Chapter 34: Synchronization Concepts

This section is designed to give you a basic understanding of synchronization as it relates to

Pro Tools.

If you are using a device that sends or receives time code (such as a video tape recorder or MIDI interface), refer that device’s documentation for information on how it generates or receives time code and how to configure its options.

Synchronization Peripherals

Synchronization peripherals include a wide variety of devices, ranging from simple MTC-capable MIDI interfaces, to high precision time code synchronizers capable of handling many professional time code and clock standards used in the music, television, and film industries.

Though many devices are capable of providing synchronization, only the Digidesign SYNC I/O and USD peripherals deliver frame-edge accuracy when synchronizing Pro Tools TDM systems to external time code sources.

Synchronization

Requirements

Synchronization requirements and support vary among different Pro Tools hardware systems.

This guide notes whenever a feature requires a synchronization peripheral (such as a Digidesign SYNC I/O, or equivalent). See the guides for these peripherals for details on their installation and configuration.

We strongly recommend that you research your synchronization requirements thoroughly. For example, if you are using Pro Tools for audio post production work for video, consult your video engineer or editor to determine what time code format will be used. Also, there may be additional time code issues that affect how you use synchronization (such as pull-down). Your

Digidesign dealer may be able to offer suggestions about working with synchronization in your studio.

Aspects of Synchronization

Synchronization in a digital audio workstation has two concepts that need to be independently considered:

◆ “Where are we?” This is called the Positional

Reference.

◆ “How fast are we going?” This is called the

Clock Reference.

To synchronize Pro Tools to another device

(such as a tape machine or VTR) accurately over an extended period of time, Pro Tools needs to know where the device is and at what speed it is running. Some peripherals can provide only one

Chapter 34: Synchronization Concepts 599

of these references; for example, a black burst generator provides only a clock reference. Some peripherals, such as the Digidesign SYNC I/O, can provide both.

Synchronizing Pro Tools

Pro Tools TDM systems require a clock reference, in addition to time code, to maintain correct synchronization once it has been achieved.

In addition, transfers (recording) between digital devices must be performed to a resolved clock source.

You can resolve Pro Tools to an external clock reference without locking it to a positional reference. For example, you can use any

Pro Tools|HD audio interface to resolve the

Pro Tools sample clock to an incoming digital signal (S/PDIF, or Word Clock, for example) without synchronizing Pro Tools to external time code.

In the following example, Pro Tools uses time code for positional information, and a clock reference to maintain synchronization. Pro Tools is slaved, using the SYNC I/O or USD, to a video tape recorder, with Pro Tools and the VTR referenced to the same house video reference source

(house sync).

Example: Pro Tools slaved, through a SYNC I/O or

USD, to a video tape recorder

In this example, house sync is provided to the

SYNC I/O, which provides the master Loop Sync signal to the Pro Tools|HD-series system through a Pro Tools|HD audio interface. House sync is also provided to the VTR.

1 When you start the video tape, time code (LTC or VITC) is read from the tape and routed to the

SYNC I/O or USD. The SYNC I/O (or USD) reads the time code position and continuously forwards the positional reference information down the SYNC I/O serial connection to the Pro Tools card.

2 Pro Tools takes the first time code address it receives, and calculates the sample location in the session that corresponds to the address. Though you can work with Pro Tools using any standard time code format, it calculates internally in sample numbers.

3 Assuming that the time code address corresponds to a sample number that is within the

Pro Tools session, Pro Tools converts the time code address to a sample number within the session, and begins playing from that point. The point from which playback starts is the trigger point . The clock reference is used by Pro Tools and the VTR to keep them running at the same speed.

4 At this point, if the video tape is stopped, rewound, and started again, the process repeats, based upon a newly calculated trigger point.

About Positional References

Time code is positional information in an analog signal or a digital streaming signal that can be recorded on magnetic video or audio tape.

Time code can be used as a positional reference, to synchronize the playback and recording of your Pro Tools system with another machine’s time code signal, such as an analog multitrack tape machine or a video tape recorder (VTR).

600 Pro Tools Reference Guide

SMPTE and ISO/EBU Time Code

The “Where are we?” question refers to relative position. To describe position, many professional audio, video, and multimedia devices and programs use SMPTE (Society of Motion Picture

& Television Engineers) time code. In Europe, a standard called ISO (International Standards Organization) time code, formerly called EBU (European Broadcasters Union) time code, is generally used. These time code types are almost exactly the same in terms of how they are represented electronically. However, ISO/EBU time code works at a fixed rate of 25 fps (frames per second) and SMPTE includes provisions for several different frame rates. For the purposes of explanation, this chapter will generally refer to frame-measured timing information as “SMPTE time code.” This description should be understood to include SMPTE and ISO/EBU time code.

