SGI Audio Tools

Introduction
TODO

Prerequisites
This article assumes you're familiar with the following terms/concepts: If you're a bit unfamiliar, a quick search, tutorial, or Wikipedia skim should suffice.
 * Audio samples
 * ADSR and envelopes
 * Sample rate
 * MIDI
 * Linear predictive coding
 * AIFF format
 * the PATH environment variable (for program quick access)

This article assumes you have the SGI Audio Tools as part of the Nintendo 64 SDK. The programs in particular you're going to need are:

This article also assumes that you're using the aforementioned programs in a Windows 95-like environment. An emulator, such as Oracle VirtualBox works fine too.

The n64decomp project has decompiled  and   here. It's possible to build those two particular programs yourself and run them in the environment of your choice, which might make your life a little easier!

A good warm-up for this article might be to compile and run the  NuSystem sample, as it's more or less a "hello world" that plays the sort of audio files we're looking to generate. Keep note of the Makefile including the audio library and the spec file adding the,  , and   files to the ROM. If you're able to compile/run, even in an emulator, you'll be in a good place to test/debug/iterate an issues that pop up in your program.

Converting Your MIDI File(s)
MIDI files are generally either Type 0 or Type 1. The former specifies all of the notes in a single "track" while the latter has multiple tracks, typically for each instrument. The SGI tools require MIDI files to be in Type 0 and provides the  tool to convert to it.

Programs such as MuseScore likely export in Type 1, so it's usually a good idea to convert to Type 0 before continuing.



For each of your original MIDI files, run the following:

is the name of the converted file in this example.

Compressing Your MIDI File(s)
Once your MIDI files have been converted to Type 0, we'll be converting them to a compressed sequence format specialized for embedded playback on the Nintendo 64.

The NuSystem library is written to use compressed MIDI files for songs. If you inspect the library, you'll notice that it uses a  for storing/playing songs. It is possible to use uncompressed Type 0 MIDI, but you'll need to look into editing/rebuilding NuSystem or your own audio code with Nintendo's core audio library.



For each of your converted MIDI files, run the following:

You'll now have various  files for each of your songs.

Compiling Your MIDI File(s)
Now that we've compressed each MIDI file, its time to compile them into one "song bank". This will be added to your ROM and loaded in at runtime. To do this, we'll be using the  tool.

Run the following command with each of your  files as parameters.



The ordering is important here! Keep note of the order of each parameter, as when you're selecting your songs in your game's source code, you'll be indexing them as they're ordered here (eg:  will be , second_song_compressed.cmf will be  , etc.).

Note the lack of space between the  flag and the output file name. This seems to be intended. 🤷

Converting samples with SoX
SoX bills itself as the Swiss Army knife of sound processing programs. Its uses include (but aren't limited to) converting audio between formats, providing effects, and even recording. Given that SoX is an open-source tool, it's well worth including into any game developer's setup.

The  and   tools require audio samples to be in AIFF or AIFC. If the samples you're using are in a different format, such as WAV, you can use SoX to batch-convert your samples. It's also a good idea to resample each effect to the same sample rate, such as 32000Hz. If you're generating your instrument bank file via a script, you can hardcode the sample rate which will let you spend less time coding/debugging.

If we want to convert an arbitrary WAV file to AIFF with a sample rate of 32000 and in mono we can enter:

This article assumes that the reader is converting their files to AIFF with SoX.

Creating a code book for each file
You'll want to create a code book for each AIFF sample you want to use in your song. To do this, you'll run the  command on each of your samples and save the output of that program to a file.

For clarity, we'll be suffix-ing each code book with  but it's not necessary to do.

On each of your samples, run the following:

It's worth noting that by default  will print to. The  operator for writing to a file should work both on Unix-like and Windows here.

Compressing each sample
Once we've created our code book(s), we'll want to convert our AIFF samples to Nintendo's compressed AIFC formats. To do that, we'll be using.

