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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.
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 HDSPe MADI's word clock input can be high-impedance or terminated internally, ensuring
maximum flexibility. If termination is necessary (e.g. because the card is the last device in the
chain), activate the switch TERM between the BNC jacks on the Expansion Board so that the
yellow TERM LED lights up (see chapter 24.1).
In case the HDSPe MADI 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 (avail-
able as short BNC plug). Of course devices with internal termination do not need T-adaptor and
terminator plug.
Due to the outstanding SteadyClock technology of the HDSPe MADI, we recommend not
!
to pass the input signal via T-adapter, but to use the card's word clock output instead.
Thanks to SteadyClock, the input signal will both be freed from jitter and - in case of loss or
drop out – be reset to a valid frequency.
19.4 Operation
The HDSPe MADI's word clock input is active when Pref. Sync Ref in the Settings dialog has
been set to Word Clock, the clock mode AutoSync has been activated, and a valid word clock
signal is present. The signal at the BNC input can be Single, Double or Quad Speed, the HDSP
automatically adapts to it. As soon as a valid signal is detected, the green LED at the bracket is
lit, and the Settings dialog shows either Lock or Sync (see chapter 30.2). In the status display
AutoSync Ref the display changes to Word. This message has the same function as the green
Lock LED, 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.
User's Guide HDSPe MADI © RME
37
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