AGC2 RNN VAD: Spectral features extraction.

This CL defines SpectralFeaturesExtractor which is responsible for
computing the spectral features used as input for the RNN.

Bug: webrtc:9076
Change-Id: I5e1396b89eca9c13bb268e8419a16436a9c3450f
Reviewed-on: https://webrtc-review.googlesource.com/73760
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23206}

modules/audio_processing/agc2/rnn_vad/BUILD.gn

@@ -30,6 +30,8 @@ source_set("lib") {
     "rnn.cc",
     "rnn.h",
     "sequence_buffer.h",
+    "spectral_features.cc",
+    "spectral_features.h",
     "spectral_features_internal.cc",
     "spectral_features_internal.h",
     "symmetric_matrix_buffer.h",
@@ -90,6 +92,7 @@ if (rtc_include_tests) {
       "rnn_unittest.cc",
       "sequence_buffer_unittest.cc",
       "spectral_features_internal_unittest.cc",
+      "spectral_features_unittest.cc",
       "symmetric_matrix_buffer_unittest.cc",
     ]
     deps = [

modules/audio_processing/agc2/rnn_vad/common.h

@@ -51,6 +51,14 @@ constexpr int kBandFrequencyBoundaries[kNumBands] = {
     0,    200,  400,  600,  800,  1000, 1200, 1400, 1600,  2000,  2400,
     2800, 3200, 4000, 4800, 5600, 6800, 8000, 9600, 12000, 15600, 20000};
 
+// Feature extraction parameters.
+constexpr size_t kNumLowerBands = 6;
+static_assert((0 < kNumLowerBands) && (kNumLowerBands < kNumBands), "");
+constexpr size_t kSpectralCoeffsHistorySize = 8;
+static_assert(kSpectralCoeffsHistorySize > 2,
+              "The history size must at least be 3 to compute first and second "
+              "derivatives.");
+
 constexpr size_t kFeatureVectorSize = 42;
 
 }  // namespace rnn_vad

modules/audio_processing/agc2/rnn_vad/pitch_search.cc

@@ -14,6 +14,10 @@
 namespace webrtc {
 namespace rnn_vad {
 
+// TODO(bugs.webrtc.org/9076): To decrease the stack size, add a class that uses
+// std::vector instances instead of the local arrays used in PitchSearch(). It
+// is also useful once https://webrtc-review.googlesource.com/c/src/+/73366
+// lands.
 PitchInfo PitchSearch(rtc::ArrayView<const float, kBufSize24kHz> pitch_buf,
                       PitchInfo prev_pitch_48kHz,
                       RealFourier* fft) {

modules/audio_processing/agc2/rnn_vad/sequence_buffer.h

@@ -72,6 +72,7 @@ class SequenceBuffer {
   }
 
  private:
+  // TODO(bugs.webrtc.org/9076): Switch to std::vector to decrease stack size.
   std::array<T, S> buffer_;
 };
 

