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MOD refers to 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.

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 almost exactly the same from one player to another.

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" and "Oktalyzer" being written from scratch. Such programs are called trackers in general.

Table of contents
1 Popular formats

Popular formats

Each format builds on concepts introduced in its predecessors.

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

The format that started it all. Uses inverse-frequency note numbers. Up to 8 voices. Pattern data is not packed. Instruments are simple volume levels; samples and instruments correspond one-to-one. Up to 31 instruments. 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 FastTracker)
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.