Round-trip latency detector

RT Latency

var bufferLength = 16384;
var threshold = 0.125;  // -18dB
var gain = audio.createGain();

// input comes from a getUserMedia request
input.connect(gain);

function endTest(outTimes, inTimes) {
	if (inTimes.length === 0) {
		toolApp.problem = 'No input has been detected. Have you connected an output to an input?';
		return;
	}

	if (outTimes.length < inTimes.length) {
		toolApp.problem = inTimes.length + ' signals were detected, but only ' + outTimes.length + ' were sent. Is there a lot of noise in your system? Try and keep background noise below -18dB.';
		return;
	}

	if (outTimes.length > inTimes.length) {
		toolApp.problem = 'Only ' + inTimes.length + ' signals were detected, where ' + outTimes.length + ' were sent. Have you got your input gain turned up enough?';
		return;
	}

	console.log(outTimes, inTimes);

	var latencyTimes = [];
	var n = outTimes.length;

	while (n--) {
		latencyTimes[n] = inTimes[n] - outTimes[n];
	}

	var min = Math.min.apply(Math, latencyTimes);
	var max = Math.max.apply(Math, latencyTimes);
	var range = max - min;

	if (range > 128) {
		// That's a lot of variance in the resulting times. Looks a bit suspect.
		toolApp.problem = 'There\'s a LOT of variance in those results. They could be dodgy. Are you waving a mic around? Keep still! Try running the test again.';
		return;
	}

	console.log(min, max);

	var n = latencyTimes.length;
	var avg = 0;

	while (n--) {
		avg += latencyTimes[n] / latencyTimes.length;
	}

	console.log('Average round-trip latency samples:', avg, 'ms:', avg / toolApp.audio.sampleRate);

	toolApp.latency = Math.round(avg);
	toolApp.problem = false;
}

function testLatency() {
	var node = audio.createScriptProcessor(bufferLength, 1, 1);
	var frame = -1;
	var inputTimes = [];
	var outputTimes = [];
	
	// Keep a reference to the node around to avoid Chrome's garbage
	// collection.
	window.hfiuxw4i8mwxvhmlu = node;
	 
	node.onaudioprocess = function(e){
		var inputBuffer = e.inputBuffer;
		var outputBuffer = e.outputBuffer;
		var inputSamples = inputBuffer.getChannelData(0);
		var outputSamples = outputBuffer.getChannelData(0);
		var first = false;
	
		++frame;
	
		if (frame > 12) {
			// Last frame. End the test...
			endTest(outputTimes, inputTimes);
			gain.disconnect();
			node.disconnect();
		}
	
		if (frame % 3 - 1 === 0) {
			// Every third frame, give the samples an impulse
			outputSamples[0] = 1;
			outputSamples[1] = 1;
			first = true;
			outputTimes.push(bufferLength * frame + bufferLength);
		}
		else {
			// The rest of the time send silence
			outputSamples[0] = 0;
			outputSamples[1] = 0;
		}
		
		var n = -1;
		var l = inputSamples.length;
	
		// Detect an impulse. Leave frame 0 out to avoid click noise.
		if (frame > 0) {
			while (++n < l) {
				if (Math.abs(inputSamples[n]) > threshold) {
					inputTimes.push(bufferLength * frame + n - bufferLength);
					// Dont detect any more than one peak per frame.
					return;
				}
			}
		}
	}
	
	gain.connect(node);
	node.connect(audio.destination);
}