MOD Audio

A MOD file, or module, is the Amiga tracker format: it carries its own instrument samples and a pattern-based score that sequences them. Because the sounds travel with the song, a module plays consistently on any machine that can mix samples, which is why the format spread far beyond the Amiga.

MOD Audio

Overview

A module has three parts: a set of samples (the instruments), a list of patterns (the score), and an order list that says which patterns play in what sequence. A player steps through the order list, reads each pattern row by row, and triggers samples on the channels. The classic ProTracker MOD has up to 31 samples and four channels; later trackers raised both.

Property	Value
Channels	4 (classic), 6, 8 and more in variants
Samples	Up to 31 instruments
Sample format	8-bit signed PCM with loop
Pattern	64 rows by channel count
Structure	Samples, patterns and an order list

Samples and instruments

Each instrument is a digitised sample stored in the file, the same 8-bit signed PCM the Amiga plays natively. A sample carries a length, an optional loop start and length for sustained notes, a default volume and a finetune value that nudges its pitch. Playing one recording at different periods gives different notes, so a single sample covers a whole instrument's range.

Field	Role
Sample data	8-bit signed PCM.
Length	Sample length in words.
Loop start / length	Region repeated to sustain a note.
Volume	Default level (0 to 64).
Finetune	Small pitch adjustment.

Patterns and rows

A pattern is a grid of 64 rows by one column per channel. Each cell holds a note (as a period or note name), the sample number to play, and an effect command with a parameter. Reading down the rows at the current speed plays the music, and an empty field leaves that channel as it was. The order list chains patterns into the full song and can repeat them.

Cell field	Role
Note	Pitch as a period, or blank.
Sample number	Which instrument to trigger.
Effect	Command applied on this row.
Parameter	Argument for the effect.

Effect commands

Effects are what make trackers expressive. Each is a single command with a byte parameter, applied per channel per row: pitch slides, vibrato, volume changes, sample offset, pattern jumps, and a speed and tempo control. Most are processed on every tick between rows, so a slide or vibrato moves smoothly rather than jumping once per row.

Effect	Role
Arpeggio (0)	Rapidly cycle two extra notes for a chord effect.
Portamento (1, 2, 3)	Slide pitch up, down, or toward a target note.
Vibrato (4)	Waver the pitch around the note.
Volume slide (A)	Ramp the channel volume up or down.
Sample offset (9)	Start the sample partway in.
Position jump / break (B, D)	Move within the order list or pattern.
Set speed / tempo (F)	Change ticks per row, or the beats per minute.

Channels and variants

The four-channel ProTracker MOD is the baseline, mapped onto the Amiga's four hardware voices, two per side. Later trackers on the PC widened the format: six, eight and more channels, and the XM, S3M and IT formats added more samples, instruments with their own envelopes, and finer effects. All of them keep the same idea of samples plus patterns, so the MOD is the common denominator the others grew from.

Format	Notes
MOD (ProTracker)	4 channels, up to 31 samples, the original.
Multichannel MOD	6, 8 or more channels.
XM, S3M, IT	More channels and samples, instrument envelopes, richer effects.

Sequencer role

A module maps onto the sequencer directly. The note-and-sample in a cell is a one-shot trigger, and the effect column is per-tick automation, the same latched-versus-driven split the machines show, but expressed in one portable format that carries its own sounds.

Cell part	Kind
Note and sample	Trigger (one-shot)
Effect column	Driven every tick