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19.2 Technical Description and Usage
In the analog domain one can connect any device to another device, a synchronisation is not
necessary. Digital audio is different. It uses a clock, the sample frequency. The signal can only
be processed and transmitted when all participating devices share the same clock. If not, the
signal will suffer from wrong samples, distortion, crackle sounds and drop outs.
AES/EBU, SPDIF, ADAT and MADI are self-clocking, an additional word clock connection in
principle isn't necessary. But when using more than one device simultaneously problems are
likely to happen. For example any self-clocking will not work in a loop cabling, when there is no
'master' (main clock) inside the loop. Additionally the clock of all participating devices has to be
synchronous. This is often impossible with devices limited to playback, for example CD players,
as these have no SPDIF input, thus can't use the self clocking technique as clock reference.
In a digital studio synchronisation is maintained by connecting all devices to a central sync
source. For example the mixing desk works as master and sends a reference signal, the word
clock, to all other devices. Of course this will only work as long as all other devices are equipped
with a word clock or sync input, thus being able to work as slave (some professional CD players
indeed have a word clock input). Then all devices get the same clock and will work in every pos-
sible combination with each other. .
!
A digital system can only have one master! If the card's clock mode is set to 'Internal', all
other devices must be set to 'Slave'.
But word clock is not only the 'great problem solver', it also has some disadvantages. The word
clock is based on a fraction of the really needed clock. For example SPDIF: 44.1 kHz word clock
(a simple square wave signal) has to be multiplied by 256 inside the device using a special PLL
(to about 11.2 MHz). This signal then replaces the one from the quartz crystal. Big disadvantage:
because of the high multiplication factor the reconstructed clock will have great deviations called
jitter. The jitter of a word clock is multiple times higher than the one of a quartz based clock.
The actual end of these problems is offered by the SteadyClock technology of the HDSPe
RayDAT. Combining the advantages of modern and fastest digital technology with analog filter
techniques, re-gaining a low jitter clock signal of 22 MHz from a slow word clock of 44.1 kHz is
no problem anymore. Additionally, jitter on the input signal is highly rejected, so that even in real
world usage the re-gained clock signal is of highest quality.
19.3 Cabling and Termination
Word clock signals are usually distributed in the form of a network, split with BNC T-adapters
and terminated with resistors. We recommend using off-the-shelf BNC cables to connect all
devices, as this type of cable is used for most computer networks. You will find all the necessary
components (T-adapters, terminators, cables) in most electronics and/or computer stores. The
latter usually carries 50 Ohms components. The 75 Ohms components used for word clock are
part of video technology (RG59).
Ideally, the word clock signal is a 5 Volt square wave with the frequency of the sample rate, of
which the harmonics go up to far above 500 kHz. To avoid voltage loss and reflections, both the
cable itself and the terminating resistor at the end of the chain should have an impedance of 75
Ohm. If the voltage is too low, synchronization will fail. High frequency reflection effects can
cause both jitter and sync failure.
User's Guide HDSPe RayDAT © RME
37
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