (VO)
Multilingual Services
Each audio bitstream can be in any language. To provide audio services in multiple
languages, individual Main Services can be created for each language. This is the
artistically preferred method because it allows unrestricted placement of dialog along
with the dialog reverberation. The disadvantage of this method is that each language
requires as much as the maximum data rate for a full 5.1-channel service (currently
384 kb/s). One way to reduce the data rate is to restrict the number of audio channels
for languages with a limited audience. For instance, alternate language versions in
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Applications and Formats
two-channel stereo could be provided at a data rate of 128–192 kb/s. A mono version
could be supplied at a data rate of 64–96 kb/s.
Another way to offer multiple-language service is to provide Music and Effects
service (ME), which does not contain dialog. Multiple single-channel Dialog services
(D) can then be provided, each at a data rate of 64–96 kb/s. Formation of a complete
audio program requires that the appropriate language D service be simultaneously
decoded and mixed into the ME service. This method allows a large number of
languages to be efficiently provided, but with artistic limitations. The single channel
of dialog would be mixed into the Center reproduction channel, and could not be
panned. Also, reverberation would be confined to the Center channel, which is not
optimal. This method results in a substantial data rate savings, making it ideal for
some types of programming. Some receivers may not have the capability to
simultaneously decode a ME and a D service.
When transmitting a two-channel stereo ME service along with two-channel stereo D
services, multiple languages are delivered efficiently without compromising artistic
content. The D service and appropriate language ME service are combined in the
receiver into a complete two-channel stereo program. Dialog can be panned, and
reverberation can be included in both channels. A two-channel stereo ME service can
be sent with high quality at 192 kb/s, while the two-channel stereo D services (voice
only) can make use of lower data rates, such as 128 or 96 kb/s per language. Some
receivers may not have the capability to simultaneously decode a ME and a D service.
During those times when dialog is not present, the D services can be momentarily
removed, and their data capacity used for other purposes.
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5.4.4
Applications and Formats
Detailed Description of Service Types
Complete Main (CM)
The Complete Main service (CM) is the normal mode of operation. It contains a
complete audio program, with dialog, music, and effects. This is the type of audio
service typically provided. The CM service can contain from one to 5.1 audio
channels. It can be further enhanced with the VI, HI, C, E, or VO services described
below. To provide audio service in multiple languages, individual CM services can be
created for each language.
Music and Effects (ME)
The Music and Effects (ME) type of Main Service contains the music and effects for
an audio program, but not the dialog. The ME service can contain from one to 5.1
audio channels. The primary program dialog is missing and, if any exists, is supplied
by providing a D service. Multiple D services in different languages can be associated
with a single ME service.
Visually-Impaired (VI)
The Visually-Impaired type of Associated Service typically contains a narrative
description of the visual program content. In this case, the VI service is a single audio
channel. Simultaneous reproduction of the VI service and the Main Service allows the
visually-impaired user to enjoy the main multichannel audio program, as well as to
follow the on-screen activity. The VI service can be mixed into one of the main
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Applications and Formats
reproduction channels; the choice of channel can be left to the listener or be provided
as a separate output. The separate output might then be delivered to the VI user via
open-air headphones or other means.
The Dynamic Range Control (DRC) data in this type of VI service is intended for use
by the audio decoder to modify the level of the main audio program. Thus the level of
the Main Service is under the control of the VI service provider. The provider can
signal the decoder by altering the DRC words embedded in the VI audio elementary
bitstream to reduce the level of the main audio service by up to 24 dB assuring that
the narrative description is intelligible.
Besides being provided as a single narrative channel, the VI service can be provided
as a complete program mix containing music, effects, dialog, and narration. In this
case, the service can be coded using any number of channels, up to 5.1, and the
Dynamic Range Control word applies only to this service. The fact that the service is
a complete mix is indicated in the Dolby Digital descriptor. Refer to ATSC A/52,
Digital Audio Compression Standard (AC-3), Annex A, for more information.
Hearing-Impaired (HI)
The Hearing-Impaired type of Associated Service typically contains only a single
channel of dialog and is intended for use by those whose hearing impairments make it
difficult to understand the dialog in the presence of music and sound effects. The
dialog can be processed for increased intelligibility by the hearing impaired. The
hearing-impaired listener may wish to listen to a mixture of the single-channel HI
dialog track and the main program audio. Simultaneous reproduction of the HI
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Applications and Formats
service along with the CM service allows the HI listener to adjust the mixture to
control the emphasis on dialog over music and effects. The HI channel is typically
mixed into the Center channel. An alternative is to deliver the HI signal to a discrete
output that could be fed to a set of open-air headphones or other device, worn only by
the HI listener.
Besides being provided as a single narrative channel, the HI service can be provided
as a complete program mix containing music, effects, and dialog with enhanced
intelligibility. In this case, the service can be coded using any number of channels, up
to 5.1. The fact that the service is a complete mix is indicated in the AC-3 descriptor.
Refer to ATSC A/52, Digital Audio Compression Standard (AC-3), Annex A, for
more information.
Dialog (D)
The Dialog type of Associated Service is employed to most efficiently offer
multichannel audio in several languages simultaneously when the program material is
such that the restrictions (no panning, no multichannel reverberation) of a single
dialog channel can be tolerated. When the D service is used, the Main Service is type
ME. If the D service contains a single channel, simultaneously decoding the ME
service allows a complete audio program to be formed by mixing the D channel into
the Center channel. Typically, when the Main audio Service is of type ME, there are
several different language D services available. The transport demultiplexer can be
designed to select the appropriate D service to deliver to the audio decoder based on
the listener’s language preference as defined by data stored in the receiver memory.
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Or, the listener can override the default selection by instructing the receiver to select a
particular language D service.
If the ME service contains more than two audio channels, the D service is
monophonic (1/0 mode). If the ME service contains two channels, the D service can
contain two channels (2/0 mode). In this case, a complete audio program is formed by
simultaneously decoding the D service and the ME service. The Left channel of the
ME service is mixed with the Left channel of the D service, and the Right channel of
the ME service is mixed with the Right channel of the D service. The result is a twochannel stereo signal containing music, effects, and dialog.
Commentary (C)
The Commentary type of Associated Service is similar to the D service, except that
instead of conveying primary program dialog, the C service conveys optional
program commentary. When C service(s) are provided, the receiver can notify the
listener of their presence. The listener should be able to inquire about the various
available C services, and select one for decoding along with the Main Service. The C
service can be added to any loudspeaker channel under listener control. Typical uses
for the C service are optional added commentary during a sporting event, or different
levels (novice, intermediate, and advanced) of commentary available to accompany
documentary or educational programming.
The C service can be a single audio channel containing only the commentary content.
In this case, simultaneous reproduction of a C service and a CM service allows the
listener to hear the added program commentary.
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The Dynamic Range Control data in the single-channel C service is intended for use
by the audio decoder to modify the level of the main audio program. Thus, the level
of the Main Service is controlled by the C service provider. The provider can signal
the decoder, by altering the Dynamic Range Control words embedded in the C service
audio elementary bitstream, to reduce the level of the Main Service by up to 24 dB in
order to ensure intelligible commentary.
Besides providing the C service as a single commentary channel, the C service can be
provided as a complete program mix containing music, effects, dialog, and the
commentary. In this case the service can be provided using any number of channels
(up to 5.1). The fact that the service is a complete mix is indicated in the Dolby
Digital descriptor. Refer to ATSC A/52, Digital Audio Compression Standard (AC3), Annex A, for more information.
Emergency (E)
The Emergency type of Associated Service is intended to allow the insertion of
emergency announcements. The normal audio services do not necessarily have to be
replaced to present the emergency message. The transport demultiplexer gives priority
to this type of audio service. Whenever an E service is present, it is delivered to the
audio decoder by the transport subsystem. When the audio decoder receives an E-type
Associated Service, it stops reproducing any Main Service being received and only
reproduces the E service. The E service may also be used for non-emergency
applications. It may be used whenever the broadcaster wishes to force all decoders to quit
reproducing the main audio program and substitute a higher priority single channel.
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Voice-Over (VO)
It is possible to use the E service for announcements, but the use of the E service
leads to a complete substitution of the voice-over for the main program audio. The
Voice-Over type of Associated Service is similar to the E service, except that it is
intended for reproduction along with the Main Service. The systems demultiplexer
gives priority to this type of associated service, second only to an E service. The VO
service is intended to be simultaneously decoded and mixed into the Center channel
of the main audio service that is being decoded.
The Dynamic Range Control data in the VO service is intended for use by the audio
decoder to modify the level of the main audio program. Thus the level of the Main
Service is under the control of the broadcaster. The broadcaster may signal the
decoder by altering the Dynamic Range Control words embedded in the VO audio
bitstream, to reduce the level of the Main Service by up to 24 dB during the voiceover. The VO service allows typical voice-overs to be added to an already encoded
audio bitstream, without requiring the audio to be decoded back to baseband and then
re-encoded. Space however, must be available within the transport multiplex for the
insertion of the VO service.
