AES/EBU digital audio interfaces

Audio signals in digital form, when uncompressed, are transmitted as a continuous bit stream (as opposed to packet transmission) between two pieces of equipment. To facilitate equipment interconnection, several standardization bodies have advocated various formats for transferring digital audio data streams. These standardization bodies include the following: Audio Engineering Society (AES), European Broadcasting Union (EBU), International Telecommunication Union (ITU), Society of Motion Picture and Television Engineers (SMPTE), International Electrotechnical Commission (IEC).

In addition to these organizations, there are also national bodies (e.g. ANSI, DIN). Some standards exist in several virtually identical versions when they have been adopted simultaneously or successively by several of these bodies.

Firstly, we're interested in the AES/EBU standard, which corresponds to a two-channel digital audio interface. This interface is monodirectional; it defines a link between a transmitter and a receiver. First, we present the symmetrical (in the electrical sense) version, using a pair of conductors. This version corresponds to the AES3 standard.

An important variant of this standard bears the number AES-3id; it corresponds to the same type of link, but over coaxial cable, and therefore not symmetrical. This variant is mainly used for broadcasting. In the same context, the audio signal is naturally carried in conjunction with the video signal. We are studying the standards that specify the insertion of the AES/EBU audio signal into a digital video stream. The standard has been published by the ITU-R as BT.1305.

A third type of link derived from the AES/EBU model has recently been standardized by the AES under the reference AES42; this standard concerns the transport of a digital audio signal from a microphone. Compared with the AES3 standard, the AES42 standard mainly adds the possibility of sending power from the AES/EBU receiver to the transmitter (i.e. to the microphone) in the form of a phantom DC voltage. It also adds the possibility of sending commands from the receiver (which could be a mixer, for example) to modify the behavior of the microphone (or any other digital source).

We therefore cover techniques for conveying the sound signal alone, and those for combining the digital audio signal with digital video. On the other hand, we exclude telecommunications techniques for conveying these signals over networks. Likewise, we do not cover the various audio file formats used for storage on computer media, nor for links with computers (e.g. USB, FireWire). This document is intended as an introduction to the various standards, and not as a summary of them. Engineers who have to implement...