User's Guide
WCM MKII © RME
10
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 necessa-
ry 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 impedan-
ce of 75 Ohm. If the voltage is too low, synchronization will fail. High frequency reflection ef-
fects 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 input of the
WCM
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), set S1 to ON. This activates an internal 75 Ohm termina-
tion resistor.
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
terminator plug as described in the previous paragraph. Some devices (like the WCM MKII)
have switchable 75 Ohm resistors, which saves both T-adapter and terminator.