5.4.5
Splicing Bitstreams
In some broadcast applications it is likely that encoded bitstreams will be spliced. The
ideal place to splice encoded audio bitstreams is at the boundary of a sync frame. If a
bitstream splice is performed at the sync frame boundary, the audio decoding proceeds
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without interruption. If a bitstream splice is performed randomly, an audio interruption
results. The frame that is incomplete does not pass the error detection test within the
decoder and causes it to mute. The decoder does not find sync in its proper place in the
next frame, and enters a sync search mode. Once the sync code of the new bitstream is
found, synchronization is achieved, and audio reproduction can begin once again. The
outage may be on the order of two frames, about 64 milliseconds at the 48 kHz sample
rate. The actual outage depends on the specific audio decoder implementation, and the
implementation of the demultiplexing system that precedes the audio decoder. In some
implementations the actual mute could be longer than 64 milliseconds. When the audio
goes to mute, there may be a gentle fade down over a period of 2.6 milliseconds due to
the windowing process of the filter bank. When the audio is recovered, it may fade up
over a period of 2.6 milliseconds. Except for the approximately 64 milliseconds (or
longer) of time that the audio is muted, the effect of a random splice of a Dolby Digital
elementary bitstream can be relatively benign.
SMPTE is developing a standard for splicing MPEG-2 transport bitstreams. Splices
performed according to this standard will occur on Dolby Digital frame boundaries
and on picture frame boundaries. Since audio frame boundaries and picture frame
boundaries are not synchronous, however, splicing inevitably leaves a gap in the
audio, and causes an interruption in the audio frame sequence. The first frame after the
splice has an MPEG-2 presentation time stamp (PTS) value that is greater than 32
milliseconds relative to the frame prior to the splice. The behavior of equipment in this
situation is not well defined. Ideally, a receiver would fade down the old audio,
immediately resynchronize to the new framing sequence, and fade up the new audio. In
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the worst case a receiver may go into a frame repeat type of error concealment, followed
by an extended mute, before recovering and beginning to reproduce the new audio.
Given the uncertain behavior of receivers, it is good practice to provide a moment of
silence at anticipated splice points, i.e., at the end of any program segment.
5.5
Laser Disc
5.5.1
Track Layout
When preparing the D2 master for the pressing plant, the audio and video tracks
should be conformed onto the D2 master as they will appear on the laser disc itself.
The D2 masters provided for conforming audio tracks should have the finished video
as it will appear on the laser disc with a separate D2 for each side of the disc.
Depending on the laser disc format, this is a maximum of one hour per side for CLV
discs and one half hour per side for CAV discs. These tapes are typically delivered to
the audio facility with the two-channel Lt/Rt or stereo PCM tracks already on
channels 3 and 4 of the D2 tape and a mono composite or commentary track on
channel 1. Channel 2 is reserved for the Dolby Digital bitstream. The stereo or mono
tracks should already be on the D2 tape before the Dolby Digital bitstream is
recorded. Refer to Table 5-3 for the typical audio track layout for the D2.
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Table 5-3 Typical D2 Track Layout
Track Layout
Mono or Commentary (+12 dB analog)
Dolby Digital Bitstream
PCM Lt or Left (+20 dB analog )
PCM Rt or Right (+20 dB analog )
5.5.2
D2
PCM Channel 1
PCM Channel 2
PCM Channel 3
PCM Channel 4
Laser disc
Left Analog Channel
Right Analog Channel
PCM Left (+20 dB analog)
PCM Right (+20 dB analog)
Audio/Video Synchronization
When a Dolby Digital bitstream is encoded onto the laser disc, the bitstream is given
a six-frame advance on the disc itself. The advance accounts for the access time
required to retrieve the data from the disc in the laser disc player. The common
method for preparing the master is to record the Dolby Digital bitstream in
synchronization with the picture and have the six-frame advance done at the laser disc
pressing plant. This method allows the audio editor to hear the Dolby Digital
bitstream in synchronization with picture as the disc is checked for quality control
(QC). It is acceptable to have the six-frame advance performed at the time of
encoding onto the D2 tape. If this is done, care must be taken to properly label the
tape: “The Dolby Digital bitstream on this tape has a six-frame advance.” Digital
delays can then be added to the decoded Dolby Digital audio channels to place the
audio back in sync with the picture for QC purposes. The approximate delay time for
six frames at a frame rate of 29.97 is 200 milliseconds.
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Figure 5-1 depicts a typical setup for performing a Dolby Digital laser disc encoding
session.
Modular
Digital
Multitrack
(MDM)
Dolby
Digital
Encoder
D2 Video
Tape
Machine
Dolby
Digital
Decoder
Audio
Console
Audio
Monitoring
System
Figure 5-1 Typical Dolby Digital Laser Disc Encoding Setup
The Lt/Rt or stereo tracks on the D2 channels 3 and 4 can be used as guide tracks to
confirm synchronization while listening to the decoded Dolby Digital bitstream.
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5.5.3
Applications and Formats
Important Considerations
The output format of the Dolby Digital encoder must be set to “Pro 16-bit” to encode
the Dolby Digital bitstream to a single channel of an AES/EBU pair. Pro 16-bit Ch1
is used for the odd number channels 1 and 3 while Pro 16-bit Ch2 is used for the even
numbered channels 2 and 4.
Dolby Digital laser discs are always encoded with a sample rate of 48 kHz and a data
rate of 384 kb/s.
During production the Dolby Digital bitstream is stored on a linear PCM channel.
Since it is data and not linear PCM audio, the bitstream must therefore be recorded
onto the linear PCM channel using certain guidelines.
•
•
•
•
•
Use unity gain at all stages of input and output.
Do not apply signal processing of any kind (this includes dithering).
Do not edit the bitstream and record in one continuous pass.
Use proper digital referencing to lock all digital audio and video equipment.
Only use digital pathways in the audio and data chain beginning with the input of the
Dolby Digital encoder through to the input of the Dolby Digital decoder.
• Record a pre- and post-roll of Dolby Digital silence at the beginning and end,
respectively, of the encoded material. Standard practice is to begin the bitstream 30
seconds prior to the start of program and continue it 30 seconds after the end of
program.
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Chapter 6
Professional Encoders and Decoders
6.1
Dolby Digital Professional Encoders
A licensed and approved professional encoder must be used when Dolby Digital
encoding content for DVD-Video, DVD-ROM, DVD-Audio, laser disc, and
broadcast applications. Any content created with a Dolby Digital professional
encoder is eligible to carry the Dolby Digital logo and will be compatible with any
consumer decoder bearing the same Dolby Digital logo. Refer to Section 7.3,
Trademark Usage, for more information on Dolby trademarks.
6.1.1
Software vs. Hardware
There are two fundamental types of Dolby Digital professional encoders on the market
today: software encoders and hardware encoders. Each type of encoder has certain
characteristics and advantages. The choice of encoder depends on the type of
application and intended use. Hardware encoders operate in the real-time domain,
which means that the input audio linear PCM bitstream is encoded into a Dolby Digital
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Professional Encoders and Decoders
bitstream with very little delay (a few hundred milliseconds) in the process. A realtime encoder is ideal for live broadcast applications where the signal must be encoded
and transmitted on the fly. Real-time encoders are also convenient for content
generation for DVDs and laser discs since they allow real-time monitoring of the
output using a professional Dolby Digital decoder. Software encoders, although
usually non-real-time, are ideal for batch processing and encoding where a series of
linear PCM files or bitstreams are to be encoded in a single session. It is reasonable to
expect software encoders to reach real-time performance in the near future as the
processing power of personal computers increases.
6.1.2
Licensed Dolby Digital Encoders and Quality
In addition to offering its own manufactured products, many different Dolby Digital
professional encoders are licensed and approved by Dolby Laboratories. All licensed
and approved encoders produce the same high quality audio signals. Whether an
encoder is software or hardware, stand-alone or integrated, all materials generated with
these products are eligible to carry the Dolby Digital logo indicating professional
quality content.
6.1.3
Dolby Laboratories Encoders
Dolby Laboratories designs, manufactures, and sells Dolby Digital stand-alone
hardware reference encoders, such as the Model DP569, for multichannel
applications, and the Model DP567, for two-channel applications. These real-time
Dolby Digital Professional Encoding Guidelines
Professional Encoders and Decoders
reference encoders have been designed with professional features making them ideal
for DTV broadcast applications and for DVD and laser disc content generation. The
DP569 can encode from one to 5.1 channels in Dolby Digital, while the DP567 can
encode one or two channels. For more information on the Dolby Laboratories
manufactured encoders, refer to Section 7.2, Contacting Dolby Laboratories.
