audiobuffersource-playbackrate-zero.html

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  <head>

    <title>audiobuffersource-playbackrate-zero.html</title>

    <script src="/resources/testharness.js"></script>

    <script src="/resources/testharnessreport.js"></script>

    <script src="/webaudio/resources/audit-util.js"></script>

  </head>

  <body>

    <script>

      // Sample rate should be power of 128 to observe the change of AudioParam

      // at the beginning of rendering quantum. (playbackRate is k-rate) This is

      // the minimum sample rate in the valid sample rate range.

      const sampleRate = 8192;

      // The render duration in seconds, and the length in samples.

      const renderDuration = 1.0;

      const renderLength = renderDuration * sampleRate;

      const context = new OfflineAudioContext(1, renderLength, sampleRate);

      promise_test(async t => {

        const ramp = new AudioBufferSourceNode(context);

        const rampBuffer = createLinearRampBuffer(context, renderLength);

        ramp.buffer = rampBuffer;

        ramp.connect(context.destination);

        ramp.start();

        // Leave the playbackRate as 1 for the first half, then change it

        // to zero at the exact half. The zero playback rate should hold the

        // sample value of the buffer index at the moment. (sample-and-hold)

        ramp.playbackRate.setValueAtTime(1.0, 0.0);

        ramp.playbackRate.setValueAtTime(0.0, renderDuration / 2);

        const renderedBuffer = await context.startRendering();

        const data = renderedBuffer.getChannelData(0);

        const rampData = rampBuffer.getChannelData(0);

        const half = rampData.length / 2;

        let passed = true;

        let i;

        for (i = 1; i < rampData.length; i++) {

          if (i < half) {

            if (data[i] !== rampData[i]) {

              passed = false;

              break;

          } else {

            if (data[i] !== rampData[half]) {

              passed = false;

              break;

        assert_true(

            passed,

            `The zero playbackRate should hold the sample value. Expected` +

                `${rampData[half]} but got ${data[i]} at index ${i}`);

      }, 'Synthesize and verify sample-and-hold behavior when playbackRate ' +

          'is set to zero halfway');

      promise_test(async t => {

        const context = new OfflineAudioContext(1, renderLength, sampleRate);

        const rampBuffer = new AudioBuffer({

          length: renderLength,

          sampleRate: context.sampleRate

});

        const data = new Float32Array(renderLength);

        const startValue = 5;

        for (let k = 0; k < data.length; ++k) {

          data[k] = k + startValue;

        rampBuffer.copyToChannel(data, 0);

        const src = new AudioBufferSourceNode(context, {

          buffer: rampBuffer,

          playbackRate: 0

});

        src.connect(context.destination);

        // Purposely start the source between frame boundaries

        const startFrame = 27.3;

        src.start(startFrame / context.sampleRate);

        const audioBuffer = await context.startRendering();

        const actualStartFrame = Math.ceil(startFrame);

        const audio = audioBuffer.getChannelData(0);

        assert_constant_value(

            audio.slice(0, actualStartFrame),

0,

            'output before startFrame should be silence: ');

        assert_constant_value(

            audio.slice(actualStartFrame),

            startValue,

            `output after startFrame (starting at index ${actualStartFrame}) ` +

                `should match start value: `);

      }, 'Subsample start with playbackRate 0 should produce sample-and-hold ' +

          'from exact start frame');

    </script>

  </body>

</html>