Cabling And Termination - RME Audio HDSP 9632 User Manual

10.2 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 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 neces-
sary components (T-adapters, terminators, cables) in most electronics and/or computer stores.
The ideal word clock signal should be a 5 Volt square wave having the same frequency as the
sample rate. This signal will generate harmonics up to more than 500 kHz. To avoid voltage
loss and reflections, both the cable itself and the terminating resistor should have an imped-
ance of 75 Ohm. If the voltage is too low, synchronization will fail. High frequency reflection
effects can cause both jitter and sync failure.
In practice, the situation has improved in recent years. The relatively low frequency of word
clock signals is not a problem for modern electronic circuits. Because of the higher voltage, big
word clock networks are often more stable and reliable if cables are not terminated at all. Also,
75 Ohm cable is almost impossible to find these days. 50 Ohm cable is standard - this will also
work as long as the termination resistors are 75 Ohm.
The WCM's input is a high impedance type to offer highest flexibility for the user. In case a
termination according to the standard is necessary (because the WCM is the last device in a
chain of several devices), push the switch with a small tool, so that the yellow LED lights up.
In case the WCM resides within a chain of devices receiving word clock, plug a T-adapter into
the WCM'S BNC input jack, and the cable supplying the word clock signal to one end of the
adapter (as above), but connect the free end to the next device in the chain via a further BNC
cable. The last device in the chain should be terminated using another T-adapter and a termi-
nator plug as described in the previous paragraph. Some devices (like the WCM) have
switchable 75 Ohm resistors, which saves both T-adapter and terminator.
Due to the outstanding SteadyClock technology of the HDSP 9632, we recommend not to
pass the input signal via T-adapter, but to use the WCM's two word clock outputs. Thanks
to SteadyClock, the input signal will both be freed from jitter and - in case of loss or drop out
– be held at the last valid frequency.
User's Guide HDSP 9632 WCM © RME
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