Converting Composer 669 (.669) to WAV: 8-Channel DOS Tracker Synthesis

In the genesis years of the IBM PC demoscene in 1992, tracker music was heavily dominated by the Commodore Amiga's 4-channel ProTracker format. However, as 386 and 486 PCs equipped with Creative Sound Blaster cards became widespread, PC demoscene composers demanded more channels, faster tempo controls, and dedicated artist metadata.

Enter Composer 669, created in 1992 by polish programmer and demoscener Tomasz Pytel (Tran of Renaissance) alongside UNIS 669. Modules saved in this format carry the .669 extension. Composer 669 established an 8-channel standard on MS-DOS, featuring 64-row pattern matrices, an embedded 108-byte text notepad, and raw 8-bit unsigned PCM sample banks.

This technical guide unpacks the binary architecture of .669 files, details how its 8-channel voice registers and playback lists function, and demonstrates how convrtr's 669 to WAV synthesizer renders vintage DOS chiptune modules into 16-bit 44.1kHz stereo WAV audio entirely within your browser memory.

The 669 File Header Specification

Every Composer 669 file starts with a fixed-format header defining global song parameters:

| Offset | Size (Bytes) | Field Name | Description | | :--- | :--- | :--- | :--- | | 0x00 | 2 bytes | Signature Tag | Magic bytes "if" (0x69 0x66) or "JN" (0x4A 0x4E) | | 0x02 | 108 bytes | Song Message | 3 lines of 36 ASCII characters each | | 0x6E | 1 byte | Sample Count | Number of active instruments (up to 64) | | 0x6F | 1 byte | Pattern Count | Total unique pattern matrices (up to 128) | | 0x70 | 1 byte | Loop Order | Order index where song loops after completion | | 0x71 | 128 bytes | Order List | Pattern playback sequence (0xFF indicates stop) | | 0xF1 | 128 bytes | Tempo List | Playback speed for each order position | | 0x171 | 128 bytes | P-Break List | Pattern break row for early order advancement |

The "if" and "JN" Signatures

Composer 669 files use two primary magic headers:

• if (0x69 0x66): The standard signature found in original Composer 669 files created with early versions of Tomasz Pytel's tracker.
• JN (0x4A 0x4E): The extended signature used by UNIS 669 and Renaissance tracker releases, introducing refined pitch calculation tables and smoother note slides.

The 108-Byte Artist Message Block

Rather than hiding author information in empty sample names, Composer 669 provides three dedicated 36-character lines (108 bytes total). These lines were typically used for the song title, artist handle, and demogroup affiliation:

Offset 0x02: "EUPHORIA IN C MINOR                 "
Offset 0x26: "COMPOSED BY TRAN / RENAISSANCE 1992 "
Offset 0x4A: "GREETINGS TO ALL DEMOMAKERS         "

Null bytes (\0) inside this block terminate each string, with the remainder of each line padded with ASCII spaces (0x20).

Sample Descriptors and Waveform Storage

Directly after the p-break table at offset 0x1F1, the file provides descriptor records for 64 instruments. Each descriptor is exactly 25 bytes in length:

| Offset | Size (Bytes) | Field Name | Description | | :--- | :--- | :--- | :--- | | +0x00 | 13 bytes | Filename | Null-terminated 8.3 DOS filename of sample | | +0x0D | 4 bytes (LE) | Sample Length | Total PCM length in bytes (up to 64KB) | | +0x11 | 4 bytes (LE) | Loop Start | Byte offset where sample loop begins | | +0x15 | 4 bytes (LE) | Loop End | Byte offset where loop ends (0xFFFFFFFF if non-looping) |

8-Bit Unsigned PCM Audio

Following all pattern matrices, the file stores raw waveform data. Unlike Amiga MOD files which use signed 8-bit PCM (where zero is represented by 0x00 and values range from -128 to +127), Composer 669 samples are stored as 8-bit unsigned PCM (where silence is 0x80, maximum negative is 0x00, and maximum positive is 0xFF), matching the native DAC expectations of Sound Blaster DSP hardware.

When converting 8-bit unsigned PCM to 16-bit signed audio:

const unsignedByte = sampleData[offset];
const signedSample = (unsignedByte - 128) << 8;

Pattern Matrices: 8-Channel Polyphony

Composer 669 patterns are organized into 64 rows across 8 independent polyphonic channels. Unlike formats with variable row counts, 669 patterns have a fixed size:

Pattern Size = 64 rows * 8 channels * 3 bytes = 1,536 bytes

Each 3-byte cell represents a single channel event:

Byte 0: [ N5 N4 N3 N2 N1 N0 I5 I4 ]  (6 bits Note, 2 bits Instrument MSB)
Byte 1: [ I3 I2 I1 I0 V3 V2 V1 V0 ]  (4 bits Instrument LSB, 4 bits Volume)
Byte 2: [ E3 E2 E1 E0 P3 P2 P1 P0 ]  (4 bits Effect command, 4 bits Parameter)

• Note (6 bits): Values 0 to 59 represent 5 octaves of musical semitones. A value of 0x3F (63) indicates no note event.
• Instrument (6 bits): 0 to 63 indicating the sample number to trigger.
• Volume (4 bits): Direct volume setting from 0 to 15, mapped linearly to standard 0–64 channel volume.
• Effect & Parameter: Commands such as porta up (1), porta down (2), tone portamento (3), and frequency vibrato (4).

Real-Time Audio Synthesis in convrtr

To render a .669 file into a WAV audio file, convrtr performs client-side software mixing:

1. Timer Resolution: Order positions are processed using the order-specific tempo array, converting tempo ticks into exact millisecond intervals.
2. Channel Mixing: For each audio frame, all 8 channels compute their current sample index based on the note's playback frequency:

frequency = 8363 * Math.pow(2, (note - 24) / 12)

3. Stereo Panning: Channels 0, 2, 4, 6 pan left (70% left / 30% right), while channels 1, 3, 5, 7 pan right (30% left / 70% right), recreating classic PC stereo soundcard separation.
4. WAV Header Generation: Mixed floating-point audio accumulators are clamped, converted to 16-bit signed integers, and packaged into a standard RIFF/WAVE header container.

All processing runs entirely in client-side TypeScript within the browser sandbox, guaranteeing that your demoscene module archive never leaves your device.