A word clock or wordclock (sometimes sample clock, which can have a broader meaning) is a clock signal used to synchronise other devices, such as digital audio tape machines and compact disc players, which interconnect via digital audio signals. Word clock is so named because it clocks each audio sample. Samples are represented in data words.
S/PDIF, AES/EBU, ADAT, and TDIF are some of the formats that use a word clock. Various audio over Ethernet systems use communication protocols to distribute word clock. The device which generates the word clock is the only master clock source for all the slave audio devices.
Comparison to timecode
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. Timecode is actual data (technically metadata) about the media being transmitted, and is optional, being sent in a higher layer. Time code can be used as an initial phase reference for jam sync using the word clock as the frequency reference.
Over coax cable
Professional digital audio equipment may have a word clock input or output to synchronize timing between multiple devices. 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 TTL level, 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.
The chain connection from the source (master) to the receivers (slaves) may increase jitter. Using clock distributing devices for parallel transmission is a better way. The length and quality of coaxial cables are important.
In annex B, the AES11 standard also describes common practice in transmitting and receiving a plain word clock signal. This is not an attempt to standardize it, the annex is informative only.