TR41.3.3-10-02-008-L-Skype Audio Specification v4.0.5,GHess

TR41.3.3-10-02-008-L-Skype Audio Specification v4.0.5,GHess
Document Cover Sheet
Project Number
Document Title
Skype Audio Specification v4.0.5
Source
MWM Acoustics
Contact
Name:
Glenn Hess
Suite 520
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317-596-1721
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For Incorporation Into TIA Publication
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The document to which this cover statement is attached is submitted to a Formulating Group or
sub-element thereof of the Telecommunications Industry Association (TIA) in accordance with the
provisions of Sections 6.4.1–6.4.6 inclusive of the TIA Engineering Manual dated March 2005, all of
which provisions are hereby incorporated by reference.
Abstract
The attached Skype™ specification is drawing world-wide attention by audio product manufactures. This
public domain document covers VoIP transmission test methods and performance requirements based
exclusively on the Skype™ soft client. The requirements are divided into several groups covering
handsets, headsets, speakerphones, and other audio devices such as cordless, DECT, and Bluetooth
products. Telecom audio products must meet these audio requirements to be Skype™ certified. This
specification could supersede TIA 810B and 920 for some product companies here in North America.
The Skype™ specification has three priority levels of audio performance identified as P1, P2, and P3,
where P1 is a mandatory must comply requirement, P2 a should pass, and P3 nice or desirable to meet.
The test conditions and/or requirement limits differ between the three priorities. Test parameters include
send and receive frequency response, overall sensitivity, volume level, distortion, speech-to-noise,
stability, crosstalk, echo, and ring tone loudness for normal band, wideband, and super wideband devices.
These measurements are performed on an ITU-T compatible HATS with the Type 3.3 ear simulator.
Page 1 of 1
Hardware Certification Audio Specification
Copyright © 2009 Skype. All Rights Reserved.
Last saved: 2009-04-01
Author: Markus Vaalgamaa
Ergo Esken
Status: Final
Filename: Test_SpecAudio_4.0.5.doc
Security Classification: Public
Approved by:
Ed Botterill
Version: 4.0.5
2009-04-01
Security Classification: Public
2 / 68
SUMMARY OF REVISIONS
Version
Date
Comments
Valid
4.0.5
2009-04-01
Fixed some cross-references.
2009-04-01
4.0.4
2009-03-31
Added sub categories
2009-04-01
General audio requirements - All groups:
Additional requirements for PC or Mac
accessories
Headset audio UI: Audio performance
requirements for Skype Super Wideband
Certification
Definitions and references moved to end of
document.
4.0.1
2008-11-06
Few typos corrected, more explanations
added based on comments by
HeadAcoustics
2009-04-01
4.0
2008-10-01
Specification changes frozen. Changes are
listed down in Appendix
2009-04-01
3.0
2008-01-01
Specification changes frozen.
2008-07-01
2.2
2007-12-31
List of major modifications:
Modified requirement:
2008-07-01
•
Divided Additional delay to speech
signal to receiving and sending
direction requirements
•
Priority: 1 Minimum crosstalk from
receiving to sending direction to
Headset, Handset and Other Audio
product groups
Added requirements:
•
Error!
Error!
Error!
Error!
Reference source not found.
Reference source not found.
Reference source not found.
Reference source not found.
To headset, handset and speakerphone audio
UI groups:
•
Priority: 1 Microphone - Sensitivity at
loud speech level
•
Priority: 1 Microphone – Speech to self
noise ratio during speech activity
To speakerphone UI group:
•
Priority: 1,2 & 3 Microphone – Speech
to background noise ratio
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
3 / 68
CONTENTS
1.
INTRODUCTION .................................................................................................................................... 6
1.1
PURPOSE .............................................................................................................................................. 6
1.2
AUDIO UI GROUPS ................................................................................................................................. 6
1.2.1
Headset audio UI group ............................................................................................................... 7
1.2.2
Handset audio UI group ............................................................................................................... 7
1.2.3
Speakerphone audio UI group..................................................................................................... 7
1.2.4
Other audio product group........................................................................................................... 8
1.2.5
Non-audio product group ............................................................................................................. 8
1.3
AUDIO REQUIREMENTS AND PRIORITIES – OVERVIEW ............................................................................... 9
1.3.1
Audio performance....................................................................................................................... 9
1.3.2
Quality expectation of the audio UI groups.................................................................................. 9
1.3.3
Use of the test case priorities ...................................................................................................... 9
2.
GENERAL AUDIO REQUIREMENTS VALID FOR ALL GROUPS .................................................... 10
2.1
ALL GROUPS: AUDIO PERFORMANCE REQUIREMENTS ............................................................................ 10
2.1.1
Priority: 1 Round trip delay of speech signals ........................................................................... 10
2.1.2
Priority: 1 Total quality loss in sending direction........................................................................ 10
2.1.3
Priority: 1 Total quality loss in receiving direction...................................................................... 11
2.2
ALL GROUPS: ADDITIONAL REQUIREMENTS FOR PC OR MAC ACCESSORIES ............................................ 11
2.2.1
Priority: 1 Analog gain adjustment latency ................................................................................ 11
2.2.2
Priority: 1 Device – Sampling frequency accuracy .................................................................... 12
2.3
GENERAL AUDIO TEST INSTRUCTIONS ................................................................................................... 12
2.3.1
Objective testing measurement setup ....................................................................................... 12
3.
HEADSET AUDIO UI GROUP ............................................................................................................. 14
3.1
HEADSET: AUDIO PERFORMANCE REQUIREMENTS ................................................................................. 14
3.1.1
Priority: 1 Microphone – Sensitivity at normal speech level ...................................................... 14
3.1.2
Priority: 2 Microphone – Sensitivity at lowered speech level..................................................... 14
3.1.3
Priority: 1 Microphone – Sensitivity at loud speech level .......................................................... 14
3.1.4
Priority: 1 Microphone – Frequency response........................................................................... 14
3.1.5
Priority: 2 Microphone – Frequency response........................................................................... 15
3.1.6
Priority: 1 Microphone – Speech to self noise ratio ................................................................... 16
3.1.7
Priority: 2 Microphone – Speech to self noise ratio ................................................................... 16
3.1.8
Priority: 3 Microphone – Speech to self noise ratio ................................................................... 17
3.1.9
Priority: 2 Microphone – Speech to self noise ratio during speech activity ............................... 17
3.1.10 Priority: 2 Microphone – Speech to background noise ratio ...................................................... 17
3.1.11 Priority: 1 Earpiece – Speech to self noise ratio........................................................................ 17
3.1.12 Priority: 2 Earpiece – Speech to self noise ratio........................................................................ 17
3.1.13 Priority: 3 Earpiece – Speech to self noise ratio........................................................................ 18
3.1.14 Priority: 1 Earpiece – Frequency response ............................................................................... 18
3.1.15 Priority: 2 Earpiece – Frequency response ............................................................................... 19
3.1.16 Priority: 1 Earpiece – Stability of frequency response ............................................................... 19
3.1.17 Priority: 2 Earpiece – Stability of frequency response ............................................................... 20
3.1.18 Priority: 3 Earpiece – Stability of frequency response ............................................................... 20
3.1.19 Priority: 1 Minimum crosstalk from receiving to sending direction............................................. 20
3.2
HEADSET: REQUIREMENTS FOR SKYPE SUPER W IDEBAND CERTIFICATION (OPTIONAL) ........................... 20
3.2.1
Priority: 1 Microphone – Frequency response........................................................................... 20
3.2.2
Priority: 1 Earpiece – Frequency response ............................................................................... 21
3.2.3
Priority: 1 Earpiece – Speech to noise ratio .............................................................................. 22
3.3
HEADSET: SUPPORTING AUDIO DOCUMENTATION REQUIREMENTS .......................................................... 22
3.3.1
Priority: 1 Verifying supporting documentation for Headset Audio UI group ............................. 23
3.4
HEADSET: AUDIO TEST INSTRUCTIONS .................................................................................................. 23
3.4.1
Objective testing measurement setup ....................................................................................... 23
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
4.
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HANDSET AUDIO UI GROUP ............................................................................................................. 25
4.1
HANDSET: AUDIO PERFORMANCE REQUIREMENTS ................................................................................. 25
4.1.1
Priority: 1 Microphone – Sensitivity at normal speech level ...................................................... 25
4.1.2
Priority: 2 Microphone – Sensitivity at lowered speech level..................................................... 25
4.1.3
Priority: 1 Microphone – Sensitivity at loud speech level .......................................................... 25
4.1.4
Priority: 1 Microphone – Frequency response........................................................................... 25
4.1.5
Priority: 2 Microphone – Frequency response........................................................................... 26
4.1.6
Priority: 1 Microphone – Speech to self noise ratio ................................................................... 27
4.1.7
Priority: 2 Microphone – Speech to self noise ratio ................................................................... 27
4.1.8
Priority: 3 Microphone – Speech to self noise ratio ................................................................... 27
4.1.9
Priority: 2 Microphone – Speech to self noise ratio during speech activity ............................... 28
4.1.10 Priority: 2 Microphone – Speech to background noise ratio ...................................................... 28
4.1.11 Priority: 1 Earpiece – Speech to self noise ratio........................................................................ 28
4.1.12 Priority: 2 Earpiece – Speech to self noise ratio........................................................................ 28
4.1.13 Priority: 3 Earpiece – Speech to self noise ratio........................................................................ 28
4.1.14 Priority: 1 Earpiece – Frequency response ............................................................................... 29
4.1.15 Priority: 2 Earpiece – Frequency response ............................................................................... 29
4.1.16 Priority: 3 Earpiece – Frequency response ............................................................................... 30
4.1.17 Priority: 1 Minimum crosstalk from receiving to sending direction............................................. 31
4.1.18 Priority: 1 Earpiece – Stability of frequency response ............................................................... 31
4.1.19 Priority: 2 Earpiece – Stability of frequency response ............................................................... 32
4.1.20 Priority: 3 Earpiece – Stability of frequency response ............................................................... 32
4.1.21 Priority: 1 Earpiece – Suitable volume level for office and home handset (Indoor)................... 32
4.1.22 Priority: 2 Earpiece – Suitable volume level for office and home handset (Indoor)................... 32
4.1.23 Priority: 1 Earpiece – Suitable volume level for “anywhere” handset (Outdoor) ....................... 32
4.1.24 Priority: 2 Earpiece – Suitable volume level for “anywhere” handset (Outdoor) ....................... 33
4.1.25 Priority: 1 Maximum ring tone loudness..................................................................................... 33
4.1.26 Priority: 2 Maximum ring tone loudness..................................................................................... 33
4.1.27 Priority: 3 Maximum ring tone loudness..................................................................................... 34
4.2
HANDSET: SUPPORTING AUDIO DOCUMENTATION REQUIREMENTS .......................................................... 34
4.2.1
Priority: 1 Verifying supporting documentation for Handset audio ............................................ 34
4.3
HANDSET: AUDIO TEST INSTRUCTIONS .................................................................................................. 35
4.3.1
Objective testing measurement setup ....................................................................................... 35
5.
SPEAKERPHONE AUDIO UI GROUP ................................................................................................ 37
5.1
SPEAKERPHONE: AUDIO PERFORMANCE REQUIREMENTS ....................................................................... 37
5.1.1
Priority: 1 Microphone – Sensitivity at normal speech level ...................................................... 37
5.1.2
Priority: 1 Microphone – Sensitivity at lowered speech level..................................................... 37
5.1.3
Priority: 1 Microphone – Sensitivity at loud speech level .......................................................... 37
5.1.4
Priority: 1 Microphone – Frequency response........................................................................... 37
5.1.5
Priority: 2 Microphone – Frequency response........................................................................... 38
5.1.6
Priority: 3 Microphone – Frequency response........................................................................... 39
5.1.7
Priority: 1 Microphone – Speech to self noise ratio ................................................................... 40
5.1.8
Priority: 2 Microphone – Speech to self noise ratio ................................................................... 40
5.1.9
Priority: 3 Microphone – Speech to self noise ratio ................................................................... 41
5.1.10 Priority: 2 Microphone – Speech to self noise ratio during speech activity ............................... 41
5.1.11 Priority: 1 Amount of acoustic echo ........................................................................................... 41
5.1.12 Priority: 2 Amount of acoustic echo ........................................................................................... 41
5.1.13 Priority: 3 Amount of acoustic echo ........................................................................................... 42
5.1.14 Priority: 2 Echo loss in single talk during Skype call.................................................................. 42
5.1.15 Priority: 3 Echo loss in single talk without Skype speech improvements .................................. 43
5.1.16 Priority: 1 Loudspeaker – Frequency response......................................................................... 43
5.1.17 Priority: 2 Loudspeaker – Frequency response......................................................................... 44
5.1.18 Priority: 3 Loudspeaker – Frequency response......................................................................... 44
5.1.19 Priority: 1 Loudspeaker – Suitable volume level for quiet office use ......................................... 45
5.1.20 Priority: 1 Loudspeaker – Distortion at quiet office use ............................................................. 45
5.1.21 Priority: 2 Loudspeaker – Suitable volume level for normal office use...................................... 46
5.1.22 Priority: 2 Loudspeaker – Distortion at normal office use .......................................................... 46
5.1.23 Priority: 3 Loudspeaker – Suitable volume level for noisy office use ........................................ 46
5.1.24 Priority: 3 Loudspeaker – Distortion at noisy office use............................................................. 46
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
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5.1.25 Priority: 2 Loudspeaker – Volume level at maximum operating distance.................................. 47
5.1.26 Priority: 2 Microphone – Sensitivity at maximum operating distance ........................................ 47
5.1.27 Priority: 3 Microphone – Speech to self noise ratio at maximum operating distance................ 47
5.2
SPEAKERPHONE: SUPPORTING AUDIO DOCUMENTATION REQUIREMENTS ................................................ 47
5.2.1
Priority: 1 Verifying supporting documentation for Speakerphone audio .................................. 47
5.3
SPEAKERPHONE: AUDIO TEST INSTRUCTIONS........................................................................................ 48
5.3.1
Objective testing measurement setup ....................................................................................... 48
5.3.2
Subjective testing measurement setup...................................................................................... 49
6.
OTHER AUDIO PRODUCT GROUP ................................................................................................... 51
6.1
OTHER AUDIO PRODUCT: AUDIO PERFORMANCE REQUIREMENTS............................................................ 51
6.1.1
Priority: 1 Frequency responses – sending and receiving directions ........................................ 51
6.1.2
Priority: 1 Product provides suitable levels for audio signal output ........................................... 52
6.1.3
Priority: 1 Product provides suitable levels for audio signal input ............................................. 52
6.1.4
Priority: 1 Minimum crosstalk from receiving to sending direction............................................. 52
6.2
OTHER AUDIO PRODUCT: SUPPORTING AUDIO DOCUMENTATION REQUIREMENTS..................................... 52
6.2.1
Priority: 1 Verifying supporting documentation for Other audio product.................................... 52
6.3
OTHER AUDIO PRODUCT: AUDIO TEST INSTRUCTIONS ............................................................................ 53
6.3.1
Objective testing measurement setup ....................................................................................... 53
7.
NON-AUDIO PRODUCT GROUP........................................................................................................ 54
7.1
NON-AUDIO PRODUCT: AUDIO PERFORMANCE REQUIREMENTS ............................................................... 54
7.1.1
Priority: 1 Continuous transmission of speech .......................................................................... 54
7.1.2
Priority: 2 Continuous transmission of speech .......................................................................... 54
7.2
NON-AUDIO PRODUCT: SUPPORTING AUDIO DOCUMENTATION ................................................................ 54
7.2.1
Priority: 1 Verifying supporting documentation for Non-audio product ...................................... 54
7.3
NON AUDIO PRODUCT: AUDIO TEST INSTRUCTIONS ................................................................................ 55
7.3.1
Objective testing measurement setup ....................................................................................... 55
8.
LIST OF ENVIRONMENTS .................................................................................................................. 56
8.1
LIST OF TEST PLATFORMS ................................................................................................................... 56
8.1.1
Skype Audio Test Lab................................................................................................................ 56
8.1.2
Compatible testing environment ................................................................................................ 58
9.
