Writing MIDI Files

Writing MIDI Files

Below is a basic guide to writing .mid and .rmi files.

Writing a .mid file

writeMIDIFile

Renders the sequence as a .mid file.

writeMIDIFile(midi);

• midi - the MIDI instance to export.

The returned value is an Uint8Array - a binary representation of the .mid file.

modifyMIDI

Allows to easily modify the sequence's programs and controllers.

modifyMIDI(midi, desiredProgramChanges, desiredControllerChanges, desiredChannelsToClear, desiredChannelsToTranspose);

• midi - the MIDI instance to modify.
• desiredProgramChanges - an array of objects, defined as follows:

/**
 * @typedef desiredProgramChange {Object}
 * @property {number} channel - the channel to modify. Note that this allows going over 16 if the MIDI is a multi port file
 * @property {number} program - the MIDI program to use.
 * @property {number} bank - the bank to use.
 * @property {boolean} isDrum - if the channel is a drum channel. Will add GS Use Drums System exclusive and GS on if needed
 */

• desiredControllerChanges - an array of objects, defined as follows:

/**
 * @typedef desiredControllerChange {Object}
 * @property {number} channel - same as above.
 * @property {number} controllerNumber - the MIDI CC number to use.
 * @property {number} controllerValue - the desired value of the controller.
 * 
 */

• desiredChannelsToClear - an array of numbers, indicating the channel number to effectively mute.
• desiredChannelsToTranspose - an array of objects, defined as follows:

/**
 * @typedef desiredTranspose {Object}
 * @property {number} channel - same as above.
 * @property {number} keyShift - the amount to shift the notes on this channel by. Can be negative. The decimal part will be tuned via the RPN fine tune command if provided.
 */

Warning

Clearing the channel removes the messages rather than setting volume to 0! This operation is irreversible if the original midi file is lost.

applySnapshot

Applies a SynthesizerSnapshot to the sequence in place. This means changing the programs and controllers if they are locked.

applySnapshotToMIDI(midi, snapshot);

• midi - the MIDI instance to modify.
• snapshot - the SynthesizerSnapshot to use.

For example if channel 1 has locked preset on Drawbar Organ, this will remove all program changes for channel 1 and add one at the start to change the program to Drawbar organ.

Example

Below is a basic example of writing a modified MIDI file

// create your midi and synthesizer
const midi = new MIDI(yourBufferGoesHere);
const synth = new Synthetizer(yourContext, yourSoundfontBuffer);

// ...

// get the snapshot and apply it
const snapshot = await synth.getSynthesizerSnapshot();
applySnapshotToMIDI(midi, snapshot);

// write midi 
const midiBinary = writeMIDIFile(midi);

// save the file
const blob = new Blob([midiBinary.buffer], {type: "audio/midi"});
const url = URL.createObjectURL(blob);
const a = document.createElement("a");
a.href = url;
a.download = midi.midiName + ".mid";
a.click();

Writing an .rmi file

writeRMIDI

Writes out an RMIDI file (midi + SF2). See more info about this format

const rmidiBinary = writeRMIDI(soundfontBinary, midi, soundfont, bankOffset = 1, encoding = "Shift_JIS", metadata = {});

• soundfontBinary - an Uint8Array of the soundfont to embed, created by soundfont.write().
• midi - MIDI to embed.
• soundfont - SoundFont2 - the soundfont that soundfontBinary contains. Used for correcting bank and program changes.
• bankOffset - number, optional - The bank offset to apply to the file. A value of 0 is recommended. See this for more info
• encoding - string, optional - The encoding to add to the INFO chunk of an RMID file. Make sure to pick a value that is acceptable by the TextDecoder
• metadata - Object, optional - The metadata of the file. If left undefined, some basic metadata (like song's title) will be copied from the MIDI. All of the properties below are optional.
• name - string - the name of the song.
• engineer - string - the engineer of the soundfont.
• artist - string - the artist of the song.
• album - string - the album's name.
• genre - string - the song's genre.
• comment - string - a comment about the song.
• creationDate - string - the creation date of the file. If not provided, current day is used.
• copyright - string - the copyright string. If not provided, midi.copyright is copied.
• picture - ArrayBuffer - the album cover of the song. Binary data of the image.
• midiEncoding - string - The encoding of the inner MIDI file. Make sure to pick a value that is acceptable by the TextDecoder

Caution

Providing any of the metadata fields overrides the encoding with utf-8. This behavior is forced due to lack of support of other encodings by the TextEncoder class.

Tip

use trimSoundfont to drastically reduce the file size. consider also using compression (like shown in example) to save even more space. (using these both methods I managed to cram a 1GB soundfont into a 5MB RMIDI!)

Example

Below is a simple example for exporting an RMIDI file

<label for='soundfont_upload'>Upload soundfont</label>
<input type='file' id='soundfont_upload'>
<label for='midi_upload'>Upload MIDI</label>
<input type='file' id='midi_upload'>
<button id='export'>Export</button>

const sfInput = document.getElementById("soundfont_upload");
const midiInput = document.getElementById("midi_upload");
document.getElementById("export").onchange = async () => {
    // get the files
    const soundfont = new SoundFont2(await sfInput.files[0].arrayBuffer());
    const midi = new MIDI(await midiInput.files[0].arrayBuffer());

    // trim the soundfont
    trimSoundfont(soundfont, midi);
    // write out with compression to save space (0.5 is medium quality)
    const soundfontBinary = soundfont.write({compress: true, compressionQuality: 0.5});
    // get the rmidi
    const rmidiBinary = writeRMIDI(soundfontBinary, midi, soundfont, 0, 'utf-8', {
        name: "A cool song",
        artist: "John",
        creationDate: new Date().toDateString(),
        album: "John's songs",
        genre: "Rock",
        comment: "My favorite!"
    });

    // save the file
    const blob = new Blob([rmidiBinary.buffer], { type: "audio/rmid" });
    const url = URL.createObjectURL(blob);
    const a = document.createElement("a");
    a.href = url;
    a.download = midi.midiName + ".rmi";
    a.click();
}