MACKIE HDR24/96 - Manual (Page 190)

HDR 24/96
Appendix F – Synchronization
If you had only one piece of gear in your studio that recorded and played audio, life would be dandy and
chances are you wouldn’t be reading this section. But because you are, we’ll assume that your life has
been complicated by the need to get several pieces of equipment to record and play back in sync with
each other. To many people, even some of the big name professionals, synchronization is a black art
where trial and error rather than disciplined practice often dictates how equipment is synchronized.
Additionally, up until very recently good information about synchronization principles and techniques
was not easy to find, so it is no wonder why so many people have a hard time making two pieces of audio
equipment play back in sync.
The good news is that synchronization is not a black art, and in fact is not difficult at all once you
understand the basic principles. So rather than giving you a myriad of “plug-and-play” examples showing
you how to mindlessly connect gear together under every conceivable circumstance, the examples in this
section are intended to illustrate how synchronization actually works. Armed with this knowledge, you
should be able to successfully configure any system for synchronization under almost any situation.
What Makes the HDR24/96 (and Other Devices Like it) Tick?
Inside the HDR is a clock, very much like the watch on your wrist. The difference between them is
that the wristwatch ticks in seconds, the HDR24/96 ticks in samples, thousands of times every
second. This clock, called the Sample Clock, is the engine under the hood that runs everything inside
a digital audio device, be it a recorder like the HDR24/96, a CD Player, A/D or D/A converter, or
digital mixer. Without it, nothing comes in and nothing gets out.
Unlike your wristwatch, you can change the speed of the Sample Clock to accomplish different
things. The speed at which the Sample Clock runs at is called the Sample Rate. The Sample Clock
inside a CD player runs at a Sample Rate of 44,100 Hz (samples/per second). Some people in the
audio industry like to run at 96,000 Hz because it sounds a bit better. The sample clock of a digital
audio device is like a servo control mechanism that determines how fast a tape transport runs.
Speeding up or slowing down the Sample Clock causes the audio to playback to speed up or slow
down.
As everyone knows, if you put any two clocks next to each other and start them at exactly the same
time, a week later they will not read exactly the same time. This is because there are slight tolerance
differences in the mechanisms of each clock that cause them to run at slightly different rates. The
same is true of Sample Clocks. If you put two HDR24/96s (or any other two digital audio devices)
next to each other, set them both to run at 48 kHz, and put them into play at exactly the same instant,
then over time they won’t display the same time. In a digital recorder like the HDR24/96, the time
that you see, whether displayed as SMPTE, Bars|Beats|Ticks, or Hours|Min|Sec|Millisec, is really just
the Sample Clock time converted to different scales, like converting pounds to kilograms or having a
watch with1000 divisions to a minute rather than 60.
In typical digital audio equipment the Sample Rate difference between any two Sample Clocks can by
up to 0.01%. In practical terms this means that the two Sample Clocks may drift by up to 360 ms per
hour, or 6 ms per minute. Since flamming can be heard between two percussive sounds with as little
as a few milliseconds of delay, if you start two HDR24/96s at exactly the same instant you may hear
drift in less than one minute, well before you can actually see a discrepancy between their time
displays. This rate of sample clock drift is actually quite normal. Understanding the simple fact of
clock drift puts you way ahead of most people in understanding how synchronization works.
Going back to the watch analogy, let’s suppose we were able to get two wrist watches to run at
exactly the same speed. Then if the watches started running at exactly the same instant with the
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