APPENDIX ........................................................................................................................................... 59
9.1
DEFINITIONS........................................................................................................................................ 59
9.2
REFERENCES ...................................................................................................................................... 64
9.3
CHANGES BETWEEN 4.0 AND 3.0 VERSIONS .......................................................................................... 64
9.3.1
Major changes ........................................................................................................................... 64
9.3.2
Introduction, Abbreviations and References.............................................................................. 65
9.3.3
General audio requirements ...................................................................................................... 65
9.3.4
Headset audio UI ....................................................................................................................... 66
9.3.5
Handset audio UI ....................................................................................................................... 66
9.3.6
Speakerphone audio UI ............................................................................................................. 67
9.3.7
Other audio product ................................................................................................................... 67
9.3.8
Non-audio product ..................................................................................................................... 68
9.3.9
List of environments................................................................................................................... 68
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
6 / 68
1. Introduction
This specification defines the audio requirements for Skype Certified Solutions. The requirements
are divided into several groups, based on the acoustic user interface (UI) type.
For each group there are certain audio requirements. The requirements are mostly the same for all
products that fall into one of the categories, but there can be small variances within one group,
depending on the underlying technology.
In addition to the audio requirements, any product under test must comply with general Skype
Certification Specifications which can be downloaded from Skype Developer Zone
(https://developer.skype.com/Certification/Hardware/Specs/ ). A rule to calculate the final test
result for a product is defined in Skype Certification Specifications.
1.1 Purpose
The requirements found in this test specification define the main parts of audio performance,
ergonomic topics and documentation.
The purpose of this document is not to define requirements for all aspects of audio, but rather to
concentrate on parts that affect the end user experience. Thus the tests cases based on these
audio requirements do not replace other necessary testing that a vendor should and must perform
in order to improve the end quality of the product before applying for Skype Certified label.
1.2 Audio UI groups
Skype Certified products are broken into several categories that are based on the acoustic
interface type of the product. The groups are:
•
Headset audio UI,
•
Handset audio UI,
•
Speakerphone audio UI,
•
Other audio products
•
No Acoustic UI audio product group.
One product can belong to several audio UI groups depending on possible usage scenarios of the
product. For example: Wi-Fi phone, can have Handset, Headset and Speakerphone audio UI
functionalities built into it, because it can have a handsfree feature (headset included in the
package) and speakerphone mode support. In these cases requirements and test cases for
several audio UI groups are valid.
Important point to notice is that some audio groups give actual acoustic interface to the user and
others don’t.
The groups that provide acoustic user interface are:
•
Headset audio UI,
•
Handset audio UI and
•
Speakerphone audio UI groups.
Products belonging to these groups must have microphone or similar speech pickup device or
loudspeaker / earpiece to reproduce speech, or even both.
Non-acoustic user interface groups are
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
•
Other audio products
•
Non-audio product group,
7 / 68
They include products that do not have microphone, earpiece or loudspeaker that would be used
for communication. Examples: soundcard, ATA, motherboard.
1.2.1 Headset audio UI group
Headset audio UI product consists of two main components – earpiece(s) and microphone
assembled together so that the headset can be fixed on the user’s head or ear(s). Products that
have microphone and earpieces separated physically (for example desktop microphone and
headphones) also fall into Headset audio UI group.
Skype certification specifications for Headset audio UI group are categorized as follows:
Plug-in Headsets – wired headsets. They usually have standard 3.5 mm mini-plug audio
connectors or USB cable.
Cordless Headsets – wireless headsets. They operate through radio frequencies, for example
Bluetooth, DECT or Infrared.
Headset is connected to another device, like PC or PDA that has Skype running in it. Examples of
Headset audio UI devices are illustrated below:
1.2.2 Handset audio UI group
A handset audio UI product is a handset that the user holds in his hand and puts next to his ear
when in a call, so the form factor of the device is similar to that of a landline or mobile phone. The
handset has both earpiece and microphone in the same device.
Just like the headset, handset can be wired or wireless. Skype certification specifications valid for
this category are Plug-in Handsets and Cordless Handsets.
A handset typically has a keyboard and often a display. A handset can also be mobile or
embedded device, where Skype is running inside the handset itself.
Examples of Handset audio UI devices are mobile phones and landline phones; few pictures
below illustrate the group:
1.2.3 Speakerphone audio UI group
A Speakerphone audio UI product can be speakerphone, handset with speakerphone mode
support or similar. Speakerphone audio UI product consists of two main components –
microphone(s) and loudspeaker(s), usually integrated into the same device, but separate
microphone and loudspeaker can also be viewed as a speakerphone. Often the device is placed
on the table without physical contact with the user.
Audio Requirement Specification
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From audio quality perspective, quite crucial issue is a big enough distance between microphone
and loudspeaker compared to distance to the user. This is due the need to achieve good acoustic
echo cancellation from loudspeaker to microphone.
Unlike the headset and handset audio UI devices, the speakerphone audio UI device can be
shared by several users, for example between users who sit around the table in a conference call.
Conference calls are typically what speakerphones are used for. The speakerphone system may
include several microphones or/and loudspeakers to enable picking up sound from all directions
without attenuation and providing adequate sound volume to all conference call participants.
A speakerphone audio UI device is typically connected to the USB port or soundcard of a
computer, but it can also be wireless. It can have keypad and display.
Speakerphone Skype certification specification is valid for Speakerphone audio UI products. Note
that a handset or in principle even a headset can have a speakerphone audio UI functionality, and
thus belong to Speakerphone audio UI group.
Examples of speakerphone audio UI devices are:
1.2.4 Other audio product group
This product is a part of audio signal chain in Skype environment, and it does not provide
acoustic user interface, but still it can have a strong impact upon the audio quality for the end-toend user experience. Typically it is an interface device that provides a conversion of audio from
one format to another and thus does not improve the speech quality as such. These products can
degrade the quality with additional delay, bandwidth limitation, noise, distortion, interference
problems, etc.
The products belonging to this group are for example sound cards, Analog Terminal (Telephone)
Adapters (ATA) and motherboards. As examples, here are an ATA device that turns common
landline phone into a Skype internet phone and few soundcards:
1.2.5 Non-audio product group
Group contains products that actually do not directly influence audio, like cameras without
microphone, displays, flash dongle... Such products can still have influence upon the audio quality,
by increasing delay or creating drops or distortion of audio by overloading the computer or device
in which the Skype application is running.
Below is an example of memory card that belongs to this group:
Audio Requirement Specification
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2009-04-01
Security Classification: Public
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1.3 Audio requirements and priorities – overview
Audio requirements presented in this document aim at the products that provide a good sound
quality, delight the user with great conversation experience and make communication easy.
At a high level the audio requirements and test cases in this document define the audio
performance of a product. Some audio ergonomic requirements are set in other Skype Certification
requirements.
The testing of audio quality is divided into objective and subjective testing. Objective testing
measures quality by means of technical measurement tools, whereas subjective testing requires
people to talk or/and listen and rate audio quality of the products. Audio performance requirements
defined in this document are mainly verified using objective measures, but there are few cases
where subjective measures are also involved.
1.3.1 Audio performance
The audio performance defines the audio quality of the product under test. In a high level the
attributes that affect to the performance are intelligibility, naturalness and conversational effort. In
a low level the performance consists of technical parameters such as frequency response,
sensitivity, distortion, noise and acoustic echo.
Naturalness and also intelligibility are typically measured with listening quality metrics. Intelligibility
can be difficult to measure, however a good assumption is that if user perceives the naturalness of
conversation to be good then also the intelligibility must be good. Thus the listening quality metric
that mainly concentrates to naturalness covers also enough of the intelligibility. The conversational
quality metrics measure conversational effort.
1.3.2 Quality expectation of the audio UI groups
Audio quality expectations that the end user has for the product may vary depending on the price,
advertisement promises and brand expectations, intended use of the product and experience of
other similar solutions.
The audio requirements here are set based on the audio UI groups mainly, but in addition, there
are a few technology dependent requirements. All requirements are the same for any product price
category.
An example of technology dependency is cordless headsets technology limitation compared to
plug-in headsets. Because of technology limitations the cordless headset like Bluetooth or DECT
are often frequency band limited between 300 and 3.4 kHz (narrowband), like most landline and
mobile phones are today. However Skype can provide wideband quality with frequencies between
50 and 7000 kHz. So Cordless headsets often can not benefit fully better audio quality, compared
to the plug-in headsets, i.e. headsets with analog audio or USB connection, that do not have such
limitation.
1.3.3 Use of the test case priorities
Each audio UI group has its own requirements and in addition there are General audio
requirements valid for all groups in Chapter 2. The total number of test cases in for each solution
varies between 10 and about 25. Each test case has several requirements and every requirement
has a different priority.
The priorities are mapped to Must, Should, and Nice requirements.
They are marked as:
•
Priority 1 = Must (at least 100% of Priority 1 requirements must PASS)
•
Priority 2 = Should (at least 50% of Priority 2 requirements must PASS)
•
Priority 3 = Nice to have (at least 10% of Priority 3 requirements must PASS)
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
10 / 68
2. General audio requirements valid for all groups
2.1 All groups: Audio performance requirements
Requirements below are valid for all groups: Headset, Handset, Speakerphone, Other and Nonaudio products. Some of the requirements below are not applicable for Non-audio product. Audio
test instructions in section 2.2 apply and should be followed in all requirements.
2.1.1 Priority: 1 Round trip delay of speech signals
Purpose:
To ensure that both parties can hear each other without significant delay, the round
trip acoustic end-to-end delay during Skype call must be as short as possible. When
the delay is long the potential acoustic echo coming back to the talker is very
disturbing. The interactivity of the interaction of call also suffers due to the long talk
switching times between the call participants and there is a high risk of unintended
doubletalk. The purpose of this test case is to ensure that the device under test
does not increase the round trip delay in good network conditions over a specified
limit.
Input:
Play the measurement signal – first in sending and then in receiving direction. The
delay is calculated using a cross correlation calculation. Short test signal is used for
measuring delay at given moment. Long 60 second signal is used to determine the
long term stability of the delay.
Round trip delay figure is calculated as Round trip delay = Sending direction delay +
Receiving direction delay
Output:
Note:
The average calculated round trip delay must be less than:
•
400ms – for devices connected to PC or MAC and using the software
Skype client
•
400ms – for devices with embedded Skype client and using LAN cable
•
480ms – for wireless devices with embedded Skype client
Please refer to 8.1.1 for description and specification of the measurement setup
2.1.2 Priority: 1 Total quality loss in sending direction
Purpose:
To verify that users perceive natural and intelligible speech. The Perceptual
Evaluation of Speech Quality tool (PESQ) [10] that complies with ITU-T P.862
standard is used for the analysis.
Input:
Play back speech samples in sending direction (i.e. mic direction) and record the far
end output.
Output:
Use PESQ tool to analyze the speech quality in sending direction. Verify that the
listening quality at the far end does not drop more than 1.0 MOS compared to a
good quality reference device from the same product category measured in the
same usage scenario.
If the device under test fails to meet the requirement the audio engineer will try to
determine by listening to the recordings made during the above testing, if some of
the following problems could be the cause for low MOS Listening Quality Objective
(MOS-LQO) score:
•
Speech quality is degraded by additional coding or format conversions
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Note:
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•
Drops or distortions are present in speech signals
•
Additional noises or sounds are present in speech signals
•
Interference noises are present from electric power supply
•
Interferences are present from devices with radio frequency transmission
Skype wants to point out clearly that Skype acknowledges the fact that PESQ has
not been designed and verified for acoustic interfaces therefore PESQ is not used
as a measure of a quality of acoustic interface, but only to measure problems
mentioned in the list up. Further Skype uses PESQ as a relative metric comparing
the result of an acoustic interface device to a known reference device. In other
words Skype is not using PESQ as an absolute metric in acoustic interface cases.
2.1.3 Priority: 1 Total quality loss in receiving direction
Purpose:
To verify that users perceive natural and intelligible speech The Perceptual
Evaluation of Speech Quality tool (PESQ) [10] that complies with ITU-T P.862
standard is used for the analysis.
Input:
Play back speech samples in receiving direction (i.e. loudspeaker/earpiece
direction) and record the near end output.
Output:
Use PESQ tool to analyze the speech quality in receiving direction. Verify that the
listening quality at the near end does not drop more than 1.0 MOS compared to a
good quality reference device from the same product category measured in the
same usage scenario.
If the device under test fails to meet the requirement the audio engineer will try to
determine by listening to the recordings made during the above testing, if some of
the following problems could be the cause for low MOS-LQO score:
Note:
•
Speech quality is degraded by additional coding or format conversions
•
Drops or distortions are present in speech signals
•
Additional noises or sounds are present in speech signals
•
Interference noises are present from electric power supply
•
Interferences are present from devices with radio frequency transmission
Skype wants to point out clearly that Skype acknowledges the fact that PESQ has
not been designed and verified for acoustic interfaces therefore PESQ is not used
as a measure of a quality of acoustic interface, but only to measure problems
mentioned in the list up. Further Skype uses PESQ as a relative metric comparing
the result of an acoustic interface device to a known reference device. In other
words Skype is not using PESQ as an absolute metric in acoustic interface cases.
2.2 All groups: Additional requirements for PC or Mac accessories
2.2.1 Priority: 1 Analog gain adjustment latency
Purpose:
To verify that the time to set- and get the microphone slider value does not exceed
the requirement.
Input:
Calculate the average time to set- and get the microphone slider value through
Windows audio API.
Output:
The average response time is < 50 ms
Audio Requirement Specification
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Note:
12 / 68
Only applicable to devices using PC or Mac Skype Client.
2.2.2 Priority: 1 Device – Sampling frequency accuracy
Purpose:
To ensure stable echo canceller performance the sampling frequencies of analogto-digital and digital-to-analog converters must be accurate. This will allow using
different audio interfaces for input and output during Skype call. For example: Using
built-in speakers for Skype audio playback and USB microphone for Skype audio
input.
Input:
Measure the sampling frequencies at input and output when a sampling frequency
of 48 kHz is selected. The sampling frequencies may be estimated by software
using following calculation:
Fs(input) = number of samples recorded / measurement time
Fs(output) = number of samples played out / measurement time
The measurement time is >15 minutes and high precision timer is used. The
number of samples being played out and recorded can be acquired through the
audio API.
Output:
Maximum deviation from the 48 kHz is 0.1%, i.e. 1000ppm for both play out and
recording.
Note: Only applicable to devices using PC or Mac Skype Client.
2.3 General audio test instructions
Test environment is defined in Chapter 8.
There are good quality reference devices for each Audio UI groups separately. The reference
device is chosen from the same Audio UI group from where the DUT is. Mean Opinion Scores and
other audio performance measures from these devices are used as references for DUT.
2.3.1 Objective testing measurement setup
Audio testing tools and environment are listed in 8.1.1. Objective testing is performed with the
automated audio testing system. Test practices and setups follow the principles given in ITU-T
recommendations [4]. Actual test cases are specially built for the requirements defined in this
document.
If Mean Opinion Score is mentioned in requirement, the result is judged by PESQ. Several test
speech samples are recorded from sending and receiving directions. These recordings are divided
to 10 sec length segments that are analyzed with objective speech quality tool. The speech
material consists of variety of speakers and both male and female voices. The average score is
used as the final MOS value.
In the test cases 2.1.2 – 2.1.3 MOS is first evaluated for a good quality reference device.
Reference device belongs to the same audio UI group. Next the MOS is evaluated for DUT and
the values are compared to each other. If the MOS value of DUT is lower than that of the reference
device, then the audio engineer goes through the checklist and verifies which one of the conditions
listed in the output of the test cases is not fulfilled causing the system to show lower MOS. This
manual verification is performed both by listening to and analyzing the recordings.
If DUT has acoustic interface, the instructions from Sections 3.4, 4.3, and 5.3 will be followed for
acoustic test setup.
The delay in the test case 2.1.1 is measured as follows:
•
Skype call is created between two Skype clients.
•
One Skype client runs on PC with Windows XP operating system (reference client).
Audio Requirement Specification
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•
The other Skype client is run either on another PC or is embedded into device under
test (referred to as device under test Skype client).
•
A third computer with ACQUA audio measurement system, MFE front end and HATS
connected to it is used that allows playback and recording simultaneously.
•
A test signal is played at one end of a Skype-to-Skype call and recorded at the other
end.
