Word Clock

Overview

The term "Word Clock" is used to describe a one cycle per sample period "square wave" signal used for synchronization of digital audio equipment. The signal is typically "TTL level" 5 volt p-p and is carried on 75 Ohm coaxial cable with BNC connectors.

History

Earlier digital audio systems employed a number of formats of interconnection, many of which were proprietary. Some were parallel; in which case each "bit" was carried on a separate conductor and a Word Clock signal was used to synchronize the timing of the transmission of each complete "word" of 16 bits, once per sample period.

In other systems; the left and right channel's digital audio data was transmitted in a serial manner, in parallel with a Word Clock signal which was used to synchronize the receiver with the beginning of the transmission of each serial "word."

As digital audio systems grew in complexity and the need for synchronization with video equipment arose, Word Clock was used as the system "clock" even though newer formats, such as the AES3 digital audio format were "self-clocking."

Due to the fact that the Word Clock format is a relatively "low frequency" signal compared to the serial formats with an embedded "bit-clock," with two transitions per sample period as versus hundreds of transitions per sample period; the Word Clock signal did offer advantages in terms of jitter issues. With reasonable care to use the proper cable and termination; cable reflections (one of the main sources of jitter) have the time to decay before the next transition occurs; which is not the case with serial formats.