MOD is a class of file format used to represent music on a computer. MOD files store several patterns or pages of music data in a form similar to that of a spreadsheet. These patterns contain note numbers, instrument numbers, and controller messages. The number of notes that can be played simultaneously depends on how many tracks there are per pattern. Originally, all modules had 4 tracks per pattern, to directly correspond with the Amiga sound chips' four channels.

In a MOD file the minimum time frame is 0.02 seconds, because the original software used the VSync of the monitor running at 50 Hz for timing. Every track has up to 64 rows, each row lasts 0.02 seconds times the current speed setting.

MOD files also give a list of the order in which to play the patterns. However, the biggest advantage of MOD family over standard MIDI files is that MOD includes its own audio samples and should sound exactly the same from one player to another barring interpolation methods and eventual errors in players.

MOD files are often referred to as tracker modules, and composing modules is known as tracking, simply because the first ever module creating program was Soundtracker , created by Karsten Obarski in 1987. Soundtracker was cloned many times, with programs such as Noisetracker , and Protracker being direct descendants from the original Soundtracker code, and others such as MED/OctaMED and Oktalyzer being written from scratch. Such programs are called trackers in general.

Popular formats

Each format builds on concepts introduced in its predecessors.

Sound/Pro/Noisetracker module (file extension: .mod) (originated on Amiga computers)

The format that started it all. Uses inverse-frequency note numbers. 4 voices, up to 32 in later variations of the format. Pattern data is not packed. Instruments are simple volume levels; samples and instruments correspond one-to-one. 15 instruments in the original Soundtracker, 31 in later trackers. This format was originally created to be easily playable with the Amiga hardware. The CPU has to do very little work to play these modules on an Amiga.

Oktalyzer (originated on Amiga computers)

This was an early effort to bring 8 channel sound to the Amiga.

MED/OctaMED (originated on Amiga computers)

This format is very similar to sound/pro/noisetracker, but the way the data is stored is different. MED was not a direct clone of soundtracker, and had different features and fileformats. OctaMED was an 8 channel version of MED, which eventually evolved into OctaMED soundstudio (which offers 128 channel sound, MIDI support and lots of other high-end features).

AHX (originated on Amiga computers)

This format is a synth-tracker. That is, there are no samples in the module file, rather descriptions of how to synthesize the required sound. This results in very small audio files (AHX modules are typically 1k - 4k in size), and a very characteristic sound. AHX is designed to sound as much like a Commodore 64 as possible.

.s3m (originated in ScreamTracker version 3 for PC)

Uses MIDI-like note numbers. Up to 16 or more voices. Samples can specify any playback frequency for middle c. Simple run-length packing of pattern data. Introduction of several new controllers and a dedicated "volume column" in each voice to replace volume controllers. Predictable support for stereo panning.

.xm (originated in Fast Tracker)

Introduction of instruments with volume and panning envelopes. Basic sample compression.

.it (originated in Impulse Tracker; not to be confused with the country code for Italy)

New Note Actions let the beginning of one sound in a voice. Instruments can now share a sample. Adds some new effects such as a resonant filter . Better sample compression.

.ned (Nerd Tracker II)

Designed for playback on Nintendo Entertainment System. No samples in basic format (just tone generator instrument specification); extended format uses compressed samples but limits playback frequencies to the 16 rates that the NES hardware is capable of reproducing. Each channel has its own order list.

Software

• Winamp (preferably with the Oldsk00l [1] plugin, the internal player is not very good)
• dbpoweramp
• ModPlug Tracker
• MadTracker
• Renoise
• Unix Amiga Delitracker Emulator plays most of the Amiga music formats on UNIX platforms (open source)
• XMP plays most mod variant formats on UNIX platforms (open source) [2]
• MikMod [3]
• Deliplayer for Windows supports almost all module formats for Amiga and other oldschool machines. [4]
• XMPlay

External links

• Amiga Music Preservation - Tens of thousands of Amiga music modules to download
• Exotica - Lots of music modules, dedicated to "unusual" tracker formats from the Amiga
• Chiptune.com - Lots of chiptunes
• Mod Archive - Not the largest mod file site but seems to be the hub for new music
• Scene Music - "Nectarine" - An internet radio station playing modules
• Aminet's MOD Archive - AmiNet archive's mod file section - no longer updated but was the hub of the modscene for years
• Modland - FTP server with more than 100000 modules in several formats, mostly MOD, XM, S3M and IT, sorted on author.