On each of your samples, run the following:

The following  file(s) will be compiled in to make a sound bank.

Before we Begin
This is likely the most tricky and confusing parts of the SGI Audio Tools, so be sure to take a break if you're finding yourself frustrated. Take comfort in that what you're feeling is pretty normal, and that others have been in the same spot.

Section 18.1.12 of the Nintendo 64 Programming Manual is a pretty comfortable overview of what each section of an instrument bank file does. It's not "correct" in certain areas though, and copy/pasting the shown examples won't always work with. A particular example is that the manual says to reference each instrument in your  section with   when in fact you'll need to use   instead.

If you're looking for a reference of a working instrument bank, it's best to check the example banks at  included with the SDK. They'll run through  fine and help clarify things for you.

The Instrument Bank File
An instrument bank file usually has the file extension of. Inside it contains one or more of each of the following:
 * section(s), indicating an ADSR
 * section(s), indicating the range of "piano keys" a sound occupies, as well as other data
 * section(s), indicating a sampled sound and the  and   it uses
 * section(s), indicating a "MIDI instrument" with a volume, pan, and various  s
 * A single  section, indicating the sample rate, and which  s correspond to which MIDI instrument numbers in your sequences

envelope
The SGI Audio Tools represent an ADSR with s. Volume for each of the ADSR points ranges from   to. Time is modelled in microseconds for each of the ADSR points.

Different samples and sounds can use the same, but your tracks will generally sound better if you ensure that each sample has a matching envelope. If you're unsure, it doesn't

An  looks like the following: envelope AnExampleEnvelope {       attackTime    = 10000; attackVolume = 127; decayTime  = 500000; decayVolume  = 100; releaseTime  = 200000; }

In the example above, the volume goes from  to   in 10000 microseconds (the attack), then decays to 100 over   microseconds. When the sound using is envelope ends, the sound fades out over  microseconds. This should generally match up to your sample.

For more information, review the N64 SDK Documentation at 18.1.2.5.

keymap
A  represents a range of "keys" for a sound to cover. The MIDI Standard represents each of the western music pitches from  to. can be considered middle C.

A  looks like the following: keymap AnExampleKeymap {       velocityMin = 0; velocityMax = 127; keyMin     = 0; keyMax     = 127; keyBase    = 60; detune     = 0; } The example above maps to every available pitch as  is   and   is. represents the "reference pitch" to scale when changing keys. In the example above, a sample with the pitch of middle C should be used. Samples at different frequencies will require a different  value.

For more information, review the N64 SDK Documentation at 18.1.2.4.

sound
A  combines a ,  , and a compressed sample file together into a unit. A  also has properties for stereo panning and volume from   to   each.

An example might look like: sound AnExampleSound {       use ("your/particular/path/to/compressed_song_sample.aifc"); pan   = 64; volume = 127; keymap = AnExampleKeymap; envelope = AnExampleEnvelope; }

Note how the  and   parts correspond to names of our examples above.

For more information, review the N64 SDK Documentation at 18.1.2.3.

instrument
An  models a single MIDI instrument. It consists of one or more s.

An example  might look like:

instrument AnExampleInstrument {       volume = 127; pan   = 64; sound = AnExampleSound; }

Note how  property matches the name of an existing sound above.

An  can specify multiple  s. For example,   in the N64 SDK uses four sounds for a MIDI Cello:

instrument Cello {       volume = 127; pan   = 64; vibratoType = 128;   /* 128, 129, 130, 131 */ vibratoRate = 222;   /* 0 to 255 */ vibratoDepth = 6;   /* 0 to 255 */ vibratoDelay = 1;    /* 1 to 255 */ sound = Cello00; sound = Cello01; sound = Cello02; sound = Cello03; } Each of the corresponding s have different  s and samples that cover different ranges of notes. This can produce nicer-quality audio as the pitch of a sample doesn't need to be distorted as much. The tradeoff being more audio memory required for your, especially at higher sampling frequencies such as 44100Hz.