•
The measurement signal is fed into the system either by electric connections or
acoustically via the HATS mouth, depending on the test case.
•
Delay measurements are performed in a local network with minimum number of clients
on the same subnet.
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3. Headset audio UI group
Audio test instructions in section 3.4 apply and should be followed in requirements of this Chapter.
3.1 Headset: Audio performance requirements
In all tests related to the requirements below the headset is positioned on HATS [2] as naturally as
possible. HATS [2] is placed into the anechoic room.
3.1.1 Priority: 1 Microphone – Sensitivity at normal speech level
Purpose:
To check that the DUT microphone provides speech signal strong enough for the
Skype audio engine.
Input:
Play back a speech signal from the artificial mouth [2] at a normal speech level
(check 3.4 Headset: Audio test instructions and Abbreviations). Microphone gain
level is set by Skype client.
Output:
The microphone signal level is monitored at the far end and measured with ACQUA.
The speech level is not less than -30 dBov RMS (-24 dBm0 RMS).
3.1.2 Priority: 2 Microphone – Sensitivity at lowered speech level
Purpose:
To check that the DUT microphone provides speech signal strong enough for the
Skype audio engine.
Input:
Play back a speech signal from the artificial mouth [2] at a lowered speech level
(check 3.4 Headset: Audio test instructions and Abbreviations). Microphone gain
level is set by Skype client.
Output:
The microphone signal level is monitored at the far end and measured with ACQUA.
The speech level is not less than -30 dBov RMS (-24 dBm0 RMS).
3.1.3 Priority: 1 Microphone – Sensitivity at loud speech level
Purpose:
To check that microphone circuit has enough dynamic headroom for occasions
where loud speech level is used.
Input:
Play back a speech signal from the artificial mouth [2] at a loud speech level (check
3.4 Headset: Audio test instructions and Abbreviations). Microphone gain level is set
by Skype client.
Output:
The microphone signal level is monitored at the far end and measured with ACQUA.
The speech level is not less than -30 dBov RMS (-24 dBm0 RMS). The signal must
not overload the input causing clipping.
3.1.4 Priority: 1 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes minimum
requirement.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
limited wideband tolerance window:
Audio Requirement Specification
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15 / 68
.
Exception:
Frequency
Lower limit
Upper limit
299Hz
-80,0 dB
20,0 dB
300Hz
-5,0 dB
5,0 dB
1000 Hz
-5,0 dB
5,0 dB
3400 Hz
-5,0 dB
10,0 dB
7000Hz
-5,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
In special cases an exception to this requirement can be given to products, where
technology limits the bandwidth. Such cases can be DECT or Bluetooth products.
The resulting frequency response in such cases must be at least 300 Hz – 3.4 kHz
with a maximum ±5 dB ripple.
3.1.5 Priority: 2 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes super wideband
requirement.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
wideband tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Frequency
Lower limit
Upper limit
149Hz
-80,0 dB
20,0 dB
150Hz
-5,0 dB
5,0 dB
1000 Hz
-5,0 dB
5,0 dB
3400 Hz
-5,0 dB
10,0 dB
7000Hz
-5,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
16 / 68
3.1.6 Priority: 1 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level to allow Skype to adjust the microphone gain setting to a suitable value. Then
play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 40 dB.
3.1.7 Priority: 2 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level to allow Skype to adjust the microphone gain setting to a suitable value. Then
play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 45 dB.
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3.1.8 Priority: 3 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level to allow Skype to adjust the microphone gain setting to a suitable value. Then
play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 50 dB.
3.1.9 Priority: 2 Microphone – Speech to self noise ratio during speech activity
Purpose:
To check that the self noise level of the microphone is sufficiently low during the
active speech.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level. Immediately following play a special speech type of test signal to deactivate
the possible microphone noise gating function. Record the test signal at the far end.
Output:
The recorded microphone signal is processed to separate the speech part from the
noise part. When the level of speech part is compared to the level of noise part, Aweighted RMS speech to noise ratio is at least 30 dB.
3.1.10 Priority: 2 Microphone – Speech to background noise ratio
Purpose:
To verify that the microphone does not pick too much surrounding sounds and
background noise compared to speech.
Input:
Set up 3-dimensional sound playback environment into anechoic room. (Skype uses
18.1 channel 3D loudspeaker system using DIRAC processed samples). Remove
HATS from the measurement area. Create different types of background noise
environments to a measurement position, such as car, restaurant, street and office
noises. Calibrate the A-weighted SPL level of noises to be 62 dB. Place HATS to
the center of measurement area. Play back a measurement speech signal from the
HATS artificial mouth [2] at a normal speech level and a background noise from the
loudspeaker(s).
Output:
The microphone signal is monitored at the far end output. When the speech signal
level is compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 10 dB.
3.1.11 Priority: 1 Earpiece – Speech to self noise ratio
Purpose:
To check that the self noise level of the earpiece is sufficiently low.
Input:
Play back a normal level speech signal at the far end input while on a Skype call.
And adjust the listening level at near end output to the preferred listening level.
(check 3.4 Headset: Audio test instructions and Abbreviations)
Output:
The earpiece signal is monitored at the near end. When the speech signal level is
compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 40 dB.
3.1.12 Priority: 2 Earpiece – Speech to self noise ratio
Purpose:
To check that the self noise level of the earpiece is sufficiently low.
Input:
Play back a normal level speech signal at the far end input while on a Skype call.
And adjust the listening level at near end output to the preferred listening level.
(check 3.4 Headset: Audio test instructions and Abbreviations)
Output:
The earpiece signal is monitored at the near end. When the speech signal level is
compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 45 dB.
Audio Requirement Specification
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3.1.13 Priority: 3 Earpiece – Speech to self noise ratio
Purpose:
To check that the self noise level of the earpiece is sufficiently low.
Input:
Play back a normal level speech signal at the far end input while on a Skype call.
And adjust the listening level at near end output to the preferred listening level.
(check 3.4 Headset: Audio test instructions and Abbreviations)
Output:
The earpiece signal is monitored at the near end. When the speech signal level is
compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 50 dB.
3.1.14 Priority: 1 Earpiece – Frequency response
Purpose:
To verify that the earpiece frequency response curve passes minimum requirement.
Input:
Play a speech or a measurement signal through the earpiece.
Output:
Measure frequency response of the earpiece by comparing the monitored speech
signal to the original speech. The resulting frequency response fits into a limited
wideband tolerance window:
Frequency
Lower limit
Upper limit
299Hz
-80,0 dB
20,0 dB
300Hz
-10,0 dB
10,0 dB
7000Hz
-10,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
Exception:
In special cases an exception to this requirement can be given to
products, where technology limits the bandwidth. Such cases can be DECT or
Bluetooth products. The resulting frequency response in such cases must be at
least 300 Hz – 3.4 kHz with a maximum ±10 dB ripple.
Note:
Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check test
instructions 3.4.1.
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3.1.15 Priority: 2 Earpiece – Frequency response
Purpose:
To verify that the earpiece frequency response curve passes super wideband
requirement.
Input:
Play a speech or a measurement signal through the earpiece.
Output:
Measure frequency response of the earpiece by comparing the monitored speech
signal to the original speech. The resulting frequency response fits into a wideband
tolerance window:
Note:
Frequency
Lower limit
Upper limit
149Hz
-80,0 dB
20,0 dB
150Hz
-10,0 dB
10,0 dB
7000Hz
-10,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check test
instructions 3.4.1.
3.1.16 Priority: 1 Earpiece – Stability of frequency response
Purpose:
To check that frequency characteristic of the earpiece(s) does not change too much
when its position on the ear changes, which can happen, when the user moves his
head. Basically, this test case is to test leak tolerance of the earpiece.
Input:
Play back a speech, music or measurement signal through the earpiece, Change
the position of the headset on HATS and repeat the measurement several times.
Output:
Compared to the normal position of the headset i.e. the frequency response got in
the previous requirement, check if the maximum absolute change between 500 Hz
and 1 kHz is less than 15 dB and between 1-3.4 kHz less than 10 dB.
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3.1.17 Priority: 2 Earpiece – Stability of frequency response
Purpose:
To check that the frequency characteristic of the earpiece(s) does not change too
much when its position on the ear changes, which can happen, when the user
moves his head. Basically, this test case is to test leak tolerance of the earpiece.
Input:
Play back a speech, music or measurement signal through the earpiece, Change
the position of the headset on HATS and repeat the measurement several times.
Output:
Compared to the normal position of the headset i.e. the frequency response got in
the previous requirement, check if the maximum absolute change between 300 and
1 kHz is less than 10 dB and between 1 kHz and 6 kHz less than 5 dB.
3.1.18 Priority: 3 Earpiece – Stability of frequency response
Purpose:
To check that the frequency characteristic of the earpiece(s) does not change too
much when its position on the ear changes, which can happen, when the user
moves his head. Basically, this test case is to test leak tolerance of the earpiece.
Input:
Play back a speech, music or measurement signal through the earpiece, Change
the position of the headset on HATS and repeat the measurement several times.
Output:
Compared to the normal position of the headset i.e. the frequency response got in
the previous requirement, check if the maximum absolute change between 150 and
300 Hz is less than 10 dB and between 300 Hz and 7 kHz less than 5 dB.
3.1.19 Priority: 1 Minimum crosstalk from receiving to sending direction
Purpose:
To check that crosstalk level between microphone and earpiece/loudspeaker meets
the requirement. To ensure that conversation is pleasant and smooth, the echo
must be minimized. Most of this echo is created between earpiece/ loudspeaker and
microphone, but also electric connections and wires can leak i.e. to create crosstalk.
This electric leaking is studied here.
Input:
Cover microphone and/or earpiece/loudspeaker properly to minimize acoustic echo
from earpiece/loudspeaker to microphone. Play back a test signal through device
under test earpiece / loudspeaker. At the same time monitor and analyze the
microphone signal level at the other Skype client output.
Output:
Digital crosstalk level at the far end Skype client output is less than -51 dBov Aweighted RMS (-45 dBm0 A-weighted RMS).
3.2 Headset: requirements for Skype Super Wideband Certification (optional)
3.2.1 Priority: 1 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes super wideband
requirement.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
super wideband tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Frequency
Lower limit
Upper limit
99Hz
-80,0 dB
20,0 dB
100Hz
-5,0 dB
5,0 dB
1000 Hz
-5,0 dB
5,0 dB
3400 Hz
-5,0 dB
10,0 dB
10000Hz
-5,0 dB
10,0 dB
10001Hz
-80,0 dB
20,0 dB
21 / 68
3.2.2 Priority: 1 Earpiece – Frequency response
Purpose:
To verify that the earpiece frequency response curve passes super wideband
requirement.
Input:
Play a speech or a measurement signal through the earpiece.
Output:
Measure frequency response of the earpiece by comparing the monitored speech
signal to the original speech. The resulting frequency response fits into a super
wideband tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Note:
Frequency
Lower limit
Upper limit
99Hz
-80,0 dB
20,0 dB
100Hz
-5,0 dB
5,0 dB
1000Hz
-5,0 dB
5,0 dB
3400Hz
-15,0 dB
5,0 dB
10000Hz
-15,0 dB
5,0 dB
10001Hz
-80,0 dB
20,0 dB
22 / 68
Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check test
instructions 3.4.1.
3.2.3 Priority: 1 Earpiece – Speech to noise ratio
Purpose:
To check that the self noise level of the earpiece is sufficiently low.
Input:
Play back a normal level speech signal at the far end input while on a Skype call.
And adjust the listening level at near end output to the preferred listening level.
(check 3.4 Headset: Audio test instructions and Abbreviations)
Output:
The earpiece signal is monitored at the near end. When the speech signal level is
compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 55 dB.
3.3 Headset: Supporting audio documentation requirements
In addition to the user manual (the one that comes with the product) we also ask for supporting
audio documentation (for certification testing purposes). Such documentation contains engineering
data and engineering test data for the product. Earpiece below means the acoustic output
component for sound playback to the user’s ear, for example the small loudspeaker.
Audio Requirement Specification
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3.3.1 Priority: 1 Verifying supporting documentation for Headset Audio UI group
Purpose:
Solution must come with a supporting audio documentation (only for certification
testing purposes).
Output:
DUT arrives with supporting audio documentation that contains information about:
•
Active signal processing: yes/no, if yes then:
o
Acoustic echo cancellation in sending (i.e. mic) or/and in receiving
(i.e. earpiece) directions: yes/no
o
Noise suppression in sending or/and in receiving directions: yes/no
o
Automated Gain Control in sending or/and in receiving directions:
yes/no
•
Microphone: Directionality/design principle
•
Microphone: Frequency range (lowest and highest audible frequencies)
•
Earpiece: Lowest and highest designed audible frequencies in intended use
case
3.4 Headset: Audio test instructions
Test environment is defined in Chapter 8.
Headset under test is compared to a good quality reference headset. This reference headset is
chosen from Skype Certified headsets. The sending (microphone) and receiving
(earpiece/loudspeaker) parts might be chosen from two different headsets.
3.4.1 Objective testing measurement setup
Audio performance requirements are measured with objective measurement tools. The
measurements will be performed with Head And Torso Simulator (HATS) [2] with type 3.3
anatomic ears [6] and with automated audio testing system in anechoic room. The audio testing
tools and environment are listed in 8.1.1.Test practices and setups follow the principles given in
ITU-T recommendations [4]. Actual test cases are specially built for the requirements defined in
this document.
The measurements will be performed mainly with Skype to Skype call and DUT audio drivers. For
passive headsets a reference soundcard is used as electric interface.
Frequency response results are averaged to 1/3 octave frequency resolution.
In microphone measurements the headset is attached to HATS as naturally as the user would do it
in real life scenario.
In speech to background noise ratio measurement of microphone (3.1.10) various background
noises are tested, like inside car, street and cafeteria noises. The background noises are real life
3D sound recordings that will be replayed with 3D loudspeaker setup of the test facility.
Earpiece/s in requirements 3.1.11 - 3.1.19 and 3.2.2 - 3.2.3 is/are measured with anatomic artificial
ears, ITU-T type 3.3 that have measurement microphones at Drum Reference Point [6] and using
DRP to diffuse field frequency response correction for measured earpiece responses. Note that
Skype does not consider this to be the optimum response for a headset and does not assume flat
diffuse field corrected response to be target in designing rather the diffuse field correction is
chosen here for practical purposes. If manufacturer request or Certification tester considers
appropriate some of the other standard corrections such as DRP to ERP (Ear Reference Point) or
DRP to free field can be used or taken into account when interpreting the results.
Preferred listening level is defined to be 75 dB SPL A-weighted for a headset that reproduces
speech only to one ear and 69 dB SPL A-weighted for a headset that plays speech to both ears.
Audio Requirement Specification
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The headset is adjusted manually with natural placement forces and effort to ear so that it gives
good acoustic sealing for earpiece towards the ear. Measurement can be repeated for the same or
for different pairs of headphones (if available during testing) if it seems to be appropriate for the
headset. In stability measurements headset is taken off from the head and replaced again after
each measurement. At least three different measurements are performed in this way.
The SPL level of normal speech in these tests is 62 dB SPL A-weighted at 1 m distance in front of
mouth. The level is based on ITU-T recommendations of real and artificial mouth speaking levels.
The lowered speech is about 10 dB quieter and loud speech is about 10 dB louder. Note that in
real life the speaking levels vary more than 10 dB depending on speaker, distance between people
having conversation and environment.
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4. Handset audio UI group
Audio test instructions in section 4.3 apply and should be followed in all requirements.
4.1 Handset: Audio performance requirements
In all tests related to the requirements below the handset is positioned on HATS [2] as naturally as
possible. HATS is placed into the anechoic room.
4.1.1 Priority: 1 Microphone – Sensitivity at normal speech level
Purpose:
To check that the DUT microphone provides speech signal strong enough for the
Skype audio engine.
Input:
Play back a speech signal from the artificial mouth [2] at a normal speech level
(check 4.3 Handset: Audio test instructions and Abbreviations). Microphone gain
level is controlled by the Skype audio engine.
Output:
The microphone signal level is monitored at the far end and measured with ACQUA.
The speech level is not less than -30 dBov RMS (-24 dBm0 RMS).
