48 lines
2.8 KiB
Markdown
48 lines
2.8 KiB
Markdown
Sound in SM64 consists of three parts: samples, sound banks, and sequences.
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Samples represent raw sound data, given as AIFF files with a custom ADPCM-based
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compression format that reduces file sizes by ~70% compared to uncompressed
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AIFF (with 16-bit samples). The build system automatically converts
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uncompressed AIFF files into this format.
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Samples are collected into banks, given by directories. The order in which each
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bank's samples end up in the final binary is determined by their file names.
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Thus, to keep the ROM OK, sample files should be prefixed by a number to
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maintain sort order. (Bank directories are ordered by their usages in way
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that automatically makes the ROM match.)
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Samples cannot be referred to directly from sequence files. Instead, there is
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an indirect step in the form of sound banks. Each sound bank refers to a single
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sample bank, and amends it with extra information in the form of a JSON file.
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This JSON file contains a set of "instruments", which describe how note values
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(pitches) map to samples, which pitch corrections to perform, and what ADSR
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envelopes to use. The `instrument_list` key maps indices to instruments; these
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indices are what sequence files can refer to. A special instrument `percussion`
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is also supported, as an array with (usually) 64 different entries, covering
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note values 0..63. It is referred to as instrument ID 0x7f.
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C-style comments are supported in the JSON. Similar to samples, the sound banks
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come in alphabetical order in the ROM, so they should be prefixed to maintain
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sort order.
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Sequence files are what actually controls the audio. The are in .m64 format,
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which is similar to MIDI, but Turing complete. An .m64 file has a sequence
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script that can spawn channels, which have channel scripts that can spawn
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layers, which have layer scripts that can play notes. Each note is performed
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using an instrument from a sound bank. A sequence file can use multiple banks;
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`sequences.json` describes the mapping from sequences to sound banks. Channels
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can switch between banks using a command. However, in practice most sequences
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limit themselves to a single sound bank. The main exception is sequence 0,
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which is responsible for sound effects.
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Like samples and sound banks, sequence files are included in the ROM in
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alphabetical file name order. They can be located in either `sound/sequences/`
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or `sound/sequences/<version>/`, and can optionally be given in disassembled
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form -- see `include/seq_macros.inc` for more details on the format.
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The repo gitignores .m64 and .aiff files by default, unless they include
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"custom" somewhere in the name (including in a directory name). Thus, for new
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custom-made samples and sequences it is advisable to include that substring
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in the file name (this also helps distinguish custom sounds from ones from
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the game). `git add -f` also works for adding edited existing files to git.
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