Description

This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.

38 You mean: compatible!? With all these extra audio channels? Please explain! (MPEG Audio Codec Family ("Layer 1, 2, 3"))

Compatibility has been a major topic during the MPEG-2 definition phase.
The main idea is to use the same basic bitstream format as defined in
MPEG-1, with the main data field carrying two audio signals (called L0
and R0) as before, and the ancillary data field carrying the multichannel
extension information. Without going further into details, three terms can
be explained here:
"forwards compatible": the MPEG-2 decoder has to accept any
MPEG-1 audio bitstream (that represents one or two audio channels)
"backwards compatible": the MPEG-1 decoder should be able to
decode the audio signals in the main data field (L0 and R0) of the
MPEG-2 bitstream
"Matrixing" may be used to get the surround information into L0 and
R0:
L0 = left signal + a * center signal + b * left surround signal
R0 = right signal + a * center signal + b * right surround signal
Therefore, a MPEG-1 decoder can reproduce a comprehensive downmix of
the full 5-channel information. A MPEG-2 decoder uses the multichannel
extension information (3 more audio signals) to reconstruct the five
surround channels.

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