4.1.2 Priority: 2 Microphone – Sensitivity at lowered speech level
Purpose:
To check that the DUT microphone provides speech signal strong enough for the
Skype audio engine.
Input:
Play back a speech signal from the artificial mouth [2] at a lowered level (check 4.3
Handset: Audio test instructions and Abbreviations). Microphone gain level is
controlled by the Skype audio engine.
Output:
The microphone signal level is monitored at the far end and measured with ACQUA.
The speech level is not less than -34 dBov RMS (-28 dBm0 RMS).
4.1.3 Priority: 1 Microphone – Sensitivity at loud speech level
Purpose:
To check that microphone circuit has enough dynamic headroom for occasions
where loud speech level is used.
Input:
Play back a speech signal from the artificial mouth [2] at a loud speech level (4.3
Handset: Audio test instructions and Abbreviations). Microphone gain level is set by
the Skype audio engine
Output:
The microphone signal level is monitored at the far end and measured with ACQUA.
The speech level is not less than -30 dBov RMS (-24 dBm0 RMS). The signal must
not overload the input causing clipping.
4.1.4 Priority: 1 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes minimum
requirement.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
limited wideband tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Frequency
Lower limit
Upper limit
299Hz
-80,0 dB
20,0 dB
300Hz
-5,0 dB
5,0 dB
1000 Hz
-5,0 dB
5,0 dB
3400 Hz
-5,0 dB
10,0 dB
5000Hz
-5,0 dB
10,0 dB
5001Hz
-80,0 dB
20,0 dB
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Note:
Limited wideband is used because usually form factor of handsets does not allow
positioning their microphones in front of the mouth and highest frequencies are
attenuated due to the directionality of the mouth.
Exception:
In special cases an exception to this requirement can be given to some cordless
products. Such cases can be DECT or Bluetooth products in which the protocol
limits the frequency bandwidth of speech. These are judged case by case.
4.1.5 Priority: 2 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes wideband
requirement
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
wideband tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Frequency
Lower limit
Upper limit
149Hz
-80,0 dB
20,0 dB
150Hz
-5,0 dB
5,0 dB
1000 Hz
-5,0 dB
5,0 dB
3400 Hz
-5,0 dB
10,0 dB
7000Hz
-5,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
27 / 68
4.1.6 Priority: 1 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level to allow Skype to adjust the microphone gain setting to a suitable value. Then
play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 40 dB.
4.1.7 Priority: 2 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level to allow Skype to adjust the microphone gain setting to a suitable value. Then
play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 45 dB.
4.1.8 Priority: 3 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Audio Requirement Specification
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28 / 68
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level to allow Skype to adjust the microphone gain setting to a suitable value. Then
play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 50 dB.
4.1.9 Priority: 2 Microphone – Speech to self noise ratio during speech activity
Purpose:
To check that the self noise level of the microphone is sufficiently low during the
active speech.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level. Immediately following play a special speech type of test signal to deactivate
the possible microphone noise gating function. Record the test signal at the far end.
Output:
The recorded microphone signal is processed to separate the speech part from the
noise part. When the level of speech part is compared to the level of noise part, Aweighted RMS speech to noise ratio is at least 30 dB.
4.1.10 Priority: 2 Microphone – Speech to background noise ratio
Purpose:
To verify that the microphone does not pick too much surrounding sounds and
background noise compared to speech.
Input:
Set up 3-dimensional sound playback environment into anechoic room. (Skype uses
18.1 channel 3D loudspeaker system using DIRAC processed samples). Remove
HATS from the measurement area. Create different types of background noise
environments to a measurement position, such as car, restaurant, street and office
noises. Calibrate the A-weighted SPL level of noises to be 62 dB. Place HATS to
the center of measurement area. Play back a measurement speech signal from the
HATS artificial mouth [2] at a normal speech level and a background noise from the
loudspeaker(s).
Output:
The microphone signal is monitored at the far end output. When the speech signal
level is compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 10 dB
4.1.11 Priority: 1 Earpiece – Speech to self noise ratio
Purpose:
To check that the self noise level of the earpiece is sufficiently low.
Input:
Play back a normal level speech signal at the far end input while on a Skype call.
And adjust the listening level at near end output to the preferred listening level.
(check 4.3 Handset: Audio test instructions and Abbreviations)
Output:
The earpiece signal is monitored at the near end. When the speech signal level is
compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 40 dB.
4.1.12 Priority: 2 Earpiece – Speech to self noise ratio
Purpose:
To check that the self noise level of the earpiece is sufficiently low.
Input:
Play back a normal level speech signal at the far end input while on a Skype call.
And adjust the listening level at near end output to the preferred listening level.
(check 4.3 Handset: Audio test instructions and Abbreviations)
Output:
The earpiece signal is monitored at the near end. When the speech signal level is
compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 45 dB.
4.1.13 Priority: 3 Earpiece – Speech to self noise ratio
Purpose:
To check that the self noise level of the earpiece is sufficiently low.
Audio Requirement Specification
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29 / 68
Input:
Play back a normal level speech signal at the far end input while on a Skype call.
And adjust the listening level at near end output to the preferred listening level.
(check 4.3 Handset: Audio test instructions and Abbreviations)
Output:
The earpiece signal is monitored at the near end. When the speech signal level is
compared to the noise level (noise is measured during pauses of the speech
signal), A-weighted RMS speech to noise ratio is at least 50 dB.
4.1.14 Priority: 1 Earpiece – Frequency response
Purpose:
To verify that the earpiece frequency response curve passes minimum requirement.
Input:
Play a speech or a measurement signal through the earpiece of the handset.
Output:
Measure frequency response of the earpiece by comparing the monitored speech
signal to the original speech. The resulting frequency response fits into a very
limited wideband tolerance window
Frequency
Lower limit
Upper limit
499Hz
-80,0 dB
20,0 dB
500Hz
-10,0 dB
10,0 dB
5000 Hz
-10,0 dB
10,0 dB
5001Hz
-80,0 dB
20,0 dB
Exception:
In special cases an exception to this requirement can be given to some cordless
products. Such cases can be DECT or Bluetooth products in which the protocol
limits the frequency bandwidth of speech. These are judged case by case.
Note:
Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check test
instructions 4.3.1
4.1.15 Priority: 2 Earpiece – Frequency response
Purpose:
To verify that the earpiece frequency response curve passes limited wideband
requirement.
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
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Security Classification: Public
30 / 68
Input:
Play a speech or a measurement signal through the earpiece of the handset.
Output:
Measure frequency response of the earpiece by comparing the monitored speech
signal to the original speech. The resulting frequency response fits into a limited
wideband tolerance window:
Note:
Frequency
Lower limit
Upper limit
299Hz
-80,0 dB
20,0 dB
300Hz
-10,0 dB
10,0 dB
6000Hz
-10,0 dB
10,0 dB
6001Hz
-80,0 dB
20,0 dB
Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check test
instructions 4.3.1
4.1.16 Priority: 3 Earpiece – Frequency response
Purpose:
To verify that the earpiece frequency response curve passes wideband requirement.
Input:
Play a speech or a measurement signal through the earpiece of the handset.
Output:
Measure frequency response of the earpiece by comparing the monitored speech
signal to the original speech. The resulting frequency response fits into a wideband
tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Note:
Frequency
Lower limit
Upper limit
149Hz
-80,0 dB
20,0 dB
150Hz
-10,0 dB
10,0 dB
7000Hz
-10,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
31 / 68
Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check test
instructions 4.3.1
4.1.17 Priority: 1 Minimum crosstalk from receiving to sending direction
Purpose:
To check that crosstalk level between microphone and earpiece/loudspeaker meets
the requirement. To ensure that conversation is pleasant and smooth, the echo
must be minimized. Most of this echo is created between earpiece/ loudspeaker and
microphone, but also electric connections and wires can leak i.e. to create crosstalk.
This leakage is studied here.
Input:
Cover microphone and/or earpiece / loudspeaker properly to minimize acoustic
echo from earpiece/loudspeaker to microphone. Play back a test signal to device
under test earpiece / loudspeaker. At the same time monitor and analyze the
microphone signal level at the other Skype client output.
Output:
Digital crosstalk level at other Skype client output is less than -51 dBov A-weighted
RMS (-45 dBm0 A-weighted RMS).
4.1.18 Priority: 1 Earpiece – Stability of frequency response
Purpose:
To check that the frequency characteristic of the earpiece(s) does not change too
much when its position on the ear changes, which can happen, when the user
moves his head. Basically, this test case is to test leak tolerance of the earpiece.
Input:
Play back a speech, music or measurement signal through the earpiece. Change
the position of handset on HATS and repeat the measurement several times.
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Output:
32 / 68
Compared to the normal position of the handset. Maximum absolute change
between 500 Hz and 1 kHz is less than 15 dB and between 1-3.4 kHz less than 10
dB.
4.1.19 Priority: 2 Earpiece – Stability of frequency response
Purpose:
To check that the frequency characteristic of the earpiece(s) does not change too
much when its position on the ear changes, which can happen, when the user
moves his head. Basically, this test case is to test leak tolerance of the earpiece.
Input:
Play back a speech, music or measurement signal through the earpiece. Change
the position of the handset on HATS and repeat the measurement several times.
Output:
Compared to the normal position of the handset. Maximum absolute change
between 300 and 1 kHz is less than 10 dB and between 1 kHz and 6 kHz less than
5 dB.
4.1.20 Priority: 3 Earpiece – Stability of frequency response
Purpose:
To check that the frequency characteristic of the earpiece(s) does not change too
much when its position on the ear changes, which can happen, when the user
moves his head. Basically, this test case is to test leak tolerance of the earpiece.
Input:
Play back a speech, music or measurement signal through the earpiece. Change
the position of the handset on HATS and repeat the measurement several times.
Output:
Compared to the normal position of the handset. Maximum absolute change
between 150 and 300 Hz is less than 10 dB and between 300 Hz and 7 kHz less
than 5 dB.
4.1.21 Priority: 1 Earpiece – Suitable volume level for office and home handset (Indoor)
Purpose:
To verify that user can hear and understand speech while using the handset in
normal every-day life.
Input:
Play back speech through Skype and measure level with artificial ear (or listen the
level subjectively).
Output:
Earpiece volume level can be set both below and above 70 dB SPL A-weighted
RMS (this is 5 dB below the preferred listening level).
4.1.22 Priority: 2 Earpiece – Suitable volume level for office and home handset (Indoor)
Purpose:
To verify that user can hear and understand speech while using the handset in
normal noisy office environment or home.
Input:
Play back speech through Skype and measure level with artificial ear (or listen the
level subjectively).
Output:
Earpiece volume level can be set at least to 75 dB SPL A-weighted RMS (this is the
preferred listening level).
4.1.23 Priority: 1 Earpiece – Suitable volume level for “anywhere” handset (Outdoor)
Purpose:
To verify that user can hear and understand speech while using the handset in
normal and noisy every-day-life environment.
Input:
Play back speech through Skype and measure level with artificial ear (or listen the
level subjectively).
Output:
Earpiece volume level can be set at least to 75 dB SPL A-weighted RMS (this is the
preferred listening level).
Note:
“Anywhere” handset means a portable phone that can be used on street, public
places, transportation, restaurants… where the environmental noise levels are high.
Such a device can be mobile phones as an example.
Audio Requirement Specification
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Security Classification: Public
33 / 68
4.1.24 Priority: 2 Earpiece – Suitable volume level for “anywhere” handset (Outdoor)
Purpose:
To verify that user can hear and understand speech while using the handset in
normal and noisy every-day-life environment.
Input:
Play back speech through Skype and measure level with artificial ear (or listen the
level subjectively).
Output:
Earpiece volume level can be set at least to 80 dB SPL A-weighted RMS.
4.1.25 Priority: 1 Maximum ring tone loudness
Purpose:
To verify that user can hear the ringing of incoming call in normal and noisy everyday-life environment.
Input:
Set possible volume setting for ring tones of the handset at maximum. Set phone to
the free field conditions (check Abbreviations for details). Choose calibrated
measurement microphone and recording setup or take calibrated SPL measurement
meter. Set the measurement microphone to 10 cm distance from the handset. The
location can be chosen freely, but typically a place that gives highest SPL values is
in the front of the outlets of the loudspeaker that plays the ring tones. Play ring
tones of handset one by one.
Output:
Record the ring tones with a calibrated microphone and check SPL levels offline or
check SPL levels with SPL analyzer connected to the measurement microphone.
For each tone measure the maximum hold value of SPL fast RMS (i.e. 125 ms
exponential time weighting) with A-weighting (frequency correction) applied. The
max-hold fast RMS level must be for half of the ring tones higher than 80 dB SPL.
Exception:
Lower ring tone levels can be accepted if manufacturer asks for it with a detailed
written explanation why and if Skype approves it. Such case can be for example
that due to the design of the handset and safety regulations that are valid for the
product, the highest output levels of the product needs to be limited to avoid too
high sound pressure levels to user’s ears. One such regulation is given at European
Standard EN 50332: “Sound system equipment: Headphone and earphones
associated with portable audio equipment – Maximum sound pressure level
measurement methodology and limit considerations”
4.1.26 Priority: 2 Maximum ring tone loudness
Purpose:
To verify that user can hear the ringing of incoming call in normal and noisy everyday-life environment.
Input:
Set possible volume setting for ring tones of the handset at maximum. Set phone to
the free field conditions (check Abbreviations for details). Chose calibrated
measurement microphone and recording setup or take calibrated SPL measurement
meter. Set the measurement microphone to 10 cm distance from the handset. The
location can be chosen freely, but typically a place that gives highest SPL values is
in the front of the outlets of the loudspeaker that plays the ring tones. Play ring
tones of handset one by one.
Output:
Record the ring tones with a calibrated microphone and check SPL levels offline or
check SPL levels with SPL analyzer connected to the measurement microphone.
For each tone measure the maximum hold value of SPL fast RMS (i.e. 125 ms
exponential time weighting) with A-weighting (frequency correction) applied. The
max-hold fast RMS level must be for half of the ring tones higher than 90 dB SPL.
Exception:
Lower ring tone levels can be accepted if manufacturer asks for it with a detailed
written explanation why and if Skype approves it. Such case can be for example
that due to the design of the handset and safety regulations that are valid for the
product, the highest output levels of the product needs to be limited to avoid too
high sound pressure levels to user’s ears. One such regulation is given at European
Standard EN 50332: “Sound system equipment: Headphone and earphones
Audio Requirement Specification
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2009-04-01
Security Classification: Public
34 / 68
associated with portable audio equipment – Maximum sound pressure level
measurement methodology and limit considerations”
4.1.27 Priority: 3 Maximum ring tone loudness
Purpose:
To verify that user can hear the ringing of incoming call in normal and noisy everyday-life environment.
Input:
Set possible volume setting for ring tones of the handset at maximum. Set phone to
the free field conditions (check Abbreviations for details). Chose calibrated
measurement microphone and recording setup or take calibrated SPL measurement
meter. Set the measurement microphone to 10 cm distance from the handset. The
location can be chosen freely, but typically a place that gives highest SPL values is
in the front of the outlets of the loudspeaker that plays the ring tones. Play ring
tones of handset one by one.
Output:
Record the ring tones with a calibrated microphone and check SPL levels offline or
check SPL levels with SPL analyzer connected to the measurement microphone.
For each tone measure the maximum hold value of SPL fast RMS (i.e. 125 ms
exponential time weighting) with A-weighting (frequency correction) applied. The
max-hold fast RMS level must be for half of the ring tones higher than 100 dB SPL.
Exception:
Lower ring tone levels can be accepted if manufacturer asks for it with a detailed
written explanation why and if Skype approves it. Such case can be for example
that due to the design of the handset and safety regulations that are valid for the
product, the highest output levels of the product needs to be limited to avoid too
high sound pressure levels to user’s ears. One such regulation is given at European
Standard EN 50332: “Sound system equipment: Headphone and earphones
associated with portable audio equipment – Maximum sound pressure level
measurement methodology and limit considerations”
4.2 Handset: Supporting audio documentation requirements
In addition to the user manual (the one that comes with the product) we also ask for supporting
audio documentation (only for certification testing purpose). Such documentation contains
engineering data and engineering test data for the product.