SMPTE Time Code Methods

There are two basic techniques used to record

SMPTE time code onto magnetic tape: LTC (Linear Time Code) and VITC (Vertical Interval Time

Code). LTC is recorded or generated on an audio channel or a dedicated time code track of the audio or video device. VITC is recorded within the video signal in the video “blanking area” of each video frame. VITC cannot be recorded on audio tracks, so it has no application when working with audio tape recorders, but it does offer features for post production (such as still frame and slow speed time code address reading).

There is also a non-SMPTE form of time code called MIDI Time Code (MTC) that some devices use to send timing information.

SMPTE Time Code Units

Time code describes locations in terms of hours, minutes, seconds, frames, and subframes

(1/100th of a frame). The frame is used as a unit of time measurement due to SMPTE time code’s origin in film and video applications (see also

“Frame Count and Frame Rate” on page 601.)

Depending on the SMPTE frame rate, one frame is equal to 1/24th, 1/25th, 1/29.97th, or 1/30th of a second. For example, a video tape time code reading of “01:12:27:15” would tell us that we were at a position of one hour, twelve minutes, twenty-seven seconds, and fifteen frames. However, this time address alone does not tell us frame rate information.

Because SMPTE stores an absolute time reference on the tape in the form of time code, any location on that tape can be precisely located by devices that read time code. Once the time code has been recorded or striped on a tape, it provides a permanent positional reference that allows Pro Tools to link the playback of an event to an exact tape location. For example, with time code synchronization, a gun shot sound effect can be played at the precise instant that the gun’s flash appears on-screen.

Frame Count and Frame Rate

It is important to remember the difference between frame count and frame rate.

Frame count is the amount of frames that the time code counts before ascending to the next second count. Standard frame counts are:

Frame counts per frame rate

Frame Count fps frames 0–23 frames 0–24 frames 0–29

24

25

30

Chapter 34: Synchronization Concepts 601

“Frame rate” is the rate of speed that the time code is ascending through the frames. For example, when someone refers to a time code of

29.97, they usually are referring to “using a frame count of 30 frames, but counting each frame at the speed of 29.97 frames per second.”

LTC (Longitudinal or Linear Time

Code)

LTC is time code that is recorded and played back, or generated, in the form of an analog audio signal. LTC is supported by many audio and video tape recorders.

LTC Speed Usage

LTC can be read at high tape shuttle speeds, allowing a machine’s time code reader to communicate with synchronizers at rewind or fast forward speeds exceeding 50 times playback speed

(provided the tape recorder is able to reproduce the time code at this speed). However, LTC cannot be read at very slow shuttle speeds (such as when you are “crawling” the tape frame by frame) or when the machine is paused. With

LTC, the VTR must be running (usually at a minimum speed of about 1/10th normal playback speed) in order to capture a SMPTE time address.

VITC Speed Usage

VITC’s ability to be read when moving a VTR transport at slow speeds or when the VTR is paused makes it more useful than LTC in these situations. When VITC is used, Pro Tools can capture the current SMPTE time from the VTR when it is paused or in “crawl” mode. However, if you are using additional external transport synchronizers in your setup, most synchronizers cannot read VITC at speeds exceeding approximately 10 times playback speed, preventing slaved machines from maintaining synchronization during rewind and fast forward.

LTC/VITC Auto-Switching

Many synchronizers and devices support automatic switching between LTC and VITC, depending on the speed, to get the best of both worlds (both the SYNC I/O and USD support auto-switching). For example, VITC might be used when a VTR is paused, or crawling frameby-frame, while the synchronizer might automatically switch to LTC when fast-forwarding.

VITC (Vertical Interval Time Code)

VITC is a type of time code that is recorded and played as an invisible part of a video signal.

VITC is commonly used in professional video editing and audio-for-picture applications. Because VITC is recorded as part of each video frame, it must be recorded at the same time as the video signal—it cannot be added later as

LTC can. Since VITC cannot be recorded on audio tracks, it is never used to synchronize audioonly recorders. Instead, LTC is most often used in audio-only applications.

Bi-Phase/Tach

This electronic pulse stream is used by film mag recorders, film editing stations, and film projectors. You can use this format to synchronize

Pro Tools if you have a SYNC I/O (or USD). Unlike time code, Bi-Phase/Tach doesn’t actually contain absolute location information. It simply supplies speed (based upon the frequency of the pulses) and direction, and therefore, relative position. Since the SYNC I/O can “count” both the speed and direction of the stream of pulses, it can use a Bi-Phase/Tach source to deduce positional information from a starting “address point.” The difference between Bi-Phase and

Tach formats is that Bi-Phase encodes rate and direction on a pair of signals using a format

602 Pro Tools Reference Guide

called phase-quadrature, while Tach encodes rate on one signal and direction on the other.