6.1.4
Dolby Laboratories Licensed Encoders
Dolby Laboratories licensees have produced both software and hardware Dolby
Digital professional encoders for a variety of applications. Hardware encoders are
available from Dolby licensees in many form factors such as PCI cards and integrated
audio/video encoders.
6.1.5
Software Updates
Dolby Laboratories is committed to providing its customers and licensees with the
highest quality encoders on an ongoing basis. Dolby Laboratories will continue to
furnish all its customers and licensees with upgrades of encoder software and
routines. To acquire the latest version of Dolby Digital professional encoder software,
refer to Section 7.2, Contacting Dolby Laboratories.
Dolby Digital Professional Encoding Guidelines
6.2
Professional Encoders and Decoders
Dolby Digital Professional Decoders
While a consumer decoder product may have many exciting and useful features, it
usually does not have the flexibility and capability that is essential in a production
environment. A professional decoder offers unique and useful features, such as the
ability to monitor Dolby Digital bitstream parameters. This feature is essential
anytime a production engineer is encoding audio material and needs to monitor the
encoder output for correct parameter settings. Another important feature is the ability
of a professional decoder to emulate any type of decoder on the market whether it is a
DVD player, an A/V receiver, an HDTV, or a set-top box. Since most consumer
decoders have some Dolby Digital features (Dialog Normalization, Dynamic Range
Control (DRC), downmixing, etc.) preset at the factory, it is critical that the encoding
engineer use a professional reference decoder to allow monitoring of all possible
decoding options. Other features that differentiate a professional decoder from a
consumer decoder are rack-mount style chassis and professional electrical
connections (XLR, AES/EBU).
6.2.1
Dolby Laboratories Decoder
The Model DP562 is a Dolby Digital multichannel reference decoder manufactured
by Dolby Laboratories. It is designed for high-quality monitoring of Dolby Digital
bitstream parameters in real-time, and for keeping track of any errors or faults for
quality assurance. The DP562 can emulate any Dolby Digital consumer decoder in
addition to decoding Dolby Surround material using a built-in, digitally-implemented
Dolby Digital Professional Encoding Guidelines
Professional Encoders and Decoders
Dolby Surround Pro Logic decoder. This unit is designed with professional features
for DVD and laser disc production and DTV broadcast applications. For more
information on the DP562, refer to Section 7.2, Contacting Dolby Laboratories.
6.2.2
Licensed Dolby Digital Professional Decoders
There are two types of licensed Dolby Digital professional decoders on the market
today, broadcast baseband decoders for fixed-mode broadcast monitoring, and
confidence decoders that are limited in features and are integrated with many Dolby
Digital professional encoders.
Dolby Digital Professional Encoding Guidelines
Chapter 7
Miscellaneous Information
7.1
Technical Assistance
Dolby Laboratories provides technical support to content creators and encoder users
in a variety of ways. Many technical documents are available for viewing or
downloading on the Dolby web site at www.dolby.com. Printed copies of documents
can also be obtained by sending e-mail to info@dolby.com with a description of the
desired documents and a complete mailing address.
Dolby has a staff of engineers who can assist with audio production, encoding, and
trademark usage. Dolby engineers are also available to provide on-site assistance with
room configuration and calibration, audio production, and encoding. Telephone
support is available free of charge, and local on-site support can often be provided
without cost. In situations where extensive on-site support or long distance travel is
required, standard engineering rates may apply.
If you would like technical support, please contact the nearest Dolby office at any of
the locations listed in the following section.
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Dolby Digital Professional Encoding Guidelines g
7.2
Miscellaneous Information
Contacting Dolby Laboratories
In addition to its headquarters in San Francisco, Dolby has several offices around the
world. All offices can provide information on audio production and encoding.
You may contact Dolby from anywhere in the world by e-mail using the addresses in
Table 7-1.
Table 7-1 Dolby Email Contact Addresses
Address
Use
info@dolby.com
General information and inquiries
Questions on audio encoding for DVD
dvd@dolby.com
hdtv@dolby.com
Questions on audio production and encoding for DTV
Questions on multimedia applications
Multimedia@dolby.com
EncoderLicensing@dolby.com
Questions about encoder licensing
EncoderImplementations@dolby.com Questions about encoder implementations
tsa@dolby.com
Applications for Dolby trademark agreements (TSA)
In addition, a wide variety of technical and trademark information can be found on
Dolby’s web site at www.dolby.com.
Information on local Dolby offices follows. Please contact the nearest office for direct
assistance.
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Dolby Digital Professional Encoding Guidelines g
Miscellaneous Information
Corporate Headquarters
UK Headquarters
Dolby Laboratories Inc
100 Potrero Avenue
San Francisco, CA 94103-4813
Telephone 415-558-0200
Facsimile 415-863-1373
Dolby Laboratories Inc
Wootton Bassett
Wiltshire, SN4 8QJ, England
Telephone (44) 1793-842100
Facsimile (44) 1793-842101
Los Angeles
Shanghai Office
Dolby Laboratories Inc
3375 Barham Boulevard
Los Angeles, CA 90068-1446
Telephone 323-845-1880
Facsimile 323-845-1890
Dolby Laboratories Representative Office
7/Fl. Hai Xing Plaza, Unit H
Rui Jin Road (S)
Shanghai 2000023 China
Telephone (86) 21-6418-1015
Facsimile (86) 21-6418-1013
New York
Tokyo Office
Dolby Laboratories Inc
1350 Avenue of the Americas
New York, NY 10019-4703
Telephone 212-767-1700
Facsimile 212-767-1705
Dolby Laboratories International
Services Inc
Japan Branch
Fuji Chuo Building 6F
2-1-7, Shintomi, Chuo-ku
Tokyo 104-0041 Japan
Telephone (81) 3-5542-6160
Facsimile (81) 3-5542-6158
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Dolby Digital Professional Encoding Guidelines
7.3
Miscellaneous Information
Trademark Usage
Dolby Laboratories encourages use of the Dolby Digital trademark to identify
soundtracks and other audio programs that are Dolby Digital encoded. This is an
effective way to inform listeners of the audio format, and the use of a standard logo
promotes easy recognition in the marketplace. As with any trademark, the Dolby
Digital logo may not be used without permission. Dolby Laboratories provides a
royalty-free Trademark and Standardization Agreement (TSA) for companies who
wish to use Dolby trademarks. The company that owns the program material being
produced must sign this agreement. Recording studios or production facilities that
provide audio production, encoding, or manufacturing services for outside clients
generally do not require a trademark license. We do ask that these facilities refer their
clients to us for trademark licensing information.
If you would like to use the Dolby Digital logo you can apply for a Dolby TSA by
sending e-mail to tsa@dolby.com or by contacting Dolby Laboratories at any of the
locations given in Section 7.2, Contacting Dolby Laboratories. When sending written
requests please indicate that you would like a Dolby Digital trademark license and
include your name, your company name, mailing address, and the type of media that
your soundtracks or other audio programs will be distributed on (such as DVD, DVDROM, DTV broadcast, etc.).
For detailed information on Dolby trademark licensing, please refer to the document
Use of Dolby Trademarks on Audio and Video Media, available on the Dolby web
7-4
Dolby Digital Professional Encoding Guidelines
Miscellaneous Information
site at www.dolby.com. We are also planning to make our license application form
available on-line, so check the Dolby web site in the future for the on-line version of
the Media Licensing Questionnaire.
If you are already a Dolby licensee and would like more information on trademark
use, please contact Dolby Laboratories. We are always happy to review artwork and
assist with the proper use of our trademarks. Information on trademark licensing plus
instructions for using the Dolby Digital trademark and marking audio features on
DVD can also be found on the Dolby web site.
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Dolby Digital Professional Encoding Guidelines
Appendix A
The Dolby Digital Algorithm Theory of Operations
A.1
Introduction
In the broadest sense, Dolby Digital is a complete multichannel audio system. Dolby
Digital encoders and decoders provide controls for important system features such as
Dialog Normalization, Dynamic Range Control (DRC), channel downmixing,
copyright notification, and many others discussed earlier in this manual. The most
important feature of a Dolby Digital encoder, however, is its ability to reduce the data
rate required to store or transport high-quality multichannel digital audio. Without
data rate reduction, multichannel audio would simply not be a viable option for many
applications in which data rate is scarce.
In order to accomplish this data rate reduction, Dolby Digital encoders make use of a
sophisticated audio data compression algorithm called Dolby AC-3. Dolby AC-3 is a
perceptual audio coder, also known as a lossy coder. The term lossy is used to
indicate that the audio that comes out of the decoder is not identical to the source
material that went into the encoder—some of the original information is lost. The
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Algorithm - Theory of Operation
goal of a perceptual audio coder is to ensure that whatever is lost is not perceptible. In
other words, the output of the decoder, although numerically different, sounds the
same as the original source material.