Earpiece below means the acoustic output device, for example a small loudspeaker. Ring tone
loudspeaker means the component that reproduces ring tones. In some devices it is the same
component that reproduces speech, in others it is a separate element.
4.2.1 Priority: 1 Verifying supporting documentation for Handset audio
Purpose:
Solution must come with a supporting audio documentation (only for certification
testing purposes).
Output:
DUT arrives with supporting audio documentation that contains information about:
•
Active signal processing: yes/no, if yes then:
o
Acoustic echo cancellation: yes/no, in sending (i.e. mic) or/and
receiving (i.e. earpiece) directions.
o
Noise suppression: yes/no, in sending or/and receiving directions
o
Automated Gain Control: yes/no, in sending or/and receiving
directions
o
Other: describe what, sending or/and receiving directions
•
Microphone: Directionality/design principle
•
Microphone: Frequency range (lowest and highest audible frequencies)
•
Earpiece: Lowest and highest designed audible frequencies
Audio Requirement Specification
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2009-04-01
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35 / 68
•
Earpiece: Designed acoustic SPL for receiving speech signal at the user’s
ear
•
Ring tones: Number of tones
•
Ring tones: Types of tones: MP3, Midi, wav/PCM etc…
•
Ring tones from the loudspeaker: Maximum level of the loudest ring tone at
10 cm distance from the handset in the free field conditions (SPL, fast time
weighting max hold)
•
Ring tones from the loudspeaker: Level that half of the ring tones exceed
when volume settings are on maximum. Measured at 10 cm distance from
the handset in the free field conditions (SPL, fast time weighting max hold)
4.3 Handset: Audio test instructions
Test environment is defined in Chapter 8.
Handset under test is compared to a good quality reference handset. This reference handset is
chosen from Certified Skype handsets. The sending (microphone) and receiving
(earpiece/loudspeaker) parts might be chosen from two different handsets.
4.3.1 Objective testing measurement setup
Audio performance requirements are measured with objective measurement tools. The
measurements will be performed with Head And Torso Simulator (HATS) [2] with type 3.3
anatomic ears [6] placing handset to the handset positioner and with an automated audio testing
system in anechoic room. The audio testing tools and environment are listed in 8.1.1.Test
practices and setups follow the principles given in ITU-T recommendations [4]. Actual test cases
are specially built for the requirements defined in this document.
The measurements will be performed mainly in Skype call having all speech enhancement
algorithms as they are by default in Skype and potential device audio drivers.
Frequency response results are averaged to 1/3 octave frequency resolution.
For majority of tests (expect the stability tests of earpiece) measurements the handset is placed on
head as naturally as user would do it.
In speech to background noise ratio measurement of microphone (4.1.10) – various background
noises can be tested, like inside car, street and cafeteria noises. The background noises are real
life 3D sound recordings that will be replayed with 3D loudspeaker setup of the test facility.
Earpiece of handset in requirements 4.1.11 – 4.1.24 is measured with anatomic artificial ears, ITUT type 3.3 and using a handset positioner available for ITU-T defined artificial heads [2] to place
the handset to realistic and repeatable position. The handset will be placed to a position where
phone is visually and acoustically sealed tightly to the ear but so that position is natural. In ITU-T
recommendations so called ERP position (check [7], Annex E for 3.3 type ears) is typically
proposed to be used. Unfortunately often this position does not give good and natural sealing for
small modern handsets. Thus for many handset a better position compared to ERP position is to
move the handset backwards (towards the back of head) 0.5-1 cm so that end of the handset
seals to the pinna “hill” behind the ear canal entrance, press the handset closer to the ear by 0.5-1
cm and fold handset to natural position.
DRP to diffuse field frequency response correction is applied in earpiece measurements. This is
contrary to ITU-T practice for narrowband phones to use DRP to ERP correction. Skype does not
consider DRP to diffuse field correction to be the optimum response for a handset and does not
assume flat diffuse field corrected response to be target in designing rather the diffuse field
correction is chosen here for practical purposes. It can be noted that difference between DRP to
diffuse field and DRP to ERP is relatively small compared to frequency limits given in the
requirements. If manufacturer request or Certification tester considers appropriate some of the
other standard corrections such as DRP to ERP (Ear Reference Point) or DRP to free field can be
used or taken into account when interpreting the results.
Audio Requirement Specification
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2009-04-01
Security Classification: Public
36 / 68
Earpiece measurement can be repeated for the same or for different handsets (if available during
testing) if it seems to be appropriate for the handset. In stability measurements handset position is
adjusted with the handset positioner by 2-10 millimeters to potential directions where handset
might be positioned or move during the real use. At least three different measurements are
performed in this way.
Preferred listening level is defined to be 75 dB SPL A-weighted for a handset that reproduces
speech only to one ear.
The SPL level of normal speech is defined here to be about 62 dB SPL A-weighted at 1 m
distance in the front of the mouth. The level is based on ITU-T recommendations of real and
artificial mouth speaking levels. The lowered speech is about 10 dB quieter and loud speech is
about 10 dB louder. Note that in real life the speaking levels vary more than 10 dB depending on
speaker, distance between people having conversation and environment.
Audio Requirement Specification
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2009-04-01
Security Classification: Public
37 / 68
5. Speakerphone audio UI group
In all tests related to the requirements below the solution with speakerphone audio functionality is
positioned on the table in front of the HATS [2] in a natural way, as it would be done by the end
user. Testing is done in the anechoic room. Audio test instructions in section 5.3 apply and should
be followed in all requirements.
5.1 Speakerphone: Audio performance requirements
5.1.1 Priority: 1 Microphone – Sensitivity at normal speech level
Purpose:
To check that the DUT microphone provides speech signal strong enough for the
Skype audio engine.
Input:
Place the device under test to recommended usage position. Play back a speech
signal from the artificial mouth [2] at a normal speech level (check 5.3
Speakerphone: Audio test instructions and Abbreviations). Microphone gain level is
controlled by the Skype audio engine.
Output:
The microphone signal level is monitored at the far end and measured with ACQUA
[11]. The speech level is not less than -34 dBov RMS ( -28 dBm0 RMS).
Note:
Check the exact measurement setup and positions of DUT and HATS from 5.3.1.
5.1.2 Priority: 1 Microphone – Sensitivity at lowered speech level
Purpose:
To check that the DUT microphone provides speech signal strong enough for the
Skype audio engine.
Input:
Place the device under test to recommended usage position. Play back a speech
signal from the artificial mouth [2] at quiet speech level (check 5.3 Speakerphone:
Audio test instructions and Abbreviations). Microphone gain level is controlled by
the Skype audio engine.
Output:
The microphone signal level is monitored at the far end and measured with ACQUA
[11]. The speech level is not less than -34 dBov RMS (-28 dBm0 RMS).
5.1.3 Priority: 1 Microphone – Sensitivity at loud speech level
Purpose:
To check that microphone circuit has enough dynamic headroom for occasions
where loud speech level is used.
Input:
Place the device under test to recommended usage position. Play back a speech
signal from the artificial mouth [2] at loud speech level (check 5.3 Speakerphone:
Audio test instructions and Abbreviations). Microphone gain level is controlled by
the Skype audio engine.
Output:
The microphone signal is monitored from another Skype client and measured with
ACQUA [11]. The speech level is not less than -34 dBov RMS (-28 dBm0 RMS).
The signal must not overload the input causing clipping.
5.1.4 Priority: 1 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes minimum
requirement.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
38 / 68
limited wideband tolerance window:
Exception:
Frequency
Lower limit
Upper limit
299Hz
-80,0 dB
20,0 dB
300Hz
-10,0 dB
10,0 dB
5000Hz
-10,0 dB
10,0 dB
5001Hz
-80,0 dB
20,0 dB
In special cases an exception to this requirement can be given to products, where
for example echo cancellation technology limits the bandwidth. The resulting
frequency response in such cases must be at least 300 Hz – 3.4 kHz with a
maximum ±10 dB ripple.
5.1.5 Priority: 2 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes wideband
requirement.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
wideband tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Frequency
Lower limit
Upper limit
149Hz
-80,0 dB
20,0 dB
150Hz
-10,0 dB
10,0 dB
7000Hz
-10,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
39 / 68
5.1.6 Priority: 3 Microphone – Frequency response
Purpose:
To verify that the microphone frequency response curve passes super wideband
requirement.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level.
Output:
Measure frequency response of the microphone by comparing the monitored
speech signal to the original speech. The resulting frequency response fits into a
super wideband tolerance window:
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
Frequency
Lower limit
Upper limit
149Hz
-80,0 dB
20,0 dB
150Hz
-10,0 dB
10,0 dB
10000Hz
-10,0 dB
10,0 dB
10001Hz
-80,0 dB
20,0 dB
40 / 68
5.1.7 Priority: 1 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Speakerphone is placed at the recommended usage distance from the HATS
mouth. Play back a measurement signal from the artificial mouth [2] at a normal
speech level to allow Skype to adjust the microphone gain setting to a suitable
value. Then play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 35 dB.
5.1.8 Priority: 2 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Speakerphone is placed at the recommended usage distance from the HATS
mouth. Play back a measurement signal from the artificial mouth [2] at a normal
speech level to allow Skype to adjust the microphone gain setting to a suitable
value. Then play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 40 dB.
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5.1.9 Priority: 3 Microphone – Speech to self noise ratio
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Speakerphone is placed at the recommended usage distance from the HATS
mouth. Play back a measurement signal from the artificial mouth [2] at a normal
speech level to allow Skype to adjust the microphone gain setting to a suitable
value. Then play the measurement signal again and record it at the far end.
Output:
The recorded microphone signal is analyzed. When the speech signal level is
compared to the noise level (noise is measured during pauses of speech), Aweighted RMS speech to noise ratio is at least 45 dB.
5.1.10 Priority: 2 Microphone – Speech to self noise ratio during speech activity
Purpose:
To check that the self noise level of the microphone is sufficiently low during the
active speech.
Input:
Play back a measurement signal from the artificial mouth [2] at a normal speech
level. Immediately following play a special speech type of test signal to deactivate
the possible microphone noise gating function. Record the test signal at the far end.
Output:
The recorded microphone signal is processed to separate the speech part from the
noise part. When the level of speech part is compared to the level of noise part, Aweighted RMS speech to noise ratio is at least 30 dB.
5.1.11 Priority: 1 Amount of acoustic echo
Purpose:
To verify that the other party does not hear echo during the call. This is often the
biggest problem for speakerphones.
Input:
Ask another tester to use the product under test in a quiet meeting room (with the
2
floor area of at least 10 m ). Set the distance between him/her and the
speakerphone to what the vendor specified as the recommended. If the
recommended distance is not specified then use the maximum available distance.
Pick yourself a good quality reference headset and set up a call. Ask the other party
to set speakerphone volume level so that he/she could hear you with slight
concentration effort, i.e. the audible speech level is about 5 dB below the preferred
listening level (i.e. level here is about 55 dB SPL (A)). Try interrupting each other
while talking.
Output:
Talk and at the same time listen to the echo of your own voice which might come
back to you from the other party. Echo may be audible, but if you don’t speak at the
same time the echo and other potential echo related artifacts are not annoying.
Interrupting other party might be difficult as double talk transmission of system might
be poor. Switching from talker to other is possible when there is a silence before
talking turn changes.
Note:
At Skype audio lab, the situation is simulated with an objective recording in call,
where single talk from both parties, partial and heavy double talk are used. The
recordings are analyzed by listening. The recordings and comments are included to
the Audio report of the DUT.
Note 2:
DUT is tested when Acoustic Echo Canceller of Skype is enabled.
5.1.12 Priority: 2 Amount of acoustic echo
Purpose:
To verify that the other party does not hear echo during the call. This is often the
biggest problem for speakerphones.
Input:
Ask another tester to use the product under test in a quiet meeting room (with the
2
floor area of at least 10 m ). Set the distance between him/her and the
speakerphone to what the vendor specified as the recommended. If the
recommended distance is not specified then use the maximum available distance.
Pick yourself a good quality reference headset and set up a call. Ask the other party
Audio Requirement Specification
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to set speakerphone volume level to the preferred listening level (60 dB SPL (A)).
Try interrupting each other while talking.
Output:
Talk and at the same time listen to the echo of your own voice which might come
back to you from the other party. Echo may be audible when you talk at the same
time, during this double talk the echo and other potential echo related artifacts are
on maximum only slightly annoying. If you don’t speak at the same time the echo is
not audible and not annoying. Switching from one speaker to another with
interruptions is easy.
Note:
At Skype audio lab, the situation is simulated with an objective recording in call,
where single talk from both parties, partial and heavy double talk are used. The
recordings are analyzed by listening. The recordings and comments are included to
the Audio report of the DUT.
Note 2:
DUT is tested when Acoustic Echo Canceller of Skype is enabled.
Note 3:
If there is Acoustic Echo Canceller in device, it will be tested also without Skype
Acoustic Echo Canceller.
5.1.13 Priority: 3 Amount of acoustic echo
Purpose:
To verify that the other party does not hear echo during the call. This is often the
biggest problem for speakerphones.
Input:
Ask another tester to use the product under test in a quiet meeting room (with the
2
floor area of at least 10 m ). Set the distance between him/her and the
speakerphone to what the vendor specified as the recommended. If the
recommended distance is not specified then use the maximum available distance.
Pick yourself a good quality reference headset and set up a call. Ask the other party
to set speakerphone volume level to the preferred listening level (60 dB SPL (A)).
Try interrupting each other while talking.
Output:
Talk and at the same time listen to the echo of your own voice which might come
back to you from the other party. Echo is not audible and there are no other echo
related artifacts. There are no challenges in switching from speaker to another with
interruptions.
Note:
At Skype audio lab, the situation is simulated with an objective recording in call,
where single talk from both parties, partial and heavy double talk are used. The
recordings are analyzed by listening. The recordings and comments are included to
the Audio report of the DUT.
Note 2:
DUT is tested when Acoustic Echo Canceller of Skype is enabled.
Note 3:
If there is Acoustic Echo Canceller in device, it will be tested also without Skype
Acoustic Echo Canceller.
5.1.14 Priority: 2 Echo loss in single talk during Skype call
Purpose:
To verify that the other party does not hear acoustic echo while he or she is
speaking.
Input:
Place the speakerphone to the recommended usage position. A Skype call is set up
and a test signal is sent to the loudspeaker. The output signal at the remote end is
measured and analyzed for echo presence. Levels and details are defined in ITU-T
Recommendations G.122 and G.131 [8], [9]
Output:
Echo loss must be at least 40 dB.
Note:
This requirement is based on ITU-T Recommendation G.131 [9] information of
tolerance of talker echo (figure 1 in G.131) targeting to 50 dB Talker Echo Loudness
Rating that corresponds to about 40 dB Echo Loss taking into account of potential
long delay of IP network. The measurement method is defined ITU-T
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Recommendation G.122 [8]. In this requirement the Skype speech preprocessing,
including echo canceller is enabled.
5.1.15 Priority: 3 Echo loss in single talk without Skype speech improvements
Purpose:
To verify that the other party does not hear acoustic echo while he or she is
speaking.
Input:
Place the speakerphone to the recommended usage position. A test signal is sent to
the loudspeaker. The microphone output is measured and analyzed for echo
presence. Levels and details are defined in ITU-T Recommendations G.122 and
G.131 [8], [9]. In practice the loudspeaker and microphone levels are set to the
same as they were in requirement 5.1.14.
Output:
Echo loss must be at least 40 dB.
Note:
This requirement is based on ITU-T Recommendation G.131 [9] information of
tolerance of talker echo (figure 1 in G.131) targeting to 50 dB Talker Echo Loudness
Rating that corresponds to about 40 dB Echo Loss taking into account of potential
long delay of IP network. The measurement method is defined ITU-T
Recommendation G.122 [8]. The measurement can be made during a Skype call,
but having Skype echo canceller disabled.
5.1.16 Priority: 1 Loudspeaker – Frequency response
Purpose:
To verify that the loudspeaker frequency response curve passes minimum
requirement.
Input:
Play back a measurement signal through the loudspeaker. Measure the
loudspeaker frequency response at the recommended usage position.
