Cables And Termination; General Operation - RME Audio Hammerfall HDSP 9632 User Manual

26.3 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 neces-
sary 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 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 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 al-
ready 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.
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 bigger
studios.
The HDSP 9632'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), push the switch at the back beside the BNC socket (see chapter 26.1).
In case the HDSP 9632 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 termi-
nator plug.
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 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.
26.4 Operation
The HDSP 9632'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 33.1). 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.
In the line Freq., the AutoSync Ref shows the frequency of the reference signal, measured by
the hardware. In this case the frequency of the current wordclock signal is measured and dis-
played.
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User's Guide HDSP System HDSP 9632 © RME