User`s guide

User’s Guide HDSP System HDSP 9652 © RME
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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.
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 larger
studios. 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 HDSP 9652’s word clock input can be high-impedance or terminated internally, ensuring
maximum flexibility. If termination is necessary (e.g. because HDSP 9652 is the last device in
the chain), bring jumper X105 on the Expansion Board into position 75 Ohm.
In case the HDSP 9652 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. Remove jumper
X105 from the Expansion Board. 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 terminator plug.