Cables And Termination - RME Audio Hammerfall HDSP 9632 User Manual

The end of these problems should have been the so called Superclock, which uses 256 times
the word clock frequency. This equals the internal quartz frequency, so no PLL for multiplying is
needed and the clock can be used directly. But reality was different, the Superclock proved to
be much more critical than word clock. A square wave signal of 11 MHz distributed to several
devices - this simply means to fight with high frequency technology. Reflections, cable quality,
capacitive loads - at 44.1 kHz these factors may be ignored, at 11 MHz they are the end of the
clock network. Additionally it was found that a PLL not only generates jitter, but also also rejects
disturbances. The slow PLL works like a filter for induced and modulated frequencies above
several kHz. As the Superclock is used without any filtering such a kind of jitter and noise
suppression is missing. No wonder Superclock did not become a commonly accepted standard.
The actual end of these problems is offered by the SteadyClock technology of the HDSP
9632. Combining the advantages of modern and fastest digital technology with analog filter
techniques, re-gaining a low jitter clock signal of 11 MHz from a slow word clock of 44.1 kHz is
no problem anymore. Additionally, jitter on the input signal is highly rejected, so that even in
real world usage the re-gained clock signal is of highest quality.

10.2 Cables 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.
To avoid voltage loss and reflections, both the cable itself and the terminating resistor 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.
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 examples where 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 larger studios.
Also, 75 Ohm cable is almost impossible to find these days. 50 Ohm cable is standard – it will
also work as long as the termination resistors are 75 Ohm.
The HDSP 9632's word clock input on the optional HDSP 9632 Word Clock Module can be
high-impedance or terminated, ensuring maximum flexibility. If termination is necessary (e.g.
because HDSP 9632 is the last device in the chain), push the WCM's termination switch so that
the yellow termination LED lights up.
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). Devices with internal termination do not need T-adaptor and terminator 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 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 Hammerfall DSP 9632 © RME
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