XMI / Audio Format Extraction

Decoding Miles Sound System’s .XMI extended-MIDI music and the standard .WAV sound effects that Captain Claw packs inside REZ.

Why it matters

Claw’s music is not MP3 or even plain MIDI — it is XMI (XMIDI), a 1990s Miles Sound System format that no modern player or SDL_mixer build understands directly. To get music out, you must either convert XMI to Standard MIDI (.mid) at extraction time or teach the engine to parse it. Sound effects, by contrast, are ordinary RIFF WAV and play as-is. Knowing which is which avoids writing a decoder you do not need. See music-playback-midi-xmi-conversion.

How it works

XMI is a RIFF/IFF-style chunked container; the differences from Standard MIDI matter:

Aspect	Standard MIDI (SMF)	XMI (XMIDI)
Container	MThd/MTrk chunks	FORM/XDIR/CAT/EVNT
Multiple songs	one per file	many, in one CAT
Delta time	variable-length, between events	fixed interval count
Note duration	separate note-on/note-off	duration packed into note-on

Conversion to SMF means walking EVNT, splitting each note’s packed duration back into a note-on plus a delayed note-off, and rewriting XMI’s interval timing as SMF variable-length delta times. The freely available xmi2mid tool (and the routines in scummvm/munt) does exactly this. WAV needs none of it — RIFF....WAVE is a public format; pull the bytes out of REZ and hand them straight to SDL_mixer. Playback of the converted MIDI still needs a synth (a SoundFont via FluidSynth, or hardware/OS MIDI).

Example

Extraction split inside the asset pipeline:

for entry in rez:
    if entry.type == "XMI":
        smf = xmi_to_smf(entry.bytes)   # repack EVNT -> MThd/MTrk
        write(entry.name + ".mid", smf)
    elif entry.type == "WAV":
        write(entry.name, entry.bytes)  # already RIFF, copy as-is

One XMI CAT may yield several .mid files (the menu theme, each level’s track). A 3-minute level tune is a few KB of MIDI events, versus megabytes if it were streamed audio — which is why a 1997 game shipped sequenced music.

Pitfalls

• Feeding XMI to SDL_mixer directly — it is not SMF; Mix_LoadMUS rejects or mis-plays it. Convert first.
• Dropping packed note durations — XMI encodes length in the note-on; ignore it and every note rings forever.
• No synth on the target — converted MIDI is silent without a SoundFont/synth; bundle FluidSynth + an SF2.
• Over-engineering WAV — effects are plain RIFF; do not route them through the XMI path.

See also

• music-playback-midi-xmi-conversion
• sdl-mixer-integration