15-12
A simple analogy makes this behavior easy to understand:
Synchronizing audio to SMPTE/MTC is a lot like trying to get even and
stay neck-and-neck with another car on the freeway. If the car is ahead
of you, you need to drive faster to catch up to it. If it's behind you, you
have to slow until the car catches up to you. Once the two cars are neck-
and-neck, you can simply keep going at the same speed, unless the other
car changes its speed. If the other car speeds or slows, you must speed or
slow too.
The Þrst time you play audio under SMPTE/MTC Sync, the audio clock
has to get even with the external clock. This could mean racing ahead,
which raises the pitch of the audio, or stepping on the brakes, which
lowers the pitch of the audio. These ßuctuations continue until Pro
Audio matches its playback speed to the external clock, which usually
takes no more than 30 seconds. The stable playback speed, by the way,
may be slightly faster or slower than the normal audio playback speed,
resulting in a slight change in the pitch of the audio. HereÕs the best way
to address this problem:
¥ Start each new Pro Audio session by playing some audio under
SMPTE/MTC Sync.
¥ Let the audio play for 30 seconds or until all audio pitch ßuctuations stop.
Once this procedure is complete, Pro Audio knows the difference in rates
between the external time code and the audio clock on your sound card.
For the rest of the session, Pro Audio will start playback closely in sync,
without any drastic pitch changes.
If the external timing source were 100 percent stable, the audio would
stay in sync with the external clock. Unfortunately, no timing source is
perfect. Therefore, every once in a while after playback has started, Pro
Audio may need to vary the playback speed by a tiny amount to stay
even with the time code. If the time code signal is unstable (as might be
the case from an analog source), these variations can cause noticeable
changes in audio pitch, which can in turn cause audible audio distortion.
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