"Codec" is an acronym for "compressor/decompressor". A codec is needed for compressing or decompressing the contents of an audio or video file. While the format of a file (e.g. .AVI or .WAV) is handled by an IOProc (see the previous page), the contents of a file are handled by a codec.

For example, MMPM/2 comes with an IOProc for the .AVI file format, but it does not have enough codecs to understand all compression formats which are typically used with AVI files.

The "FourCC" (four-character code) of a codec must match the one of an IOProc. The same codec can be installed several times for several IOProcs. For example, the "Intel Indeo" codec is installed for the AVI-file IOProc. But if you had a new IOProc for some other video file format (e.g. QuickTime, ".MOV"), the same codec could be installed for .MOV files as well, because the compression format is the same, and only the file format differs.