ISO/IEC 13818-3, also called MPEG-2 BC (backward compatible), extends Layers I and II to five full-bandwidth channels and an optional low-frequency effects channel. The multichannel audio is coded so that an ordinary MPEG-1 decoder still plays a stereo version.
How it works
The frame’s normal stereo pair holds a downmix of the five channels, called Lo and Ro, that any MPEG-1 decoder plays. The extra information a multichannel decoder needs to recover the other channels lives in the ancillary data field of the frame. When the ancillary field is too small, the extension continues in a separate extension stream that an MPEG-1 decoder never sees.
A multichannel decoder then undoes the mix, a step the standard calls dematrixing, using a per-subband-group transmission channel allocation and a dematrix procedure setting.
What is documented here
This page cannot list the fields of the multichannel header (commonly called
mc_header) or the extension stream. They are defined only in ISO/IEC
13818-3, a paid standard, and no public specification or reverse-engineering page
found could confirm them. Their bit widths and codes are left out rather than
guessed.
In practice these streams are rare. Almost every .mp2 file is one or two
channels. Broadcast multichannel audio moved to AC-3 and AAC instead, and common
decoders such as FFmpeg and mpg123 only decode the compatible stereo pair.