Amiga MOD Tracker Format

Overview

The Amiga MOD file format is one of the earliest formats for creating and playing music on personal computers. Originally developed for the Commodore Amiga in the late 1980s, it stores music created using tracker software, which combines pattern-based sequencing with embedded audio samples.

MOD files encapsulate everything needed to play back a song: samples (instruments), patterns (rows of note events), effects (vibrato, portamento, volume slides, etc.), and playback order. The format became a foundational building block for the demoscene, game audio, and later chiptune and retro-inspired music.

History

1987 – The MOD format was introduced with Ultimate Soundtracker on the Amiga.
1989–1990 – Popularity exploded with the release of ProTracker, which standardized the format into what is now called the “M.K.” MOD format (4 channels, 31 instruments).
1990s–present – Variants with more channels and features appeared (e.g. ScreamTracker, FastTracker, IT, XM), but the classic MOD format remains in use for its Amiga compatibility and retro authenticity.

Tracker Programs

Original Amiga Trackers

Ultimate Soundtracker – The original tracker, limited features but groundbreaking.
ProTracker – The gold standard; introduced the 4-channel “M.K.” format.
OctaMED – Supported 8-channel playback using CPU mixing.

Modern Trackers and Tools

OpenMPT (Open ModPlug Tracker) – Powerful Windows-based tracker that supports MOD and many other formats.
MilkyTracker – Cross-platform clone of FastTracker II with full MOD support.
SchismTracker – Modernized clone of Impulse Tracker with MOD import/export.
Deflemask – Multi-chip tracker supporting Amiga MOD among other vintage formats.
libxmp – A portable C library for MOD/XM/S3M/IT playback, used in many players and emulators.
Renoise – A modern digital audio workstation (DAW) that evolved from the tracker paradigm.

Advantages of MOD Files

Authenticity – Perfect for retro Amiga-style games or demos.
Portability – Self-contained, minimal overhead.
Efficiency – Extremely small file sizes (compared to WAV or MP3).
Simplicity – Easy to parse and understand compared to full MIDI or DAWs.

Format Specification (ProTracker 4-Channel)

==============================================================
                AMIGA MOD FILE FORMAT SPECIFICATION
                (ProTracker-compatible, 4 channels)
==============================================================

A standard ProTracker MOD file is a binary file with the following layout:

--------------------------------------------------------------
| Offset | Length | Description
--------------------------------------------------------------
| 0x000  | 20     | Song title (ASCII, null-padded)
--------------------------------------------------------------
| 0x014  | 30 × 31 = 930 bytes | Sample headers (31 instruments)
         Each sample header has the following layout:
         ------------------------------------------------------
         | Offset | Length | Field
         ------------------------------------------------------
         | 0x00   | 22     | Sample name (ASCII)
         | 0x16   | 2      | Sample length (in words = 2 bytes, so ×2 for bytes)
         | 0x18   | 1      | Finetune (4-bit signed nibble stored as byte: 0–15)
                           |   (0–7 = 0 to +7 semitones, 8–15 = -8 to -1)
         | 0x19   | 1      | Volume (0–64)
         | 0x1A   | 2      | Loop start (in words)
         | 0x1C   | 2      | Loop length (in words)
         ------------------------------------------------------
         (Total = 30 bytes per sample × 31 samples = 930 bytes)
--------------------------------------------------------------
| 0x3B6  | 1      | Song length (number of positions to play, 0–127)
| 0x3B7  | 1      | Restart position (usually 0)
--------------------------------------------------------------
| 0x3B8  | 128    | Pattern order table (sequence of pattern numbers)
--------------------------------------------------------------
| 0x438  | 4      | Signature (identifies format):
         - "M.K." = 4-channel MOD (ProTracker)
         - "M!K!" = alternate ProTracker
         - "6CHN", "8CHN", etc. = more channels
         NOTE: "M.K." is the most common Amiga-compatible MOD.
--------------------------------------------------------------
| 0x43C  | n × 1024 | Pattern data
         Each pattern is 64 rows × 4 channels × 4 bytes = 1024 bytes
         Each "note" is 4 bytes, packed as follows:
         ------------------------------------------------------
         Byte 0: Upper 4 bits of sample number + upper 4 bits of period
         Byte 1: Lower 8 bits of period
         Byte 2: Lower 4 bits of sample number + upper 4 bits of effect
         Byte 3: Effect parameter
         ------------------------------------------------------
         Notes are stored row-major: all 4 channels for row 0, then row 1, etc.
         Number of patterns is inferred from the highest index in the pattern table.
--------------------------------------------------------------
| After patterns | Sample data for all instruments
         The sample data is raw signed 8-bit PCM, one block per sample
         Length of each sample = sample length × 2 bytes (from header)
--------------------------------------------------------------

==============================================================
                    DETAILS AND NOTES
==============================================================

- PERIOD TABLE:
  Notes are encoded as "periods" (inverse of frequency). Common values:
    C-3 = 428, C#3 = 404, D-3 = 381, ..., C-4 = 214, etc.
  There is a standard lookup table to convert periods to notes.

- EFFECT COMMANDS:
  The effect command is 1 hex digit, and the parameter is 2 hex digits.
  Examples:
    0xy – Arpeggio
    1xy – Portamento up
    2xy – Portamento down
    3xy – Tone portamento
    4xy – Vibrato
    9xx – Sample offset
    Axy – Volume slide
    Bxx – Jump to order
    Cxx – Set volume
    Dxy – Pattern break
    Fxx – Set speed (<= 32) or tempo (> 32)

- SAMPLE DATA:
  - Each sample is raw 8-bit signed PCM audio.
  - Sample length and loop values are in *words* (2 bytes).
  - Volume is 0–64. Finetune is signed 4-bit (0–15).

- COMPATIBILITY:
  - "M.K." format is standard for Amiga MODs (4 channels, 31 samples).
  - File size is variable: grows with number of patterns and sample data.
  - Real Amiga hardware and ProTracker clones expect strict adherence to the format.

==============================================================
                    COMMON LIMITATIONS
==============================================================

- Max samples: 31
- Max patterns: 64 (though pattern table can index up to 128)
- Max channels: 4 (for "M.K." format; others exist but not Amiga-safe)
- Sample size per instrument: up to 64 KB (in bytes)
- No support for stereo, 16-bit, or compressed samples
- All samples are inline; no external sample banks

==============================================================
                    EXAMPLE PATTERN ENTRY (HEX)
==============================================================
Example 4-byte note:
  0x48 0x71 0x37 0x0F
Split:
  Byte 0: 0x48 → 0100 1000
     → Sample high nibble = 4
     → Period high nibble = 8
  Byte 1: 0x71 → Period low byte = 0x71
     → Period = 0x871 = 2161 (somewhere between C-4 and B-3)
  Byte 2: 0x37 → Sample low nibble = 7
     → Sample = (4 << 4) | 7 = 0x47 = 71
     → Effect = 3
  Byte 3: 0x0F → Effect param = 0F
     → Full effect command: 3 0F = Tone portamento with param 0F

==============================================================
                    END OF SPECIFICATION
==============================================================