modules/audio_processing/agc2/rnn_vad/spectral_features.cc

@@ -0,0 +1,210 @@
+/*
+ *  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/rnn_vad/spectral_features.h"
+
+#include <algorithm>
+#include <cmath>
+#include <limits>
+#include <numeric>
+
+#include "modules/audio_processing/agc2/rnn_vad/spectral_features_internal.h"
+#include "rtc_base/checks.h"
+
+namespace webrtc {
+namespace rnn_vad {
+namespace {
+
+constexpr float kSilenceThreshold = 0.04f;
+
+// Computes the new spectral difference stats and pushes them into the passed
+// symmetric matrix buffer.
+void UpdateSpectralDifferenceStats(
+    rtc::ArrayView<const float, kNumBands> new_spectral_coeffs,
+    const RingBuffer<float, kNumBands, kSpectralCoeffsHistorySize>& ring_buf,
+    SymmetricMatrixBuffer<float, kSpectralCoeffsHistorySize>* sym_matrix_buf) {
+  RTC_DCHECK(sym_matrix_buf);
+  // Compute the new spectral distance stats.
+  std::array<float, kSpectralCoeffsHistorySize - 1> distances;
+  for (size_t i = 0; i < kSpectralCoeffsHistorySize - 1; ++i) {
+    const size_t delay = i + 1;
+    auto old_spectral_coeffs = ring_buf.GetArrayView(delay);
+    distances[i] = 0.f;
+    for (size_t k = 0; k < kNumBands; ++k) {
+      const float c = new_spectral_coeffs[k] - old_spectral_coeffs[k];
+      distances[i] += c * c;
+    }
+  }
+  // Push the new spectral distance stats into the symmetric matrix buffer.
+  sym_matrix_buf->Push({distances.data(), distances.size()});
+}
+
+}  // namespace
+
+SpectralFeaturesView::SpectralFeaturesView(
+    rtc::ArrayView<float, kNumBands - kNumLowerBands> coeffs,
+    rtc::ArrayView<float, kNumLowerBands> average,
+    rtc::ArrayView<float, kNumLowerBands> first_derivative,
+    rtc::ArrayView<float, kNumLowerBands> second_derivative,
+    rtc::ArrayView<float, kNumLowerBands> cross_correlations,
+    float* variability)
+    : coeffs(coeffs),
+      average(average),
+      first_derivative(first_derivative),
+      second_derivative(second_derivative),
+      cross_correlations(cross_correlations),
+      variability(variability) {}
+
+SpectralFeaturesView::SpectralFeaturesView(const SpectralFeaturesView&) =
+    default;
+SpectralFeaturesView::~SpectralFeaturesView() = default;
+
+SpectralFeaturesExtractor::SpectralFeaturesExtractor()
+    : fft_(),
+      reference_frame_fft_(kFrameSize20ms24kHz),
+      lagged_frame_fft_(kFrameSize20ms24kHz),
+      band_boundaries_(
+          ComputeBandBoundaryIndexes(kSampleRate24kHz, kFrameSize20ms24kHz)),
+      dct_table_(ComputeDctTable()) {}
+
+SpectralFeaturesExtractor::~SpectralFeaturesExtractor() = default;
+
+void SpectralFeaturesExtractor::Reset() {
+  spectral_coeffs_ring_buf_.Reset();
+  spectral_diffs_buf_.Reset();
+}
+
+bool SpectralFeaturesExtractor::CheckSilenceComputeFeatures(
+    rtc::ArrayView<const float, kFrameSize20ms24kHz> reference_frame,
+    rtc::ArrayView<const float, kFrameSize20ms24kHz> lagged_frame,
+    SpectralFeaturesView spectral_features) {
