5.6 - What is S/P-DIF? What is AES/EBU?
Description
This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
5.6 - What is S/P-DIF? What is AES/EBU?
AES/EBU and S/P-DIF describe two similar protocols for communicating
two-channel digital audio information over a serial link. They are
slightly different in details, their basic format is almost identical,
but there are enough differences that the two are, for all intents and
purposes electrically incompatible. Both of these digital protocols
are described fully in an international standard, IEC 958, available
from the International Electrotechnical Commission.
AES/EBU (which stands for the joint Audio Engineering Society/European
Broadcasting Union standard) is the so-called "professional" protocol.
It uses standard 3-pin XLR connectors and 110-ohm balanced
differential cables for connection (no, standard microphone cables,
not even good quality cables, won't work, even though it seems they
might) and a 5 volt, differential signal.
S/P-DIF (which stands for Sony/Philips Digital InterFace, a now
obsolete standard superseded by IEC 958) is the so-called "consumer"
format. It uses what appears to be standard RCA connectors and cables,
but, in fact, require 75-ohm connectors and cables. Good quality video
"patch" cables have proven adequate (no, standard "audio" patch cords,
even excellent quality versions, have been shown not to work). The
signals are 0.5 volts unbalanced.
The actual datastream, are very similar. Each sample period a "frame"
is transmitted. Each frame consists of two "subframes", one each for
left and right channels, each subframe is 32 bits wide. In that
subframe, 4 bits are used for synchronization, then up to 24 bits are
usable for audio (the "consumer mode" format is limited to 16 bits).
The remaining four bits are used for parity (the first level of error
detection), validity, user status and channel status. 192 subframes
are collected, and the 192 user bits and 192 channel status bits are
collected into separate 24 8 bit status bytes for each channel.
The channel status bytes are interesting, because they contain the
important control information and the major differences between the
two protocol formats. One bit tells whether the data stream is
professional or consumer format. There are bits that specify
(optionally) the sample rate, deemphasis standards, channel usage, and
other information. The consumer format has several bits allocated to
copy protection and control: the SCMS bits.
Now, the notion that all of this is encoded in a standard may be
reassuring, but a standard is nothing but a voluntary statement of
common industry practice. There is a lot of incompatibility between
equipment out there caused directly by subtle differences between
interpretations and implementations. The result is that some
equipment simply refuses to talk to each other. Even THAT
possibility is stated in the standard! [Dick]
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