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If you're looking for game modifications and rom hacks, see Game Mod. If you're looking for the 1960s subculture, see Scooter Riding Mod. If you're coming here via an accidental wick from the abbreviation for the UK's Ministry of Defense, see Brits With Battleships.
MOD (tracker MODule) formats are a hybrid of sorts. They combine note data (like a MIDI file) with digital audio samples (typically in a format similar to WAV). To continue the analogy used on those pages: if a MIDI file is a piece of sheet music, and a WAV file an orchestra, then a MOD file is a piece of sheet music bundled with a specific trumpet for use in playing the piece.
The principle used to generate notes from the samples has been around since at least the early 1960s, where it was used by the BBC Radiophonic Workshop to create the first versions of the Doctor Who theme music. The folks at the BBC recorded individual notes on magnetic tape and generated other notes by changing the playback speed. Faster playback speeds produced higher notes and slower ones produced lower notes. By splicing together segments of tape containing these generated notes and mixing the results together, the first version of the Doctor Who theme was brought to life.
Now, what does that have to do with music on a computer? Well, when a computer plays back music in MOD format, it's essentially doing what the folks at the BBC Radiophonic Workshop did by hand over 25 years earlier. The note data in a MOD file tells the computer how fast and how loud to play back the sample data (this is a simplification, but for an introduction it will suffice).
MOD formats originated in the late 1980s on the Amiga, making use of the rather smart Paula chip in the computers. This sound chip was able to play four streams of audio at once, each with a different sample rate. Unsurprisingly, early Amiga MOD formats were limited to four channels (later software tricks boosting this to eight channels), but after migration to the PC, where more processing power was available, variations on the original format and many new formats with more channels cropped up. The generic name comes from the file extension of one of the first and most popular formats.
MOD formats have been fairly popular for use with games since the 1990s as they give a good balance between quality and file size. Several examples of games with MOD format music include Jazz Jackrabbit, Death Rally, Epic Pinball, Terminal Velocity, Star Control II, Unreal, Unreal Tournament, Deus Ex, the Crusader franchise and Seiklus.
The Demoscene has extensively used MOD formats and a number of them have also originated with Demoscene groups, with one of the best-known being S3M (Scream Tracker 3 Module). There are archive sites which have literally thousands of pieces of music in various MOD formats.
Unlike standards like General MIDI and MP 3, "MOD" refers to an entire family of formats, most of which are not well-documented, having evolved organically from the early SoundTracker format. Thanks to this, it is often difficult to determine whether any given MOD file will be playable with any given player except by experimentation. Many (but not all) of the various subformats are broadly cross-compatible within certain well-known subgroups, but even then, there are (usually) minor incompatibilities which are not always obvious. This is among the reasons that, even where MOD formats remain popular for composing music, the file is nowadays often rendered into a non-tracked format such as MP 3 for distribution.
The basic idea still exists for video games in general today, as nearly every game system from the SNES on up includes a wavetable sound chip, relieving the processor of the burden of mixing the channels itself and freeing it to spend that time on other things.
- ↑ Notable exception: the Game Boy Advance has digital audio output, but only one stereo channel, so the game engine has to do any sound mixing itself. This is why early and low-budget GBA titles tend to have fewer channels at lower quality, and why so many games make use of the included original Gameboy sound chip.