Word Clock

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Contents

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” which is nominally 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 64 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. Lavry AD converters also offer the option of synchronizing to AES sync, in addition to Word Clock.

Basics

See termination for important information on proper termination. “Proper” termination requires two things:

  1. Both the sending end and the receiving end must have termination at exactly one place.
  2. The termination must be the correct value; which is the same as the characteristic impedance of the cable.

In (1) the sending end is generally a "given" as all Word Clock outputs have fixed internal termination by design. The important issue is termination of the receiving device(s).

There are three basic approaches to using Word Clock in systems larger than two devices:

a.) Use a Word Clock source with multiple outputs. The Word Clock source must have as many outputs as there are "slave" devices with Word Clock inputs. The Lavry Synchrony-16 is an example of this type of source.

b.) Use one Word Clock source and "chain" the same signal using BNC "T" connectors on the inputs of the receiving devices. The slave devices in middle of the chain must not have fixed internal termination so the only receiving device with termination is the last slave device in the chain. The last device can either have fixed internal termination or a BNC "T" connector on its input with a BNC terminator on the "T" opposite in the incoming Word Clock cable.

c.) A "mixed" approach. This can be a combination of (a) and (b) or a combination of Word Clock connections to synchronize all external AD converters and AES synchronization of the "master" AD converter to the digital audio interface.

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