Pre-amplification in the audio processing module.

Added a new sub-module 'GainApplier'. The build target is
'modules/audio_processing/agc2:gain_applier'. A small refactoring
makes the GainApplier used in adaptive-digital AGC2.

The AGC2 now multiplies samples with a gain in 3 places. It's the
GainApplier, the GainCurveApplier, and the FixedGainController. The
GainApplier is used in AdaptiveDigitalGainApplier and will be used as
a pre-amplifier.

Bug: webrtc:9138
Change-Id: Ibc4c0ea109c6757f159d4adb6e3d8614179c9bc6
Reviewed-on: https://webrtc-review.googlesource.com/69321
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22849}

modules/audio_processing/agc2/BUILD.gn

@@ -31,6 +31,7 @@ rtc_source_set("adaptive_digital") {
 
   deps = [
     ":common",
+    ":gain_applier",
     ":noise_level_estimator",
     "..:aec_core",
     "..:apm_logging",
@@ -108,6 +109,18 @@ rtc_source_set("noise_level_estimator") {
   configs += [ "..:apm_debug_dump" ]
 }
 
+rtc_source_set("gain_applier") {
+  sources = [
+    "gain_applier.cc",
+    "gain_applier.h",
+  ]
+  deps = [
+    ":common",
+    "..:audio_frame_view",
+    "../../../rtc_base:safe_minmax",
+  ]
+}
+
 rtc_source_set("test_utils") {
   testonly = true
   visibility = [ ":*" ]
@@ -161,11 +174,13 @@ rtc_source_set("adaptive_digital_unittests") {
   sources = [
     "adaptive_digital_gain_applier_unittest.cc",
     "adaptive_mode_level_estimator_unittest.cc",
+    "gain_applier_unittest.cc",
     "saturation_protector_unittest.cc",
   ]
   deps = [
     ":adaptive_digital",
     ":common",
+    ":gain_applier",
     ":test_utils",
     "..:apm_logging",
     "..:audio_frame_view",

modules/audio_processing/agc2/adaptive_digital_gain_applier.cc

@@ -64,52 +64,13 @@ float ComputeGainChangeThisFrameDb(float target_gain_db,
   return rtc::SafeClamp(target_gain_difference_db, -kMaxGainChangePerFrameDb,
                         kMaxGainChangePerFrameDb);
 }
-
-// Returns true when the gain factor is so close to 1 that it would
-// not affect int16 samples.
-bool GainCloseToOne(float gain_factor) {
-  return 1.f - 1.f / kMaxFloatS16Value <= gain_factor &&
-         gain_factor <= 1.f + 1.f / kMaxFloatS16Value;
-}
-
-void ApplyGainWithRamping(float last_gain_linear,
-                          float gain_at_end_of_frame_linear,
-                          AudioFrameView<float> float_frame) {
-  // Do not modify the signal when input is loud.
-  if (last_gain_linear == gain_at_end_of_frame_linear &&
-      GainCloseToOne(gain_at_end_of_frame_linear)) {
-    return;
-  }
-
-  // A typical case: gain is constant and different from 1.
-  if (last_gain_linear == gain_at_end_of_frame_linear) {
-    for (size_t k = 0; k < float_frame.num_channels(); ++k) {
-      rtc::ArrayView<float> channel_view = float_frame.channel(k);
-      for (auto& sample : channel_view) {
-        sample *= gain_at_end_of_frame_linear;
-      }
-    }
-    return;
-  }
-
-  // The gain changes. We have to change slowly to avoid discontinuities.
-  const size_t samples = float_frame.samples_per_channel();
-  RTC_DCHECK_GT(samples, 0);
-  const float increment =
-      (gain_at_end_of_frame_linear - last_gain_linear) / samples;
-  float gain = last_gain_linear;
-  for (size_t i = 0; i < samples; ++i) {
-    for (size_t ch = 0; ch < float_frame.num_channels(); ++ch) {
-      float_frame.channel(ch)[i] *= gain;
-    }
-    gain += increment;
-  }
-}
 }  // namespace
 