+  // Analyze reference frame.
+  fft_.ForwardFft(reference_frame, reference_frame_fft_);
+  ComputeBandEnergies(reference_frame_fft_,
+                      {band_boundaries_.data(), band_boundaries_.size()},
+                      {reference_frame_energy_coeffs_.data(),
+                       reference_frame_energy_coeffs_.size()});
+  // Check if the reference frame has silence.
+  const float tot_energy =
+      std::accumulate(reference_frame_energy_coeffs_.begin(),
+                      reference_frame_energy_coeffs_.end(), 0.f);
+  if (tot_energy < kSilenceThreshold)
+    return true;
+  // Analyze lagged frame.
+  fft_.ForwardFft(lagged_frame, lagged_frame_fft_);
+  ComputeBandEnergies(
+      lagged_frame_fft_, {band_boundaries_.data(), band_boundaries_.size()},
+      {lagged_frame_energy_coeffs_.data(), lagged_frame_energy_coeffs_.size()});
+  // Log of the band energies for the reference frame.
+  std::array<float, kNumBands> log_band_energy_coeffs;
+  ComputeLogBandEnergiesCoefficients(
+      {reference_frame_energy_coeffs_.data(),
+       reference_frame_energy_coeffs_.size()},
+      {log_band_energy_coeffs.data(), log_band_energy_coeffs.size()});
+  // Decorrelate band-wise log energy coefficients via DCT.
+  std::array<float, kNumBands> log_band_energy_coeffs_decorrelated;
+  ComputeDct({log_band_energy_coeffs.data(), log_band_energy_coeffs.size()},
+             {dct_table_.data(), dct_table_.size()},
+             {log_band_energy_coeffs_decorrelated.data(),
+              log_band_energy_coeffs_decorrelated.size()});
+  // Normalize (based on training set stats).
+  log_band_energy_coeffs_decorrelated[0] -= 12;
+  log_band_energy_coeffs_decorrelated[1] -= 4;
+  // Update the ring buffer and the spectral difference stats.
+  spectral_coeffs_ring_buf_.Push({log_band_energy_coeffs_decorrelated.data(),
+                                  log_band_energy_coeffs_decorrelated.size()});
+  UpdateSpectralDifferenceStats({log_band_energy_coeffs_decorrelated.data(),
+                                 log_band_energy_coeffs_decorrelated.size()},
+                                spectral_coeffs_ring_buf_,
+                                &spectral_diffs_buf_);
+  // Write the higher bands spectral coefficients.
+  auto coeffs_src = spectral_coeffs_ring_buf_.GetArrayView(0);
+  RTC_DCHECK_EQ(coeffs_src.size() - kNumLowerBands,
+                spectral_features.coeffs.size());
+  std::copy(coeffs_src.begin() + kNumLowerBands, coeffs_src.end(),
+            spectral_features.coeffs.begin());
+  // Compute and write remaining features.
+  ComputeAvgAndDerivatives(spectral_features.average,
+                           spectral_features.first_derivative,
+                           spectral_features.second_derivative);
+  ComputeCrossCorrelation(spectral_features.cross_correlations);
+  RTC_DCHECK(spectral_features.variability);
+  *(spectral_features.variability) = ComputeVariability();
+  return false;
+}
+
