appendix a

Additional Synchronization Information

Video and VITC Signals

Black Burst and House Sync

A black burst signal is essentially a “position-less” video signal. As with any

“shared” video signal, you’ll want to ensure that your video feed comes from a properly buffered and distributed source, such as a video distribution amplifier, or the video reference/black burst output of another device in the house sync chain.

Resolving to video instead of house/black

There are several reasons why you would resolve the USD to a video signal rather than house sync.

When House Sync is Unavailable Resolve to a video signal whenever you are synchronizing Pro Tools (or other device) to video, and you either:

• do not have a black burst generator

– or –

• your setup includes equipment that lacks house sync/black burst input and synchronization capability (including consumer grade VCR, or some entry-level

PC-based editing systems).

Simple Setups In a modest setting with one or two VTRs, Pro Tools and the USD, using the video signal as the clock reference is often satisfactory. In these situations, proper sync can be achieved using the video signal as clock reference.

Why VITC is Unavailable for Clock Reference

VITC itself does not provide clock information directly as part of its time code information, only positional information. However, since VITC is always embedded into a video signal, that video signal can be used as a clock reference by selecting Video Input as the clock reference (or Video Ref In if your facility has house sync).

LTC Signals

Because it’s an analog audio signal, LTC can sometimes be susceptible to either tape dropouts (tape shedding), or to level mismatches between the LTC source and the

LTC input. The USD’s freewheeling feature allows you to compensate for brief time code dropouts. However, if you have serious dropouts, you may not be able to sustain accurate synchronization.

Appendix A: Additional Synchronization Information 67

If you plan to use LTC as a Clock Reference

(whether or not you are also using it as a

Positional Reference), you will need to ensure that your LTC is recorded at as high a level as possible without distortion, and that there are no dropouts longer than

1/80th of a frame.

The USD reads LTC most reliably when fed with a LTC signal of at least -12 dBu (and preferably 0 dBu to +3 dBu.)

Working with Analog Machines

It is good practice on a 24-track analog tape machine to record time code on Track 24 at a reference level of -10 dBu (or lower), with

Track 23 left blank as a “guard” track. This practice avoids crosstalk “bleed” that can occur between the time code track and otherwise adjacent audio tracks. Time code

(which is a mid-frequency alternating pitch square wave) is very sensitive to crosstalk from adjacent tracks, and conversely you don’t want audible time code leaking onto your audio tracks.

If your ATR is under the control of a synchronizer, you must make sure that the synchronizer and USD are both locked to the same reference source (such as, typically, from a video black burst generator.)

Auto-Switch LTC/VITC

Auto-Switch LTC/VITC lets USD automatically select between these two (time code) sources.

LTC and VITC both provide useful and unique capabilities. For instance, it is impossible to read LTC off a paused videotape. Consequently, using only LTC, there’s no way you can use Pro Tools to perform Auto-Spotting of regions when the tape is paused. However, VITC continues to be read as long as the picture remains visible, so it can be used as a positional reference when the VTR is paused. On the other hand, VITC cannot be read at fast winding speeds (except by broadcast-quality VTRs);

LTC can be read at fast winding speeds, as long as its signal remains within the highend frequency response of the ATR or VTR.

Examples of Auto-Switch LTC/VITC

◆ The USD will switch to LTC for positional reference during hi-speed searching and cueing, for example, or whenever the tape speed is too high to read VITC.

◆ The USD will switch to VITC if LTC stops or is unavailable. This will include, for example, if a tape is paused or parked.

◆ If both LTC and VITC are available, the

USD chooses which one to use based on the speed of playback. The switch-over point is approximately 75% of full 1x playback speed. Above 75% playback speed,

LTC is favored; below 75% speed, VITC is favored.

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If a dropout occurs, the USD waits until the freewheel duration has expired before attempting to switch over to the opposite source. If neither source is available, the

USD will stop reading time code.

Using just Word Clock, it is possible to create a “chain” of digital devices in your studio by picking one source as the Word

Clock master, and configuring other sources as Word Clock slaves.

Digital Clock Signal Types

A reference clock signal is part of any digital recording system. It is required because whenever digital audio information is mixed together or passed between devices, the playback samples must be aligned with the recording samples. In some cases (such as with AES/EBU or S/PDIF digital interfaces), the clock signal is embedded in the data stream itself. In other cases, such as

SDIF, the clock signal is carried as an entirely separate signal from the digital audio sample data.

