Steinberg Nuendo DD8 Mode D'emploi page 32

9.2 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 us-
ing 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.
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 ter-
minating resistor at then end of the chain should have an impedance of
75 Ohm. If the voltage is too low, synchronization will fail. High-fre-
quency reflection effects can cause both jitter and sync failure.
Unfortunately there are still many devices on the market, even newer
digital mixing consoles, which are supplied with a word clock output
that can only be called unsatisfactory. If the output breaks down to 3
Volts when terminating with 75 Ohms, you have to take into account
that a device, of which the input only works from 2.8 Volts and above,
does not function correctly already after 3 meters of cable. So it is not
astonishing that because of the higher voltage, word clock networks
are in some cases more stable and reliable if cables are not terminated
at all.
Ideally all outputs of word clock delivering devices are designed with
very low impedance, but all word clock inputs with high impedance, in
order to not weaken the signal on the chain. But there are also negative
examples, when the 75 Ohms are built into the device and cannot be
switched off. In this case the network load is often 2 x 75 Ohms, and the
user is forced to buy a special word clock distributor. Note that such a
device is generally recommended for larger studios.
Also, 75 Ohm cable is almost impossible to find these days. 50 Ohm ca-
ble is standard - this will also work as long as the termination resistors
are 75 Ohm.
NUENDO DD 8
English – 32