+void SpectralFeaturesExtractor::ComputeAvgAndDerivatives(
+    rtc::ArrayView<float, kNumLowerBands> average,
+    rtc::ArrayView<float, kNumLowerBands> first_derivative,
+    rtc::ArrayView<float, kNumLowerBands> second_derivative) {
+  auto curr = spectral_coeffs_ring_buf_.GetArrayView(0);
+  auto prev1 = spectral_coeffs_ring_buf_.GetArrayView(1);
+  auto prev2 = spectral_coeffs_ring_buf_.GetArrayView(2);
+  RTC_DCHECK_EQ(average.size(), first_derivative.size());
+  RTC_DCHECK_EQ(first_derivative.size(), second_derivative.size());
+  RTC_DCHECK_LE(average.size(), curr.size());
+  for (size_t i = 0; i < average.size(); ++i) {
+    // Average, kernel: [1, 1, 1].
+    average[i] = curr[i] + prev1[i] + prev2[i];
+    // First derivative, kernel: [1, 0, - 1].
+    first_derivative[i] = curr[i] - prev2[i];
+    // Second derivative, Laplacian kernel: [1, -2, 1].
+    second_derivative[i] = curr[i] - 2 * prev1[i] + prev2[i];
+  }
+}
+
+void SpectralFeaturesExtractor::ComputeCrossCorrelation(
+    rtc::ArrayView<float, kNumLowerBands> cross_correlations) {
+  const auto& x = reference_frame_fft_;
+  const auto& y = lagged_frame_fft_;
+  auto cross_corr = [x, y](const size_t freq_bin_index) -> float {
+    return (x[freq_bin_index].real() * y[freq_bin_index].real() +
+            x[freq_bin_index].imag() * y[freq_bin_index].imag());
+  };
+  std::array<float, kNumBands> cross_corr_coeffs;
+  constexpr size_t kNumFftPoints = kFrameSize20ms24kHz / 2 + 1;
+  ComputeBandCoefficients(
+      cross_corr, {band_boundaries_.data(), band_boundaries_.size()},
+      kNumFftPoints - 1, {cross_corr_coeffs.data(), cross_corr_coeffs.size()});
+  // Normalize.
+  for (size_t i = 0; i < cross_corr_coeffs.size(); ++i) {
+    cross_corr_coeffs[i] =
+        cross_corr_coeffs[i] /
+        std::sqrt(0.001f + reference_frame_energy_coeffs_[i] *
+                               lagged_frame_energy_coeffs_[i]);
+  }
+  // Decorrelate.
+  ComputeDct({cross_corr_coeffs.data(), cross_corr_coeffs.size()},
+             {dct_table_.data(), dct_table_.size()},
+             {cross_correlations.data(), cross_correlations.size()});
+  // Normalize (based on training set stats).
+  cross_correlations[0] -= 1.3f;
+  cross_correlations[1] -= 0.9f;
+}
+
+float SpectralFeaturesExtractor::ComputeVariability() {
+  // Compute spectral variability score.
+  float spec_variability = 0.f;
+  for (size_t delay1 = 0; delay1 < kSpectralCoeffsHistorySize; ++delay1) {
+    float min_dist = std::numeric_limits<float>::max();
+    for (size_t delay2 = 0; delay2 < kSpectralCoeffsHistorySize; ++delay2) {
+      if (delay1 == delay2)  // The distance would be 0.
+        continue;
+      min_dist =
+          std::min(min_dist, spectral_diffs_buf_.GetValue(delay1, delay2));
+    }
+    spec_variability += min_dist;
+  }
+  // Normalize (based on training set stats).
+  return spec_variability / kSpectralCoeffsHistorySize - 2.1f;
+}
+
+}  // namespace rnn_vad
+}  // namespace webrtc

