Digital Audio Synchronization And Timecode - Merging Pyramix 4.3 User Manual

User Manual : Digital Audio Synchronization and TimeCode

Digital Audio Synchronization and TimeCode

THERE MUST BE ONLY ONE SOURCE OF SYNC FOR AUDIO AND TIME-CODE
Digital audio relies on extremely accurate timing. In any digital audio system there can only be one
source of sync at one time. This is particularly important when planning multi-machine systems. If time-
code is not locked to the same sync source as the digital audio then either the audio will work properly,
or the time-code. But NOT BOTH AT THE SAME TIME.
Ideally, in any system with more than one device, there will also be an independent source of sync. E.g.
a word-clock generator with multiple outputs. Each device is fed by a single output and configured to
use this source as its sync reference.
Example:
A location digital recorder records at a nominal 44.1kHz sampling rate generated by its internal crystal
oscillator and also records time-code derived from the same oscillator. Although the machine may be
running slightly slow or fast the digital audio and time-code will vary by exactly the same percentage.
When this location recording is played back on a machine locked to a stable sync source, digital audio
will play at the same rate as the workstation and the time-code will be correct.
Consider an alternative scenario:
A digital multi-track is used as a location recorder, synced to its internal oscillator. Time-code is
recorded on an audio track sourced from, say, a camcorder. When the resulting tape is played back on
a machine locked to a stable sync source, the audio will be at the correct rate but the time-code will
'drift' in relation to it. The amount of this error is known as 'DELTA'. Delta is simply the result of the fol-
lowing formula: Internal TC minus External TC minus Offset = Delta. Where such a recording exists and
it is imperative the time-code on tape is the master reference there are several solutions. The preferred
options are:
Play back the tape with the machine chase-synchronized to the recorded time-code. Since the digital
audio is not locked to the time-code the sample rate will drift. If recorded directly, this would result in
missed or duplicated samples. I.e. unpleasant audible artefacts. Therefore, in order to record the audio
in Pyramix it must go via a digital audio synchronizer/sample rate converter synchronized to the master
word-clock source. This will then present Pyramix with digital audio at the correct rate.
Alternatively, the audio could be converted to analogue then fed into Pyramix via an analogue to digital
converter.
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