Converting Miles Sound System XMI to WAV: 1990s PC Game Soundtracks

During the MS-DOS PC gaming boom of the 1990s, the Miles Sound System (MSS)—originally created by programmer John Miles under the name Audio Interface Library (AIL)—was the industry gold standard middleware for interactive video game sound.

Titles developed by Sierra On-Line, Origin Systems, LucasArts, Bullfrog Productions, and BioWare—including Wing Commander, Ultima Underworld, Theme Park, Master of Magic, Star Wars: X-Wing, and Baldur's Gate—relied on the Extended MIDI (.xmi) format to drive music playback across AdLib, Sound Blaster, Roland MT-32, and General MIDI hardware.

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1. The Structure of an IFF XMID File

Unlike standard .mid files which follow standard MIDI chunk conventions (MThd / MTrk), XMI files are organized into Electronic Arts-style Interchange File Format (IFF) chunk structures:

• Top-Level FORM Chunk: Declares the overall envelope with an XDIR (sequence catalog) or XMID (single extended song) form type FourCC.
• Timbre Bank (TIMB) Chunk: Catalogs which hardware patch voices, Roland MT-32 timbres, or General MIDI program numbers are utilized by the sequence tracks.
• Event Stream (EVNT) Chunk: Encapsulates the compressed MIDI note events. Rather than standard MIDI variable-length quantities, XMI represents delta time through sequential accumulator bytes with values under 128 (0x00..0x7F).
• Embedded Note Duration: In XMI, Note On events (0x90) explicitly embed note duration counts directly within the command packet, simplifying polyphony tracking on early CPUs.

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2. In-Browser Synthesis to 16-bit Linear WAV

Because contemporary operating systems and media players do not ship with 1990s Miles Sound System drivers, playing .xmi files requires emulating the sequence scheduler and instrument voice synthesizer:

1. Delta Interval Accumulation: Accumulates sequential sub-128 timing bytes to schedule note onset and release events precisely against tempo markings.
2. Harmonic Voice Generation: Emulates multi-harmonic oscillator instruments across channels with dynamic ADSR amplitude envelopes.
3. Stereo Spatialization: Evaluates pan controller events (CC 10) to recreate wide stereo imaging.
4. Uncompressed WAV Export: Writes standard 16-bit 44.1 kHz linear PCM stereo audio via writeWav.

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3. Preservation with 100% Offline Client-Side Privacy

Preserving retro computer gaming culture should never depend on cloud services or remote server conversions. convrtr synthesizes your .xmi soundtrack files entirely inside your browser sandbox. 100% offline, private, and instant.