The USD is able to resolve to three different types of digital audio clock information:

AES/EBU, Word Clock, and Super Clock

(256x Slave Clock).

Word Clock

Many professional digital audio products— including open-reel multitrack tape recorders, digital mixing consoles, and the Tascam DA-88 modular digital multitrack— have Word Clock (1x sample rate) connectors.

Word Clock allows the DA-88 (and other

Word Clock-compatible devices) to send or receive external clock information which controls the sample rate, which in turn

(where applicable) controls the play and record speed.

AES/EBU

Some professional digital audio products use AES/EBU “null clock” (which is an

AES/EBU data stream that contains only clock information only and no audio information) as a system clock reference source.

These systems rely upon a single AES/EBU master clock source that is distributed throughout a digital audio facility, in much the same way that house sync is distributed throughout a video facility. If you are connecting the USD to such a system, you will want to use the USD AES/EBU input as the clock reference connection, so that all system components are referenced to the same time base.

In some cases (such as using the USD as a standalone clock resolver or time code generator without a digital audio workstation), you may wish to use an audio DAT machine (or other similar device) as a source of AES/EBU null clock, and resolve your system to this reference source. In this case, the audio sample data in the AES/EBU data stream is stripped off, and only the clock information is used.

Super Clock

Super Clock (or Slave Clock) is Digidesign’s proprietary hi-speed Word Clock that runs at 256x the sample rate. It is required by

Digidesign Pro Tools systems to synchronize audio interfaces (up to the limits of each system) to a single, highly-accurate

Appendix A: Additional Synchronization Information 69

clock from USD. Pro Tools audio interfaces must always be connected to this separate

Super Clock signal chain.

Bi-Phase/Tach

Strictly speaking, Bi-Phase/Tach signals are clock reference signals, and do not contain positional information of their own. However, they do contain enough information for the USD to calculate positional information.

Bi-Phase/Tach signals use two square waves to generate pulses that can function as a clock reference. The two square waves are

90° out-of-phase, in a pattern that resembles this:

Calculating the direction of a Tach signal is slightly different. As you may recall, Tach also uses two signals. The “A” signal is a square wave that provides clock information; the “B” signal is in a steady state (high or low) that indicates the direction. Unfortunately, not all Tach-generating equipment uses the B signal in the same way.

Fortunately, the USD’s Input Signals options allows you to choose the appropriate method (Tach: Fwd = B is Low, or Tach:

Fwd = B is High).

This explains how the USD can use a Bi-

Phase/Tach signal to deduce the direction, and how it also uses the signal as a clock reference—as long as USD is told the starting frame of the first clock signal.

Square Wave A

Square Wave B

With a Bi-Phase signal, the USD can deduce the direction (forward or reverse) of the signal based upon which wave is read “high” relative to the other. For instance, with some film equipment, when the device is running forward, it will generate a Bi-Phase signal where the “A” wave leads the “B” wave—that is, where the A wave peaks before the B wave peaks. When the device is in reverse, the B wave will lead the A wave.

However, some film equipment works in the opposite manner, which is why the

USD’s Input Signals option lets you make the appropriate selection (Fwd = A leads B, or Fwd = B leads A).

70 Universal Slave Driver

Pilot Tone

The USD can resolve to an external Pilot

Tone signal for synchronizing to (or transferring audio from) certain types of openreel audio tape recorders.

In general, Pilot Tone is a sine wave reference signal running at the “line frequency” or “mains frequency,” meaning the same frequency transmitted by the AC line voltage from the local power utility.

Pilot Tone is used on location film shoots to establish a common sync reference between a film or video camera with a portable 1/4” analog ATR (such as those made by

Nagra or Stellavox). On location, Pilot

Tone is derived by clock referencing the camera to the local AC line frequency

(which is 60Hz or 50Hz depending on the country of origin), and this same frequency

is then used to clock-reference the ATR.

The result is that both the camera and the

ATR will run at the same speed.

You can think of Pilot Tone as a kind of inexpensive and readily available “house sync” for location production. Increasingly, it’s being replaced by time code, since new-generation film cameras as well as many portable DAT recorders are time code-capable.

Please note that Pilot Tone contains no positional information; it is simply a clock reference. Most 1/4-inch machines have a center track for time code or pilot.

Appendix A: Additional Synchronization Information 71

72 Universal Slave Driver