New module for the adaptive level controlling functionality in the audio processing module

webrtc/modules/audio_processing/level_controller/gain_selector.cc

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/modules/audio_processing/level_controller/gain_selector.h"
+
+#include <math.h>
+#include <algorithm>
+
+#include "webrtc/base/checks.h"
+#include "webrtc/modules/audio_processing/include/audio_processing.h"
+#include "webrtc/modules/audio_processing/level_controller/lc_constants.h"
+
+namespace webrtc {
+
+GainSelector::GainSelector() {
+  Initialize(AudioProcessing::kSampleRate48kHz);
+}
+
+void GainSelector::Initialize(int sample_rate_hz) {
+  gain_ = 1.f;
+  frame_length_ = rtc::CheckedDivExact(sample_rate_hz, 100);
+  highly_nonstationary_signal_hold_counter_ = 0;
+}
+
+// Chooses the gain to apply by the level controller such that
+// 1) The level of the stationary noise does not exceed
+//    a predefined threshold.
+// 2) The gain does not exceed the gain that has been found
+//    to saturate the signal.
+// 3) The peak level achieves the target peak level.
+// 4) The gain is not below 1.
+// 4) The gain is 1 if the signal has been classified as stationary
+//    for a long time.
+// 5) The gain is not above the maximum gain.
+float GainSelector::GetNewGain(float peak_level,
+                               float noise_energy,
+                               float saturating_gain,
+                               SignalClassifier::SignalType signal_type) {
+  RTC_DCHECK_LT(0.f, peak_level);
+
+  if (signal_type == SignalClassifier::SignalType::kHighlyNonStationary) {
+    highly_nonstationary_signal_hold_counter_ = 10000;
+  } else {
+    highly_nonstationary_signal_hold_counter_ =
+        std::max(0, highly_nonstationary_signal_hold_counter_ - 1);
+  }
+
+  float desired_gain;
+  if (highly_nonstationary_signal_hold_counter_ > 0) {
+    // Compute a desired gain that ensures that the peak level is amplified to
+    // the target level.
+    desired_gain = kTargetLcPeakLevel / peak_level;
+
+    // Limit the desired gain so that it does not amplify the noise too much.
+    float max_noise_energy = kMaxLcNoisePower * frame_length_;
+    if (noise_energy * desired_gain * desired_gain > max_noise_energy) {
+      RTC_DCHECK_LE(0.f, noise_energy);
+      desired_gain = sqrtf(max_noise_energy / noise_energy);
+    }
+  } else {
+    // If the signal has been stationary for a long while, apply a gain of 1 to
+    // avoid amplifying pure noise.
+    desired_gain = 1.0f;
+  }
+
+  // Smootly update the gain towards the desired gain.
+  gain_ += 0.2f * (desired_gain - gain_);
+
+  // Limit the gain to not exceed the maximum and the saturating gains, and to
+  // ensure that the lowest possible gain is 1.
+  gain_ = std::min(gain_, saturating_gain);
+  gain_ = std::min(gain_, kMaxLcGain);
+  gain_ = std::max(gain_, 1.f);
+
+  return gain_;
+}
+
+}  // namespace webrtc