For more information on Bi-Phase/Tach, see the

SYNC I/O Guide .

SMPTE Frame Formats

Several different formats of SMPTE time code exist, and Pro Tools can synchronize to all common formats with a compatible synchronization peripheral.

When you work with NTSC video (the standard in North America and Japan), you will generally work with the NTSC color video standard: either 29.97 fps Non-Drop or

29.97 fps Drop frame. If you are working with PAL, your frame rate is 25 fps.

Pro Tools supports the following SMPTE frame rates:

30 fps Frame Format

This is the original SMPTE format developed for monochrome (black & white) video, and is commonly used in audio-only applications. This format is often referred to as 30 Non-Drop frame format.

30 fps Drop Frame Format

Some field film recordings are done at 30df so when they get pulled down after the telecine transfer, they will end up as 29.97df.

29.97 Non-Drop Frame Format

This format is used with NTSC color video. It runs at a rate of 29.97 fps.

29.97 Drop Frame Format

NTSC color video has an actual frame rate of

29.97 fps, so an hour’s worth of frames

(108,000) running at 29.97 fps Non-Drop will take slightly longer than one hour of real time to play. This makes calculating the actual length of a program difficult when using 29.97 Non-

Drop time code. A program that spans one hour of 29.97 Non-Drop time code addresses (for example, from 1:00:00:00 to 2:00:00:00) is actually

60 minutes, 3 seconds and 18 frames long.

To make working with 29.97 time code easier for broadcasters, the SMPTE committee created

29.97 Drop Frame time code, which runs at exactly the same speed as 29.97 Non-Drop (nondrop frame) time code, but compensates for the slower speed by “dropping” (omitting) two frames at the top of each minute, with the exception of every 10th minute. For example, the time code address of 1:01:00:00 does not exist in drop frame code because it has been skipped.

Note that even though time code addresses are skipped in drop frame format, actual frames of video material are not dropped.

At the end of a program that spans precisely one hour of drop frame time code (for example,

1:00:00;00 to 2:00:00;00), exactly one hour of real time has elapsed.

Although it sounds complicated, drop frame time code allows broadcasters to rely on time code values when calculating the true length of programs, facilitating accurate program scheduling.

Chapter 34: Synchronization Concepts 603

25 fps Frame Format

This format is used with the European PAL video standard, which runs at a 25 fps frame rate. This format is also called the EBU (European Broadcast Union) format because it’s used by broadcasters throughout most of Europe.

24 fps Frame Format

This format is used for high-definition video and film applications. Film is typically photographed and projected at a 24 fps frame rate, so this SMPTE format is useful when one time code frame should equal one film frame

23.976 fps Frame Format

This format is used for high-definition digital video production using NTSC video equipment, and film applications.

down, or pulling down by 0.1%, the production sound to compensate for the change in speed

from film to NTSC video. (See Figure 46 on page 606.)

Guide Tracks and Conforming

In the Video editing process, the audio track produced by the video editor (the “guide track”) is rough and needs to be enhanced and improved by the audio engineer. For this reason, the audio engineer will need to re-assemble the original sound elements in a process known as conforming .

Telecine machine converts film frames to video fields using

3:2 Transfer

Working with Film-Originated

Material

When you do post production work in

Pro Tools, you will usually work with video material. However, it is possible that the video you are working on was shot on film.

Film footage and production sound go through separate conversion processes before they reach video, and the audio post production stage. The film is transferred to video using a process called

Telecine, using a method called 3:2 Pulldown.

Audio can also be pulled down during the transfer, or you might end up working with audio that has not been adjusted (production sound).

Typically, during the Telecine process, a master digital video tape is created, along with a work copy on Betacam or 3/4-inch analog video tape for the picture editor to use. At the same time, a new audio master may be created by slowing

604 Pro Tools Reference Guide

Digital video master

29.97

fps

Analog audio

Analog video

“work tape”

29.97

fps

Production audio

30 fps Non-Drop

48 kHz

Telecine

“simo” or “simul” DAT

Sample rate conversion or A-D-A (analog stage)

29.97

fps

48 kHz

The Telecine stage of video post production

3:2 Pulldown

A film clip that lasts 1000 seconds consists of

24,000 film frames (pictures). If you want to transfer that film to 1000 seconds worth of

NTSC color video, you have to fit 24,000 film frames into 29,970.02997 video frames.

