Syncing Digital Audio Devices - MOTU HDX-SDI User Manual

SYNCING DIGITAL AUDIO DEVICES

The HDX-SDI's stereo BNC AES/EBU input is
equipped with sample rate converters (SRC). This
allows the digital audio data to be transferred
cleanly, with no dropped samples, clicks, or pops ,
even when the two devices are not resolved to one
another. However, it is still a good idea to resolve
digital audio devices with the HDX-SDI during
digital audio transfers to prevent audio from
drifting out of sync with picture.
The 8-channel DB25 AES/EBU connection is not
equipped with sample rate conversion and
therefore requires that the HDX-SDI is
synchronized with the connected digital audio
device.
Audio clock synchronization is described in the
following sections, starting with a general
discussion of digital audio phase lock, and why it is
essential for clean and successful digital audio
transfers.
Digital audio phase lock
Without sample rate conversion, when you transfer
digital audio between two devices, their audio
clocks must be in phase with one another — or
phase-locked — as demonstrated below in
Figure 3-10. Otherwise, you'll hear clicks, pops,
and distortion in the audio, or perhaps no audio at
all.
Not phase-locked
Device A
Device B
Figure 3-10: When transferring audio without sample rate conver-
sion, two devices must have phased-locked audio clocks to prevent
clicks, pops or other artifacts.
Without sample rate conversion, there are two ways
to achieve phase lock: slave one device to the other,
or slave both devices to a third master clock. If you
have three or more digital audio devices, you need
to slave them all to a single master audio clock.
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
Figure 3-11: Without sample rate conversion, you need to choose a
clock master to which all other devices slave. Each slaved device
remains continuously resolved to the master, meaning that there will
be no drift over time.
Audio phase lock as shown above in Figure 3-11
can be achieved independently of time code
(location). For example, one device can be the time
code master while another is the clock master. But
only one device can be the audio clock master.
Another benefit of direct master/slave clocking
(without sample rate conversion) is that each
slaved device remains continuously resolved to the
master, which means that there will be no gradual
drift over time. This form of synchronization is
best for audio that must remain resolved to picture.
Audio synchronization when capturing or
playing video
The HDX-SDI audio clock always resolves to its
video clock. The HDX-SDI has three possible
sources for video clock:
■
(Figure 6-3 on page 34)
■
Phase-locked
■
Resolving to the current video source
If the HDX-SDI is transmitting or receiving digital
audio from the current video source, the video
genlock between the two devices ensures that their
digital audio clocks remain resolved and phase
locked.
Master
Slave Slave
The currently chosen video input source
The VIDEO REF IN jack
The HDX-SDI's own internal video clock
Master
Slave
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