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A word clock or wordclock (sometimes sample clock, which can have a broader meaning) is a clock signal (not the actual device) used to synchronise other devices, such as digital audio tape machines and compact disc players, which interconnect via digital audio. S/PDIF, AES/EBU, ADAT, TDIF and other formats use a word clock. Various audio over Ethernet protocols use broadcast packets for the word clock. The device which maintains the word clock on a network is the master clock.
Word clock should not be confused with timecode; word clock is used entirely to keep a perfectly-timed and constant bitrate to avoid data errors. The word clock generator, usually built-in to analog-to-digital converters, creates digital pulses which contain no other data, and is considered essential to avoid frequency drift between the internal oscillators of each device. Timecode is actual data (technically metadata) about the media content being transmitted, and is optional, being sent in a higher layer.
Most professional digital audio equipment have a "Word Clock" input and/or output to synchronize timing between multiple devices. Word clock is similar to the left/right clock (LRCK) in a hardware audio codec as they both run at the sample frequency. And although the electrical characteristics of the word clock signal have not been completely standardized some characteristics should always apply. Things that should remain consistent are a 75ohm output impedance, 75ohm cables and a 75ohm terminating resistor at the end of a chain or cable.
Proper termination of the word clock signal with a 75ohm resistor is important. It prevents the clock signal from reflecting back into the cable and causing false detection of extra 1's and 0's. Some digital equipment include a switchable terminator, some include a hardwired terminator and others have no terminator at all. The unfortunate part is that some equipment manuals do not indicate whether a hardwired terminator is included or not. [1]
The phase of the digital word clock signal is another important factor. The wrong phase can cause the order of the left and right inputs of a stereo pair or the order of paired input channels to be reversed.
Further technical references can be found on the Audio Engineering Society website [2] and in AES Standard AES11-2009 (Revision of AES11-2003).
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