Techniques & Technology

MOD Format

The tracker music revolution

The MOD format combined samples with pattern-based sequencing in a single self-contained file, becoming the standard for game and demo music on the Amiga and across the chiptune scene.

commodore-amigaibm-pcaudiotrackerdemo-sceneformatprotracker1987–present

Overview

The MOD format, introduced with Ultimate Soundtracker (Karsten Obarski, Amiga, 1987) and refined into ProTracker, revolutionised game and demo music. It stores both digitised audio samples and the pattern data that tells the player when to play them — a self-contained music format far smaller than recorded audio. A typical 4-channel MOD is 50-200 KB; a comparable WAV recording would be megabytes.

The format’s simplicity made it portable. Within a few years of the Amiga release, MOD players existed for PC, Atari ST, Acorn Archimedes, and even Linux — anywhere with a sample-capable sound chip. Demo coders, BBS sysops, and bedroom musicians spread MODs across modems and floppies; the format became the default soundtrack of the demoscene.

Fast facts

  • Origin: Ultimate Soundtracker (Amiga, 1987); refined by ProTracker (Amiga, 1990).
  • Channels: 4 in the canonical MOD (one per Paula channel); extensions add more.
  • Samples: 8-bit PCM, up to 31 instruments per file (15 in the earliest variants).
  • Patterns: 64 rows × 4 channels of note data, indexed and chained into a song sequence.
  • Effects: 16 effect commands per note (volume slide, portamento, vibrato, etc.).
  • Typical size: 100 KB – 1 MB.
  • Extensions: S3M (Scream Tracker), XM (FastTracker II), IT (Impulse Tracker) — each adding channels, finer envelopes, more samples.

File structure

A standard 31-instrument 4-channel MOD has a fixed layout:

Offset Size Field
$0000 20 bytes Song name (zero-padded ASCII)
$0014 31 × 30 bytes Instrument records (see below)
$03B6 1 byte Number of song positions (1-128)
$03B7 1 byte Restart byte (legacy field, usually 127)
$03B8 128 bytes Pattern order table (which pattern plays at each song position)
$0438 4 bytes Format signature: M.K. (4-channel), 4CHN, 6CHN, 8CHN, etc.
$043C N × 1024 bytes Pattern data (N = highest pattern index + 1)
(after) variable Sample data — raw 8-bit signed PCM

The 1084-byte ($043C) header is followed by patterns and then samples. Older 15-instrument MODs lack the format signature and start at offset $01D8.

Instrument record (30 bytes)

Offset Size Field
0 22 Instrument name (zero-padded ASCII)
22 2 Sample length in words (so byte length = ×2)
24 1 Finetune (-8 to +7, signed nibble)
25 1 Volume (0-64)
26 2 Loop start in words
28 2 Loop length in words (1 if no loop)

All multi-byte values are big-endian (Motorola 68000 native).

Pattern data (1024 bytes per pattern)

Each pattern is 64 rows × 4 channels × 4 bytes:

Row 0: ch0 ch1 ch2 ch3
Row 1: ch0 ch1 ch2 ch3
...
Row 63: ch0 ch1 ch2 ch3

Each cell is 4 bytes encoding the note, sample, and effect:

   Byte 0          Byte 1          Byte 2          Byte 3
+------------+ +------------+ +------------+ +------------+
| sample.hi  | | period.hi  | | sample.lo  | | effect.hi  |
| period.hi  | | period.lo  | | effect.hi  | | effect.lo  |
+------------+ +------------+ +------------+ +------------+

Decoded:
- sample number: bits (byte0 & $F0) | (byte2 >> 4)
- period:        ((byte0 & $0F) << 8) | byte1
- effect command: (byte2 & $0F)
- effect param:   byte3

The period is the Paula playback period — directly written to AUDxPER. Smaller period = higher pitch. A note table maps musical notes to periods based on the sample’s finetune.

Effect commands

The effect nibble selects one of 16 commands:

Code Command Behaviour
0 Arpeggio Cycle between three notes within the row
1 Slide up Decrease period (higher pitch)
2 Slide down Increase period (lower pitch)
3 Tone portamento Glide toward target note
4 Vibrato Sine modulation of period
5 Tone portamento + volume slide
6 Vibrato + volume slide
7 Tremolo Sine modulation of volume
8 (unused in original)
9 Set sample offset Skip into sample by param × 256
A Volume slide Add/subtract from volume
B Position jump Jump to song position
C Set volume Direct volume set
D Pattern break Jump to next pattern at row param
E Extended Sub-effects in upper nibble of param
F Set speed/tempo Speed (1-31) or BPM (32-255)

Paula playback

A MOD player runs at the pattern’s tempo (default 50 ticks/second on PAL Amiga, divided by the speed value). Each tick:

  1. Process effect for each active channel (volume slide, portamento, etc.)
  2. Every speed ticks, advance to the next row
  3. New row: load sample, period, volume into AUDxLC/AUDxLEN/AUDxPER/AUDxVOL, enable DMA

The four MOD channels map directly to Paula’s four DMA channels:

MOD ch Paula ch Stereo side
1 0 Left
2 1 Right
3 2 Right
4 3 Left

This stereo split is hardwired in Paula and gives MODs their distinctive “channels 1+4 left, 2+3 right” panning. Some players add a software mixer to soften it.

Format extensions

Format Year Origin Key additions
MOD 1987 Ultimate Soundtracker (Amiga) Original
S3M 1993 Scream Tracker 3 (PC) Up to 32 channels, multiple effect columns, GM/MIDI mapping
XM 1994 FastTracker II (PC) Volume/panning envelopes, instrument multi-sample, up to 32 channels
IT 1996 Impulse Tracker (PC) New volume column, NNA (new note actions), 64 channels, finer effects

Modern players (libxmp, OpenMPT) handle all four, often with sample-accurate playback. The chiptune scene continues to produce work in all these formats.

See also