modules/audio_processing/agc2/rnn_vad/spectral_features.h

@@ -0,0 +1,92 @@
+/*
+ *  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_RNN_VAD_SPECTRAL_FEATURES_H_
+#define MODULES_AUDIO_PROCESSING_AGC2_RNN_VAD_SPECTRAL_FEATURES_H_
+
+#include <array>
+#include <complex>
+#include <vector>
+
+#include "api/array_view.h"
+#include "modules/audio_processing/agc2/rnn_vad/common.h"
+#include "modules/audio_processing/agc2/rnn_vad/fft_util.h"
+#include "modules/audio_processing/agc2/rnn_vad/ring_buffer.h"
+#include "modules/audio_processing/agc2/rnn_vad/symmetric_matrix_buffer.h"
+
+namespace webrtc {
+namespace rnn_vad {
+
+// View on spectral features.
+class SpectralFeaturesView {
+ public:
+  SpectralFeaturesView(rtc::ArrayView<float, kNumBands - kNumLowerBands> coeffs,
+                       rtc::ArrayView<float, kNumLowerBands> average,
+                       rtc::ArrayView<float, kNumLowerBands> first_derivative,
+                       rtc::ArrayView<float, kNumLowerBands> second_derivative,
+                       rtc::ArrayView<float, kNumLowerBands> cross_correlations,
+                       float* variability);
+  SpectralFeaturesView(const SpectralFeaturesView&);
+  ~SpectralFeaturesView();
+  // Higher bands spectral coefficients.
+  const rtc::ArrayView<float, kNumBands - kNumLowerBands> coeffs;
+  // Average and first and second derivative over time for the lower bands.
+  const rtc::ArrayView<float, kNumLowerBands> average;
+  const rtc::ArrayView<float, kNumLowerBands> first_derivative;
+  const rtc::ArrayView<float, kNumLowerBands> second_derivative;
+  // Spectral cross-correlation for the lower bands.
+  const rtc::ArrayView<float, kNumLowerBands> cross_correlations;
+  // Spectral variability score.
+  float* const variability;
+};
+
+// Class to compute spectral features.
+class SpectralFeaturesExtractor {
+ public:
+  SpectralFeaturesExtractor();
+  SpectralFeaturesExtractor(const SpectralFeaturesExtractor&) = delete;
+  SpectralFeaturesExtractor& operator=(const SpectralFeaturesExtractor&) =
+      delete;
+  ~SpectralFeaturesExtractor();
+  // Resets the internal state of the feature extractor.
+  void Reset();
+  // Analyzes a pair of reference and lagged frames from the pitch buffer,
+  // detects silence and computes features. If silence is detected, the output
+  // is neither computed nor written.
+  bool CheckSilenceComputeFeatures(
+      rtc::ArrayView<const float, kFrameSize20ms24kHz> reference_frame,
+      rtc::ArrayView<const float, kFrameSize20ms24kHz> lagged_frame,
+      SpectralFeaturesView spectral_features);
+
+ private:
+  void ComputeAvgAndDerivatives(
+      rtc::ArrayView<float, kNumLowerBands> average,
+      rtc::ArrayView<float, kNumLowerBands> first_derivative,
+      rtc::ArrayView<float, kNumLowerBands> second_derivative);
+  void ComputeCrossCorrelation(
+      rtc::ArrayView<float, kNumLowerBands> cross_correlations);
+  float ComputeVariability();
+
+  BandAnalysisFft fft_;
+  std::vector<std::complex<float>> reference_frame_fft_;
+  std::vector<std::complex<float>> lagged_frame_fft_;
+  std::array<float, kNumBands> reference_frame_energy_coeffs_{};
+  std::array<float, kNumBands> lagged_frame_energy_coeffs_{};
+  const std::array<size_t, kNumBands> band_boundaries_;
+  const std::array<float, kNumBands * kNumBands> dct_table_;
+  RingBuffer<float, kNumBands, kSpectralCoeffsHistorySize>
+      spectral_coeffs_ring_buf_;
+  SymmetricMatrixBuffer<float, kSpectralCoeffsHistorySize> spectral_diffs_buf_;
+};
+
+}  // namespace rnn_vad
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AGC2_RNN_VAD_SPECTRAL_FEATURES_H_

modules/audio_processing/agc2/rnn_vad/spectral_features_internal.cc

@@ -14,7 +14,6 @@
 #include <cmath>
 
 #include "rtc_base/checks.h"
-#include "rtc_base/function_view.h"
 
 namespace webrtc {
 namespace rnn_vad {
@@ -23,11 +22,19 @@ namespace {
 // DCT scaling factor.
 const float kDctScalingFactor = std::sqrt(2.f / kNumBands);
 
-// Iterates through frequency bands and computes coefficients via |functor| for
-// triangular bands with peak response at each band boundary. |functor| returns
-// a floating point value for the FFT coefficient having index equal to the
-// argument passed to |functor|; that argument is in the range {0, ...
-// |max_freq_bin_index| - 1}.
+}  // namespace
+
+std::array<size_t, kNumBands> ComputeBandBoundaryIndexes(
+    size_t sample_rate_hz,
+    size_t frame_size_samples) {
+  std::array<size_t, kNumBands> indexes;
+  for (size_t i = 0; i < kNumBands; ++i) {
+    indexes[i] =
+        kBandFrequencyBoundaries[i] * frame_size_samples / sample_rate_hz;
+  }
+  return indexes;
+}
+
 void ComputeBandCoefficients(
     rtc::FunctionView<float(size_t)> functor,
     rtc::ArrayView<const size_t, kNumBands> band_boundaries,
@@ -64,25 +71,12 @@ void ComputeBandCoefficients(
   // (*): "size_t i" must be declared before the main loop.
 }
 
