MOTU HDX-SDI User Manual page 27

Camera
Video out
Video in
AES/EBU BNC in
AES/EBU out
Figure 3-14: Capturing digital audio from a device that is not resolved
with the HDX-SDI. In this case, sample rate conversion is used.
A caution about using rate conversion
Rate conversion can be a life saver in situations
where resolving digital audio clocks is difficult or
impossible. As long as you are capturing audio and
video together, audio will remain perfectly in sync
with picture.
However, if you use sample rate conversion when
capturing audio separately from its corresponding
video, you run the risk of the audio drifting apart
from picture over time. If the audio clip being
captured is short, this may not be an issue. But for
longer record/playback passes (more than a few
minutes), you are much better off resolving the
HDX-SDI to picture during the digital audio
transfer.
As a general rule of thumb, most devices employ a
clock crystal that has an accuracy of about ±50
parts per million, which produces drift that
amounts to approximately one frame every five
minutes at 60 fps.
I N S T A L L I N G T H E H D X - S D I H A R D W A R E
HDX-SDI clock source
setting: Input Source
Digital mixer running under
its own digital audio clock
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