RME Fireface UCX II 40-Channel USB-B Audio/MIDI Interface - Manual (Page 48)

48 User's Guide Fireface UCX II © RME
22.3 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.
Unfortunately there are still devices on the market which provide 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 meter cable length. 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. In some cases 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 (such a device is generally recommended for big-
ger studios).
Therefore it seems of advantage to design outputs of word clock as low impedance types, but all
word clock inputs as high impedance types, in order to not weaken the signal on the chain.
The Fireface's word clock input can be high-impedance or terminated internally, ensuring maxi-
mum flexibility. If termination is necessary (e.g. because the Fireface is the last device in the
chain), check the option
Word Clock In Term. under Options (see chapter 7.1 / 14.1).
In case the UCX II resides within a chain of devices receiving word clock, plug a T-adapter into
its BNC input jack, and the cable supplying the word clock signal to one end of the adapter.
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 75 Ohm resistor (available as
short BNC plug). Of course devices with internal termination do not need T-adaptor and termina-
tor plug.
22.4 Operation
The field WCK on the display will light up green as soon as a word clock signal is detected. To
change to word clock as clock source, activate the Clock Source Word in the field Clock Mode
within the Settings dialog. The status display
Current changes to Word as soon as a valid signal
is present at the BNC jack. This message has the same meaning as the green State field, but
appears on the monitor, i.e. the user can check immediately whether a valid word clock signal is
present and is currently being used.