How do I play arbitrary MIDI notes with javascript

问题:

To clarify: I don't want to generate a MIDI file nor do I want to play a MIDI file, I wish to play MIDI notes on the fly.

I tried using https://github.com/mudcube/MIDI.js as the MIDI library, and it works somewhat.

I am able to play notes by calling MIDI.noteOn(0,midiNumber,100);. However, this plays a note for a couple seconds and then tapers off even if I never call MIDI.noteOff.

I don't believe this is how MIDI is intended to work. I wish to be able to call noteOn and have a note play and sustain until noteOff is called.

Intended browsers: modern firefox/chrome.

回答1:

It's a bug your version of MIDI.js:

var playChannel = function (id) {
    var note = notes[id];
    if (!note) return;
    var nid = (channel_nid + 1) % channels.length;
    var time = (new Date()).getTime();
    var audio = channels[nid];
    channel_map[note.id] = audio;
    audio.src = MIDI.Soundfont[note.id];
    audio.volume = volume;
    audio.play();
    channel_nid = nid;
};

As you can see playChannel will load a given note and play it. Since there is no autoloop attribute it won't repeat, so the call of noteOff isn't necessary. You could fix this yourself if you set the audio element to auto-loop.

回答2:

Alternatively, http://mohayonao.github.io/timbre.js/ provides various sound generators for which noteOn and noteOff can be called.

回答3:

Different instruments behave differently. A piano has an "intrinsic" note off, but an organ doesn't.

Also, noteOn and noteOff depend on the implementation. modcu.be in the HTML5 implementation plays a OGG file for each note, and it doesn't care of noteOff at all. When the OGG ends, the sound stops. And autoloop wouldn't help for a piano in this case.

回答4:

Google for Web MIDI API. It is not implemented in all browsers yet, but there is a polyfill by Chris Wilson at GitHub.

回答5:

This is right behavour. You need a better samples library to play long sounds.

See church organ example of endless sound.

this exanple uses WebAudioFont to play more then 1500 instruments and drums. This lib supports ADSR, reverberation and loop points.