Output:
The resulting frequency response fits into a limited narrowband tolerance window:
Frequency
Lower limit
Upper limit
499Hz
-80,0 dB
20,0 dB
500Hz
-10,0 dB
10,0 dB
3400Hz
-10,0 dB
10,0 dB
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3401Hz
-80,0 dB
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20,0 dB
Note: Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check
test instructions 5.3.1
5.1.17 Priority: 2 Loudspeaker – Frequency response
Purpose:
To verify that the loudspeaker frequency response curve passes wideband
requirement.
Input:
Play back a measurement signal through the loudspeaker. Measure the
loudspeaker frequency response at the recommended usage position.
Output:
The resulting frequency response fits into a limited wideband tolerance window:
Frequency
Lower limit
Upper limit
299Hz
-80,0 dB
20,0 dB
300Hz
-10,0 dB
10,0 dB
7000Hz
-10,0 dB
10,0 dB
7001Hz
-80,0 dB
20,0 dB
Note: Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check
test instructions 5.3.1.
5.1.18 Priority: 3 Loudspeaker – Frequency response
Purpose:
To verify that the loudspeaker frequency response curve passes super wideband
requirement.
Input:
Play a speech or a measurement signal through the loudspeaker. Measure the
loudspeaker frequency response at the recommended usage position.
Output:
The resulting frequency response fits into a limited super wideband tolerance
window:
Audio Requirement Specification
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Note:
Frequency
Lower limit
Upper limit
149Hz
-80,0 dB
20,0 dB
150Hz
-10,0 dB
10,0 dB
10000Hz
-10,0 dB
10,0 dB
10001Hz
-80,0 dB
20,0 dB
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Skype uses ITU-T type 3.3 ear and DRP to diffuse field correction, check test
instructions 5.3.1.
5.1.19 Priority: 1 Loudspeaker – Suitable volume level for quiet office use
Purpose:
To verify that the speakerphone output level fulfills requirement for recommended
operating distance in quiet office environment.
Input:
Place the speakerphone at the recommended operating distance from the
measurement microphone. Play back a speech signal through the loudspeaker. Set
the speakerphone volume to loud and measure the loudspeaker output.
Output:
Measured output is at least 55 dB SPL A-weighted (this is 5 dB below the preferred
listening level).
5.1.20 Priority: 1 Loudspeaker – Distortion at quiet office use
Purpose:
To verify that device does not create too much distortion to degrade speech quality
and produce audible echo at the far end.
Input:
Place the speakerphone to the recommended usage position. Make a Skype call
and play a speech signal from other party side. Set the speakerphone volume to 55
dB SPL A-weighted. THD is then measured with stepped sine signals set to -9 dBov
RMS (-15 dBm0 RMS) at other Skype client (equals to about -6 dBov peak level).
Distortion is measured starting from the lowest corner frequency of cases 5.1.165.1.18 that DUT passed and up to 3.4 kHz. Speech level and distortion are
measured with measurement microphone which is placed at the intended position of
listener’s head.
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Output:
Measured Total Harmonic Distortion (THD) of all measurement points is below 3%
(equals to -30 dB).
Example:
How to calculate the measurement bandwidth? For example if DUT passed cases
5.1.16 and 5.1.17, which have low corner frequencies of 500 and 300 Hz
respectively, then the lower one of these is chosen, meaning that distortion will be
measured from 300 to 3400 Hz.
5.1.21 Priority: 2 Loudspeaker – Suitable volume level for normal office use
Purpose:
To verify that the speakerphone output level fulfills requirement for recommended
operating distance in normal office environment.
Input:
Place the speakerphone at the recommended operating distance from the
measurement microphone. Play back a speech signal through the loudspeaker. Set
the speakerphone volume to loud and measure its loudspeaker output.
Output:
Measured output is at least 60 dB SPL A-weighted.
5.1.22 Priority: 2 Loudspeaker – Distortion at normal office use
Purpose:
To verify that device does not create too much distortion to degrade speech quality
and produce audible echo at the far end.
Input:
Place the speakerphone to the recommended usage position. Make a Skype call
and play a speech signal from other party side. Set the speakerphone volume to 60
dB SPL A-weighted. THD is then measured with stepped sine signals set to -9 dBov
RMS (-15 dBm0 RMS) at other Skype client (equals to about -6 dBov peak level).
Distortion is measured starting from the lowest corner frequency of cases 5.1.165.1.18 that DUT passed and up to 3.4 kHz. Speech level and distortion are
measured with measurement microphone which is placed at the intended position of
listener’s head.
Output:
Measured Total Harmonic Distortion (THD) of all measurement points is below 3%
(equals to -30 dB).
5.1.23 Priority: 3 Loudspeaker – Suitable volume level for noisy office use
Purpose:
To verify that the speakerphone output level fulfills requirement for recommended
operating distance in noisy office environment.
Input:
Place the speakerphone at the recommended operating distance from the
measurement microphone. Play back a speech signal through the loudspeaker. Set
the speakerphone volume to loud and measure the loudspeaker output.
Output:
Measured output is at least 65 dB SPL A-weighted.
5.1.24 Priority: 3 Loudspeaker – Distortion at noisy office use
Purpose:
To verify that device does not create too much distortion to degrade speech quality
and produce audible echo at the far end.
Input:
Place the speakerphone to the recommended usage position. Make a Skype call
and play a speech signal from other party side. Set the speakerphone volume to 65
dB SPL A-weighted. THD is then measured with stepped sine signals set to -9 dBov
RMS (-15 dBm0 RMS) at other Skype client (equals to about -6 dBov peak level).
Distortion is measured starting from the lowest corner frequency of cases 5.1.165.1.18 that DUT passed and up to 3.4 kHz. Speech level and distortion are
measured with measurement microphone which is placed at the intended position of
listener’s head.
Output:
Measured Total Harmonic Distortion (THD) of all measurement points is below 3%
(equals to -30 dB).
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5.1.25 Priority: 2 Loudspeaker – Volume level at maximum operating distance
Purpose:
To verify that the speakerphone output level fulfills requirement for recommended
maximum operating distance in the office environment.
Input:
Place the speakerphone at the recommended maximum operating distance from the
measurement microphone. Play back a speech signal through the loudspeaker. Set
the speakerphone volume to loud and measure the loudspeaker output.
Output:
Measured output is at least 55 dB SPL A-weighted (this is 5 dB below the preferred
listening level).
5.1.26 Priority: 2 Microphone – Sensitivity at maximum operating distance
Purpose:
To verify that the DUT microphone provides strong speech signal for Skype
application, when speakerphone is tested at the maximum operating distance that
has been specified by he manufacturer. The distance is measured between the
microphone and the mouth.
Input:
Place the speakerphone to the maximum operating distance. Play back a speech
signal from an artificial mouth [2] at a normal speech level from intended usage
distance of the speakerphone.
Output:
The microphone signal is monitored from another Skype client and measured. The
speech level is not less than -34 dBov RMS (-28 dBm0 RMS).
5.1.27 Priority: 3 Microphone – Speech to self noise ratio at maximum operating distance
Purpose:
To check that the self noise level of the microphone is sufficiently low.
Input:
Place the speakerphone to the recommended usage position. Play back a
measurement signal from the artificial mouth [2] at a normal speech level.
Output:
The microphone signal is monitored at the far end and measured with ACQUA [11].
When the speech signal level is compared to the noise level (noise is measured
during pauses of speech), A-weighted RMS speech to noise ratio is at least 35 dB.
5.2 Speakerphone: Supporting audio documentation requirements
In addition to the user manual (the one that comes with the product) in Certification testing we also
ask for supporting audio documentation. Such documentation contains engineering data and
engineering test data of the product.
5.2.1 Priority: 1 Verifying supporting documentation for Speakerphone audio
Purpose:
Solution must come with a supporting audio documentation (only for certification
testing purposes).
Output:
DUT arrives with supporting audio documentation that contains the following
information:
•
•
Usage related info:
o
Recommended operating distance
o
Maximum operating distance
Active signal processing: yes/no, if yes then:
o
Active beam forming microphone and/or loudspeaker: yes/no
o
In built acoustic echo cancellation: yes/no
o
Echo cancellation operating bandwidth (narrowband, wideband,
super wideband)
o
Noise suppression: yes/no, in sending or/and receiving directions
Audio Requirement Specification
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•
•
o
Automated Gain Control: yes/no, in sending or/and receiving
directions
o
Other: describe what, sending or/and receiving directions
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Microphone/s:
o
Frequency range (lowest and highest audible frequencies)
o
Directionality/design principle of a microphone
o
Number of microphones / microphone inputs
o
Microphone phantom power yes/no, supply voltage (if applicable)
o
Microphone input connector type (balanced, unbalanced) (if
applicable)
Loudspeaker/s:
o
Frequency range (lowest and highest audible frequencies)
o
Number of loudspeaker / line outputs (if applicable)
o
Loudspeaker design principle (one/multiway, open/closed box/bass
reflex)
5.3 Speakerphone: Audio test instructions
Test environment is defined in Chapter 8.
Device under test that provides Speakerphone acoustic UI is compared to a good quality reference
speakerphone. This reference speakerphone is chosen from Skype Certified speakerphones. The
sending (microphone) and receiving (loudspeaker) parts might be chosen from two different
products.
5.3.1 Objective testing measurement setup
Audio performance requirements are measured with objective measurement tools. The
measurements will be performed with Head And Torso Simulator (HATS) [2] or/and measurement
microphone and with automated audio testing system. The measurements are performed in
anechoic and/or in quiet office room. The audio testing tools and environment are listed in
8.1.1.Test practices and setups follow the principles given in ITU-T recommendations [4]. Actual
test cases are specially built for the requirements defined in this document.
The measurements will be performed mainly during a Skype to Skype call. If device is connected
to PC the default audio drivers for DUT are used.
Frequency response results are averaged to 1/3 octave frequency resolution.
For hand-held speakerphones (such as small phones) the recommended usage position is in front
of the user -30˚ below the mouth at a recommended usage distance, specified by the
manufacturer.
For non-handheld i.e. desktop devices, recommended usage position specified by the
manufacturer shall be used. If there is no manufacturer’s recommendation provided, the test
arrangement as per ITU-T recommendation P.340 [5] shall be used (see figure below).
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Preferred listening level is defined to be 60 dB SPL A-weighted RMS for a speakerphone.
The SPL level of normal speech in these tests is 62 dB SPL A-weighted at 1 m distance in the
front of mouth. The level is based on ITU-T recommendations of real and artificial mouth speaking
levels. The lowered speech is about 10 dB quieter and loud speech is about 10 dB louder. Note
that in real life the speaking levels vary more than 10 dB depending on speaker, distance between
people having conversation and environment.
5.3.2 Subjective testing measurement setup
A speakerphone is tested by a tester under normal conditions as defined in the requirements. The
speakerphone is placed on the table next to the user in the office space or/and in a meeting room
(with the floor area of at least 10 m2).
If the speakerphone is not a standalone product, then it is tested in a typical environment, for
example on a PC with a normal sound card and audio connectors. The speakerphone will be
tested on several PCs or/and operation systems if necessary.
The testing positions are set such as defined in Objective testing measurement setup in the
previous section. The tester on the other side should use a good quality headset.
Subjective testing is applied primarily for Echo requirements 5.1.11 – 5.1.13. First create a Skype
call and play/speak test signal from the other party side. The both sides of call should talk at
normal levels. The volume level of speakerphone loudspeaker is set as is defined in the
requirements - either to a lowered speech or preferred speech level. It is recommended that call is
recorded and listened to detect potential echo.
During the test call testers are recommended to talk both single and double talk. At speakerphone
side a tester can talk partly at the same time as the other party is speaking.
Very important issues with a speakerphone are the distance from the user and hard surfaces
proximity, such as wall or computers. The hard surfaces create strong reflections and acoustic
echo back to the microphone. This can be tested by using the device at the maximum distance
and placing it closer to the wall(s) or computer(s).
Test case judgments are based on comparison of tester’s perception with the requirements. The
recorded samples can also be analyzed with a normal sound editor program available for
Audio Requirement Specification
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computers, check Section 8.1.1. Testing is by nature informal meaning that it does not have blind
testing of multiple people and related statistical analysis of judgments. If judgment of some
requirement is difficult, then two additional testers will perform the test case, and if two or more
testers judge the case to be failed, then requirement is failed.
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6. Other audio product group
Audio test instructions in section 6.3 apply and should be followed in all requirements.
6.1 Other audio product: Audio performance requirements
6.1.1 Priority: 1 Frequency responses – sending and receiving directions
Purpose:
To verify that the sending and receiving direction frequency response curves pass
the minimum requirements.
Input:
Play back a measurement signal in sending and receiving directions.
Output:
Measure frequency responses of sending and receiving directions by comparing the
monitored speech signals to the original speech signals. The resulting frequency
responses fit into a wideband tolerance window:
Exception:
Frequency
Lower limit
Upper limit
99Hz
-80,0 dB
20,0 dB
100Hz
-3,0 dB
3,0 dB
7000Hz
-3,0 dB
3,0 dB
7001Hz
-80,0 dB
20,0 dB
In special cases an exception to this requirement can be given for some cordless
and Analog Telephony Adapter (ATA) products, like DECT or Bluetooth products in
which the protocol limits the frequency bandwidth. These are judged case by case.
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6.1.2 Priority: 1 Product provides suitable levels for audio signal output
Purpose:
To verify that the output level that the product provides is suitable for other devices
in the signal chain. For example a sound card needs to provide suitable (high
enough) signal levels for reference headphones and ATA must provide suitable
output levels for a reference handset.
Input:
Using the product set up a Skype call. Depending on the product interface, connect
corresponding reference device:
• Sennheiser HD650 Headphones
• Siemens Euroset 802 Deskset
Perform the level measurement test.
Output:
Output volume level is at least 70 dB SPL A-weighted RMS (this is 5 dB below the
preferred listening level on one ear listening case)..
6.1.3 Priority: 1 Product provides suitable levels for audio signal input
Purpose:
To verify that the input level that the product provides is suitable for other devices in
the signal chain. For example a sound card needs to provide suitable (high enough)
signal levels for reference microphone and ATA must provide suitable input levels
for a reference handset.
Input:
Using the product set up a Skype call. Depending on the product interface, connect
corresponding reference device:
• Microphone EMM-8
• Siemens Euroset 802 Deskset
Perform the level measurement test.
Output:
Input volume level is not less than -30 dBov RMS (-24 dBm0 RMS).
6.1.4 Priority: 1 Minimum crosstalk from receiving to sending direction
Purpose:
To verify if crosstalk level passes the minimum requirement.
Input:
Disconnect the acoustic interface from the device. Play back a test signal to device
under test output i.e. receiving direction. At the same time monitor and analyze the
input i.e. sending direction signal level at the other Skype client output.
Output:
Digital crosstalk level at other Skype client output is less than -51 dBov A-weighted
RMS (-45 dBm0 A-weighted RMS).
6.2 Other audio product: Supporting audio documentation requirements
In addition to the user manual (the one that comes with the product) we also ask for supporting
audio documentation (for certification testing purposes). Such documentation contains engineering
data and engineering test data for the product.
6.2.1 Priority: 1 Verifying supporting documentation for Other audio product
Purpose:
Solution must come with a supporting audio documentation (only for certification
testing purposes).
Output:
DUT arrives with supporting audio documentation that contains the following
information:
•
Sending:
o
Speech signal delay from input to output (if above 5 ms)
Audio Requirement Specification
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o
•
•
•
•
53 / 68
Usable frequency bandwidth
Receiving:
o
Speech signal delay from input to output (if above 5 ms)
o
Usable frequency bandwidth
Connectors (if applicable):
o
Type/s & electric connections (ground, signals, bias voltages…)
o
Target input and output levels/voltages
o
Maximum input and output levels/voltages
o
Maximum and minimum impedances for external connection
Volume control (if applicable):
o
Range in dB
o
Minimum volume (dBV, dBSPL or similar RMS)
o
Maximum volume (dBV, dBSPL or similar RMS)
Active signal processing: yes/no
o
if yes then what?
6.3 Other audio product: Audio test instructions
6.3.1 Objective testing measurement setup
Objective testing arrangement depends on if the testing is performed with or without acoustic
interface. Example of the earlier is a sound card that can be tested together with headset. The
example of latter is an audio processing algorithm that does not give direct signal to acoustic
interface device. In a case the device is tested together with acoustic interface, the testing setup
can be picked from headset, handset or speakerphone test instructions in the previous chapters.