If we use the black and white NTSC video standard (30 fps) instead of 29.97 fps, the process of converting film frames to video frames is greatly simplified. Now instead of any fractional frames, we have 24,000 film frames going into

30,000 video frames (60,000 video fields). In the

Telecine process (for NTSC color video), each odd film frame is copied to two video fields, and each even film frame is copied to three video fields, creating what is called a 3:2 Pulldown.

The speed of the film is also “pulled down” to

23.976 fps in order to accommodate the slower speed of NTSC color video compared to NTSC black and white video (29.97 fps compared to

30 fps).

Film frames

1 1 2 2 2 3 3 4 4 4

Video fields

1 2 3

Video frames

4 5

How film frames translate to video fields and frames in a Telecine transfer

Film Speed Differs from NTSC

Video Speed

When spotting audio to video that was transferred from film to NTSC video, there are two important terms to keep in mind: film speed and video speed.

Film Speed Film speed refers to audio that was recorded and plays back in synchronization with the original film material. This audio often comes from production reels recorded on a Nagra® recorder or a field DAT recorder, and is usually striped with 30 Non-Drop time code. Film must be pulled down 0.1% when being transferred to 29.97 NTSC. Film must be pulled up by

4.16667% when film is being transferred to PAL.

Video Speed Video speed refers to audio that is running at the NTSC color standard of

29.97 fps. Video speed is 0.1% slower than film speed, so audio that is still at film speed will be out of sync with the video.

In Figure 46, note the following:

• The vertical arrow at the left (“faster”

“slower”) represents program speed or rate.

• Telecine is a frame count conversion (for example, 24 to 30).

• The speed you play the result back is a frame rate conversion (for example, 30 to 29.97).

The following table lists the applications for each of the frame rates and counts shown in

Figure 46 on page 606.

Frame Rates and their Applications

Frame Rate Applications

25 fps

24 fps

23.976 fps

30 fps

29.97 fps

PAL

Film, and 24P HD

24P pull down, for NTSC broadcast, and HD digital video

30 frame music production, and

30 frame recording for film

Broadcast NTSC video

Chapter 34: Synchronization Concepts 605

Frame Rates and Relative Playback Speeds

The following diagram illustrates the relative playback speeds of SMPTE formats. faster 25 fps

x104.1667% up x96.0% down

(“4%” pull up) (“4%” pull down)

24 fps x100.1% up

(0.1% pull up) x99.9% down

(0.1% pull down)

Telecine 30 fps x100.1% up

(0.1% pull up) x99.9% down

(0.1% pull down)

23.976 fps Telecine 29.97 fps slower same speed,

less frames

Figure 46. Frame rates, telecine transfers, and relative playback speeds

Pull Up and Pull Down

Pull Up and Pull Down are terms used to refer to the deliberate recalibration of the audio sample rate clock (speed, or musical pitch) in order to compensate for a speed change. Pro Tools can be used to pull down or pull up audio or video playback rates. Pro Tools 5.3.1 and higher support pull rates known as “4% factors” (4.0% down, or 4.167% up to be exact), applicable to audio and video playback, to support

PAL/film/NTSC conversion requirements.

Pull Down allows you to play back film-originated material at video speed (–0.1%). Pull Up allows you to play back video speed material at film speed (+0.1%).

same speed, more frames

Using Pro Tools in Pull Up or Pull Down modes requires a SYNC I/O, USD, or third party synchronizer.

When to Pull Up or Pull Down

There are many ways to get audio into Pro Tools for post production. Consider your source audio and your final destination format carefully. In some cases, audio will already be pulled down for you. In other cases, audio will have to be temporarily pulled down. In still other cases, you may choose to pull down your audio source, like a DAT deck, then use a a D-A-D (digital-to-analog-to-digital) process, or the Sample

Rate Conversion option on the 192 I/O, to record the audio into Pro Tools at the proper sample rate. Or you may choose only to pull

Pro Tools up or down on delivery of the audio.

606 Pro Tools Reference Guide

Because mistakes in pull factors can be expensive, good communication with other participants in the project is critical. Find out as soon as possible how your task relates to the rest of the project.

Final Audio Destination: Film

If your final destination is film, your source audio is at film speed, and your goal is to edit and mix audio in Pro Tools and then lay back to a device that runs at film speed (such as mag or time code DAT), you can temporarily pull down the audio in Pro Tools for NTSC video work, then return the audio back to film speed when you're finished (by disengaging pull down).

