RME OCTAMIC II [21/32] Cabling and termination

User’s Guide OctaMic II © RME

12.2 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. Actually you will find all the

necessary components (T-adapters, terminators, cables) in most electronics and computer

stores. The latter usually carries 50 Ohm components. The 75 Ohm 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 as low impedance types, 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 larger

studios.

The OctaMic II's word clock input can be high-impedance or terminated internally, ensuring

maximum flexibility. If termination is necessary (e.g. because the OctaMic II is the last device in

the chain), push the switch at the back (see chapter 9.1).

In case the OctaMic 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 termi-

nator plug.