In another case when acoustic interface is not used, the electric to electric tests between two
Skype clients and the DUT can be performed.
The measurements will be performed mainly in Skype call having all speech enhancement
algorithms as they are by default in Skype and potential device audio drivers.
Frequency response results are averaged to 1/3 octave frequency resolution.
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7. Non-audio product group
Audio test instructions in section 7.3 apply and should be followed in all requirements.
7.1 Non-audio product: Audio performance requirements
7.1.1 Priority: 1 Continuous transmission of speech
Purpose:
To verify that users hear continuous audio transmission without short or long
temporal drops or distortions while product or solution is under normal and heavy
load.
Input:
Connect the product to the Skype or in between Skype signal path. Set up a call
and play back speech samples in sending and receiving directions. Record both
near and far end Skype outputs. During the recording create load to the product,
solution or/and PC or phone to where Skype application is installed. Use for
example other available Skype features, such as file sharing and video, open
browser and open a video playback etc.
Output:
Use PESQ tool to analyze the speech quality in both sending and receiving
directions. The biggest MOS-LQO drop must be smaller than 1.0 compared to
average MOS score of Skype call without the product in use over periods of 10
secs.
7.1.2 Priority: 2 Continuous transmission of speech
Purpose:
To verify that users hear continuous audio transmission without short or long
temporal drops or distortions while product or solution is under normal and heavy
load.
Input:
Connect the product to the Skype or in between Skype signal path. Set up a call
and play back speech samples in sending and receiving directions. Record both
near and far end Skype outputs. During the recording create load to the product,
solution or/and PC or phone to where Skype application is installed. Use for
example other available Skype features, such as file sharing and video, open
browser and open a video playback etc.
Output:
Use PESQ tool to analyze the speech quality in both sending and receiving
directions. The biggest MOS-LQO drop must be smaller than 0.5 compared to
average MOS score of Skype call without the product in use over periods of 10
secs.
7.2 Non-audio product: Supporting audio documentation
In addition to the user manual (the one that comes with the product) in Certification testing we also
ask for supporting audio documentation. Such documentation contains engineering data and
engineering test data of the product.
7.2.1 Priority: 1 Verifying supporting documentation for Non-audio product
Purpose:
Solution must come with a supporting audio documentation (only for certification
testing purposes).
Output:
DUT arrives with supporting audio documentation that contains the following
information:
•
Solution speech signal delay (if above 5 ms)
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•
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Influence on the audio quality of Skype call under normal and heavy
loading of product/solution
7.3 Non audio product: Audio test instructions
7.3.1 Objective testing measurement setup
Audio performance requirements can be measured with objective measurement tools.
Measurement tool and Skype clients will be connected electrically as acoustic interface is not
needed.(electric to electric measurement).
Mean Opinion Score results are judged by PESQ or similar advanced objective speech quality
metric. Several test speech samples are recorded from sending and receiving directions. These
recordings are divided to about 10 sec length segments that are analyzed with objective speech
quality tool. The speech material consists of variety of speakers and both male and female voices.
MOS values are calculated without and with the DUT being connected. The drop is calculated as a
difference between individual MOS values.
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8. List of environments
8.1 List of Test Platforms
Solutions are tested in the following environments. This list will be extended in future:
8.1.1 Skype Audio Test Lab
Skype Audio Test Lab consists of state of the art audio testing tools for VoIP and
telecommunication:
•
Objective testing is performed in a wideband audio measurement capable anechoic room,
with a build-in and acoustically „invisible” 18 channel and subwoofer-loudspeaker setup for
real and artificial 3D sound reproduction.
•
Measurement setup consisting of ACQUA audio testing tool from HEAD Acoustics, with
VoIP option and with MFE VI.1 measurement front-end.
•
HATS and handset positioner [2] from Brüel & Kjær, HATS model 4128C.
•
The actual tests performed by ACQUA system are customized by Skype staff and
arranged into test macros, which automate the test process.
•
PESQ and other similar advanced objective speech quality models are used in ACQUA
system. Skype uses mainly Opticom version of PESQ that has been integrated to ACQUA
by HeadAcoustics.
•
Free and pressure field measurement microphones and cables from G.R.A.S. Sound and
Vibration
•
Reference Skype Client on PC with high quality sound card and customizable Skype
version
•
Several other tools, such as reference headphones: Sennheiser HD650 and HD25,
reference low- noise microphone Rode NT2, EMM-8 cal Calibrated measurement
microphone, reference loudspeakers: Genelec 8020A, microphone preamps, loudspeaker
processor, headphone amp, PC with professional audio editing softwares: Adobe Audition
and Audicity and sound cards.
•
Skype Certified products will be used as the reference products in the tests.
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Measurement Setup
•
Head Acoustics ACQUA software – automated audio testing system
http://www.head-acoustics.de/eng/telecom_acqua.htm
http://www.head-acoustics.de/downloads/eng/acqua/acqua18e_mail.pdf
•
Head Acoustics MFE VI.I Measurement Front End
http://www.head-acoustics.de/eng/telecom_acqua_mfe_VI_1.htm
http://www.head-acoustics.de/downloads/eng/mfe/D6462e1_MFE_VI_1.pdf
•
Opticom PESQ (Perceptual Evaluation Voice of Speech Quality) software
http://www.opticom.de/download/SpecSheet_PESQ_05-11-14.pdf
•
Bruel and Kjaer Head and Torso Simulator – model 4128C
http://www.bksv.com/1650.asp
http://www.bksv.com/pdf/Bp0521.pdf
•
Bruel and Kjaer Head and Torso Simulator – handset positioner 4606
http://www.bksv.com/pdf/Bp0521.pdf
•
Soundcard in Reference Skype Client PC – ECHO Audio MIAMIDI
http://www.echoaudio.com/Products/PCI/MiaMIDI/specs.php
•
DUT Skype Client and Reference Skype Client PC specification
Intel DG965SS motherboard with BIOS version MQ96510J.86A.1666.2007.0327.2349
The processor in all PC-s: Intel 630 P4 FSB800 2MB 3.0GHz
1GB (2x512Mb 533MHz DDR2 NON-ECC CL4 Kingston DIMM modules)
Samsung 40Gb SATAII NCQ 7200 RPM 8Mb Hard drive
Samsung DVD ROM
•
Etherfast router Linksys BEFSR81 ver 3.1
•
WiFi access point for wireless devices – Cisco Aironet AIR-AP1131AG
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8.1.2 Compatible testing environment
Manufacturer or audio laboratory willing to test products in their premises need to fulfill at least
following conditions and tools:
•
ITU-T compatible HATS with type 3.3 anatomic ear, for example from Bruel and Kjaer or
Head Acoustics
•
Calibrated acoustic and electric measurement system: microphones, amplifiers, wiring…
•
Skype recommends to perform measurements mainly at anechoic room as defined in
ITU-T documents, for example at ITU-T P.341 recommendation requirement for Test
rooms (A.3.1.1), that defines anechoic conditions, sizes of room and noise floor to be
below 24 dBA SPL rms
o
•
It can be possible to use quiet and non-echoic environment/room for headset
and handset measurements if the setup is built with care and acoustic
knowledge and measurements are performed professionally. In such case
Skype considers that noise floor must be below 30 dB SPL A especially for
echo and noise floor measurements.
Care must be put in design and usage of the facility to temporal and stationary noises
from: cars, doors, talking, walking, water and drain pipes, ventilation, electricity and
radio frequency interferences.
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9. Appendix
9.1 Definitions
A-weighting
A frequency weighting curve defined in IEC179 and various other
standards for use widely in sound level meters. A-weighting is an inverse
curve for an equal loudness contour of human hearing at quiet levels
(precisely based on the 40-phon Fletcher-Munson curves). A-weighted
measurements estimate how people would perceive loudness of a sound
taking into account that hearing has different sensitivities for different
frequencies.
Acoustic echo
Signal leaking from earpiece or loudspeaker to microphone. For a good
call quality this should be as small as possible. If it is too strong it makes
communication difficult.
Acoustic user
interface
Allows user to hear or speak over the communication system. Products
providing acoustic UI have microphone, earpiece or/and loudspeaker.
Check Section 1.2.4.
ACQUA
Advanced Communication Quality Analysis system from HEAD Acoustics
[11]
Anechoic Chamber /
Room
Anechoic chambers are commonly used in acoustics to perform
experiments in nominally free field conditions. This means that all sound
energy will be traveling away from the source with almost none being
reflected back. Anechoic chamber is a room in which there are no
echoes.
Annoying sound
A sound that is so clearly audible that it distracts user’s attention from the
conversation. It can be an unintended consequence of the intended
sound or just unwanted sound that irritates the user.
Audible
Means that user can hear certain sound both in quiet environment and in
presence of other sounds. Check also slightly audible.
Audio ergonomics
Defines how comfortable and meaningful a product is for user from audio
perspective, check Section 1.3.
Audio performance
Audible, perceivable performance of a product as judged by the user,
check Section 1.3. Consists on sub-parameters – intelligibility,
naturalness and conversational effort.
Certification
An endorsement from Skype that a third-party vendor meets Skype’s own
criteria for a co-labeled solution.
Conversational effort
How little or how much concentration a conversation requires from the
user. It is a sub-parameter of audio performance. Check technical
parameters from Conversational quality.
Conversational
quality
Defines how good or how bad are perceived audio parameters that affect
conversational quality. Typical parameters are: delay, acoustic echo,
noise and continuity of transmission.
Cordless handset
Handset that operates through radio frequencies without wired
connection to a PC or other device. Examples are Bluetooth and DECT
handset.
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Cordless headset
Headset that operates through radio frequencies without wired
connection to a PC or other device, for example Bluetooth headset.
Crosstalk
Undesired leaking of receiving i.e. earpiece/loudspeaker signal into
sending i.e. microphone signal at transmission circuits.
dB / Decibel
Decibel is a logarithmic representation of a number or ratio between
numbers. dB SPL is Sound Pressure Level in decibels (check SPL), dBV
is a voltage in dB, so that 1 V (root mean square) equals to 0 dBV.
dBFS
dBFS is a commonly used measure of signal level in digital system
compared to decibels full scale. Two different definitions exist, where in
both the highest peak level of digital system is 0 dBFS, but RMS level
definition varies. The Audio Engineering Society has defined the highest
scaled sine signal to have RMS level of 0 dBFS, whereas the other
definition sets the same signal RMS level to be -3.1 dBFS. The latter
definition is used in some software audio editors. Due to the existence of
the two definitions the dBov definition is used for digital levels in this
document.
dBm0
Abbreviation for the power in dBm measured at a zero transmission level
point. In practice the conversion from dBov is as Y dBm0 ≈ X dBov + 6
dB.
dBov
Measure of a signal level compared to overload point of digital system.
Defined in [1] in section 5.7. For a maximum scale digital sine signal the
peak level is 0dBov and RMS is -3.1 dBov. dBov definition here is the
same as the square wave scaled dBFS definition.
Delay
Delay of speech signal(s) between users
Diffuse field
correction
Defined at ITU-T recommendation P.58 [2]. This is the preferred
frequency correction of Skype for earpiece and loudspeaker
measurements when using HATS. It is a difference, in dB, between the
third-octave spectrum level of the acoustic pressure at the ear-Drum
Reference Point (DRP) and the third-octave spectrum level of the
acoustic pressure at the HATS Reference Point (HRP) in a diffuse sound
field with the HATS absent.
Double talk
Situation where two or more parties of a call are talking at the same time.
Electric to electric
Skype call
Skype call between two Skype clients that are measured from electric
outputs and inputs of good quality sound cards. Acoustic interfaces are
not present.
Far end
The other side of the call compared to a local user using device under
test. Opposite for the near end.
Free field conditions
Audio measurement environment, with no reflecting surfaces. Such
conditions are reachable in an anechoic audio measurement room.
Good quality
reference device
A device in every audio UI group that has shown or proven to have good
audio quality in various aspects. This device serves as a reference for
Device Under Test in some test cases, such as MOS evaluations.
Handset
Product that the user keeps in his hand and puts next to his ear, when in
a call, like mobile phone or landline phone, check Section 1.2.2.
HATS
Head and Torso Simulator. Skype Audio Lab HATS is B&K 4128C
Head and torso
simulator
Measurement device modeling the head, ear, mouth and upper part of
the torso of an average user. Defined in [2].
Headset
Product consisting of earpiece(s) and microphone that the user puts on
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his head or ear(s) during a Skype call. Check Section 1.2.2.
Intelligibility
Ability to recognize words and their meanings and also transmission of
non verbal information such as emotions, emphasis, identity of speaker. It
is a sub-parameter of audio performance.
ITU-T
International Telecommunication Union – Telecom sector,
http://www.itu.int/ITU-T/ main standardization organization of speech
transmission and quality.
Listening quality
Defines what the perceived quality of speech is. It covers naturalness and
partly intelligibility parts of audio performance.
Local user
The person who is using a product under test.
Loudness
Defines a loudness of sound perceived by a listener, other non-scientific
everyday life terms are volume, volume level or speech level.
Loudspeaker
Product that converts electric audio signal to acoustic signal – plays back
a speech to the user.
Loud speech level
In noisy environment and in situation when people do not hear properly
the other participants, people talk at louder level. Technically the level
can be even 10 dB A-weighted louder than the normal speaking level. In
this document such test signal is used, though the low frequencies have
not been amplified with the full 10 dB and crest factor is limited compared
to normal speech level.
Lowered speech level
People speak at lowered speech level, when they do not want to disturb
other people in the same room. In technical terms the level is around 10
dB SPL lower than the normal speech level, thus the average level in
most cases is around 52 dB SPL when measured from 1 m in front of the
listener.
Mean Opinion Score
Check definition for MOS
MOS
Mean Opinion Score, a numerical indication of the perceived quality of
speech or audio. Typically an average of several listeners who have
performed a specific MOS test in controlled and formal way. MOS scale
is defined in ITU-T recommendation P.800 [3] for speech quality as:
MOS
Quality
5
Excellent
4
Good
3
Fair
2
Poor
1
Bad
In standardized tests, a good quality narrowband call, for example
between mobile phones, can reach MOS slightly above 4. Wideband call
can reach close to 5 in good conditions. MOS below 3 is generally
considered to be too low.
MOS-LQO
Mean Opinion Score – Listening Quality measured with Objective tools
(such as PESQ measurement)
Narrowband speech
Typical landline or mobile phone speech, with a frequency bandwidth
between 300 and 3400 Hz.
Naturalness
How natural is listening (and speaking) in conversation. Technical
parameters are: adequate loudness, natural frequency content, low noise
and distortion. This is a sub-parameter of audio performance.
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Near end
The side of the call where the device under test is used. Opposite for the
far end.
Non-audio product
group
Group of products that do not directly influence Skype audio quality
during a call, check Section 1.2.5.
Non-acoustic user
interface
Products that do not provide acoustic interface.
PESQ
Perceptual Evaluation of Speech Quality tool [10], complies with ITU-T
P.862 recommendation. PESQ is used to identify following artifacts:
distortion, additional coding, temporal artifacts, additional noise and delay
changes, but not acoustic interface artifacts. Skype wants to point out
clearly that Skype acknowledges the fact that PESQ has not been
designed and verified for acoustic interfaces therefore PESQ is not used
as a measure of a quality of acoustic interface. Further Skype uses
PESQ as a relative metric comparing the result of an acoustic interface
device to a known reference device. In other words Skype is not using
PESQ as an absolute metric in acoustic interface cases.
Preferred listening
level
Preferred listening levels are defined to be:
•
75 dB SPL A-weighted for handset and headset that reproduces
speech only to one ear
•
69 dB SPL A-weighted for a headset that plays speech to both
ears and
•
60 dB SPL A-weighted for speakerphone
All levels are measured with artificial ear of Head And Torso Simulator [2]
when diffuse field frequency correction is applied. The levels are set here
based on calculations from ITU-T recommendations for Sending and
Receiving Loudness Ratings and other available listening level data. Note
that in real life the preferred listening levels between persons can vary up
to +/- 10 dB.