For example, film speed audio from a field audio recorder that is referenced to 30 fps time code is recorded into your Pro Tools system at a sample rate of 44.1 or 48 kHz. Keeping in mind that film speed is faster than video speed, select

30 fps in your Session Setup window in

Pro Tools, and record in your audio online and referenced to the time code on the field audio recorder.

Most “simul-dats” or “simo-dats” already have their audio pulled down, and should not be pulled down further.

Once all the audio has been recorded, and you are locked to a video work print (at video speed), enable Pull Down. If you are using a Digidesign

SYNC I/O, select Pull Down in the Session Setup window. If your synchronizer is not a Digidesign product, select Pull Down on the front of your synchronizer, then enable Pull Down in the Pro Tools Session Setup window. At this point, it is highly recommended that you verify whether the video you’re working with is striped with 29.97 Drop Frame or Non-Drop

Frame time code. While in Pull Down mode, you can work with your reference video and everything will remain synchronized and run at the proper speed (assuming your system is completely resolved).

Once you are ready to lay back your completed project to an audio device running at film speed, deselect Pull Down in the Session Setup window, and from your synchronizer if it is not a

SYNC I/O. Then change your time code frame rate in the Pro Tools session back to 30 fps.

Once Pull Down has been deselected, the audio played back from Pro Tools will synchronize perfectly with the edited film.

Alternatively, you can pull down the source audio deck while recording audio into Pro Tools, work at 29.97 fps with no Pull-Down selected in

Pro Tools, and then switch to 30 Non-Drop frame format, and select Pull Up during the delivery stage. You will have to perform a sample rate conversion on the audio either digitally or by using an analog stage (D-A-D).

Only the inputs on the Digital I/O card (on the

192 I/O) feature real-time sample rate conversion.

You will also need to select the Audio Rate

Pull Up/Down option in the Session Setup window.

Final Audio Destination: Video

If you are working with video that was transferred from film, your audio source is at film speed, and the final layback destination is NTSC video (or television), and you would like to provide a digital transfer to your clients, you will need to alter the above recipe slightly. Keep in mind that when you are working in Pull Down mode, your active sample rate is 44.056 kHz (if the audio was recorded at 44.1 kHz) or

47.952 kHz (if the audio was recorded at

48 kHz).

Chapter 34: Synchronization Concepts 607

Pull Down the Audio Source

Some professional DAT machines will let you pull down the sample rate to 44.056 kHz (and

47.952 kHz). You can record this audio into

Pro Tools using a D-A-D (digital-to-analog-todigital) process, or Digidesign’s 192 I/O Sample

Rate Conversion option. Then your audio will be at the correct speed for the remainder of the project, since the final destination is video, and no Pull Down or Pull Up is necessary.

Pull Up Pro Tools While Recording

If you cannot pull down your source, you can accomplish the same thing by Pulling Up

Pro Tools, and setting your SMPTE frame format to 30 fps, before recording in the production sound. First select 30 fps as the frame format in the Session Setup window. Then select an Audio

Rate Pull Up option in the Session Setup window (and on your synchronizer if it isn’t a SYNC

I/O) before you record in the production audio.

In this case, while the production audio is running at 44.1 or 48 kHz, Pro Tools is running

(and recording) at a rate of 44.144 or 48.048 kHz. After all the production audio has been recorded into Pro Tools, reset the Audio Rate Pull to none in the Session Setup window, and on your hardware synchronizer if it is not a

SYNC I/O. After you deselect Pull Up, the recorded audio will play back 0.1% slower, synchronized with the video, while achieving a true playback sample rate of 44.1 or 48 kHz. Note that this process is designed for a final destination of video; to bring this audio back up to film speed you would have to pull up Pro Tools and record to a destination that is not Pulled Up.

Note on Sample Rate Conversion

In many cases, you have to perform a sample rate conversion at some point, either digitally, or by recording in audio using an analog stage

(D-A-D). The only situation where sample rate conversion never has to be performed is when you are working with film speed audio and your final destination format is film. Then you can simply pull down Pro Tools while you work with the video, then deselect Pull Down to set the audio back to film speed. (On the 192 I/O and 192

Digital I/O, the inputs on the Digital I/O card feature real-time sample rate conversion.)

Using Digital Input

If working with Pull Up or Pull Down, do not

“resolve” to any digital inputs that are used as audio sources in Pro Tools. This would override the use of the SYNC I/O as the clock reference.

Any equipment providing digital audio sources to Pro Tools should be synchronized externally.

You will also need to select the Audio Rate

Pull Up/Down option in the Session Setup window.

608 Pro Tools Reference Guide