MUS

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The MUS format is a slightly altered MIDI format used in the Doom engine soundtracks. MUS files are nearly identical to MIDI files, only slightly more compact. The file format has a size limit of 65536 bytes.

The later Vanilla Doom versions added support for MIDI files, which meant WAD authors no longer needed to convert MIDI files to MUS.

[edit] Limitations

The MUS format can only use the first 9 MIDI channels using the first 8 for music instruments and the 9th for drums. The reason this is forced upon the format is that older soundcards had an FM sound chip with only 9 FM channels. Although some at the time had 18 channels, these were less common.