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.
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 OPL2 cards (Adlib, Sound Blaster 1.0, SB 1.5, SB 2.0) had only nine channels. FM cards with dual OPL2 (SB Pro, Pro AudioSpectrum) or OPL3 (SB Pro 2, SB 16) had 18 channels, and any card fully compliant with the General MIDI standard had to support at least 16 channels; but the success of the earlier 9-channel FM cards required going with the lower 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.
The MIDI2MUS converter does not create perfect conversions; sometimes the created files can sound distorted. This can be most audibly heard with MIDI files that use pitch bend effects, such as "Oww!" from Rise of the Triad.
The MIDI format uses the value 127 as the maximum possible volume in volume change events; MUS allows for higher values. A MUS->MIDI converter that does not take this into account might end up glitching the volume on certain channels. "Smells like Burning Corpses" is known to be affected by this problem.