.MOD File Format

Introduction

The ProTracker module is the standard Amiga music module: a song with 31 samples played on 4 channels, the four voices of the Amiga’s sound chip. It holds the samples themselves, as 8-bit sound, and patterns of 64 rows that say which sample to play on each channel, at what pitch and with what effect, with an order list that strings the patterns into a song. The letters M.K. at offset 1080 mark it.

The layout first appeared in Soundtracker 2.2 or 2.3, in 1988 or 1989 (sources disagree), and became the standard through NoiseTracker (1989) and ProTracker (from 1990). Almost every tracker since, on the Amiga and the PC, reads and writes it. The versions after the first only added to it: NoiseTracker 1.1 gave the restart byte a meaning, ProTracker 1.0c put a finetune in the low half of a byte that had been zero, new effects took unused numbers, and ProTracker 2.3 added the signature M!K! for songs of more than 64 patterns. A reader of the first files reads the later ones, so all of them are this one format. No maker ever published a specification; the format is known from players’ source code, ProTracker 2.3D’s replay routine above all.

Files are named .mod. On the Amiga the name was often a prefix instead, as in mod.songname. Modules with other channel counts, StarTrekker’s 8-channel modules and the older 15-sample modules are separate formats.

A .mod file dropped on any page of this site opens in the explorer, which shows the header field by field, the samples, the order list and every pattern note by note, and plays the song as ProTracker 2.3D plays it.

Structure

A module is a fixed 1,084-byte header, then the patterns, then the sample data, with nothing between them. There are no chunks and no lengths to follow: the number of patterns comes from the order table, and the size of each sample from its header. All numbers are big-endian.

The number of patterns stored is one more than the highest pattern number in the order table, so the sample data of a module with patterns 0 to 9 starts at 1084 + 10 × 1,024 = 11,324. Each pattern cell gives a sample number, an Amiga period for the pitch and an effect; how a player turns all this into sound, tick by tick, is on How it plays.

Telling it apart

The explorer’s parser for this format checks two things: the file is at least 1,084 bytes long, and the four bytes at offset 1080 are one of the 4-channel signatures:

Everything after the signature is read as it comes. Problems such as a song length of 0, a volume above 64, a loop that runs past the end of its sample or a file cut short are reported, but are not grounds for turning the file down. A file with another signature at 1080 is turned down, and the explorer tries the parsers for the other module formats.

Files from unrelated programs are named .mod too, Go and Fortran module files among them. Without one of these signatures at 1080 a file is not read as this format, whatever its name.

Variants

• ProTracker module (M.K.) – 31 samples and 4 channels, marked M.K. at offset 1080: the standard Amiga module
• Multichannel module (xCHN, xxCH) – 31 samples and 1 to 32 channels, marked 6CHN, 8CHN, 16CH and so on
• StarTrekker 8-channel module (FLT8) – 8 channels, each pattern stored as two 4-channel patterns
• Soundtracker module (15 samples) – the first modules, 1987 to 1989: 15 samples and no signature

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