While a complete explanation of the inner workings of Dolby AC-3 is beyond the
scope of this document, this section introduces the basic principles of the encoding
and decoding algorithm. For more detailed information, please consult Document
A/52 of the Advanced Television Systems Committee, Digital Audio Compression
Standard (AC-3), available at www.atsc.org, or on the Dolby Laboratories web site at
www.dolby.com.
A.2
Perceptual Coding Principles
The primary task of a perceptual audio coder is to reduce the data rate necessary to
represent a digital audio signal without introducing any audible differences. To
achieve this, perceptual audio coders take advantage of several physiological
limitations of the human hearing system. In other words, a perceptual coder predicts
which sounds your ears will and will not hear, and only encodes audible sounds.
The human ear is a remarkably sensitive and accurate instrument, however, it does
have limitations. One example is the well-known hearing threshold phenomenon.
Simply stated, the ear is not equally sensitive at all frequencies. Our ears are easily
able to detect quiet signals in the 2 kHz–4 kHz midrange, while they are much less
sensitive to quiet signals at very low or very high frequencies. To varying degrees,
Dolby Digital Professional Encoding Guidelines
Algorithm - Theory of Operation
this phenomenon occurs across the entire audible dynamic range including the
threshold of hearing. The absolute hearing threshold varies as a function of frequency
and sounds below the threshold are inaudible. Refer to Figure A-1.
AUDIBLE SIGNAL
dB
INAUDIBLE SIGNAL
HEARING
THRESHOLD
Frequency
Figure A-1 Hearing Threshold
When the human hearing mechanism is stimulated by a complex acoustical
waveform, regions along the basilar membrane within the inner ear are excited.
Behaving as a filter bank, these regions correlate to frequency bands of varying
widths known as critical bands. All aural processing within the brain is performed on
the neural output from these regions.
The basilar membrane vibrates such that individual frequency components are not
localized to a single point. Areas near the point of excitation also vibrate, resulting in
a phenomenon known as frequency-domain masking. Frequency-domain masking
occurs when two different signals are located close to one another in frequency. If
Dolby Digital Professional Encoding Guidelines
Algorithm - Theory of Operation
one of the signals is much louder then the other, it is possible that the softer signal is
not audible at all. Rather, the loud signal can obscure, or mask, any softer signals that
are nearby in frequency. Refer to Figure A–2.
DOMINANT
SIGNAL
dB
DOWNWARD
MASKING
UPWARD
MASKING
INAUDIBLE
SIGNAL
INAUDIBLE
SIGNAL
Frequency
Figure A–2 Effect of Masking
Perceptual coders take advantage of these and other limitations of human hearing in
order to remove inaudible information from the coded audio signal. By predicting
how the ear will react to a complex audio signal, these coders are able to identify and
remove a substantial amount of unnecessary information, and as a result achieve
substantial data rate reduction while maintaining very high quality. Coding
(quantization) noise is minimized through constraint to near the dominant frequency
components in the audio signal. As a result, the subjective audio quality of the
original signal is preserved. Refer to Figure A-3.
Dolby Digital Professional Encoding Guidelines
Algorithm - Theory of Operation
ORIGINAL
SPECTRUM
dB
MASKING
CURVE
CODING
(QUANTIZATION)
NOISE
Frequency
Figure A-3 Coding (Quantization) Noise Below the Masking Curve
Dolby Digital Professional Encoding Guidelines
Appendix B
Bitstream Format
Dolby Digital information can be conveyed as a bitstream when formatted onto
S/PDIF (also referred to as IEC-958 or in the case of data, such as Dolby Digital, IEC
61937), or ATSC A/52 Annex B in one of several ways. In addition, a file may be
recorded to a hard drive in either big or little endian format, and a byte-reversal
process must be used to convert from one to the other.
The fundamental difference between a Dolby Digital bitstream transmitted through an
S/PDIF connection and a bitstream saved as an .ac3 file is that the S/PDIF transmitted
bitstream is padded with "zero" data. This is done to fill in the difference between the
Dolby Digital data rate (e.g., 384 or 448 kb/s) and the serial transmission data rate
(i.e., 48 kHz * 16 bits *2 channels = 1.536 Mb/s per channel).
B.1
Output Mode
There are four ways of placing the Dolby Digital data within the S/PDIF bitstream (Output
Mode):
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Dolby Digital Professional Encoding Guidelines
Bitstream Format
1. Pro 32-bit—Place alternate 16-bit words of Dolby Digital data in left and right
channels of the audio bitstream, and pad with zeros until the next frame is available
(Professional).
2. Pro 16-bit Ch1—Place consecutive 16-bit words of Dolby Digital data in the left
channel only, leaving the right channel available for conveying PCM data
(Professional, Left).
3. Pro 16-bit Ch2—Same as 2, except Dolby Digital data is packed in the right channel.
(Professional, Right).
4. Consumer Mode—Same as 1, except used in consumer applications.
The Dolby Digital Recorder program available from Dolby Laboratories works with any of
these formats. The program accepts the 1.536 Mb/s bitstream and ignores the zero padding,
saving only the relevant data at the Dolby Digital data rate (e.g., 384 kb/s). The resulting
file consists of 1536 16-bit data words per Dolby Digital frame. These 16-bit words may be
written to disc either high-order byte first, or low-order byte first, and this depends on the
processor and operating system being used.
A Dolby Digital bitstream can also be recorded into a computer using a digital soundcard
and audio capture software. This results in a data file marked “Audio,” which if played
back as audio, results in a bursty noise.
Figure B-1 shows a screen capture of Dolby Digital data with zero padding.
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Dolby Digital Professional Encoding Guidelines
Bitstream Format
Figure B-1 Screen Capture of Dolby Digital Data with Zero Padding
A file saved on a personal computer may also have to be byte-swapped, or the order
of the high-order and low-order bytes reversed, in order to be read correctly by a
UNIX based workstation, for example. Consult a computer software vendor for
commercially available programs that can perform this task.
B.2
Audio/Non-Audio Bit
Most, if not all, digital audio cards for computers supply the S/PDIF digital audio
bitstream marked as “Audio” data. Many consumer decoders use this indicator to
determine if the serial data bitstream contains linear PCM or Dolby Digital data. A
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Dolby Digital Professional Encoding Guidelines
Bitstream Format
consumer decoder receiving input from an audio card may fail to decode the Dolby
Digital data, reproducing the data as bursts of full-level noise at the Dolby Digital frame
rate (32 Hz).
In order to avoid this situation, the “Audio/Non-Audio” bit within the S/PDIF bitstream
from the digital audio card must be set to indicate non-audio data. Commercial products
are available that can accomplish this, such as the Lexicon LFI-10, or a simple circuit can
be constructed to read the bitstream and invert the “Audio/Non-Audio” bit.
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Dolby Digital Professional Encoding Guidelines
Appendix C
Dynamic Range Control (DRC)
C.1
Background
Dolby Digital uses a novel approach to applying Dynamic Range Control (DRC) to
audio program material. Rather than compressing the dynamic range of the audio in
an irreversible way, Dolby Digital encoders generate compression gain (also referred
to as control) words that are carried in the Dolby Digital bitstream. When the
bitstream is decoded, the compression gain words are applied to the audio material
according to user settings. Dolby Digital decoders can be commanded to provide full,
reduced, or even no dynamic range compression at all. This allows end users to adjust
the amount of dynamic range compression to suit individual tastes and needs.
The compression gain words are computed based on a number of separate input
parameters, including the audio program material, the program Dialog Normalization
(dialnorm) value, and the selected Dynamic Range Compression profile (also referred
to as a preset). Although these words are referred to as gain words, they can take on
negative values and thus actually represent program attenuation.
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Dolby Digital Professional Encoding Guidelines
Dolby Digital Dynamic Range Compression
An accurate setting of the dialnorm value is crucial to the proper operation of DRC.
The dialnorm value is a Dolby Digital parameter setting that describes the long-term
average dialog level of the associated program. It may also describe the long-term
average level of programs that do not contain dialog, such as music. This level is
specified on an absolute scale ranging from -1 dBFS to -31 dBFS. Dolby Digital
decoders attenuate programs based on the dialnorm value in order to achieve uniform
playback level. This feature of Dolby Digital is termed Dialog Normalization (also
referred to as volume normalization). The amount of adjustment is determined by the
difference between the dialnorm value and the relevant reference playback level (the
dialnorm reference playback level is -31 dBFS for Line mode; +11 dB raises this
level to -20 dBFS for RF mode). As a result, all DRC calculations consider the input
level relative to the dialnorm value, and not in an absolute sense. Thus, setting the
dialnorm value properly is a critical step in calibrating the DRC system.
There are currently six Dynamic Range Compression profiles, described in detail in
Table C-1: Film Standard, Film Light, Music Standard, Music Light, Speech, or None.
