Taken from Annex B of AES11. “AES recommended practice for digital audio engineering – Synchronization of digital audio equipment in studio operations”
Annex B (Informative) Word Clock
It is possible to meet all the timing requirements of AES11 by means of a square wave at sampling frequency basic rate, commonly called word clock. It is used between different pieces of equipment to provide sampling frequency clocking of various sources.
This signal is not standardised and the parameters quoted are merely examples. The signal is commonly carried on coaxial cable, so that a single output can synchronise several receiving equipments by looping the signal through each in turn, and possibly terminating the cable with a 75 Ohm resistor at the far end.
The transmitted signal may vary in peak-to-peak amplitude from 1 V up to 5 V, and be either AC or DC coupled. It may be required to sink the current for a number of transistor-transistor-logic (TTL) inputs, or drive a 75 Ohm resistive load.
Receiving devices may require AC or DC coupling, and any peak-to-peak level from 1 V to 5 V, TTL logic levels being quite common. These require a logic ‘low’ of less that 0.4 V and a logic ‘high’ greater than 2.4 V. Some receivers are known to fail when the peak-to-peak signal exceeds 3 V, and others are known to require at least 4 V. Making it impossible to design a driver guaranteed to operate with all receivers.
The driver level most likely to provide the functionality required is a full 5 V, DC coupled, and able to drive a 75 Ohm load. Then adapters containing capacitors or attenuators may be used to suit individual receivers. Any practical technique may be used to detect the edges of the signal.
It should be noted that TTL signals are difficult to distribute using distribution amplifiers, and no common distribution amplifiers (video, AES3 or audio etc.) handle this signal, so that the loop-through architecture is the usual option. Video amplifiers can usually handle a peak-to-peak signal of 2 V, AC coupled or rather more than 1 V DC coupled and drive a terminating load of 75 Ohms.
Where new equipment is designed to use a word-clock signal, it is recommended that the rising edge is treated as the timing reference point.
The expression ‘word clock’ is also used at circuit-board level to describe various sampling-frequency logic signals.