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diff --git a/webrtc/modules/audio_processing/agc2/rnn_vad/spectral_features_internal.h b/webrtc/modules/audio_processing/agc2/rnn_vad/spectral_features_internal.h
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+/*
+ *  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_INTERNAL_H_
+#define MODULES_AUDIO_PROCESSING_AGC2_RNN_VAD_SPECTRAL_FEATURES_INTERNAL_H_
+
+#include <stddef.h>
+
+#include <array>
+#include <vector>
+
+#include "api/array_view.h"
+#include "modules/audio_processing/agc2/rnn_vad/common.h"
+
+namespace webrtc {
+namespace rnn_vad {
+
+// At a sample rate of 24 kHz, the last 3 Opus bands are beyond the Nyquist
+// frequency. However, band #19 gets the contributions from band #18 because
+// of the symmetric triangular filter with peak response at 12 kHz.
+constexpr size_t kOpusBands24kHz = 20;
+static_assert(kOpusBands24kHz < kNumBands,
+              "The number of bands at 24 kHz must be less than those defined "
+              "in the Opus scale at 48 kHz.");
+
+// Number of FFT frequency bins covered by each band in the Opus scale at a
+// sample rate of 24 kHz for 20 ms frames.
+// Declared here for unit testing.
+constexpr std::array<int, kOpusBands24kHz - 1> GetOpusScaleNumBins24kHz20ms() {
+  return {4, 4, 4, 4, 4, 4, 4, 4, 8, 8, 8, 8, 16, 16, 16, 24, 24, 32, 48};
+}
+
+// TODO(bugs.webrtc.org/10480): Move to a separate file.
+// Class to compute band-wise spectral features in the Opus perceptual scale
+// for 20 ms frames sampled at 24 kHz. The analysis methods apply triangular
+// filters with peak response at the each band boundary.
+class SpectralCorrelator {
+ public:
+  // Ctor.
+  SpectralCorrelator();
+  SpectralCorrelator(const SpectralCorrelator&) = delete;
+  SpectralCorrelator& operator=(const SpectralCorrelator&) = delete;
+  ~SpectralCorrelator();
+
+  // Computes the band-wise spectral auto-correlations.
+  // |x| must:
+  //  - have size equal to |kFrameSize20ms24kHz|;
+  //  - be encoded as vectors of interleaved real-complex FFT coefficients
+  //    where x[1] = y[1] = 0 (the Nyquist frequency coefficient is omitted).
+  void ComputeAutoCorrelation(
+      rtc::ArrayView<const float> x,
+      rtc::ArrayView<float, kOpusBands24kHz> auto_corr) const;
+
+  // Computes the band-wise spectral cross-correlations.
+  // |x| and |y| must:
+  //  - have size equal to |kFrameSize20ms24kHz|;
+  //  - be encoded as vectors of interleaved real-complex FFT coefficients where
+  //    x[1] = y[1] = 0 (the Nyquist frequency coefficient is omitted).
+  void ComputeCrossCorrelation(
+      rtc::ArrayView<const float> x,
+      rtc::ArrayView<const float> y,
+      rtc::ArrayView<float, kOpusBands24kHz> cross_corr) const;
+
+ private:
+  const std::vector<float> weights_;  // Weights for each Fourier coefficient.
+};
+
+// TODO(bugs.webrtc.org/10480): Move to anonymous namespace in
+// spectral_features.cc. Given a vector of Opus-bands energy coefficients,
+// computes the log magnitude spectrum applying smoothing both over time and
+// over frequency. Declared here for unit testing.
+void ComputeSmoothedLogMagnitudeSpectrum(
+    rtc::ArrayView<const float> bands_energy,
+    rtc::ArrayView<float, kNumBands> log_bands_energy);
+
+// TODO(bugs.webrtc.org/10480): Move to anonymous namespace in
+// spectral_features.cc. Creates a DCT table for arrays having size equal to
+// |kNumBands|. Declared here for unit testing.
+std::array<float, kNumBands * kNumBands> ComputeDctTable();
+
+// TODO(bugs.webrtc.org/10480): Move to anonymous namespace in
+// spectral_features.cc. Computes DCT for |in| given a pre-computed DCT table.
+// In-place computation is not allowed and |out| can be smaller than |in| in
+// order to only compute the first DCT coefficients. Declared here for unit
+// testing.
+void ComputeDct(rtc::ArrayView<const float> in,
+                rtc::ArrayView<const float, kNumBands * kNumBands> dct_table,
+                rtc::ArrayView<float> out);
+
+}  // namespace rnn_vad
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AGC2_RNN_VAD_SPECTRAL_FEATURES_INTERNAL_H_