User`s guide

User's Guide HDSPe MADI FX © RME

24.3 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.

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 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 as high impedance types, 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 big-

ger studios.

The HDSPe MADI FX word clock input can be high-impedance or terminated internally, ensur-

ing maximum flexibility. If termination is necessary (e.g. because the card is the last device in

the chain), activate the option Word Clock In Term. in the Settings dialog.

In case the HDSPe MADI FX 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 FX, 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.