Converting id Software IMF Audio to WAV: Yamaha OPL2 FM Synthesis in the Browser

Before digital audio CD tracks and general MIDI wavetable synthesis dominated PC gaming in the mid-1990s, the AdLib and Sound Blaster sound cards reigned supreme. At their beating heart was the Yamaha YM3812 (commonly known as the OPL2)—a dedicated frequency modulation (FM) synthesis sound chip capable of producing nine simultaneous melodic channels.

To bring fast, low-overhead music to early 286 and 386 MS-DOS machines, id Software founders and legendary game composer Bobby Prince created the id Software Music Format (IMF). IMF powered the unforgettable soundtracks of Commander Keen 4–6, Wolfenstein 3D, Spear of Destiny, Bio Menace, Blake Stone, and Corridor 7.

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1. The Structure of an IMF File

Unlike tracker modules (MOD, S3M) or MIDI files, an IMF file contains no note abstractions, instruments, or waveform samples. Instead, it is a sequential stream of hardware register commands written directly to the Yamaha YM3812 chip:

• Register (1 byte): The OPL2 register index (e.g. 0x20 for operator characteristics, 0x40 for total level/attenuation, 0xA0 for frequency number low).
• Data (1 byte): The value byte to write into the specified register.
• Delay / Ticks (2 bytes LE): The number of timer interrupt ticks to wait before executing the next register-data command.

A 4-byte command packet is thus represented as:

[Reg: 1 byte] [Data: 1 byte] [Ticks: 2 bytes Little-Endian]

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2. The Game Clock Rates: 560 Hz vs 700 Hz

A common mystery for retro audio enthusiasts is why Wolfenstein 3D music played too fast or too slow when played in different source ports. The answer lies in the MS-DOS programmable interval timer (PIT):

1. Wolfenstein 3D & Spear of Destiny: The game loop hooked the timer interrupt at 560 Hz (each tick $\approx 1.786$ ms).
2. Commander Keen (Goodbye Galaxy & Aliens Ate My Babysitter): The music driver ran at 700 Hz (each tick $\approx 1.428$ ms).
3. Duke Nukem II: Used a slower timer tick rate of 280 Hz.

convrtr provides customizable clock rate presets so you can synthesize songs at their authentic intended tempos.

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3. Pure In-Browser Yamaha OPL2 Emulation

convrtr emulates the Yamaha YM3812 OPL2 architecture directly in TypeScript:

• Phase Generators & Sine Wave Transforms: Computes carrier and modulator frequencies from frequency number and octave blocks.
• Operator Envelope Generators: Models Attack, Decay, Sustain, and Release (ADSR) state machines for 2-operator FM pairs.
• Feedback & Feedback Modulation: Simulates self-feedback loops on operator 1 for rich brass and percussion textures.
• 16-bit Stereo PCM WAV Rendering: Blends the channels and encodes clean, lossless 44.1 kHz stereo audio.

All synthesis happens 100% client-side in your browser with zero external servers.