Five compression regions including a null band are defined for each profile. The null
band is positioned relative to the dialnorm reference playback level with its upper and
lower limits determined by the Dynamic Range Compression profile. Audio lying within
the null band is unaffected whereas excursions above and below are subject to dynamic
range compression, with softer material being amplified and louder material attenuated.
There are two forms of compression gain words in the Dolby Digital bitstream:
dynrng words and compr words. The dynrng words are used in Line Mode and occur
in the Dolby Digital bitstream once every 5.3 milliseconds (48 kHz sample rate). The
compr words are intended primarily for use in DRC for set-top boxes that are
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Dolby Digital Professional Encoding Guidelines
Dolby Digital Dynamic Range Compression
connected to the RF (antenna) input of a television set (RF Mode). These words occur
once every 32 milliseconds (48 kHz sample rate), and generally yield greater negative
values due to more aggressive overload protection than the dynrng words.
Even though these gain words are sent as discrete values, and can change abruptly
from one word to the next, the application of the gain is always applied smoothly
from sample to sample in the Dolby Digital decoder.
C.2
Dynamic Range Control (DRC) Algorithm
Overview
The Dolby Digital encoder generates the compression gain words from the input
parameters using a sophisticated algorithm. In addition to computing the desired
dynamic range adjustment, this algorithm also includes an overload protection
component that ensures that decoders applying the compression gain words do not
result in overload, even when downmixing multiple coded channels to fewer output
channels. The high-level overview of this algorithm is shown in Figure C-1.
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Dolby Digital Professional Encoding Guidelines
Dolby Digital Dynamic Range Compression
Multichannel
audio program
material
Compression
profile
Downmix
overload
protection
Measure
input level
Predict
worst-case
downmix
Adjust for
dialnorm
Compute desired
compression gain
words
Dialog
Normalization
value
Desired
compression
profile
Adjust for
dialnorm
Compute maximum
gain
allowable
compression
wor ds
gain words
Compute actual
compression
gain words
gain words
Compression
gain words
Figure C-1 Overview of Dynamic Range Control Algorithm
The multichannel audio program is first analyzed to determine the overall input level,
as well as to determine the worst-case linear PCM downmix. Both of these measures
are then adjusted as necessary by the Dialog Normalization (dialnorm) value.
Next, the dialnorm-adjusted input level measurement is used to compute the desired
compression gain words, based on the selected Dynamic Range Compression profile.
Separately, the dialnorm-adjusted worst-case downmix value is used to compute the
maximum allowable compression gain word, i.e., the largest gain word that can be
accommodated by the decoder without causing overload.
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Dolby Digital Professional Encoding Guidelines
Dolby Digital Dynamic Range Compression
Finally, the desired compression gain word is compared to the maximum allowable gain
word to determine the actual compression gain word that can be inserted into the Dolby
Digital bitstream. If the desired gain word will result in decoder overload, then the actual
gain word is limited to a value no larger than the maximum allowable gain word.
C.3
Compression Characteristic
The compression characteristic used by the Dynamic Range Compression profiles is
made up of five regions that encompass the entire audio dynamic range. The regions
are: constant boost for very soft signals, variable boost for moderately soft signals, no
compression action for average signals (the null band), variable cut for moderately
loud signals, and constant cut for very loud signals.
These five regions are defined using five key parameters: the maximum boost and
maximum cut, the boost ratio and cut ratio, and the null band width as shown in the
center portion of Figure C-2. A sixth parameter, not shown in Figure C-2, specifies
where the null band is located relative to the dialnorm reference playback level—
from this location, the location of all other regions can be determined.
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Dolby Digital Professional Encoding Guidelines
-
Time
constant
selector
Adjusted
input level
Smoothing
filter
boost ratio
compression gain
+
Dolby Digital Dynamic Range Compression
max
boost
null band
cut ratio
input level
Smoothing
filter
Compression
gain words
Figure C-2 Dynamic Range Compression Core
The purpose of the null band is to provide an area in which no compression action is
applied to the program material. This is useful if the program has already been
compressed, or is of limited dynamic range. The location of this null band is very
important: It is located relative to the dialnorm reference playback level so that
normalized signals are not further amplified or attenuated. For some of the Dynamic
Range Compression profiles, the null band is positioned so its lower edge coincides with
the reference level. This is done to minimize the compression action for excursions above
the dialnorm reference playback level. Excursions below the reference level are already
minimally processed due to the rather slow time constants for compression decay.
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Dolby Digital Professional Encoding Guidelines
C.4
Dolby Digital Dynamic Range Compression
Dynamic Range Compression Profiles
All of the parameters that describe the dynamic range compression core are specified
by the selected Dynamic Range Compression profile. This includes the time constant
selection parameters (decay and attack thresholds, fast and slow time constants for
both attack and decay) as well as the compression characteristic (max boost and cut,
boost and cut ratio, null bandwidth and position).
Table C-1 provides the specific parameter values for each of the Dynamic Range
Compression profiles (in Line Mode). These profiles are available in version 6.3.0 of
the Dolby Digital professional encoder software. The type of profiles and the values
presented are subject to revision in future software releases. For the compression
characteristic fields, the range of input level is shown for each of the regions,
measured in dB relative to a full-scale sine wave.
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Dolby Digital Dynamic Range Compression
Table C-1 Dynamic Range Compression Profile Parameter Values
Film
Standard
time constant (tc) selection
attack threshold 15 dB
decay threshold 20 dB
fast attack tc
10 ms
slow attack tc
100 ms
slow decay tc
3s
fast decay tc
1s
10 blocks
hold off period
(53 ms)
Compression characteristic
max boost
6 dB
(abs range)
(-43 dBFS)
boost ratio
2:1
(abs range)
(-43 to -31)
null band width 10 dB
(abs range)
(-31 to -21)
cut ratio
20:1
(abs range)
(-21 to +4)
max cut
24 dB
(abs range)
(+4 dBFS)
Profile Name
Film Light
Music
Standard
Music
Light
Speech
15 dB
20 dB
10 ms
100 ms
3s
1s
10 blocks
(53 ms)
15 dB
20 dB
10 ms
100 ms
10 s
1s
10 blocks
(53 ms)
15 dB
20 dB
10 ms
100 ms
3s
1s
10 blocks
(53 ms)
10 dB
10 dB
10 ms
100 ms
1s
200 ms
10 blocks
(53 ms)
6 dB
(-53 dBFS)
2:1
(-53 to -41)
20 dB
(-41 to -21)
20:1
(-21 to +4)
24 dB
(+4 dBFS)
12 dB
(-55 dBFS)
2:1
(-55 to -31)
10 dB
(-31 to -21)
20:1
(-21 to +4)
24 dB
(+4 dBFS)
12 dB
(-65 dBFS)
2:1
(-65 to -41)
20 dB
(-41 to -21)
2:1
(-21 to +9)
15 dB
(+9 dBFS)
15 dB
(-50 dBFS)
5:1
(-50 to -31)
10 dB
(-31 to -21)
20:1
(-21 to +4)
24 dB
(+4 dBFS)
See notes to table on next page.
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Dolby Digital Professional Encoding Guidelines
•
•
•
Dolby Digital Dynamic Range Compression
The dialnorm reference playback level of -31 dBFS (Line mode) provides ample headroom to accommodate program material
with a very wide dynamic range (signal peaks being possibly 20 dB or greater than the average level). In many cases, there is
sufficient headroom for downmixed signals as well. The absolute levels shown in the compression characteristic fields are
derived from the dialnorm reference playback level and are therefore lower than typical for traditional compressors or limiters.
Some absolute ranges extend higher than 0 dBFS. Since a full-scale sine wave cannot exceed 0 dBFS, these ranges should be
interpreted as extrapolated extensions of the allowable range. As a result, it may not be possible in practice to achieve the
maximum cut compression gain words.
If the "None" profile is selected, the dynamic range compression core generates desired compression gain words that are set to
0 dB gain, i.e., no boost or cut. Selecting the "None" profile however, does not disable the downmix overload protection
function and it is possible that the actual compression gain words in the Dolby Digital bitstream will be less than 0 dB.
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Dolby Digital Professional Encoding Guidelines
Appendix D
Dolby Digital Time-Domain Filters
There are five time-domain filters available in Dolby Digital professional encoders.
They are the 90-Degree Phase Shift filter, the Digital Deemphasis filter (not
supported in the Dolby Model DP561 encoder), the DC Highpass Filter, the Channel
Bandwidth Lowpass Filter (not supported in the Dolby Model DP561 encoder), and
the LFE Lowpass Filter.