Normal speech level
The level/volume/loudness of speech in normal communication between
people, in technical terms it is around 62 dB SPL A-weighted when
measured 1 m from the user’s mouth. At 25 mm in front of the mouth, in
so called Mouth Reference Point, the level is defined to be -4.7 dBPa that
is 89.3 dB SPL in ITU-T recommendations [4]. In real life this level can
vary easily +/-5 dB depending on the person.
Objective testing
Measures quality by means of technical measurement tools.
One-way delay
Delay of acoustic signal from the user to the other party, expressed in
milliseconds. If it is below 100 ms then it considered to be good. 400-500
ms delay makes normal conversation difficult.
Other audio product
group
Group of products that allow transmission of audio from one system to
another; also products that process audio signal, but do not provide
acoustic interface to the user, check Section 1.2.4. Examples are: electric
audio switch, sound card, Bluetooth dongle.
Other party
The user in a call with a local user (typically not physically located in the
same place)
Priority 1
(Must) requirements: Priority 1 level requirements are the absolute
minimum requirements that the product must pass. For Skype
Certification Audio Specification 100% of Priority 1 requirements must
pass.
Priority 2
(Should) requirements: Priority 2 level requirements are more
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demanding. For Skype Certification Audio Specification no less than 50%
of Priority 2 requirements must pass.
Priority 3
(Nice-to-have) requirements: Priority 3 level requirements show what
quality level we would like a Skype Certified product to have. For Skype
Certification Audio Specification no less than 10% of Priority 3
requirements must pass.
Product
Part of a solution provided by submitting vendor, includes for example
Handset, Cradle, Base station, dongle, application, and drivers, as
opposed to a full solution which also includes the latest Skype version.
Purpose
Statement of the requirement that the test case supports.
Receiving
Shortening for Receiving direction or Receiving side, meaning the audio
signal path coming from the network to the product and played through
the earpiece or loudspeaker to the end user. Simply receiving speech
from the other party and playing it to the product user.
Recommended
usage position
(speakerphone)
For hand-held devices, (such as small phones) the position is in front of
the user -30 degree below the mouth at a recommended usage distance,
specified by the manufacturer. For non-handheld i.e. desktop devices,
recommended usage position is in the middle of a table defined in ITU-T
recommendation P.340 [5].
Reference device
Check Good quality reference device
Reference soundcard
Either Echo MIAMIDI or Edirol UA-25 soundcards are used.
RMS
Root Mean Square – a calculation method for average power of signal
http://en.wikipedia.org/wiki/Root_mean_square
Round trip delay
Overall acoustic delay of signal from user to the other party and back,
synonym to a two-way delay.
Sending
Shortening for Sending direction or Sending side, meaning the audio
signal path from the mouth of the user to a microphone of product under
test and then transmitted to the other party. Simply sending user’s voice
to the other party with the product.
Slightly audible
Means that user can barely hear certain sound in a quiet environment. If
the user doesn’t put any effort he/she might not even notice the sound.
Solution
The product + the latest Skype version
Speakerphone
Product with loudspeaker and microphone, which is usually placed on the
table, next to the user during a call. Check Section 1.2.3.
SPL
Sound Pressure Level, expressed in decibels (dB). 0 dB SPL equals to a
hearing threshold of silence at 1 kHz tone. SPL is defined as: LP = 20
log10(p/p0 ), where p is a sound pressure and p0 is the reference level of
20 µPa. In this document if not else is mentioned SPL refers to RMS
power of the signal. SPL of normal conversation varies between 50-75 dB
SPL.
Subjective testing
Quality rating based on judgments of test subjects. This requires people
to talk or/and listen and rate the quality.
Super wideband
speech
Speech transmission, where the audible frequency range is wider than
what it is in wideband speech transmission. The sampling frequency is
equal or higher than 24 kHz. The bandwidth of signal is between about
50 and 11000 Hz.
THD
Total Harmonic Distortion, a measure of distortion of an audio product.
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THD+N
Total Harmonic Distortion plus Noise, a measure of distortion of an audio
product. Value is typically expressed as percentage, where value 1% (or 40 dB) is known as a limit for inaudibility of distortion of a loudspeaker.
Two-way delay
Overall acoustic delay of signal from user to the other party and back,
synonym to round trip delay.
User of the product
User of the product under study.
Vendor
Manufacturer who is submitting a solution for Skype Certification.
Wideband speech
Speech transmission, used in majority of Skype calls, where the audible
frequency range is wider than what it is in narrowband speech
transmission in PSTN and mobile calls. The sampling frequency is equal
to 16 kHz. The bandwidth of signal is between about 50 and 7500 Hz.
9.2 References
[1]
ITU-T Recommendation G.100.1: The use of the decibel and of relative levels in speech
band telecommunications http://www.itu.int/rec/T-REC-G.100.1/en
[2]
ITU-T Recommendation P.58: Head And Torso Simulator (HATS) http://www.itu.int/rec/TREC-P.58/en
[3]
ITU-T Recommendation P.800: Methods for subjective determination of transmission quality
http://www.itu.int/rec/T-REC-P.800/en
[4]
ITU-T Recommendations P-sector. http://www.itu.int/rec/T-REC-P/en
[5]
ITU-T Recommendation P.340: Transmission characteristics and speech quality parameters
of hands-free terminals http://www.itu.int/rec/T-REC-P.340/en
[6]
ITU-T Recommendation P.57: Artificial Ears http://www.itu.int/rec/T-REC-P.57/en
[7]
ITU-T Recommendation P.64: Determination of sensitivity frequency characteristics of local
telephone systems http://www.itu.int/rec/T-REC-P.64/en
[8]
ITU-T Recommendation G.122: Influence of national systems on stability and talker echo in
international connections http://www.itu.int/rec/T-REC-G.122/en
[9]
ITU-T Recommendation G.131: Talker echo and its control http://www.itu.int/rec/T-RECG.131/en
[10]
Perceptual Evaluation of Speech Quality tool, PESQ that complies with ITU-T P.862
recommendation http://www.opticom.de/download/SpecSheet_PESQ_05-11-14.pdf Skype
uses Opticom version of PESQ that has been integrated into HeadAcoustic ACQUA system
[[11]]
[11]
ACQUA Advanced Communication Quality Analysis system by HeadAcoustics
http://www.head-acoustics.de/eng/telecom_acqua.htm
9.3 Changes between 4.0 and 3.0 versions
9.3.1 Major changes
Audio ergonomics requirements are removed from the 4.0 version and similar requirements are
added to the other Certification documents. Thus in all requirement chapters of the version 4.0 the
Sections “Audio ergonomics requirements” have been removed.
The final pass criteria of device tested against this document has been relaxed for the 4.0 version.
In version 4.0 it is that 100% of Priority 1 requirements must be passed, 50% of Priority 2 and 10%
of Priority 3. The corresponding percentages in the version 3.0 and earlier versions are 100%,
75% and 25%. The reason for the relaxation is that the old pass criteria required a product to pass
a considerable amount of Priority 2 and 3 requirements, and if the product fails these that would
mean considerable and time-consuming hardware changes for product in a middle of the
development cycle.
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The specification has started to use Diffuse field correction instead of Free field correction in
earpiece and loudspeaker measurements in all acoustic measurements when using artificial ear of
HATS. This is changed to reflect the latest information in research regarding the preferred
frequency response targets in subjective user tests and the latest developments in ETSI and ITU-T
standardization forums. However Skype does not state that flat diffuse field corrected frequency
response is the optimal when measured with HATS, the frequency mask allow plenty of room for
manufacturers to optimize the response.
Super wideband frequency response requirements have been added aiming for frequency ranges
beyond wideband to allow very high quality speech, music and multimedia delivery.
9.3.2 Introduction, Abbreviations and References
Few minor text editions added to “Introduction” chapter. Example pictures for Acoustic UI groups
have been updated, for example sound cards added to “Other audio product group” section.
Text in “Audio requirements and priorities – overview” section has been modified to reflect the fact
that Audio ergonomic requirements have been removed from this document.
The new final pass criterion is presented in the end of section “Use of the test case priorities”.
New abbreviations are added: ACQUA, Diffuse field correction, Far end, MOS-LQO, Near end,
PESQ, RMS, and Super wideband speech. Also few abbreviations have been updated, for
example: preferred listening level, Priority 1, 2 and 3 and SPL.
Two more references have been added to “References” section: PESQ and ACQUA.
9.3.3 General audio requirements
Major updates have been made to this Chapter.
“Additional delay to speech signal…” requirements for sending and receiving directions in Version
3.0 have been combined to new “Round trip delay of speech signals” requirements. The
“Additional delay…” requirements have been removed. The measurement method has been
redefined and requirements updated.
Several requirements in the version 3.0 have been combined into only two test requirements:
“Total quality loss in sending direction” and “Total quality loss in receiving direction”. The combined
old requirements are:
•
Format and additional coding of speech – all priorities here
•
No drops and distortions in speech signals – all priorities here
•
No additional noises or sounds in speech signals – all priorities here
•
No interference noises from electric power supply – all priorities here
•
No interference noises from devices with radio frequency transmission – all priorities here
All of these combined requirements are removed from the 4.0 version.
“Frequency bandwidth…” requirements have been removed from the 4.0 version. The bandwidths
are tested in Frequency response requirements in the version 4.0.
“Device and driver response time from the audio mixer” requirement has been relaxed from 1 ms
in version 3.0 to a practical 50 ms in the version 4.0. Also the text has been updated.
“Sampling frequency accuracy” requirement has been relaxed from 0.01% i.e. 100 ppm deviation
in the version 3.0 to 0.1% i.e. 1000 ppm in the version 4.0. Also the text has been clarified.
“General audio test instructions” section has been updated to reflect changes in the test cases.
The “subjective way” section has been removed as all tests in the Chapter are measured with
objective tools.
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9.3.4 Headset audio UI
The “Microphone - frequency response” requirements have been relaxed from high frequencies by
adding 5dB more headroom to the tolerance window in the version 4.0. For Priority 2 requirement
the low end corner frequency of the tolerance window has been raised from 100 Hz to 150 Hz.
Priority 3 requirement has been added to test a super wideband compatibility of the microphone.
Priority 2 “Microphone – Speech to background noise ratio” requirement has been rewritten for
better clarity. Priority 1 and 3 requirements of the same test have been removed.
All priorities of “Earpiece – Speech to self noise ratio” requirements have been tightened by 5 dB
in the version 4.0 compared to the version 3.0.
In “Earpiece – Frequency response” Priority 2 requirement, the low and the high corner
frequencies have been both lowered from 300 and 15000 Hz to 100 and 7000 Hz to the version
4.0. Priority 3 requirement tolerance mask is also modified: high frequency has been dropped to
10 kHz in the version 4.0 compared to the previous value of 20 kHz, due inaccuracies of practical
measurement with artificial ears. On the other hand the low frequency tolerance mask has been
tightened due change to diffuse field correction and more knowledge about the preferred
frequency response there.
In “Earpiece – Stability of frequency response” the frequency limits have been slightly changed to
reflect the modified tolerance mask frequencies of Frequency response requirements. The
modified limits here are: Pr 2: the highest frequency is lowered from 7 to 6 kHz, Pr 3: the highest
frequency is lowered from 8 to 7 kHz, and Pr 3: the lowest frequency is raised from 100 to 150 Hz.
“Minimum crosstalk from receiving to sending direction” requirement has been modified to use Afrequency weighted values in the version 4.0 instead of not-weighted values in the previous
versions.
“Headset: Audio ergonomic requirements” section has been removed from the version 4.0.
“Headset: Supporting audio documentation requirements” has been updated and only the most
important information for Certification testing purposes has been left.
“Headset: Audio test instructions” section has been updated to reflect changes in test cases. The
“Subjective way” section has been removed as all tests in the Chapter are measured with objective
tools.
9.3.5 Handset audio UI
The “Microphone - frequency response” requirements have been relaxed from high frequencies by
adding 5dB more headroom to the tolerance window in the version 4.0. For Priority 2 requirement
the low end corner frequency of the tolerance window has been raised from 100 Hz to 150 Hz.
Priority 2 “Microphone – Speech to background noise ratio” requirement has been rewritten for
better clarity. Priority 1 and 3 requirements of the same test have been removed.
All priorities of “Earpiece – Speech to self noise ratio” requirements have been tightened by 5 dB
in the version 4.0 compared to the version 3.0.
“Earpiece – Frequency response” requirements are moved before “Earpiece – Suitable volume
level…” requirements in the version 4.0.
In “Earpiece – Stability of frequency response” the frequency limits have been slightly changed to
reflect the modified tolerance mask frequencies of Frequency response requirements. The
modified limits here are: Pr 2: the highest frequency is lowered from 7 to 6 kHz, Pr 3: the highest
frequency is lowered from 8 to 7 kHz, and Pr 3: the lowest frequency is raised from 100 to 150 Hz.
“Minimum crosstalk from receiving to sending direction” requirement has been modified to use Afrequency weighted values in the version 4.0 instead of not-weighted values in the previous
versions.
Priority 3 requirement for “Suitable volume level for office and home handset (Indoor)” has been
removed from the version 4.0.
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
67 / 68
“Handset: Audio ergonomic requirements” section has been removed from the version 4.0.
“Handset: Supporting audio documentation requirements” has been updated and only the most
important information for Certification testing purposes has been left.
“Handset: Audio test instructions” section has been updated to reflect changes in test cases. The
“Subjective way” section has been removed as all tests in the Chapter are measured with objective
tools.
9.3.6 Speakerphone audio UI
Microphone and echo requirements are moved to be before the loudspeaker requirements in the
version 4.0.
“Priority: 2 Microphone – Frequency response” the low frequency of the tolerance mask has been
increased from 100 to 150 Hz for the version 4.0. Priority 3 requirement has been added to test a
super wideband compatibility of the microphone.
“Microphone – Speech to self noise ratio during speech activity” requirement has been made
stricter by increasing A-weighted RMS speech to noise ratio to be at least 30 dB in the version 4.0
compared to 25 dB in the version 3.0.
Texts in “Amount of acoustic echo” requirements have been clarified and Notes have been
updated.
In “Loudspeaker – Frequency response” requirements for frequencies in tolerance limits have
been modified: Priority 1: the high frequency limit is reduced from 3.5 to 3.4 kHz, Priority 2: the
high frequency limit is reduced from 7.5 to 7 kHz, and Priority 3, the high frequency limit is reduced
from 15 to 10 kHz.
For “Loudspeaker – Distortion…” requirements, the text has been clarified and example is given
how to define the measurement bandwidth for a distortion measurement.
“Microphone – Sensitivity at maximum operating distance” requirement has been clarified.
“Sampling frequency accuracy – absolute” requirement in the version 3.0 has been removed from
the version 4.0.
“Speakerphone: Audio ergonomic requirements” section has been removed from the version 4.0.
“Speakerphone: Supporting audio documentation requirements” has been updated and only the
most important information for Certification testing purposes has been left.
“Speakerphone: Audio test instructions” section has been updated to reflect changes in test cases.
A graph defining the measurement setup has been added.
9.3.7 Other audio product
“Product provides suitable levels for audio signal output” requirement has been updated and
reference acoustic UI products have been added.
A new requirement has been added: “Product provides suitable levels for audio signal input”. It
also includes reference acoustic UI products.
“Minimum crosstalk from receiving to sending direction” requirement has been modified to use Afrequency weighted values in the version 4.0 instead of not-weighted values in the previous
versions.
“Other audio product: Audio ergonomic requirements” section has been removed from the version
4.0.
“Other audio product: Supporting audio documentation requirements” has been updated and only
the most important information for Certification testing purposes has been left.
“Other audio product: Audio test instructions” section has been updated to reflect changes in test
cases. The “Subjective way” section has been removed as all tests in the Chapter are measured
with objective tools.
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
2009-04-01
Security Classification: Public
68 / 68
9.3.8 Non-audio product
Texts in both requirements of “Continuous transmission of speech” have been clarified.
The “Subjective way” section has been removed as all tests in the Chapter are measured with
objective tools.
9.3.9 List of environments
“List of test tools and material” section has been removed due less need to use subjective
measurement and listening setups in grading requirements in the version 4.0 of Certification audio
requirements.
“Skype Audio Test Lab” has been updated. More details of equipment used in testing are defined
also a graph describing the objective measurement setup.
Audio Requirement Specification
Copyright © 2009 Skype Inc. All Rights Reserved.
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