.MOD File Format

Introduction

The Soundtracker module is the first tracker module. Ultimate Soundtracker, written by Karsten Obarski and sold by EAS Computer Technik for the Commodore Amiga from 1987, saved songs in it, and so did the Soundtrackers that followed it: TJC Soundtracker II, the D.O.C. Soundtrackers III to IX and 2.0, Master Soundtracker and others, until the 31-sample layout replaced it in 1989. A module holds a song title, 15 samples of 8-bit sound, an order list and patterns of 64 rows by 4 channels.

It differs from the later ProTracker module in what is already there, not only in what was added: 15 sample headers instead of 31, so everything after offset 470 sits elsewhere; a tempo in the byte after the song length; a loop start counted in bytes rather than words; and no signature. It is therefore a separate format. The versions of Soundtracker differ in their effects too, but not in the layout.

Files are named .mod, or on the Amiga with a mod. prefix, as in mod.songname; OpenMPT also uses .stk for them. Sample names often carry the name of the Soundtracker sample disk they came from, as in st-02:snare14.

A .mod or .stk 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.

Structure

A 600-byte header, then the patterns, then the sample data, with nothing between them. All numbers are big-endian.

Telling it apart

There is no signature, so the explorer’s parser recognises the format by a header that makes sense, following the checks of OpenMPT’s Soundtracker loader. Before it looks, the file must be named like a module (.mod, .stk, or a name starting mod.) and be at least 1,624 bytes long, the header and one pattern. Then:

• the four bytes at 1080 are not a known 31-sample signature such as M.K.;
• every sample volume is 64 or less, and every sample length at most 37,000 words;
• the title and sample names are mostly text: bytes that are neither zero nor printable ASCII are counted, a finetune byte that is not zero counts 16, and a total above 48 turns the file down;
• the song length is 1 to 128, and the tempo byte at most 220;
• all 128 order entries are 63 or less;
• at least one sample has both data and a volume;
• no more than 16 of the 256 cells of the first pattern are not Soundtracker notes (a sample number above 15, or a period outside 100 to 1,800).

A file that fails any check is turned down. Files named .mod that are not modules at all, such as Go and Fortran module files, fail these checks.

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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