content/renderer/media/stream/media_stream_audio_level_calculator.cc

// Copyright 2014 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 "content/renderer/media/stream/media_stream_audio_level_calculator.h"

#include <cmath>
#include <limits>

#include "base/logging.h"
#include "base/stl_util.h"
#include "media/base/audio_bus.h"

namespace content {

namespace {

// Calculates the maximum absolute amplitude of the audio data.
float MaxAmplitude(const float* audio_data, int length) {
  float max = 0.0f;
  for (int i = 0; i < length; ++i) {
    const float absolute = fabsf(audio_data[i]);
    if (absolute > max)
      max = absolute;
  }
  DCHECK(std::isfinite(max));
  return max;
}

}  // namespace

MediaStreamAudioLevelCalculator::Level::Level() : level_(0.0f) {}

MediaStreamAudioLevelCalculator::Level::~Level() {}

float MediaStreamAudioLevelCalculator::Level::GetCurrent() const {
  base::AutoLock auto_lock(lock_);
  return level_;
}

void MediaStreamAudioLevelCalculator::Level::Set(float level) {
  base::AutoLock auto_lock(lock_);
  level_ = level;
}

MediaStreamAudioLevelCalculator::MediaStreamAudioLevelCalculator()
    : counter_(0), max_amplitude_(0.0f), level_(new Level()) {}

MediaStreamAudioLevelCalculator::~MediaStreamAudioLevelCalculator() {
  level_->Set(0.0f);
}

void MediaStreamAudioLevelCalculator::Calculate(
    const media::AudioBus& audio_bus,
    bool assume_nonzero_energy) {
  // |level_| is updated every 10 callbacks. For the case where callback comes
  // every 10ms, |level_| will be updated approximately every 100ms.
  static const int kUpdateFrequency = 10;

  float max =
      assume_nonzero_energy ? 1.0f / std::numeric_limits<int16_t>::max() : 0.0f;
  for (int i = 0; i < audio_bus.channels(); ++i) {
    const float max_this_channel =
        MaxAmplitude(audio_bus.channel(i), audio_bus.frames());
    if (max_this_channel > max)
      max = max_this_channel;
  }
  max_amplitude_ = std::max(max_amplitude_, max);

  if (counter_++ == kUpdateFrequency) {
    // Clip the exposed signal level to make sure it is in the range [0.0,1.0].
    level_->Set(std::min(1.0f, max_amplitude_));

    // Decay the absolute maximum amplitude by 1/4.
    max_amplitude_ /= 4.0f;

    // Reset the counter.
    counter_ = 0;
  }
}

}  // namespace content