envelope-dx7.js

// Based on http://wiki.music-synthesizer-for-android.googlecode.com/git/img/env.html
var ENV_OFF = 4;
var outputlevel = [0, 5, 9, 13, 17, 20, 23, 25, 27, 29, 31, 33, 35, 37, 39,
	41, 42, 43, 45, 46, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61,
	62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80,
	81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99,
	100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114,
	115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127];

var outputLUT = [];
for (var i = 0; i < 4096; i++) {
	var dB = (i - 3824) * 0.0235;
	outputLUT[i] = Math.pow(20, (dB/20));
}

function EnvelopeDX7(levels, rates) {
	this.levels = levels;
	this.rates = rates;
	this.level = 0; // should start here
	this.i = 0;
	this.down = true;
	this.decayIncrement = 0;
	this.advance(0);
}

EnvelopeDX7.prototype.render = function() {
	if (this.state < 3 || (this.state < 4 && !this.down)) {
		var lev;
		lev = this.level;
		if (this.rising) {
			lev += this.decayIncrement * (2 + (this.targetlevel - lev) / 256);
			if (lev >= this.targetlevel) {
				lev = this.targetlevel;
				this.advance(this.state + 1);
			}
		} else {
			lev -= this.decayIncrement;
			if (lev <= this.targetlevel) {
				lev = this.targetlevel;
				this.advance(this.state + 1);
			}
		}
		this.level = lev;
	}
	this.i++;

	// Convert DX7 level -> dB -> amplitude
	return outputLUT[Math.floor(this.level)];
//		if (this.pitchMode) {
//			return Math.pow(pitchModDepth, amp);
//		}
};

EnvelopeDX7.prototype.advance = function(newstate) {
	this.state = newstate;
	if (this.state < 4) {
		var newlevel = this.levels[this.state];
		this.targetlevel = Math.max(0, (outputlevel[newlevel] << 5) - 224); // 1 -> -192; 99 -> 127 -> 3840
		this.rising = (this.targetlevel - this.level) > 0;
		var rate_scaling = 0;
		this.qr = Math.min(63, rate_scaling + ((this.rates[this.state] * 41) >> 6)); // 5 -> 3; 49 -> 31; 99 -> 63
		this.decayIncrement = Math.pow(2, this.qr/4) / 2048;
//      console.log("decayIncrement (", this.state, "): ", this.decayIncrement);
	}
//		console.log("advance state="+this.state+", qr="+this.qr+", target="+this.targetlevel+", rising="+this.rising);
};

EnvelopeDX7.prototype.noteOff = function() {
	this.down = false;
	this.advance(3);
};

EnvelopeDX7.prototype.isFinished = function() {
	return this.state == ENV_OFF;
};

module.exports = EnvelopeDX7;