 AdaptiveDigitalGainApplier::AdaptiveDigitalGainApplier(
     ApmDataDumper* apm_data_dumper)
-    : apm_data_dumper_(apm_data_dumper) {}
+    : gain_applier_(false, 1.f),  // Initial gain is 1, and we do not
+                                  // clip after gain.
+      apm_data_dumper_(apm_data_dumper) {}
 
 void AdaptiveDigitalGainApplier::Process(
     float input_level_dbfs,
@@ -151,15 +112,13 @@ void AdaptiveDigitalGainApplier::Process(
 
   // Optimization: avoid calling math functions if gain does not
   // change.
-  const float gain_at_end_of_frame =
+  if (gain_change_this_frame_db != 0.f) {
-      gain_change_this_frame_db == 0.f
+    gain_applier_.SetGainFactor(
-          ? last_gain_linear_
+        DbToRatio(last_gain_db_ + gain_change_this_frame_db));
-          : DbToRatio(last_gain_db_ + gain_change_this_frame_db);
+  }
-
+  gain_applier_.ApplyGain(float_frame);
-  ApplyGainWithRamping(last_gain_linear_, gain_at_end_of_frame, float_frame);
 
   // Remember that the gain has changed for the next iteration.
-  last_gain_linear_ = gain_at_end_of_frame;
   last_gain_db_ = last_gain_db_ + gain_change_this_frame_db;
   apm_data_dumper_->DumpRaw("agc2_applied_gain_db", last_gain_db_);
 }

modules/audio_processing/agc2/adaptive_digital_gain_applier.h

@@ -11,6 +11,7 @@
 #ifndef MODULES_AUDIO_PROCESSING_AGC2_ADAPTIVE_DIGITAL_GAIN_APPLIER_H_
 #define MODULES_AUDIO_PROCESSING_AGC2_ADAPTIVE_DIGITAL_GAIN_APPLIER_H_
 
+#include "modules/audio_processing/agc2/gain_applier.h"
 #include "modules/audio_processing/include/audio_frame_view.h"
 #include "modules/audio_processing/vad/vad_with_level.h"
 
@@ -29,11 +30,8 @@ class AdaptiveDigitalGainApplier {
       AudioFrameView<float> float_frame);
 
  private:
-  // Keep track of current gain for ramping up and down and
-  // logging. This member variable is redundant together with
-  // last_gain_db_. Both are kept as an optimization.
-  float last_gain_linear_ = 1.f;
   float last_gain_db_ = 0.f;
+  GainApplier gain_applier_;
 
   // For some combinations of noise and speech probability, increasing
   // the level is not allowed. Since we may get VAD results in bursts,

modules/audio_processing/agc2/fixed_gain_controller.cc

@@ -55,7 +55,7 @@ void FixedGainController::SetSampleRate(size_t sample_rate_hz) {
 }
 
 void FixedGainController::Process(AudioFrameView<float> signal) {
-  // Apply fixed digital gain; interpolate if necessary. One of the
+  // Apply fixed digital gain. One of the
   // planned usages of the FGC is to only use the limiter. In that
   // case, the gain would be 1.0. Not doing the multiplications speeds
   // it up considerably. Hence the check.

