Format Content
A lot of audio formats have similar structural contents that can be grouped into the following categories. These categories aren't highly technical, and some formats could easily be classified in multiple categories, but for sanity's sake, they are placed in the category that best describes their content.
Notational
Notational content is structured like sheet music. Because of this, the format can easily be adjusted to play on any type of instrument or even a different output hardware all together. The downside of notational content is that it's difficult to get it to sound similar across various devices and usually requires special sound drivers for each. This includes formats like MIDI and MUS as well as MOD-like formats with built-in instruments like XM and RMI.
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Instructional
Instructional content is that which contains program instructions for a specific audio chip. Log formats are very similar to instructional formats, however, instructional formats usually have additional attributes like loop instructions and meta data that is lost in the logging process. Examples include ROL, CMF, and IMF.
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Log
Formats with logged content are usually not created by developers, but by people who want to easily rip a game's music. The instructions that are sent to an audio chip are captured in a log and saved to a file so that it may be sent to the chip again at any time. While this yields an identical sound to the original format (provided it is sent to the same chip), it loses all of the original structure of the format. Instructions like loop pointers, metadata, and any notational structure will not be found in log files. Examples include VGM, SPC, and DRO.
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Programmatic
Formats with programmatic content are those which house extracted machine code and data from a program targeted to a specific CPU and audio chip, both of which are required for proper audio playback. Examples include NSF, SID, and AY.
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PCM
PCM content is found in all digital audio formats regardless of whether they are compressed. Examples include WAV, OGG, and MP3.
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Instrument
Formats that contain instrument definitions are in this category. Examples include BNK, SYX, and PATCH.
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Unknown
Those formats that have yet to have their content identified have an unknown structure.
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N/A
This is for formats that are too broad to have their content categorized. Examples include MML.
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