4.4 Sampling Rate Related to Data Rate

The standard two-channel interface was originally designed to accommodate digital audio signals with sampling rates between 32 and 48 kHz, with a margin of 12.5% to allow for varispeed operations. Since the interface carries audio data in real time, normally transferring two audio samples (channel 1 and channel 2) in the time of one sampling period, the data rate of the interface depends on the audio sampling rate. It is normally 64 times the sampling rate, since there are 64 bits in a frame (= two subframes). At a sampling rate of 48 kHz the data rate is 64 times 48 000, which is 3.072 Mb/s, whereas at 32 kHz it is only 2.048 Mb/s. If the source is varispeeded by a certain percentage then the data rate will change by the same percentage, and although it can usually be tracked by a receiver this presents problems in a system where all devices must be locked to a common, fixed sampling frequency reference (see Chapter 6), since the receiver may not change its sampling rate to follow a varispeeded source.

In recent years there has been a demand for interfaces capable of handling audio at increased sampling frequencies up to 192 kHz. For this reason a situation can arise in which one AES3 interface is used in a single-channel-double-sampling-frequency mode. Here the two subframes within a single AES frame carry successive samples of the same audio channel, making the audio sampling frequency twice the AES frame rate. The sampling frequency indicated in byte 0 of channel status (see below) remains as if the interface was operating at normal sampling frequency, because this is the frame rate of the interface. For example, in this mode a single interface could carry a single channel of audio at 96 kHz sampling frequency whilst continuing to operate at the same data transmission rate as a stereo interface running at 48 kHz. This is a useful alternative to doubling the overall interface transmission rate to accommodate the higher audio sampling frequency (giving rise to the need for better and possibly shorter cables). These modes are indicated in channel status byte 1, described below.