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

webrtc/modules/audio_processing/level_controller/noise_level_estimator.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/noise_level_estimator.h"
+
+#include <algorithm>
+
+#include "webrtc/modules/audio_processing/audio_buffer.h"
+#include "webrtc/modules/audio_processing/logging/apm_data_dumper.h"
+
+namespace webrtc {
+
+NoiseLevelEstimator::NoiseLevelEstimator() {
+  Initialize(AudioProcessing::kSampleRate48kHz);
+}
+
+NoiseLevelEstimator::~NoiseLevelEstimator() {}
+
+void NoiseLevelEstimator::Initialize(int sample_rate_hz) {
+  noise_energy_ = 1.f;
+  first_update_ = true;
+  min_noise_energy_ = sample_rate_hz * 2.f * 2.f / 100.f;
+  noise_energy_hold_counter_ = 0;
+}
+
+float NoiseLevelEstimator::Analyze(SignalClassifier::SignalType signal_type,
+                                   float frame_energy) {
+  if (frame_energy <= 0.f) {
+    return noise_energy_;
+  }
+
+  if (first_update_) {
+    // Initialize the noise energy to the frame energy.
+    first_update_ = false;
+    return noise_energy_ = std::max(frame_energy, min_noise_energy_);
+  }
+
+  // Update the noise estimate in a minimum statistics-type manner.
+  if (signal_type == SignalClassifier::SignalType::kStationary) {
+    if (frame_energy > noise_energy_) {
+      // Leak the estimate upwards towards the frame energy if no recent
+      // downward update.
+      noise_energy_hold_counter_ = std::max(noise_energy_hold_counter_ - 1, 0);
+
+      if (noise_energy_hold_counter_ == 0) {
+        noise_energy_ = std::min(noise_energy_ * 1.01f, frame_energy);
+      }
+    } else {
+      // Update smoothly downwards with a limited maximum update magnitude.
+      noise_energy_ =
+          std::max(noise_energy_ * 0.9f,
+                   noise_energy_ + 0.05f * (frame_energy - noise_energy_));
+      noise_energy_hold_counter_ = 1000;
+    }
+  } else {
+    // For a non-stationary signal, leak the estimate downwards in order to
+    // avoid estimate locking due to incorrect signal classification.
+    noise_energy_ = noise_energy_ * 0.99f;
+  }
+
+  // Ensure a minimum of the estimate.
+  return noise_energy_ = std::max(noise_energy_, min_noise_energy_);
+}
+
+}  // namespace webrtc