For more information, review the N64 SDK Documentation at 18.1.2.2.

bank
The  section is a collection of   and the final piece of the puzzle for our file. Each MIDI instrument number gets assigned a particular.

An example  might look like: bank SongBank {       sampleRate = 32000; percussionDefault = Percussion_Kit;

instrument [0] = AnExampleSound; instrument [65] = AnExampleAltoSax; instrument [107] = AnExampleKoto; } Here we associate various s with different MIDI instrument numbers. represents MIDI notes that are played with an Acoustic Grand Piano, and we've told the audio library that we should use  as the voice of Acoustic Grand Piano. MIDI notes that use instrument  get associated with an instrument called.

You'll want the value for  to match the frequency your sample files are tuned to. This is the reason we converted all of our samples to the same rate with SoX above.

is explained in the following section.

Note that you aren't required to have an  associated with every MIDI instrument number. If your song, for example, is only a solo piano piece then you'd only need to worry about an  for. It's best to only include sounds for MIDI instruments that you need. Anything else is audio memory that could be better spent elsewhere, such as sound effects or higher-frequency samples.

For more information, review the N64 SDK Documentation at 18.1.2.1. Note that the example for  in the manual says to use   for referencing an. This isn't correct and the  tool will give you an error if   is used.

Percussion Sounds
Percussion in MIDI is a bit unique in that channel 10 is reserved for percussion and that each note maps to a specific instrument. "Middle C" has a note number of 60 which is always a high bongo sound on the percussion channel.

To accommodate this, we create a special  for percussive sounds. Each different instrument will have its own,  , and   that only covers its corresponding key.

An example percussion setup for an Electric Base Drum (MIDI key ) might look like the following: keymap Percussive_Bass_Drum_1Keymap {       velocityMin = 0; velocityMax = 127; keyMin     = 36; keyMax     = 36; keyBase    = 36; detune     = 0; }

sound Percussive_Bass_Drum_1Sound {       use ("electric_bass_drum_sample.aifc"); pan   = 64; volume = 127; keymap = Percussive_Bass_Drum_1Keymap; envelope = SomeBassDrumEnvelope; }

Which would then integrate into an example percussion : instrument Percussion_Kit {       volume = 127; pan   = 64; sound = Percussive_Bass_Drum_1Sound; sound = Percussive_Acoustic_SnareSound; sound = Percussive_Low_TomSound; sound = Percussive_Open_Hi_HatSound; sound = Percussive_High_Mid_TomSound; sound = Percussive_Crash_Cymbal_1Sound; sound = Percussive_High_TomSound; sound = Percussive_Ride_Cymbal_1Sound; sound = Percussive_High_BongoSound; sound = Percussive_Low_BongoSound; sound = Percussive_CabasaSound; sound = Percussive_MaracasSound; sound = Percussive_ShakerSound; }

The  would then set   to be.

N64 SDK Example Instrument Bank
The N64 SDK has reference Instrument Banks at. If you're ever stuck on how something should look or are getting errors with, they can be a helpful guide to see how things are done.

Compiling the Instrument Bank
We use the instrument compiler program to turn our Instrument Bank file into   and   files for running ingame.

Run  on your   file like the following: ic -OSongBank SongBank.ins Note that  in this case whatever you called your   file. Also, note the lack of space before the  argument. This seems to be correct for the tool.

If your  file doesn't have any errors, you should see an output like this:



You should also then have  and   files in the same directory with the name you put before the   parameter.



If there are syntax or other errors in your  file, you might get an error message like this:



Even though the output looks garbled, try not to be discouraged! The final line output will show the line number of the error. The first place to look is often the associated line.



In this case, the example error message was caused by a missing semicolon on line 28/29.

Finishing up
Once we've completed the steps above, we should now have the following:
 * A  that consists of our converted/compressed MIDI sequences
 * and  files for our samples

We'll be including the above files into our ROM's spec file, then requesting the audio library to load and play them.