User’s Guide DIGI9636
© RME
15
10. Word Clock
10.1 Technical Description and Usage
Correct interpretation of digital audio data is dependent upon a definite sample frequency.
Signals can only be correctly processed or transferred between devices if these all share the
same clock, otherwise digital signals are misinterpreted, causing distortion, clicks/crackle and
even dropouts.
AES/EBU, SPDIF and ADAT are self-clocking, so an additional line for word clock could be
considered redundant. In practice however, using several devices at the same time can cause
problems. For example, if devices are connected in a loop without there being a defined
‘master’ device, self-clocking may break down. Besides, the clocks of all devices must be
synchronized from a single source. Devices without SPDIF inputs (typically playback devices
such as CD players) cannot be synchronized via self-clocking.
In digital studios, synchronization requirements can be met by connecting all devices to a
central sync source. For instance, the master device could be a mixing desk, sending a
reference signal - word clock - to all other devices. However, this will only work if all the other
devices have word clock inputs (e.g. some professional CD players) allowing them to run as
slaves. This being the case, all devices will receive the same clock signal, so there is no
fundamental reason for sync problems when they are connected together.
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.
In practice, the situation has improved in recent years. The relatively low frequency of word
clock signals is not a problem for modern electronic circuits. Because of the higher voltage,
word clock networks are often more stable and reliable if cables are not terminated at all. Also,
75 Ohm cable is almost impossible to find these days. 50 Ohm cable is standard - this will also
work as long as the termination resistors are 75 Ohm.
The word clock input on Hammerfall’s Expansion Board is a high-impedance type ensuring
maximum flexibility, and is therefore not terminated. If normal termination is necessary (e.g.
because Hammerfall is the last device in the chain), simply connect a T-adapter to its BNC
input jack, connect the cable supplying the word clock signal to one arm of the T-adapter and
terminate the other with a 75 Ohm resistor (as a short BNC plug).
In case the Hammerfall resides within a chain of devices receiving word clock, plug a T-adapter
into Hammerfall’s BNC input jack and the cable supplying the word clock signal to one end of
the adapter (as above), but 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
terminator plug as described in the previous paragraph.
