14
User's Guide
ADI-8 PRO © RME
The TDIF format is especially critical with respect to word clock. We have mentioned this in
different places of this manual:
When the ADI-8 DD is slave no additional word clock connection is necessary. In case DA88
and/or DA38 are slave the word clock output of the ADI-8 DD has to be connected to the word
clock input of the first (master) recorder. When using more than one recorder a special sync
cable (Tascam part number PW-88S) is needed.
What you do not need to know: the
ADI-8 PRO
takes care of the first DTRS machine's
properties, the DA-88, and it can be used together with this device without further settings.
8.2 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. You will find all the neces-
sary components (T-adapters, terminators, cables) in most electronics and/or computer stores.
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, synchroniza-
tion 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
already 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 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
ADI-8 PRO
's word clock input can be high-impedance or terminated internally, ensur-
ing maximum flexibility. If termination is necessary (e.g. because
ADI-8 PRO
is the last
device in the chain), push the switch at the back (see chapter 8.3).
In case the
ADI-8 PRO
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.