-}  // namespace
-
-std::array<size_t, kNumBands> ComputeBandBoundaryIndexes(
-    size_t sample_rate_hz,
-    size_t frame_size_samples) {
-  std::array<size_t, kNumBands> indexes;
-  for (size_t i = 0; i < kNumBands; ++i) {
-    indexes[i] =
-        kBandFrequencyBoundaries[i] * frame_size_samples / sample_rate_hz;
-  }
-  return indexes;
-}
-
 void ComputeBandEnergies(
     rtc::ArrayView<const std::complex<float>> fft_coeffs,
     rtc::ArrayView<const size_t, kNumBands> band_boundaries,
     rtc::ArrayView<float, kNumBands> band_energies) {
   RTC_DCHECK_EQ(band_boundaries.size(), band_energies.size());
-  auto functor = [fft_coeffs](const size_t freq_bin_index) {
+  auto functor = [fft_coeffs](const size_t freq_bin_index) -> float {
     return std::norm(fft_coeffs[freq_bin_index]);
   };
   ComputeBandCoefficients(functor, band_boundaries, fft_coeffs.size() - 1,

modules/audio_processing/agc2/rnn_vad/spectral_features_internal.h

@@ -16,6 +16,7 @@
 
 #include "api/array_view.h"
 #include "modules/audio_processing/agc2/rnn_vad/common.h"
+#include "rtc_base/function_view.h"
 
 namespace webrtc {
 namespace rnn_vad {
@@ -25,6 +26,17 @@ std::array<size_t, kNumBands> ComputeBandBoundaryIndexes(
     size_t sample_rate_hz,
     size_t frame_size_samples);
 
+// Iterates through frequency bands and computes coefficients via |functor| for
+// triangular bands with peak response at each band boundary. |functor| returns
+// a floating point value for the FFT coefficient having index equal to the
+// argument passed to |functor|; that argument is in the range {0, ...
+// |max_freq_bin_index| - 1}.
+void ComputeBandCoefficients(
+    rtc::FunctionView<float(size_t)> functor,
+    rtc::ArrayView<const size_t, kNumBands> band_boundaries,
+    const size_t max_freq_bin_index,
+    rtc::ArrayView<float, kNumBands> coefficients);
+
 // Given an array of FFT coefficients and a vector of band boundary indexes,
 // computes band energy coefficients.
 void ComputeBandEnergies(

