MOD Format

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:

Process effect for each active channel (volume slide, portamento, etc.)
Every speed ticks, advance to the next row
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.

The tracker music revolution