Bit clock

Overview

The term "bit clock" is used to describe a one cycle per sample "square wave" signal used for internal synchronization within digital audio equipment. One example of such a system is the I2S format used for transmission of digital audio information from one IC to another on a PC board.

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.

Due to the fact that the Word Clock format is a relatively "low frequency" signal compared to the serial formats, with two transitions per sample period as versus hundreds of transitions per sample period; the Word Clock signal does 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.

Digital audio formats designed explicitly for transmission between pieces of equipment have an "embedded" bit clock as part of the electrical waveform, and the coding of the electrical waveform allows for these very high frequency signals to be transmitted with signal transformers; which allows them to work. Formats like I2s do not have the means to address serious issues that arise when transmitting very high frequency signals more than short distance because it requires DC transmission. The discussion of these issues is beyond the scope of this subject.

Basics

Bit clock signals are used in serial digital audio formats like I2S as a means of synchronizing the transmission of signals from one IC to another on the same PC board. Please see serial data more information.