D.1
90-Degree Phase-Shift Filter
Purpose
The 90-Degree Phase Shift filter provides a means for an encoding engineer to create
a multichannel Dolby Digital bitstream that can be downmixed to a Dolby Surroundcompatible Lt/Rt output. Without this filter, point-source elements panned from
Surround to Center in the multichannel mix would seem to pan from Surround to Left
and then to Center when downmixed to Lt/Rt and reproduced using a Dolby Surround
Pro Logic decoder.
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Dolby Digital Professional Encoding Guidelines ng
Dolby Digital Time Domain Filters
This filter should generally be used whenever encoding a multichannel signal unless
it is known that the 5.1-channel source does not contain point-source element pans.
For example, if the source was recorded using five discrete microphones placed in the
corners of an auditorium, there is no panning between channels and the filter could be
safely disabled. If in doubt, use a DP562 to downmix the 5.1-channel program to
Lt/Rt, Dolby Surround Pro Logic decode the Lt/Rt signals, and then set the filter to
the setting that sounds best.
Description
The 90-degree phase-shift is created using a very long FIR filter. Since this filter
introduces a significant time delay, the other four channels are delayed using a PCM
delay line so that all six channels are kept in sample alignment. This filter has exactly
90-degree phase shift at all frequencies. The magnitude response is flat across most of
the spectrum, rolling off at the lower edge of the audio band (-3 dB below 30 Hz).
D.2
Digital Deemphasis Filter
Purpose
The Digital Deemphasis filter is used to de-emphasize any 50/15 µs pre-emphasized
linear PCM signals that may be presented to the inputs of the Dolby Digital encoder.
Pre-emphasis is a technique that was once commonly used to reduce the harshness of
A/D and D/A converters. In a system using pre-emphasis, the analog input signal is
Dolby Digital Professional Encoding Guidelines ng
Dolby Digital Time Domain Filters
passed through a high-frequency-boost shelf filter and then A/D converted. At the
output of the playback system, the D/A converter reconstructs the analog output
signal and passes it through a high-frequency-cut shelf filter. Any high-frequency
noise introduced in the A/D or D/A would be attenuated by the shelf filter cut amount
(10 dB). Pre-emphasized linear PCM signals are identified using a special channel
status subside bit in the S/PDIF or AES/EBU digital audio bitstream. Depending on
the state of this bit, the D/A converter knows whether or not to pass the analog signal
through the high-frequency-cut shelf filter.
Dolby Digital bitstreams do not carry this pre-emphasis bit, and thus all Dolby Digital
bitstreams are assumed to be encoded without pre-emphasis. If pre-emphasized linear
PCM samples are to be encoded with a Dolby Digital encoder, they must first be deemphasized otherwise the output of the decoder sounds unnaturally bright. This can
be done by using the Digital Deemphasis filter that is built into the Dolby Digital
encoder. The preferred method for this is to have the filter automatically switch in
whenever the pre-emphasis bit is detected in the S/PDIF or AES-EBU channel status
subcode fields.
Description
The Digital Deemphasis filter is a first-order high-frequency-cut shelf filter designed
to match the required analog filter at all sample rates. The Dolby Model DP561
encoder does not support this filter.
Dolby Digital Professional Encoding Guidelines ng
D.3
Dolby Digital Time Domain Filters
DC Highpass Filter
Purpose
The DC Highpass Filter is used to block DC from being Dolby Digital encoded. This
is important, as a DC offset requires some amount of data rate to encode even though
it is not audible, thereby wasting bits. Another benefit of using this filter is that the
meter values do not get stuck at the DC offset level during very quiet passages (DC
offset can easily be greater than -60 dBFS). The DC Highpass Filter should always be
enabled unless the encoding engineer is absolutely sure that there is no DC in the
input audio. To check for DC, disable the filter and view the meters during very quiet
passages to see if they remain fixed above -60 dBFS.
Description
The DC Highpass Filter is a first-order high-pass filter with unity gain across the
entire audible spectrum, and a rolloff at very low frequencies (-3 dB below 1 Hz).
D.4
Channel Bandwidth Lowpass Filter
Purpose
The Channel Bandwidth Lowpass Filter is used to roll off the high frequency content in
the input signal at a frequency just below that specified by the Dolby Digital audio
Dolby Digital Professional Encoding Guidelines ng
Dolby Digital Time Domain Filters
bandwidth boundary. Using this filter ensures that the audio signal is completely
contained within the Dolby Digital audio bandwidth. By providing a smooth transition at
the upper bandwidth edge, this filter helps to minimize artifacts that may arise if the input
signal contains significant high-frequency energy. In general, this filter should be enabled
unless the encoding engineer is confident that the input signal does not contain
appreciable high-frequency energy above the Dolby Digital audio bandwidth.
Description
The Channel Bandwidth Lowpass Filter is a sixth-order low-pass filter with unity
gain over most of the audio spectrum and a steep rolloff just below the Dolby Digital
audio bandwidth boundary. Note that the Dolby Digital audio bandwidth is variable,
and depends on the selected Data Rate, Audio Coding Mode (also referred to as
Channel Mode), and input audio sample rate. The Dolby Model DP561 encoder does
not support this filter.
D.5
LFE Lowpass Filter
Purpose
The LFE Lowpass Filter is used to band-limit the LFE channel input signal in the
Dolby Digital encoder. If this filter is not enabled, wideband signal content fed to the
LFE channel input of the encoder will produce significant audible artifacts at the
Dolby Digital Professional Encoding Guidelines ng
Dolby Digital Time Domain Filters
output of the decoder. This filter should always be enabled unless it is known that the
LFE channel input signal has already been properly band-limited.
Description
The LFE Lowpass Filter is an eighth-order low-pass filter with unity gain at low
frequencies and a steep rolloff at the upper edge of the LFE bandwidth (-3 dB above
120 Hz).
Dolby Digital Professional Encoding Guidelines
Appendix E
Mix and Mastering Data Sheets
S99/11930/XXXXX
E-7
Dolby Digital Professional Encoding Guidelines
Mix and Mastering Data Sheets
Mix Data Sheet
Date
Project
Client
Studio
/
/
Project No.
Producer
Engineer
Sampling Frequency
32 kHz
44.1 kHz
48 kHz
Bit Resolution
16-bit
18-bit
20-bit
24-bit
Time Code Format
25 fps
29.97 NDF
Tape Format
Surround Level SPL
Calibration
ADAT
DA-88
½" Digital
Data Cartridge (JAZ)
Equal to Front
-3 dB to Front
CHI
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
CH2
CH3
CH4
CH5
Time Code
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
CH6
Program
Notes:
E-1
CH7
CH8
Dolby Digital Professional Encoding Guidelines
Mix and Mastering Data Sheets
Mastering Information
/
Date
Project
Client
Studio
/
Project No.
Producer
Engineer
Sampling Frequency
32 kHz
44.1 kHz
48 kHz
Bit Resolution
16-bit
18-bit
20-bit
24-bit
Time Code Format
25 fps
29.97 NDF
Tape Format
ADAT
DA-88
½" Digital
Data Cartridge (JAZ)
Dolby Digital Encoding Information
Bitstream Information
Processing
Audio Service Configuration
Default
Digital Deemphasis
LFE Filter
Copyright Bit
DC Highpass Filter
ON
OFF
Dial Norm Setting: _______________
Center Mix Level: ___________
Bandwidth Lowpass
Mix Level: _____________________
Surround Mix Level: _________
LFE Lowpass Filter
Data Rate: ______________________
Surround Channel Processing
Dynamic Range Compression
90-Degree Phase-Shift
None
Speech
3 dB Attenuation
Mode
:
:
:
:
:
:
:
:
:
:
:
:
:
CHI
CH2
CH3
CH4
Time Code
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
:
;
CH5
Program
E-2
Film:
Music:
Std.
Std.
Light
Light
CH6
CH7
CH8
Dolby Digital Professional Encoding Guidelines
Appendix F
Glossary
Note: The items with the 2 in the definition are Dolby terms.
AC-3
Audio Coding algorithm number 3. Refer to Dolby Digital. 2
ADPCM (Adaptive
Differential Pulse Code
Modulation)
A pulse code modulation (PCM) system typically operating at a
high sampling rate whereby coding is based on a prior knowledge
of the signal to be processed (i.e., greater than, equal to, or less
than the previous sample). The system is adaptive in that digital
bits of code signify different sizes of signal change depending on
the magnitude of the signal. Refer to PCM.
AES (Audio Engineering
Society)
The official association of technical personnel, scientists,
engineers, and executives in the audio field.
F-1
Dolby Digital Professional Encoding Guidelines
Glossary
AES/EBU (Audio
Engineering
Society/European
Broadcasting Union)
interface
The serial transmission format standardized for professional
digital audio signals (AES3-1992 AES Recommended Practice
for Digital Audio Engineering – Serial Transmission Format for
Two-Channel Linearly Represented Digital Audio Data). A
specification using time division multiplex for data, and balanced
line drivers to transmit two channels of digital audio data on a
single twisted-pair cable using 3-pin (XLR) connectors. Peak-topeak values are between 3 and 10V with driver and cable
impedance specified as 110 ohms.
