Add support for WaveShaperNode.oversample

LayoutTests/webaudio/resources/waveshaper-testing.js

+var context;
+var lengthInSeconds = 2;
+
+// Skip this many frames before comparing against reference to allow
+// a steady-state to be reached in the up-sampling filters.
+var filterStabilizeSkipFrames = 2048;
+
+var numberOfCurveFrames = 65536;
+var waveShapingCurve;
+
+var waveshaper;
+
+// FIXME: test at more frequencies.
+// When using the up-sampling filters (2x, 4x) any significant aliasing components
+// should be at very high frequencies near Nyquist.  These tests could be improved
+// to allow for a higher acceptable amount of aliasing near Nyquist, but then
+// become more stringent for lower frequencies.
+
+// These test parameters are set in runWaveShaperOversamplingTest().
+var sampleRate;
+var nyquist;
+var oversample;
+var fundamentalFrequency;
+var acceptableAliasingThresholdDecibels;
+
+var kScale = 0.25;
+
+// Chebyshev Polynomials.
+// Given an input sinusoid, returns an output sinusoid of the given frequency multiple.
+function T0(x) { return 1; }
+function T1(x) { return x; }
+function T2(x) { return 2*x*x - 1; }
+function T3(x) { return 4*x*x*x - 3*x; }
+function T4(x) { return 8*x*x*x*x - 8*x*x + 1; }
+
+function generateWaveShapingCurve() {
+    var n = 65536;
+    var n2 = n / 2;
+    var curve = new Float32Array(n);
+
+    // The shaping curve uses Chebyshev Polynomial such that an input sinusoid
+    // at frequency f will generate an output of four sinusoids of frequencies:
+    // f, 2*f, 3*f, 4*f
+    // each of which is scaled.
+    for (var i = 0; i < n; ++i) {
+        var x = (i - n2) / n2;
+        var y = kScale * (T1(x) + T2(x) + T3(x) + T4(x));
+        curve[i] = y;
+    }
+
+    return curve;
+}
+
+function checkShapedCurve(event) {
+    var buffer = event.renderedBuffer;
+
+    var outputData = buffer.getChannelData(0);
+    var n = buffer.length;
+
+    // The WaveShaperNode will have a processing latency if oversampling is used,
+    // so we should account for it.
+
+    // FIXME: .latency should be exposed as an attribute of the node
+    // var waveShaperLatencyFrames = waveshaper.latency * sampleRate;
+    // But for now we'll use the hard-coded values corresponding to the actual latencies:
+    var waveShaperLatencyFrames = 0;
+    if (oversample == "2x")
+        waveShaperLatencyFrames = 128;
+    else if (oversample == "4x")
+        waveShaperLatencyFrames = 192;
+
+    var waveShaperLatencyFrames = waveshaper.latency * sampleRate;
+
+    var worstDeltaInDecibels = -1000;
+
+    for (var i = waveShaperLatencyFrames; i < n; ++i) {
+        var actual = outputData[i];
+
+        // Account for the expected processing latency.
+        var j = i - waveShaperLatencyFrames;
+
+        // Compute reference sinusoids.
+        var phaseInc = 2 * Math.PI * fundamentalFrequency / sampleRate;
+
+        // Generate an idealized reference based on the four generated frequencies truncated
+        // to the Nyquist rate.  Ideally, we'd like the waveshaper's oversampling to perfectly
+        // remove all frequencies above Nyquist to avoid aliasing.  In reality the oversampling filters are not
+        // quite perfect, so there will be a (hopefully small) amount of aliasing.  We should
+        // be close to the ideal.
+        var reference = 0;
+
+        // Sum in fundamental frequency.
+        if (fundamentalFrequency < nyquist)
+            reference += Math.sin(phaseInc * j);
+
+        // Note that the phase of each of the expected generated harmonics is different.
+        if (fundamentalFrequency * 2 < nyquist)
+            reference += -Math.cos(phaseInc * j * 2);
+        if (fundamentalFrequency * 3 < nyquist)
+            reference += -Math.sin(phaseInc * j * 3);
+        if (fundamentalFrequency * 4 < nyquist)
+            reference += Math.cos(phaseInc * j * 4);
+
+        // Scale the reference the same as the waveshaping curve itself.
+        reference *= kScale;
+
+        var delta = Math.abs(actual - reference);
+        var deltaInDecibels = delta > 0 ? 20 * Math.log(delta)/Math.log(10) : -200;
+
+        if (j >= filterStabilizeSkipFrames) {
+            if (deltaInDecibels > worstDeltaInDecibels) {
+                worstDeltaInDecibels = deltaInDecibels;
+            }
+        }
+    }
+
+    // console.log("worstDeltaInDecibels: " + worstDeltaInDecibels);
+
+    var success = worstDeltaInDecibels < acceptableAliasingThresholdDecibels;
+
+    if (success) {
+        testPassed(oversample + " WaveShaperNode oversampling within acceptable tolerance.");
+    } else {
+        testFailed(oversample + " WaveShaperNode oversampling not within acceptable tolerance.");
+    }
+
+    finishJSTest();
+}
+
+function createImpulseBuffer(context, sampleFrameLength) {
+    var audioBuffer = context.createBuffer(1, sampleFrameLength, context.sampleRate);
+    var n = audioBuffer.length;
+    var dataL = audioBuffer.getChannelData(0);
+
+    for (var k = 0; k < n; ++k)
+        dataL[k] = 0;
+
+    dataL[0] = 1;
+
+    return audioBuffer;
+}
+
+function runWaveShaperOversamplingTest(testParams) {
+    sampleRate = testParams.sampleRate;
+    nyquist = 0.5 * sampleRate;
+    oversample = testParams.oversample;
+    fundamentalFrequency = testParams.fundamentalFrequency;
+    acceptableAliasingThresholdDecibels = testParams.acceptableAliasingThresholdDecibels;
+
+    if (window.testRunner) {
+        testRunner.dumpAsText();
+        testRunner.waitUntilDone();
+    }
+
+    window.jsTestIsAsync = true;
+
+    // Create offline audio context.
+    var numberOfRenderFrames = sampleRate * lengthInSeconds;
+    context = new webkitOfflineAudioContext(1, numberOfRenderFrames, sampleRate);
+
+    // source -> waveshaper -> destination
+    var source = context.createBufferSource();
+    source.buffer = createToneBuffer(context, fundamentalFrequency, lengthInSeconds, 1);
+
+    // Apply a non-linear distortion curve.
+    waveshaper = context.createWaveShaper();
+    waveshaper.curve = generateWaveShapingCurve();
+    waveshaper.oversample = oversample;
+
+    source.connect(waveshaper);
+    waveshaper.connect(context.destination);
+
+    source.start(0);
+
+    context.oncomplete = checkShapedCurve;
+    context.startRendering();
+}