 Description
The id music format (the name is a guess) is an AdLib audio structure developed by id Software. It's was designed to be play via FM Synthesis on an AdLib card. In fact, the format is actually very similar to to the AdLib command syntax because it stores the raw data that needs to be sent to the OPL chip. The Winamp plugin AdPlug gives the most accurate playback of IMF.
There are two types of IMF. The first games to use IMF format are in what is now called Type-0. Type-0 IMF doesn't include any header information and is sent straight to the OPL chip. Later games use Type-1 which has a short header that includes the length of the song, and allows for meta data in a footer.
IMF has one major flaw due to the fact that different games play IMF files at different speeds, but the playback rate is not part of the IMF. Because of this, extra precautions must be made when playing back IMF files. Bio Menace, Commander Keen, and Monster Bash all play at 560 Hz, but Cosmo's Cosmic Adventures, Wolfenstein 3-D, and games using the Wolf 3D engine (Spear of Destiny, Blake Stone 1 & 2, Operation Body Count, and Corridor 7) all run at 700 Hz. Then, there's Duke Nukem II which runs at 280 Hz. To fix this problem, the Wolf 3D games are given the *.WLF extension and Duke Nukem II delays are simply doubled to match the 560 Hz tempo.
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