modules/audio_processing/agc2/gain_applier.cc

@@ -0,0 +1,101 @@
+/*
+ *  Copyright (c) 2018 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 "modules/audio_processing/agc2/gain_applier.h"
+
+#include "modules/audio_processing/agc2/agc2_common.h"
+#include "rtc_base/numerics/safe_minmax.h"
+
+namespace webrtc {
+namespace {
+
+// Returns true when the gain factor is so close to 1 that it would
+// not affect int16 samples.
+bool GainCloseToOne(float gain_factor) {
+  return 1.f - 1.f / kMaxFloatS16Value <= gain_factor &&
+         gain_factor <= 1.f + 1.f / kMaxFloatS16Value;
+}
+
+void ClipSignal(AudioFrameView<float> signal) {
+  for (size_t k = 0; k < signal.num_channels(); ++k) {
+    rtc::ArrayView<float> channel_view = signal.channel(k);
+    for (auto& sample : channel_view) {
+      sample = rtc::SafeClamp(sample, kMinFloatS16Value, kMaxFloatS16Value);
+    }
+  }
+}
+
+void ApplyGainWithRamping(float last_gain_linear,
+                          float gain_at_end_of_frame_linear,
+                          float inverse_samples_per_channel,
+                          AudioFrameView<float> float_frame) {
+  // Do not modify the signal.
+  if (last_gain_linear == gain_at_end_of_frame_linear &&
+      GainCloseToOne(gain_at_end_of_frame_linear)) {
+    return;
+  }
+
+  // Gain is constant and different from 1.
+  if (last_gain_linear == gain_at_end_of_frame_linear) {
+    for (size_t k = 0; k < float_frame.num_channels(); ++k) {
+      rtc::ArrayView<float> channel_view = float_frame.channel(k);
+      for (auto& sample : channel_view) {
+        sample *= gain_at_end_of_frame_linear;
+      }
+    }
+    return;
+  }
+
+  // The gain changes. We have to change slowly to avoid discontinuities.
+  const float increment = (gain_at_end_of_frame_linear - last_gain_linear) *
+                          inverse_samples_per_channel;
+  float gain = last_gain_linear;
+  for (size_t i = 0; i < float_frame.samples_per_channel(); ++i) {
+    for (size_t ch = 0; ch < float_frame.num_channels(); ++ch) {
+      float_frame.channel(ch)[i] *= gain;
+    }
+    gain += increment;
+  }
+}
+
+}  // namespace
+
+GainApplier::GainApplier(bool hard_clip_samples, float initial_gain_factor)
+    : hard_clip_samples_(hard_clip_samples),
+      last_gain_factor_(initial_gain_factor),
+      current_gain_factor_(initial_gain_factor) {}
+
+void GainApplier::ApplyGain(AudioFrameView<float> signal) {
+  if (static_cast<int>(signal.samples_per_channel()) != samples_per_channel_) {
+    Initialize(signal.samples_per_channel());
+  }
+
+  ApplyGainWithRamping(last_gain_factor_, current_gain_factor_,
+                       inverse_samples_per_channel_, signal);
+
+  last_gain_factor_ = current_gain_factor_;
+
+  if (hard_clip_samples_) {
+    ClipSignal(signal);
+  }
+}
+
+void GainApplier::SetGainFactor(float gain_factor) {
+  RTC_DCHECK_GT(gain_factor, 0.f);
+  current_gain_factor_ = gain_factor;
+}
+
+void GainApplier::Initialize(size_t samples_per_channel) {
+  RTC_DCHECK_GT(samples_per_channel, 0);
+  samples_per_channel_ = static_cast<int>(samples_per_channel);
+  inverse_samples_per_channel_ = 1.f / samples_per_channel_;
+}
+
+}  // namespace webrtc

modules/audio_processing/agc2/gain_applier.h

@@ -0,0 +1,41 @@
+/*
+ *  Copyright (c) 2018 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.
+ */
+
+#ifndef MODULES_AUDIO_PROCESSING_AGC2_GAIN_APPLIER_H_
+#define MODULES_AUDIO_PROCESSING_AGC2_GAIN_APPLIER_H_
+
+#include "modules/audio_processing/include/audio_frame_view.h"
+
+namespace webrtc {
+class GainApplier {
+ public:
+  GainApplier(bool hard_clip_samples, float initial_gain_factor);
+
+  void ApplyGain(AudioFrameView<float> signal);
+  void SetGainFactor(float gain_factor);
+
+ private:
+  void Initialize(size_t samples_per_channel);
+
+  // Whether to clip samples after gain is applied. If 'true', result
+  // will fit in FloatS16 range.
+  const bool hard_clip_samples_;
+  float last_gain_factor_;
+
+  // If this value is not equal to 'last_gain_factor', gain will be
+  // ramped from 'last_gain_factor_' to this value during the next
+  // 'ApplyGain'.
+  float current_gain_factor_;
+  int samples_per_channel_ = -1;
+  float inverse_samples_per_channel_ = -1.f;
+};
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AGC2_GAIN_APPLIER_H_

