MACKIE HDR24/96 - Manual (Page 198)

HDR 24/96
In the case of word clock synchronization, the rising edge of each device’s sample clock is always
phase-aligned such that digital audio signals are always transmitted and received within the
acceptable time window. In the case of video synchronization, the phase relationship of the sample
clocks drifts randomly such that, at times, digital audio signals may fall outside the acceptable time
window. Additionally the phase relationships often vary over time due to the nature of the video
resolving circuitry.
To solve this problem, whenever video is used to synchronize analog audio/video and digital audio
equipment together, all digital audio equipment should be synchronized to word clock from a single
master clock source that is resolved to the video signal.
Since the HDR24/96 can resolve its sample clock to video and generate a resolved word clock output,
it offers a straightforward solution to this problem. The illustration below shows the proper way to
make digital audio devices synchronize to picture under all circumstances.
One can also distribute word clock to all the digital audio devices from an external source (such as the
Aardsync II from Aardvark depicted above) that generates both a master video blackburst and a
resolved word clock output.
Video / Word Clock
Generator
Video Slave
Clock Slave Video Slave /
Clock Master
Blackburst
VTR
Video-Resolved
Word Clock
Blackburst
For smaller home setups inexpensive video blackburst generators are available for under $300. If you
setup will not allow external video or clock synchronization (such as a consumer VHS machine) you
can synchronize the HDR24/96 directly to the composite video output of the device. Although this is
not ideal - the video output from a typical consumer VHS machine is not very stable - it will work in
a pinch.
Working with Time Code
One situation that arises often is the need to lock a digital audio system to a “wild” time code source
that is not referenced to either word clock or video. In the picture below, the HDR24/96 is locked to
LTC that has been recorded onto one track of an analog multitrack tape machine.
In this example, the tape machine and the HDR24/96 will not stay in sync with each other because the
setup is in effect identical to the example given at the beginning of section 3. Even though there is no
sample clock in the multitrack machine, the net effect of the tape machine servo control mechanism
and the LTC generator that originally striped tape is that of a sample clock with a slightly different
tolerance than the HDR24/96 sample clock. The problem of synchronizing to wild time code sources
can be dealt with in several different ways.
HDR 24/96 198