MUS

The MUS format is a slightly altered MIDI format, created for DMX and 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.

Support for standard MIDI files in vanilla Doom was added in version 1.5, which meant WAD authors no longer needed to convert MIDI files to MUS. However, since this was achieved by the game running MIDI2MUS internally when needed, authors looking for vanilla compatibility should make sure that the MIDI file they include can be successfully converted. Vanilla Heretic and Hexen, derived from version 1.2 of the Doom code, still require that music be converted to MUS format before inclusion in a WAD.

In the DMX source code, the MUS format is referred alternatively as MUS, RTM, Real Time Music, and Game Music Format.

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 reasoning for this implementation detail in the DMX library seems to be that OPL2 cards had only nine channels.

FM cards with dual OPL2 or OPL3  had 18 channels, and any card fully compliant with the  standard had to support at least 16 channels; but the success of the earlier 9-channel FM cards dictated going with the lowest common denominator for compatibility reasons.

The size of MUS files is limited to 64 kilobytes, due to use of 16-bit fields inside the file format.