media/audio/audio_power_monitor.cc

   1 // Copyright 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "media/audio/audio_power_monitor.h"
   6
   7 #include <algorithm>
   8 #include <cmath>
   9
  10 #include "base/float_util.h"
  11 #include "base/logging.h"
  12 #include "base/time/time.h"
  13 #include "media/base/audio_bus.h"
  14 #include "media/base/vector_math.h"
  15
  16 namespace media {
  17
  18 AudioPowerMonitor::AudioPowerMonitor(
  19     int sample_rate, const base::TimeDelta& time_constant)
  20     : sample_weight_(
  21           1.0f - expf(-1.0f / (sample_rate * time_constant.InSecondsF()))) {
  22   Reset();
  23 }
  24
  25 AudioPowerMonitor::~AudioPowerMonitor() {
  26 }
  27
  28 void AudioPowerMonitor::Reset() {
  29   power_reading_ = average_power_ = 0.0f;
  30   clipped_reading_ = has_clipped_ = false;
  31 }
  32
  33 void AudioPowerMonitor::Scan(const AudioBus& buffer, int num_frames) {
  34   DCHECK_LE(num_frames, buffer.frames());
  35   const int num_channels = buffer.channels();
  36   if (num_frames <= 0 || num_channels <= 0)
  37     return;
  38
  39   // Calculate a new average power by applying a first-order low-pass filter
  40   // (a.k.a. an exponentially-weighted moving average) over the audio samples in
  41   // each channel in |buffer|.
  42   float sum_power = 0.0f;
  43   for (int i = 0; i < num_channels; ++i) {
  44     const std::pair<float, float> ewma_and_max = vector_math::EWMAAndMaxPower(
  45         average_power_, buffer.channel(i), num_frames, sample_weight_);
  46     // If data in audio buffer is garbage, ignore its effect on the result.
  47     if (!base::IsFinite(ewma_and_max.first)) {
  48       sum_power += average_power_;
  49     } else {
  50       sum_power += ewma_and_max.first;
  51       has_clipped_ |= (ewma_and_max.second > 1.0f);
  52     }
  53   }
  54
  55   // Update accumulated results, with clamping for sanity.
  56   average_power_ = std::max(0.0f, std::min(1.0f, sum_power / num_channels));
  57
  58   // Push results for reading by other threads, non-blocking.
  59   if (reading_lock_.Try()) {
  60     power_reading_ = average_power_;
  61     if (has_clipped_) {
  62       clipped_reading_ = true;
  63       has_clipped_ = false;
  64     }
  65     reading_lock_.Release();
  66   }
  67 }
  68
  69 std::pair<float, bool> AudioPowerMonitor::ReadCurrentPowerAndClip() {
  70   base::AutoLock for_reading(reading_lock_);
  71
  72   // Convert power level to dBFS units, and pin it down to zero if it is
  73   // insignificantly small.
  74   const float kInsignificantPower = 1.0e-10f;  // -100 dBFS
  75   const float power_dbfs = power_reading_ < kInsignificantPower ? zero_power() :
  76       10.0f * log10f(power_reading_);
  77
  78   const bool clipped = clipped_reading_;
  79   clipped_reading_ = false;
  80
  81   return std::make_pair(power_dbfs, clipped);
  82 }
  83
  84 }  // namespace media