modules/audio_processing/agc2/gain_applier_unittest.cc

@@ -0,0 +1,94 @@
+/*
+ *  Copyright (c) 2018 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 "modules/audio_processing/agc2/gain_applier.h"
+
+#include <math.h>
+
+#include <algorithm>
+#include <limits>
+
+#include "modules/audio_processing/agc2/vector_float_frame.h"
+#include "rtc_base/gunit.h"
+
+namespace webrtc {
+TEST(AutomaticGainController2GainApplier, InitialGainIsRespected) {
+  constexpr float initial_signal_level = 123.f;
+  constexpr float gain_factor = 10.f;
+  VectorFloatFrame fake_audio(1, 1, initial_signal_level);
+  GainApplier gain_applier(true, gain_factor);
+
+  gain_applier.ApplyGain(fake_audio.float_frame_view());
+  EXPECT_NEAR(fake_audio.float_frame_view().channel(0)[0],
+              initial_signal_level * gain_factor, 0.1f);
+}
+
+TEST(AutomaticGainController2GainApplier, ClippingIsDone) {
+  constexpr float initial_signal_level = 30000.f;
+  constexpr float gain_factor = 10.f;
+  VectorFloatFrame fake_audio(1, 1, initial_signal_level);
+  GainApplier gain_applier(true, gain_factor);
+
+  gain_applier.ApplyGain(fake_audio.float_frame_view());
+  EXPECT_NEAR(fake_audio.float_frame_view().channel(0)[0],
+              std::numeric_limits<int16_t>::max(), 0.1f);
+}
+
+TEST(AutomaticGainController2GainApplier, ClippingIsNotDone) {
+  constexpr float initial_signal_level = 30000.f;
+  constexpr float gain_factor = 10.f;
+  VectorFloatFrame fake_audio(1, 1, initial_signal_level);
+  GainApplier gain_applier(false, gain_factor);
+
+  gain_applier.ApplyGain(fake_audio.float_frame_view());
+
+  EXPECT_NEAR(fake_audio.float_frame_view().channel(0)[0],
+              initial_signal_level * gain_factor, 0.1f);
+}
+
+TEST(AutomaticGainController2GainApplier, RampingIsDone) {
+  constexpr float initial_signal_level = 30000.f;
+  constexpr float initial_gain_factor = 1.f;
+  constexpr float target_gain_factor = 0.5f;
+  constexpr int num_channels = 3;
+  constexpr int samples_per_channel = 4;
+  VectorFloatFrame fake_audio(num_channels, samples_per_channel,
+                              initial_signal_level);
+  GainApplier gain_applier(false, initial_gain_factor);
+
+  gain_applier.SetGainFactor(target_gain_factor);
+  gain_applier.ApplyGain(fake_audio.float_frame_view());
+
+  // The maximal gain change should be close to that in linear interpolation.
+  for (size_t channel = 0; channel < num_channels; ++channel) {
+    float max_signal_change = 0.f;
+    float last_signal_level = initial_signal_level;
+    for (const auto sample : fake_audio.float_frame_view().channel(channel)) {
+      const float current_change = fabs(last_signal_level - sample);
+      max_signal_change =
+          std::max(max_signal_change, current_change);
+      last_signal_level = sample;
+    }
+    const float total_gain_change =
+        fabs((initial_gain_factor - target_gain_factor) * initial_signal_level);
+    EXPECT_NEAR(max_signal_change, total_gain_change / samples_per_channel,
+                0.1f);
+  }
+
+  // Next frame should have the desired level.
+  VectorFloatFrame next_fake_audio_frame(num_channels, samples_per_channel,
+                                         initial_signal_level);
+  gain_applier.ApplyGain(next_fake_audio_frame.float_frame_view());
+
+  // The last sample should have the new gain.
+  EXPECT_NEAR(next_fake_audio_frame.float_frame_view().channel(0)[0],
+              initial_signal_level * target_gain_factor, 0.1f);
+}
+}  // namespace webrtc