a new algorithm

js/pitchdetect.js

@@ -201,7 +201,7 @@ function togglePlayback() {
 
 var rafID = null;
 var tracks = null;
-var buflen = 1024;
+var buflen = 2048;
 var buf = new Float32Array( buflen );
 
 var noteStrings = ["C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"];
@@ -219,42 +219,8 @@ function centsOffFromPitch( frequency, note ) {
 	return Math.floor( 1200 * Math.log( frequency / frequencyFromNoteNumber( note ))/Math.log(2) );
 }
 
-// this is a float version of the algorithm below - but it's not currently used.
+// this is the previously used pitch detection algorithm.
 /*
-function autoCorrelateFloat( buf, sampleRate ) {
-	var MIN_SAMPLES = 4;	// corresponds to an 11kHz signal
-	var MAX_SAMPLES = 1000; // corresponds to a 44Hz signal
-	var SIZE = 1000;
-	var best_offset = -1;
-	var best_correlation = 0;
-	var rms = 0;
-
-	if (buf.length < (SIZE + MAX_SAMPLES - MIN_SAMPLES))
-		return -1;  // Not enough data
-
-	for (var i=0;i<SIZE;i++)
-		rms += buf[i]*buf[i];
-	rms = Math.sqrt(rms/SIZE);
-
-	for (var offset = MIN_SAMPLES; offset <= MAX_SAMPLES; offset++) {
-		var correlation = 0;
-
-		for (var i=0; i<SIZE; i++) {
-			correlation += Math.abs(buf[i]-buf[i+offset]);
-		}
-		correlation = 1 - (correlation/SIZE);
-		if (correlation > best_correlation) {
-			best_correlation = correlation;
-			best_offset = offset;
-		}
-	}
-	if ((rms>0.1)&&(best_correlation > 0.1)) {
-		console.log("f = " + sampleRate/best_offset + "Hz (rms: " + rms + " confidence: " + best_correlation + ")");
-	}
-//	var best_frequency = sampleRate/best_offset;
-}
-*/
-
 var MIN_SAMPLES = 0;  // will be initialized when AudioContext is created.
 var GOOD_ENOUGH_CORRELATION = 0.9; // this is the "bar" for how close a correlation needs to be
 
@@ -312,6 +278,52 @@ function autoCorrelate( buf, sampleRate ) {
 	return -1;
 //	var best_frequency = sampleRate/best_offset;
 }
+*/
+
+function autoCorrelate( buf, sampleRate ) {
+	// Implements the ACF2+ algorithm
+	var SIZE = buf.length;
+	var rms = 0;
+
+	for (var i=0;i<SIZE;i++) {
+		var val = buf[i];
+		rms += val*val;
+	}
+	rms = Math.sqrt(rms/SIZE);
+	if (rms<0.01) // not enough signal
+		return -1;
+
+	var r1=0, r2=SIZE-1, thres=0.2;
+	for (var i=0; i<SIZE/2; i++)
+		if (Math.abs(buf[i])<thres) { r1=i; break; }
+	for (var i=1; i<SIZE/2; i++)
+		if (Math.abs(buf[SIZE-i])<thres) { r2=SIZE-i; break; }
+
+	buf = buf.slice(r1,r2);
+	SIZE = buf.length;
+
+	var c = new Array(SIZE).fill(0);
+	for (var i=0; i<SIZE; i++)
+		for (var j=0; j<SIZE-i; j++)
+			c[i] = c[i] + buf[j]*buf[j+i];
+
+	var d=0; while (c[d]>c[d+1]) d++;
+	var maxval=-1, maxpos=-1;
+	for (var i=d; i<SIZE; i++) {
+		if (c[i] > maxval) {
+			maxval = c[i];
+			maxpos = i;
+		}
+	}
+	var T0 = maxpos;
+
+	var x1=c[T0-1], x2=c[T0], x3=c[T0+1];
+	a = (x1 + x3 - 2*x2)/2;
+	b = (x3 - x1)/2;
+	if (a) T0 = T0 - b/(2*a);
+
+	return sampleRate/T0;
+}
 
 function updatePitch( time ) {
 	var cycles = new Array;