Clock Distribution Theory And Practice - Apogee BIG BEN User Manual

BIG BEN – User's Guide

Clock Distribution Theory and Practice

Master Clock and Conversion
To understand the importance of the clock signal during the A-to-D and D-to-A conversion, it's
useful to examine the similarities between film and digital audio, two media which capture reality by
recording and reproducing small slices of that reality.
Much in the same way that a film captures fluid motion with discrete, regularly timed still pictures, digital
audio technology captures audio waveforms with regularly timed snapshots called samples. In each
case, the timing regularity of the slices is paramount to successfully reproducing the original event.
For example, just as a hand-cranked projector will cause a perfectly normal film to appear jerky and
unnatural, timing inconsistencies (known as jitter) in a digital-to-analog converter add inaccuracies to
an otherwise good digital audio signal. Worse, if a hand-cranked film camera were used to capture a
performance, even the most stable projector couldn't remove the instability of the movement recorded
on film. Likewise, jitter anomalies encoded during the analog-to-digital stage can never be removed
further down the digital audio chain. The overall quality of a digital system is determined by the quality
of the A-to-D and D-to-A converters; the most stable clock possible ensures the most consistent
timing, and therefore the highest quality, for these stages.
Master Clock and Device Synchronization
A Master Clock also ensures the synchronization of all devices in a studio, allowing any possible
digital connection to be made seamlessly without clicks, pops or digital hash. To understand the
requirements of a clock signal, consider the analogy between the digital studio and a musical
ensemble.
In order for a musical ensemble to play together successfully, each member must perform his part at
a common tempo. Likewise, each device in a digital studio must transmit and receive digital bits as
dictated by a common clock signal. The larger the ensemble, the more difficult it is to play together
without a conductor - imagine the resulting cacophony if each player in an orchestra followed the
tempo of his neighbor. A similar situation arises in the digital studio when each device receives a clock
signal from its neighbor - timing differences between devices can cause digital bits to be dropped
when the studio must "play together", for example while transferring or mixing digitally.
On the other hand, when each member of the orchestra follows the tempo set by a conductor, even a
large group of musicians can play precisely in unison. Likewise, when each device receives a robust
and precise clock signal directly from Big Ben, even a large digital studio can operate in perfect
synchronization.
Word Clock Termination
As the frequency of a word clock signal increases, termination requirements become more critical to
precise transmission. In order to ensure the integrity of the clock waveform, each connection should
be terminated by a 75 ohm load impedance. Over-termination (a load impedance lower than 75
ohms) will attenuate the signal excessively, while under-termination ((a load impedance greater than
75 ohms) introduces overshoot and other waveform distortions. Both conditions compromise clock
accuracy, and are indicated by Big Ben's termination sensing LEDs.
(continued on p.14)
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