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 of 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.

Technology advanced; and as the speed of circuitry increased it became commonplace for digital systems to use high-speed serial transmission with the obvious advantage of fewer conductors needed to move the data from device to device. As wordlength increased from 16, to 32, to 64 bits; the advantages of serial transmission became even greater. Contemporary digital audio equipment commonly uses serial data transmission internally as well as externally; and this makes it necessary to have an internal bit clock which is in the range of 62 to 128 times the sample frequency.

Due to advances in serial transmission technology and the fact that contemporary digital audio systems operate with a much higher frequency bit clock, the internal clock frequency is significantly higher than the one cycle per sample Word Clock frequency and better methods are available to deal with issues related to very high frequency transmission. The result is that many contemporary digital audio devices do at least as good a job, if not a better, sync’ing to an AES digital audio input than to a lower frequency Word Clock signal that requires generation of a synchronized much higher frequency bit clock. For these reasons, Lavry DA converters all “lock” to the incoming digital audio and use sophisticated methods of effectively “de-coupling” the internal clock used for conversion from the jitter in the incoming digital audio signal.