ATSC (Advanced
Television Systems
Committee)
The Advanced Television Systems Committee was formed to
establish voluntary technical standards for advanced television
systems, including high definition digital television (HDTV).
Bitstream (also bit
stream)
A binary signal without regard to grouping according to
character. A continuous series of bits transmitted on a line.
Codec
A device for converting signals from analog to coded digital and
then back again for use in digital transmission schemes. Most
codecs employ proprietary coding algorithms for data
compression.
DAB (Digital Audio
Broadcasting)
NRSC (National Radio Systems Committee) term for the next
generation of digital radio broadcast.
Data rate
The speed at which digital information is transmitted, typically
expressed in hertz (Hz), bits/second (b/s), or bytes/sec (B/s).
Dolby Digital Professional Encoding Guidelines
Glossary
Data reduction (also
referred to as data
compression)
The process of reducing the number of recorded or transmitted
digital data samples through the exclusion of redundant or
unessential samples.
DBS
Direct Broadcast by Satellite.
De-emphasis
A change in frequency response characteristic complementary to
that introduced by pre-emphasis. Refer to Pre-emphasis.
Dialog Normalization
Refer to Chapter 4.
Dialog Normalization
value (dialnorm)
The Dialog Normalization value is a Dolby Digital parameter that
describes the long-term average dialog level of the associated
program. It may also describe the long-term average level of
programs that do not contain dialog, such as music. This level is
specified on an absolute scale ranging from -1 dBFS to -31 dBFS.
Dolby Digital decoders attenuate programs based on the Dialog
Normalization value in order to achieve uniform playback level. 2
Dolby Digital (formerly
AC-3)
A perceptual audio coding system based upon transform coding
techniques and psycho-acoustic principles. Frequency-domain
processing takes full advantage of noise masking by confining
quantization noise to narrow spectral regions where it will be
masked by the audio signal. Designed as an emissions (delivery)
system, Dolby Digital provides flexible coding of up to 5.1 audio
channels at a variety of data rates. In addition, Dolby Digital
bitstreams carry informational data about the associated audio
Refer to Metadata. 2
Dolby Digital Professional Encoding Guidelines
Glossary
Dolby Surround
A passive system that matrix encodes four channels of audio into
a standard two-channel format (Lt/Rt). When the signal is
decoded using a Dolby Surround Pro Logic decoder, the left,
center and right signals are recovered for playback over three
front speakers and the surround signal is distributed over the rear
speakers. Refer to Lt/Rt and Dolby Surround Pro Logic. 2
Dolby Surround Pro
Logic (DSPL)
An active decoding process designed to enhance the sound
localization of Dolby Surround encoded programs through the
use of high-separation techniques. Dolby Surround Pro Logic
decoders continuously monitor the encoded audio program and
evaluate the inherent soundfield dominance, applying
enhancement in the same direction and in proportion to that
dominance. Refer to Dolby Surround and Lt/Rt. 2
Downmix
A process wherein multiple channels are summed to a lesser
number of channels.
DRC (Dynamic Range
Control)
A feature of Dolby Digital that allows the end user to retain or
modify the dynamic range of a Dolby Digital encoded program
upon playback. The amount of control is dictated by encoder
parameter settings and decoder user options. 2
DSS
Direct Satellite System.
DTV (Digital Television)
A term used for all types of digital television including High
Definition Television (HDTV) and Standard Definition
Television (SDTV).
Dolby Digital Professional Encoding Guidelines
Glossary
DVB
Digital Video Broadcasting.
DVD (Digital Versatile
Disc)
A type of optical disc. There are two possible sizes: 12 cm
(standard) and 8 cm. There are five types of 12 cm discs: DVD-5,
single-sided, single-layer 4.7 billion byte capacity; DVD-9,
single-sided, dual-layer, 8.54 billion byte capacity; DVD-10,
dual-sided, single-layer, 9.4 billion byte capacity; DVD-14, dualsided, single/dual-layer, 13.24 billion byte capacity; DVD-18,
dual-sided, dual-layer, 17.08 billion byte capacity. There are three
formats of read-only discs: DVD-ROM (games and computer
use), DVD-Video (movies), and DVD-Audio (music-only). There
are also write-once and rewritable disc formats.
Dynamic Range
The ratio, expressed in decibels (dB), of the maximum to the
minimum signal level, whether alone or in relation to a device or
system.
Dynamic Range
Compression
Level adjustment applied to an audio signal in order to limit the
difference, or range of the loudest to the softest sounds.
Dolby Digital Professional Encoding Guidelines
Glossary
EBU (European
Broadcasting Union)
Created in 1950 and headquartered in Geneva, Switzerland, the
EBU is the world's largest professional association of national
broadcasters. The EBU assists its members in all areas of
broadcasting, briefing them on developments in the audiovisual
sector, providing advice, and defending their interests via
international bodies. The Union has active members in European
and Mediterranean countries and associate members in countries
elsewhere in Africa, the Americas, and Asia.
HDTV (High Definition
Television)
The standard for digital television in North America that includes
a definition for picture quality of at least two million pixels
(compared to 336,000 pixels for NTSC). The audio standard is
Dolby Digital.
Headroom
The difference (in dB) between the nominal level (average) and
the maximum operating level (just prior to “unacceptable”
distortion) in any system or device. Because it is a pure ratio,
there is no unit or reference-level qualifier associated with
headroom—simply "dB"; headroom expressed in dB accurately
refers to both voltage and power.
IEC (International
Electrotechnical
Commission)
A European organization (headquartered in Geneva, Switzerland)
involved in international standardization within the electrical and
electronics fields.
Dolby Digital Professional Encoding Guidelines
Glossary
ITU (International
Telecommunications
Union)
Headquartered in Geneva, Switzerland, this international
organization is involved in coordinating global telecom networks
and services. Both governmental and private sector
representatives are included.
JPEG (Joint
Photographic Experts
Group)
This is a group of experts nominated by national standards bodies
and major companies to work to produce standards for continuous
tone image coding. The 'joint' refers to its status as a committee
working on both ISO and ITU-T standards. The 'official' title of
the committee is ISO/IEC JTC1 SC29 Working Group 1, and is
responsible for both JPEG and JBIG standards.
LAeq
An Leq measurement using A weighting. Refer to Leq and
Weighting.
Leq
Leq represents the continuous noise level, equivalent in loudness
and energy, to the fluctuating sound signal under consideration.
Refer to LAeq.
LFE (Low-Frequency
Effects)
The optional LFE channel (also referred to as the “boom”
channel) carries a separate, limited, frequency bandwidth signal
that complements the main channels. It delivers bass energy
specifically created for subwoofer effects or low-frequency
information derived from the other channels. The LFE channel is
the “.1” in 5.1-channel audio.
Dolby Digital Professional Encoding Guidelines
Glossary
Line Mode
A Dolby Digital decoder operational mode. The dialnorm
reference playback level is -31 dBFS and dynrng words are used
in dynamic range compression. Refer to Dynamic Range
Compression. 2
Lo/Ro (Left only, Right
only)
A type of two-channel downmix for multichannel audio
programs. Lo/Ro downmixes are intended for applications where
surround playback is neither desired nor required. 2
LPCM (Linear Pulse
Code Modulation)
A pulse code modulation (PCM) system in which the signal is
converted directly to a PCM word without companding, or other
processing. Refer to PCM.
LS/RS (Left Surround,
Right Surround)
The actual channels or speakers delivering discrete surround
program material. 2
Lt/Rt (Left total, Right
total)
Two-channel delivery format for Dolby Surround. Four channels
of audio, Left, Center, Right, and Surround (LCRS) are matrix
encoded for two-channel delivery (Lt/Rt). Lt/Rt encoded
programs are decoded using Dolby Surround and Dolby Surround
Pro Logic decoders. Refer to Dolby Surround and Dolby
Surround Pro Logic. 2
Metadata (“data about
the data”)
The descriptive and supporting data that is connected to the
program or the program elements. It is intended to both aid the
direct use of program content and support the retrieval of content
as needed during the post-production process.
Dolby Digital Professional Encoding Guidelines
Glossary
MPEG (Moving Picture
Experts Group)
A working group within SMPTE who set, among other things,
specifications for compression schemes for audio and video
transmission. A term commonly used when referring to their
associated data compression technologies (MPEG).
NTSC (National
Television System
Committee)
Named after a committee that worked with the FCC in
formulating standards for the current United States analog color
television system. Now describes the American system of color
telecasting consisting of 525 lines transmitted at 29.97interlaced
frames per second. It is a composite of red, green, and blue
signals for color and includes a FM frequency for audio and an
MTS signal for stereo.
