Audio Engineering

568 Chapter 18

In fact, both these interfaces are very similar, with variations being more due to electrical
differences than between differences in data format.

18.1.1 AES/EBU or IEC958 Type 1 Interface

This electrically balanced version of the standard digital interface was originally defi ned
in documents produced by the Audio Engineering Society (AES) and the European
Broadcasting Union (EBU) and is, consequently, usually referred to as the AES/EBU
standard. This is the standard adopted mainly by professional and broadcast installations.
Mechanically, this interface employs the ubiquitous XLR connector and adopts normal
convention for female and male versions for inputs and outputs, respectively. Electrically,
pin 1 is specifi ed as shield and pins 2 and 3 for balanced signal. One of the advantages of
the digital audio interface over its analogue predecessor is that polarity is not important,
so it is not necessary to specify which pin of 2 and 3 is “ hot. ” The balanced signal is
intended to be carried by balanced, twisted-pair, and screen microphone-style cable and
voltage levels are allowed to be between 3 and 8 V pk-pk (EMF, measured differentially).
Both inputs and outputs are specifi ed as transformer coupled and earth free. The output
impedance of the interface is defi ned as 110 ohms, and a standard input must always
terminate in 110 ohms. A drawing for the electrical standard for this interface is given in
Figure 18.1.

18.1.2 The SPDIF or IEC985 Type 2 Interface

This consumer version of the two-channel, serial digital interface is very different
electrically from the AES/EBU interface described earlier. It is a 75-ohm, matched
termination interface intended for use with coaxial cable. It therefore has more in
common with an analogue video signal interface than with any analogue audio
counterpart. Mechanically the connector style recommended for the SPDIF interface
is RCA style phono with sockets always being of the isolated type. Voltage levels are

110

TX 2 110 RX

1 Cable 1

2

33

Figure 18.1 : AES/EBU interface.