modules/audio_processing/agc2/rnn_vad/spectral_features_unittest.cc

@@ -0,0 +1,129 @@
+/*
+ *  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/rnn_vad/spectral_features.h"
+
+#include <algorithm>
+
+#include "modules/audio_processing/agc2/rnn_vad/test_utils.h"
+#include "rtc_base/checks.h"
+// TODO(bugs.webrtc.org/8948): Add when the issue is fixed.
+// #include "test/fpe_observer.h"
+#include "test/gtest.h"
+
+namespace webrtc {
+namespace rnn_vad {
+namespace test {
+namespace {
+
+constexpr size_t kTestFeatureVectorSize = kNumBands + 3 * kNumLowerBands + 1;
+
+// Writes non-zero sample values.
+void WriteTestData(rtc::ArrayView<float> samples) {
+  for (size_t i = 0; i < samples.size(); ++i) {
+    samples[i] = i % 100;
+  }
+}
+
+SpectralFeaturesView GetSpectralFeaturesView(
+    std::array<float, kTestFeatureVectorSize>* feature_vector) {
+  return {
+      {feature_vector->data() + kNumLowerBands, kNumBands - kNumLowerBands},
+      {feature_vector->data(), kNumLowerBands},
+      {feature_vector->data() + kNumBands, kNumLowerBands},
+      {feature_vector->data() + kNumBands + kNumLowerBands, kNumLowerBands},
+      {feature_vector->data() + kNumBands + 2 * kNumLowerBands, kNumLowerBands},
+      &(*feature_vector)[kNumBands + 3 * kNumLowerBands]};
+}
+
+constexpr float kInitialFeatureVal = -9999.f;
+
+}  // namespace
+
+TEST(RnnVadTest, SpectralFeaturesWithAndWithoutSilence) {
+  // Initialize.
+  SpectralFeaturesExtractor sfe;
+  std::array<float, kFrameSize20ms24kHz> samples;
+  rtc::ArrayView<float, kFrameSize20ms24kHz> samples_view(samples.data(),
+                                                          samples.size());
+  bool is_silence;
+  std::array<float, kTestFeatureVectorSize> feature_vector;
+  auto feature_vector_view = GetSpectralFeaturesView(&feature_vector);
+
+  // Write an initial value in the feature vector to detect changes.
+  std::fill(feature_vector.begin(), feature_vector.end(), kInitialFeatureVal);
+
+  // TODO(bugs.webrtc.org/8948): Add when the issue is fixed.
+  // FloatingPointExceptionObserver fpe_observer;
+
+  // With silence.
+  std::fill(samples.begin(), samples.end(), 0.f);
+  is_silence = sfe.CheckSilenceComputeFeatures(samples_view, samples_view,
+                                               feature_vector_view);
+  // Silence is expected, the output won't be overwritten.
+  EXPECT_TRUE(is_silence);
+  EXPECT_TRUE(std::all_of(feature_vector.begin(), feature_vector.end(),
+                          [](float x) { return x == kInitialFeatureVal; }));
+
+  // With no silence.
+  WriteTestData(samples);
+  is_silence = sfe.CheckSilenceComputeFeatures(samples_view, samples_view,
+                                               feature_vector_view);
+  // Silence is not expected, the output will be overwritten.
+  EXPECT_FALSE(is_silence);
+  EXPECT_FALSE(std::all_of(feature_vector.begin(), feature_vector.end(),
+                           [](float x) { return x == kInitialFeatureVal; }));
+}
+
+// When the input signal does not change, the spectral coefficients average does
+// not change and the derivatives are zero. Similarly, the spectral variability
+// score does not change either.
+TEST(RnnVadTest, SpectralFeaturesConstantAverageZeroDerivative) {
+  // Initialize.
+  SpectralFeaturesExtractor sfe;
+  std::array<float, kFrameSize20ms24kHz> samples;
+  rtc::ArrayView<float, kFrameSize20ms24kHz> samples_view(samples.data(),
+                                                          samples.size());
+  WriteTestData(samples);
+  bool is_silence;
+
+  // Fill the spectral features with test data.
+  std::array<float, kTestFeatureVectorSize> feature_vector;
+  auto feature_vector_view = GetSpectralFeaturesView(&feature_vector);
+  for (size_t i = 0; i < kSpectralCoeffsHistorySize; ++i) {
+    is_silence = sfe.CheckSilenceComputeFeatures(samples_view, samples_view,
+                                                 feature_vector_view);
+  }
+
+  // Feed the test data one last time but using a different output vector.
+  std::array<float, kTestFeatureVectorSize> feature_vector_last;
+  auto feature_vector_last_view = GetSpectralFeaturesView(&feature_vector_last);
+  is_silence = sfe.CheckSilenceComputeFeatures(samples_view, samples_view,
+                                               feature_vector_last_view);
+
+  // Average is unchanged.
+  ExpectEqualFloatArray({feature_vector.data(), kNumLowerBands},
+                        {feature_vector_last.data(), kNumLowerBands});
+  // First and second derivatives are zero.
+  constexpr std::array<float, kNumLowerBands> zeros{};
+  ExpectEqualFloatArray(
+      {feature_vector_last.data() + kNumBands, kNumLowerBands},
+      {zeros.data(), zeros.size()});
+  ExpectEqualFloatArray(
+      {feature_vector_last.data() + kNumBands + kNumLowerBands, kNumLowerBands},
+      {zeros.data(), zeros.size()});
+  // Spectral variability is unchanged.
+  EXPECT_FLOAT_EQ(feature_vector[kNumBands + 3 * kNumLowerBands],
+                  feature_vector_last[kNumBands + 3 * kNumLowerBands]);
+}
+
+}  // namespace test
+}  // namespace rnn_vad
+}  // namespace webrtc