Spirit Digital 328, User Manual, Page: 10 / 20

Wordclock is the glue at the heart of digital sys-
tems, and understanding a little about it will avoid
the frustration of not being able to solve some
basic and simple problems. Symptoms of word-
clock-related problems include unwanted pops or
clicks, and a metallic edge or other distortion on
digital sources.
In any situation where several digital audio
devices are connected via their digital rather than
their analogue audio connections, all the devices
must be wordclock synchronised to avoid data
transfer problems. This means not only that the
different devices send and receive their data at the
same sample rate – 48kHz, for example – but
that their internal clocks run precisely in sync.
This ensures that all units send, receive and
process their data streams in a precisely simulta-
neous manner, avoiding a major source of distor-
tion and poor audio quality. To understand how
this distortion arises, consider a simple situation
in which a DAT recorder’s digital output feeds a
mixer's digital input. They are both using a
44.1kHz sampling rate, but their clocks are not
synchronised. The mixer’s input should have
44,100 ‘slots’ every second that have to be filled
by samples, and the DAT recorder should be send-
ing it 44,100 samples every second. But if there is
only a slight drift in one of the clocks, some of
those samples will be missed or will ‘jump’ a
place, and that means distortion.
So, in any system that sends digital audio
between several different devices, one unit should
be the wordclock ‘master’, and the others should
be ‘slaves’. Setting this up is usually very easy, as
the slaves can generally derive their wordclock
timing from their digital audio inputs – SPDIF,
AES/EBU, ADAT or whatever. Wordclock can also
be carried via a separate connection. In either
case, the wordclock signal is present whether
audio is sent or not.
SECTION
3: W
or
dclock
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WORDCLOCK A. Choosing a Wordclock Master
If you are using several digital connections at
once, you may not be left with much choice as to
which device should be your wordclock master. If,
for example, you are using a sampler with a digi-
tal output, you may find that it does not have any
facility to slave to an external wordclock input,
and must therefore be the wordclock master. On
the other hand, any digital recorder in the system
should be the master, though some recorders are
quite happy to slave to external wordclock. As a
rule, PCI cards installed in computers behave best
when they are wordclock masters rather than
slave. Trial and error will tell you which
master/slave combinations works best.
B. Where Does Timecode Fit In?
Timecode synchronisation is different to word-
clock synchronisation, but equally important in
another way. While wordclock enables digital
audio to be transferred without problems, by
keeping the various audio inputs and outputs run-
ning at the same frequency, timecode enables
recording and playback devices to run in sync
with one another, and with sequencers and desk
automation. Timecode is a signal that carries a
regular series of absolute time values
(hrs:mins:secs:frames). There are different frame
rates (hinting at timecode’s origins in the need for
a means to synchronise film equipment), of 24,
25, 30 (drop and non-drop) frames per second.
As in wordclock, one device is defined as the mas-
ter, and its timecode keeps every other recorder
running along in synchronisation. The two timing
systems are quite independent, and a recorder
that is the wordclock master in a system could be
the timecode slave of another device (the 328, for
example, which can generate all timecode frame
rates).
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