Converting HMI DOS Game Audio to WAV: Emulating 1990s PC Soundtracks

In the golden era of 1990s MS-DOS gaming, developing reliable audio drivers for hundreds of competing sound cards—from AdLib and Sound Blaster Pro to the Gravis UltraSound and Roland Sound Canvas—was an immense technical hurdle.

To solve this, many top game studios (including Interplay, Blizzard Entertainment, Parallax Software, and Midway) turned to the Human Machine Interfaces (HMI) Sound Operating System. HMI .hmi (and .hmp) music files provided a unified multi-track sequence container that powered the unforgettable soundtracks of Descent, Descent II, Warcraft II: Tides of Darkness, Mortal Kombat, Shattered Steel, and Battle Arena Toshinden.

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1. The Architecture of an HMI File

An HMI file is an extended, optimized MIDI container designed for real-time interrupt-driven game playback:

• Signature Header: Begins with ASCII string HMI-MIDIFILE0115 or HMI-MIDISONG0615 in the first 32 bytes.
• Track Table & Offsets: Stores track count, division (typically 120 ticks per quarter note), and offsets to individual channel track streams.
• Track Stream Packets: Encapsulates standard MIDI messages (Note On 0x90, Note Off 0x80, Program Change 0xC0, Control Change 0xB0) preceded by variable-length delta tick intervals.
• Multi-Track Voice Priority: Allowed game engines to dynamically reprioritize music channels when intense combat sound effects were triggered.

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2. In-Browser Multi-Voice Synthesis

Because modern operating systems and mobile devices do not include vintage HMI sound drivers, playback requires decoding the sequence events and synthesizing the instrument voices into a standard digital audio stream:

1. Delta-Time Scheduling: Maps tick durations against tempo markings (BPM) to construct a high-precision audio timeline.
2. Harmonic Voice Generation: Emulates multi-channel synthesizer instruments with parameterized attack, decay, sustain, and release (ADSR) amplitude envelopes.
3. Stereo Spatialization: Applies channel pan controllers (CC 10) and master volume (CC 7) to preserve the wide dynamic stereo mix.
4. 16-bit Linear PCM WAV Encoding: Emits standard uncompressed RIFF WAV files that play seamlessly on any modern phone, browser, or media player.

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3. Pure Offline In-Memory Conversion

Preserving retro video game music should never require sending files to untrusted cloud servers. convrtr synthesizes .hmi and .hmp files 100% client-side inside your browser sandbox. Zero data ever leaves your device.