PAL (Phase Alternating
Line)
A European television standard that uses 625 lines of resolution
(100 more than NTSC) transmitted at 25 interlaced frames (50
fields) per second.
PCM (Pulse Code
Modulation)
Pulsed modulation in which the analog signal is sampled
periodically and each sample is quantized and transmitted as a
digital binary code.
Peak value
The maximum numerical value reached whether the signal is
positive or negative.
Perceptual Coding
Refer to Appendix A, The Dolby Digital Algorithm - Theory of
Operations.
Dolby Digital Professional Encoding Guidelines
Glossary
Pink noise
A type of noise whose amplitude is inversely proportional to
frequency over a specified range. Pink noise is characterized by a
flat amplitude response per octave band of frequency (or any
constant percentage bandwidth), i.e., it has equal energy, or
constant power, per octave. Pink noise can be created by passing
white noise through a filter having a 3 dB/octave slope.
Pre-emphasis
An intentional change made in the frequency response of a
recording system to improve the signal-to-noise (S/N) ratio or to
reduce distortion. Typically, a high-frequency boost is used
during recording, followed by complementary de-emphasis (a
high-frequency cut) during playback.
RF Mode
A Dolby Digital decoder operational mode intended primarily for
cable set-top boxes that are connected to the RF (antenna) input
of a television set. The dialnorm reference playback level is -20
dBFS and compr words are used in dynamic range compression.
Refer to Dynamic Range Compression. 2
RMS (Root Mean
Square)
The value assigned to an alternating current or voltage that results
in the same power dissipation in a given resistance as dc current
or voltage of the same numerical value. Calculated as 0.707 of
peak amplitude of a sine wave at a given frequency.
SDTV
Standard Definition Television.
Dolby Digital Professional Encoding Guidelines
Glossary
SECAM (“Séquentiel
Couleur avec Memoire”
sequential color with
memory)
A color television system with 625 lines per frame and 50 fields
per second developed by France and the former USSR and used
in some countries that do not use either NTSC or PAL.
S/N
Refer to SNR.
SNR (Signal to Noise
Ratio)
The ratio of the magnitude of the signal to that of the noise,
generally expressed in decibels. An audio measurement of the
residual noise of a unit, stated as the ratio of signal level (or
power) to noise level (or power), normally expressed in decibels.
The "signal" reference level must be stated. Typically this is
either the expected nominal operating level, +4 dBu for
professional audio, or the maximum output level, usually around
+20 dBu. The noise is measured using a true RMS type voltmeter
over a specified bandwidth, and sometimes using weighting
filters. All of these criteria must be stated for an SNR
specification to have meaning.
Dolby Digital Professional Encoding Guidelines
SMPTE (Society of
Motion Picture and
Television Engineers)
S/PDIF (Sony/Philips
Digital Interface Format)
Glossary
An international technical society devoted to advancing the
theory and application of motion-imaging technology including
film, television, video, computer imaging, and
telecommunications. Members of the Society are engineers,
executives, technical directors, cameramen, editors, consultants,
and specialists in film processing, film and television production
and post-production, and practitioners from almost every other
discipline in the motion-imaging industry. The Society was
founded in 1916, as the Society of Motion Picture Engineers. The
T was added in 1950 to embrace the emerging television industry.
The SMPTE is recognized around the globe as a leader in the
development of standards and authoritative, consensus-based
recommended practices (RPs), and engineering guidelines (EGs).
The Society serves all branches motion imaging including film,
video, and multimedia.
A consumer version of the AES/EBU digital audio
interconnection standard. The format uses a 75-ohm coaxial cable
with RCA connectors and has nominal peak-to-peak values of
0.5V. The frame ordering differs slightly than that of AES/EBU,
specifically in the channel status information. Refer to AES/EBU
interface.
S/PDIF is equivalent to IEC 61937 when used for data, as in
Dolby Digital.
Dolby Digital Professional Encoding Guidelines
Glossary
SPL (Sound Pressure
Level)
The SPL of a sound is equal to twenty times the logarithm (base
10) of the ratio of the root-mean-square (RMS) sound pressure to
the reference sound pressure. As a point of reference, 0 dB-SPL
equals the threshold of hearing, while 140 dB-SPL produces
irreparable hearing damage.
Weighting
In a sound level meter, this is a filter that creates a response that
corresponds to the ear’s varying sensitivity at different loudness
levels. A weighting corresponds to the sensitivity of the ear at
lower listening levels. The filter design weights or is more
sensitive in certain frequency bands than others. The goal is to
obtain measurements that correlate well with the subjective
perception of noise.
ANSI A-weighting
The A-curve is a wide bandpass filter centered at 2.5 kHz, with
~20 dB attenuation at 100 Hz, and ~10 dB attenuation at 20 kHz.
Therefore, it tends to heavily roll off the low end, with a more
modest effect on high frequencies. It is essentially the inverse of
the 30-phon (or 30 dB-SPL) equal-loudness curve of a FletcherMunson.
ANSI B-weighting
The B-weighting curve is used for intermediate level sounds and
has the same upper corner as the C-weighting, but the lower
amplitude corner is 120 Hz.
ANSI C-weighting
The C-curve is basically "flat," with -3 dB corners of 31.5 Hz and
8 kHz, respectively.
Dolby Digital Professional Encoding Guidelines
CCIR 468-weighting
Glossary
This filter was designed to maximize its response to the types of
impulsive noise often coupled into audio cables as they pass
through telephone switching facilities. The CCIR 468-curve
peaks at 6.3 kHz, where it has 12 dB of gain (relative to 1 kHz).
From here, it gently rolls off low frequencies at a 6 dB/octave
rate, but it quickly attenuates high frequencies at ~30 dB/octave
(it is down -22.5 dB at 20 kHz, relative to +12 dB at 6.3 kHz).
This curve derives from the CCIR 468-curve above. Dolby
Laboratories proposed using an average-response meter with the
CCIR 468-curve instead of the costly true quasi-peak meters used
CCIR ARM-weighting or by the Europeans in specifying their equipment. They further
proposed shifting the 0 dB reference point from 1 kHz to 2 kHz
CCIR 2 kHz-weighting
(in essence, sliding the curve down 6 dB). This became known as
the CCIR ARM (average response meter), as well as the CCIR 2
kHz-weighting curve.
White Noise
A random signal having the same energy level at all frequencies
(in contrast to pink noise which has constant power per octave
band of frequency).
Dolby Digital Professional Encoding Guidelines
Appendix G
Bibliography
Davidson, Grant A., Fielder, Louis D., and Link, Brian D., “Parametric Bit Allocation in a
Perceptual Audio Coder,” AES 97th Convention, November 1994, Preprint 3921, available on the
Dolby web site at www.dolby.com/tech/.
Davis, Mark F., “The AC-3 Multichannel Coder,” AES 95th Convention, October, 1993,
available on the Dolby web site at www.dolby.com/tech.
Davis, Mark F., “The Big Squeeze, The Theory and Practice of Dolby Digital,“ Audio, Vol. 81,
No. 7, July 1997.
Davis, Mark F. and Todd, Craig C., “AC-3 Operation, Bitstream Syntax, and Features,” AES
97th Convention, November, 1994, Preprint 3910.
Dolby Laboratories, Dolby Surround Mixing Manual, available on the Dolby web site at
www.dolby.com/tech/.
Dressler, Roger, "Dolby Pro Logic Surround Decoder, Principles of Operation,” available on the
Dolby web site at www.dolby.com/tech.
G-1
Dolby Digital Professional Encoding Guidelines
Bibliography
Gundry, Kenneth, “Keeping the Audio Unchanged,” Surround Professional, Vol. 1, No. 2,
January 1999.
Todd, Craig C., "Distribution and Emission of Multichannel Audio Programs, and Implications
for Post-Production," 139th SMPTE Technical Conference, November 1997, Paper 139-15; also
“Film and Video Origination in the Era of DTV Broadcasting and Distribution,” SMPTE 1997;
also SMPTE Journal, October 1998.
Todd, Craig C., “Loudness Uniformity and Dynamic Range Control for Digital Multichannel
Audio Broadcasting,” IBC, 1995, pp. 145-150.
Todd, Craig C., Davidson, Grant A., Davis, Mark F., Fielder, Louis D., Link, Brian D., and
Vernon, Steve, “AC-3: Flexible Perceptual Coding for Audio Transmission and Storage,” AES
96th Convention, February-March 1994, available on the Dolby web site at www.dolby.com/tech/.
Vernon, Steve, “Design and Implementation of AC-3 Coders,” IEEE Transactions on Consumer
Electronics, Vol. 41, No. 3, August 1995.
“The Digital Audio Compression Standard-ATSC A/52,” available from the Dolby web site at
www.dolby.com/tech/ or the ATSC web site at www.atsc.org.
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