User`s guide

INSTALLING THE V4HD HARDWARE
29
SYNCING DIGITAL AUDIO DEVICES
If you would like to transfer audio digitally between
the V4HD and another device that has AES/EBU or
ADAT optical digital I/O, there are three basic
scenarios in which the V4HD could make a digital
audio transfer:
■ The V4HD video features are enabled and it is
therefore resolved to its video clock.
■ The V4HD video features are disabled, and you
are running the V4HD as an audio interface (see
“Au d i o o n l y ” o n p a g e 3 9 ) .
■ Regardless of the V4HD’s current operational
mode, you would like to use the V4HD’s digital
audio sample rate conversion feature.
The audio clock synchronization requirements for
these three scenarios are 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 3-14. Otherwise, you’ll hear clicks, pops,
and distortion in the audio, or perhaps no audio at
all.
Figure 3-14: 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.
Figure 3-15: 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 3-15
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,
converting or playing video
When the V4HD is capturing, converting or
playing video (as further explained later in “Five
modes” on page 37), its audio clock (see “Clock
Source” on page 73) resolves to its video clock. The
V4HD has three possible sources for video clock:
■ The currently chosen video source (Figure 6-3
on page 43 and Figure 6-5 on page 44)
■ The VIDEO REF IN jack
■ The V4HD’s own internal video clock, when it is
playing from the computer, or in Playback only
mode (page 38)
Resolving to the current video source
If the V4HD 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, as demonstrated in Figure 3-16:
Not phase-locked Phase-locked
Device A
Device B
Master
Slave
Master
Slave Slave