Converting Reality Adlib Tracker (RAD) to WAV: 9-Channel OPL2 FM Synthesis

In the mid-1990s, when PC soundcards were shifting toward sample playback, MS-DOS demoscene musicians celebrated the raw mathematical purity of frequency modulation chiptunes. The Reality Adlib Tracker (RAD), written by Reality (Jan Kiszka), was one of the most revered tools for composing multi-channel songs using the legendary Yamaha YM3812 (OPL2) sound chip.

Today, RAD modules are historical artifacts of computer music history. In this guide, we analyze the binary structure of .rad files and how convrtr emulates 9-channel 2-operator FM synthesis directly in your browser to render 16-bit stereo WAV audio.

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1. The AdLib YM3812 (OPL2) FM Sound Architecture

Unlike sample-based formats like MOD or XM that rely on pre-recorded PCM audio data, RAD songs store zero audio samples. Instead, all sounds are synthesized mathematically in real time by modulating carrier sine waves with modulator sine waves:

s(t) = Ac(t) * sin(2 * PI * fc * t + Im(t) * sin(2 * PI * fm * t))

Where:

• fc is the carrier frequency calculated from musical pitch.
• fm is the modulator frequency determined by the harmonic multiplier.
• Ac(t) is the carrier amplitude governed by an ADSR volume envelope.
• Im(t) is the modulation index controlling timbral brightness and harmonic richness.

Because only register numbers and note events are stored, a complete three-minute song typically occupies just 4 KB to 12 KB on disk.

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2. RAD Binary File Layout

A valid .rad file starts with a 16-byte ASCII signature followed by global playback parameters and instrument tables:

| Offset | Length | Description | |---|---|---| | 0x00 | 16 bytes | Magic string (RAD by REALITY or RAD by Reality!!) | | 0x10 | 1 byte | Format version (0x10, 0x11, 0x20, 0x21) | | 0x11 | 1 byte | Default speed (ticks per row, typically 6) | | 0x12 | 1 byte | Number of instruments (1 to 64) | | 0x13 | Variable | Instrument definitions (11 bytes OPL2 registers + optional name) | | Next | 1 byte | Number of orders in playback sequence | | Next | Variable | Order list (pattern indices) | | Next | Variable | Pattern event stream (64 rows x 9 channels) |

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3. Emulating OPL2 FM Synthesis in Pure TypeScript

To convert .rad into standard PCM WAV audio in the browser without plugins or native binaries, convrtr runs a lightweight OPL2 digital signal processing loop:

1. Pitch Calculation: Converts tracker note semitones into frequency in Hertz using standard equal temperament:

   const freq = 440 * Math.pow(2, (midiNote - 69) / 12);
   

2. ADSR Envelope Generator: Computes instantaneous attack, decay, sustain, and release levels for each operator on every audio sample.
3. Phase Accumulator: Advances carrier and modulator phases by angular frequency step (2 * PI * f) / sampleRate.
4. Channel Panning & Soft Clipping: Pans the 9 independent FM voices across a wide stereo field and maps floating-point amplitudes into 16-bit signed linear PCM values (-32768 to +32767).

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4. Converting RAD to WAV Online with Zero Uploads

With convrtr, you can listen to and preserve your vintage AdLib chiptunes instantly:

1. Drag your .rad file onto RAD to WAV.
2. Customize the output sample rate (up to 48,000 Hz) and stereo separation percentage.
3. convrtr synthesizes all 9 OPL2 channels in memory and generates an uncompressed 16-bit WAV file ready for playback and archival.