User`s guide

User's Guide TEB
7
7. Word Clock
In the analogue domain one can connect any device to another device, a synchronization is not
necessary. Digital audio is different. Correct interpretation of digital audio data is dependent
upon a definite sample frequency. Signals can only be correctly processed or transferred be-
tween 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 optical are self-clocking (seen from a non-technical view TDIF
too, as word clock is embedded inside the TDIF cable), 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 de-
vices such as CD- players) cannot be synchronized via self-clocking. Finally there are 'prob-
lematic' devices, which are nearly un-usable without a word clock attached anyway.
TDIF plays a special role in the world of digital audio formats. Although all necessary signals
are present inside the D-sub connector, most devices of the company Tascam require an addi-
tional word clock connection, providing a word clock signal shifted by 90°. Also many devices
with TDIF interface suffer from an incomplete clock design, causing clock noise by wrong syn-
chronisation when in master mode. The latter forced us to implement a clock select switch
which solves any compatibility problem.
In digital studios, synchronization requirements can be met by connecting all devices to a cen-
tral 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 or sync 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 fun-
damental reason for sync problems when they are connected together.
The TDIF format is especially critical with respect to word clock:
When the TEB is slave, no additional word clock connection is necessary. In case DA88
and/or DA38 are slave, the word clock output of the TEB 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.
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.
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.
The last device in a wordclock chain should be terminated using a 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.