Replace AudioSilenceDetector with an AudioPowerMonitor.

media/audio/audio_power_monitor.cc

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "media/audio/audio_power_monitor.h"
+
+#include <cmath>
+#include <limits>
+
+#include "base/bind.h"
+#include "base/float_util.h"
+#include "base/message_loop.h"
+#include "base/time.h"
+#include "media/base/audio_bus.h"
+
+namespace media {
+
+// dBFS reference level is 1.0f (the maximum possible value of a sample).
+static const float kReferenceLevel = 1.0f;

[DaleCurtis, 2013/05/16 18:24:45]

AudioBus doesn't guarantee 1.0 is maximum.  See audio_bus.h:

"Warning: Do not rely on having sane (i.e. not inf, nan, or between [-1.0, 1.0])
values in the channel data."

[miu, 2013/05/16 21:44:11]

This is the way dBFS units are defined, not a restriction on what we expect to
see in AudioBus data.  But, good point on sanity-checking: I have the
DCHECK(is_finite(x)) already, but the program shouldn't crash on bad data at
run-time.  I've also added clipping as another protection.

+
+const float AudioPowerMonitor::kZeroPowerDBFS =
+    -std::numeric_limits<float>::infinity();
+
+const float AudioPowerMonitor::kMaxPowerDBFS = 0.0f;
+
+AudioPowerMonitor::AudioPowerMonitor(
+    int sample_rate,
+    const base::TimeDelta& measurement_period,
+    base::MessageLoop* message_loop,
+    const PowerMeasurementCallback& callback)
+    : num_frames_per_callback_(sample_rate * measurement_period.InSecondsF()),
+      message_loop_(message_loop),
+      notify_power_level_(callback) {
+  DCHECK(message_loop_);
+  DCHECK(!notify_power_level_.is_null());
+  ResetScanAccumulations();
+}
+
+AudioPowerMonitor::~AudioPowerMonitor() {
+}
+
+void AudioPowerMonitor::Scan(const AudioBus* buffer, int num_frames) {
+  if (!buffer)

[DaleCurtis, 2013/05/16 18:24:45]

Is this ever true?

[miu, 2013/05/16 21:44:11]

Good point.  (Left over from AudioSilenceDetector code.)  I also changed the
signature of the Scan() method to take a const AudioBus& instead.

+    return;
+  DCHECK_LE(num_frames, buffer->frames());
+  const int num_channels = buffer->channels();
+  if (num_frames <= 0 || num_channels <= 0)
+    return;
+
+  // The overall goal here is to compute the RMS of the amplitudes over all

[DaleCurtis, 2013/05/16 18:24:45]

You say RMS, but I see no sqrt() below :)

[miu, 2013/05/16 21:44:11]

The subtlety: RMS is root-mean-square amplitude.  dBFS uses amplitude^2 (i.e.,
power = amplitude^2).  So, the sqrt() operation was unnecessary when converting
to dBFS units.

+  // channels.  An accumulation of sum-of-squares is maintained over one or more
+  // calls to this method.  Then, once a sufficient number of frames have been
+  // scanned, the accumulated values are converted into dBFS units and a
+  // callback is attempted.
+
+  // Compute the sum of squared amplitudes for the audio signal in |buffer|.
+  //
+  // TODO(miu): Implement optimized SSE/NEON to efficiently calculate
+  // sum-of-squares (or dot product, for more general-purpose use?) in
+  // media/base/vector_math.
+  float sum_of_squares = 0.0f;
+  for (int i = 0; i < num_channels; ++i) {
+    const float* p = buffer->channel(i);
+    const float* const end_of_samples = p + num_frames;
+    for (; p < end_of_samples; ++p)
+      sum_of_squares += (*p) * (*p);
+  }
+  DCHECK(base::IsFinite(sum_of_squares));
+
+  // Accumulate with existing values.
+  sum_of_squares_so_far_ += sum_of_squares / num_channels;
+  frames_so_far_ += num_frames;
+
+  // Once enough frames have been scanned, try to post a task to run the
+  // callback with the dBFS result.  The posting of the task is guaranteed to be
+  // non-blocking, and therefore could fail.  If that happens, the accumulated
+  // values will be retained.
+  if (frames_so_far_ >= num_frames_per_callback_) {

[DaleCurtis, 2013/05/16 18:24:45]

The way this is written you end up with a stretched sample space if TryPostTask
fails.  Can you elaborate on why you shouldn't just throw away the results for
sample ranges which were unpostable?

[miu, 2013/05/16 21:44:11]

I put the answer to this in the form of comments in the code, since it's a good
thing to have explained for future code readers:

  // ...  The posting of the task is guaranteed to be
  // non-blocking, and therefore could fail.  If that happens, the accumulated
  // values will be retained.  We expect this to be a good policy since
  // TryPostTask() should rarely fail.  Therefore, only rarely will the
  // measurement period be stretched, and only rarely will it be stretched by
  // more than one extra AudioBus' worth of frames.

Also note: If we throw away results, we have to start over.  That literally
means an additional "measurement period" must elapse before we attempt to post
another task to run the callback.  Users might see this in the form of
"jitteriness" in the smoothness of the tab UI animation.

+    const float average_power = sum_of_squares_so_far_ / frames_so_far_;
+    const float measurement_in_dbfs =
+        10.0f * log10(average_power / (kReferenceLevel * kReferenceLevel));

[DaleCurtis, 2013/05/16 18:24:45]

10.0? http://en.wikipedia.org/wiki/DBFS says 20.

[miu, 2013/05/16 21:44:11]

20 is for amplitudes.  10 is for power (i.e., amplitude^2).  This is mentioned
in http://en.wikipedia.org/wiki/Decibel#Power_quantities

+    if (message_loop_->TryPostTask(
+            FROM_HERE,
+            base::Bind(notify_power_level_, measurement_in_dbfs))) {
+      ResetScanAccumulations();
+    }
+  }
+}
+
+void AudioPowerMonitor::ResetScanAccumulations() {
+  sum_of_squares_so_far_ = 0.0f;
+  frames_so_far_ = 0;
+}
+
+}  // namespace media