Adds Analog Gain Control (AGC) to the WebRTC client.

media/audio/audio_input_stream_impl.cc

+// Copyright (c) 2012 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_input_stream_impl.h"
+
+static const int kMinIntervalBetweenVolumeUpdatesMs = 1000;
+
+AudioInputStreamImpl::AudioInputStreamImpl()
+    : agc_is_enabled_(false),
+      volume_(0.0) {
+}
+
+AudioInputStreamImpl::~AudioInputStreamImpl() {
+}
+
+void AudioInputStreamImpl::SetAutomaticGainControl(bool enabled) {
+  agc_is_enabled_ = enabled;
+}
+
+bool AudioInputStreamImpl::GetAutomaticGainControl() {
+  return agc_is_enabled_;
+}
+
+void AudioInputStreamImpl::UpdateAgcVolume() {
+  base::AutoLock lock(lock_);
+
+  // We take new volume samples once every second when the AGC is enabled.
+  // To ensure that a new setting has an immediate effect, the new volume
+  // setting is cached here. It will ensure that the next OnData() callback
+  // will contain a new valid volume level. If this approach was not taken,
+  // we could report invalid volume levels to the client for a time period
+  // of up to one second.
+  if (agc_is_enabled_) {
+    volume_ = GetVolume();
+  }
+}
+
+void AudioInputStreamImpl::QueryAgcVolume(double* volume) {
+  base::AutoLock lock(lock_);
+
+  // Only modify the |volume| output reference if AGC is enabled and if
+  // more than one second has passed since the volume was updated the last time.
+  if (agc_is_enabled_) {
+    base::Time now = base::Time::Now();
+    if ((now - last_volume_update_time_).InMilliseconds() >
+      kMinIntervalBetweenVolumeUpdatesMs) {
+        // Retrieve the current volume level by asking the audio hardware.
+        // Range is [0.0,1.0].
+        *volume = static_cast<double>(GetVolume());
+
+        // Store copy of latest volume level.
+        volume_ = *volume;
+
+        // Ensure that we don't ask for a new volume level at each call.
+        last_volume_update_time_ = now;
+    } else {
+      *volume = volume_;
+    }
+  }
+}
+