1 files changed, 141 insertions, 0 deletions
diff --git a/third_party/libwebrtc/modules/audio_processing/agc2/rnn_vad/lp_residual.cc b/third_party/libwebrtc/modules/audio_processing/agc2/rnn_vad/lp_residual.cc
new file mode 100644
index 0000000000..484bfba459
--- /dev/null
+++ b/third_party/libwebrtc/modules/audio_processing/agc2/rnn_vad/lp_residual.cc
@@ -0,0 +1,141 @@
+/*
+ *  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/lp_residual.h"
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <numeric>
+
+#include "rtc_base/checks.h"
+#include "rtc_base/numerics/safe_compare.h"
+
+namespace webrtc {
+namespace rnn_vad {
+namespace {
+
+// Computes auto-correlation coefficients for `x` and writes them in
+// `auto_corr`. The lag values are in {0, ..., max_lag - 1}, where max_lag
+// equals the size of `auto_corr`.
+void ComputeAutoCorrelation(
+    rtc::ArrayView<const float> x,
+    rtc::ArrayView<float, kNumLpcCoefficients> auto_corr) {
+  constexpr int max_lag = auto_corr.size();
+  RTC_DCHECK_LT(max_lag, x.size());
+  for (int lag = 0; lag < max_lag; ++lag) {
+    auto_corr[lag] =
+        std::inner_product(x.begin(), x.end() - lag, x.begin() + lag, 0.f);
+  }
+}
+
+// Applies denoising to the auto-correlation coefficients.
+void DenoiseAutoCorrelation(
+    rtc::ArrayView<float, kNumLpcCoefficients> auto_corr) {
+  // Assume -40 dB white noise floor.
+  auto_corr[0] *= 1.0001f;
+  // Hard-coded values obtained as
+  // [np.float32((0.008*0.008*i*i)) for i in range(1,5)].
+  auto_corr[1] -= auto_corr[1] * 0.000064f;
+  auto_corr[2] -= auto_corr[2] * 0.000256f;
+  auto_corr[3] -= auto_corr[3] * 0.000576f;
+  auto_corr[4] -= auto_corr[4] * 0.001024f;
+  static_assert(kNumLpcCoefficients == 5, "Update `auto_corr`.");
+}
+
+// Computes the initial inverse filter coefficients given the auto-correlation
+// coefficients of an input frame.
+void ComputeInitialInverseFilterCoefficients(
+    rtc::ArrayView<const float, kNumLpcCoefficients> auto_corr,
+    rtc::ArrayView<float, kNumLpcCoefficients - 1> lpc_coeffs) {
+  float error = auto_corr[0];
+  for (int i = 0; i < kNumLpcCoefficients - 1; ++i) {
+    float reflection_coeff = 0.f;
+    for (int j = 0; j < i; ++j) {
+      reflection_coeff += lpc_coeffs[j] * auto_corr[i - j];
+    }
+    reflection_coeff += auto_corr[i + 1];
+
+    // Avoid division by numbers close to zero.
+    constexpr float kMinErrorMagnitude = 1e-6f;
+    if (std::fabs(error) < kMinErrorMagnitude) {
+      error = std::copysign(kMinErrorMagnitude, error);
+    }
+
+    reflection_coeff /= -error;
+    // Update LPC coefficients and total error.
+    lpc_coeffs[i] = reflection_coeff;
+    for (int j = 0; j < ((i + 1) >> 1); ++j) {
+      const float tmp1 = lpc_coeffs[j];
+      const float tmp2 = lpc_coeffs[i - 1 - j];
+      lpc_coeffs[j] = tmp1 + reflection_coeff * tmp2;
+      lpc_coeffs[i - 1 - j] = tmp2 + reflection_coeff * tmp1;
+    }
+    error -= reflection_coeff * reflection_coeff * error;
+    if (error < 0.001f * auto_corr[0]) {
+      break;
+    }
+  }
+}
+
+}  // namespace
+
+void ComputeAndPostProcessLpcCoefficients(
+    rtc::ArrayView<const float> x,
+    rtc::ArrayView<float, kNumLpcCoefficients> lpc_coeffs) {
+  std::array<float, kNumLpcCoefficients> auto_corr;
+  ComputeAutoCorrelation(x, auto_corr);
+  if (auto_corr[0] == 0.f) {  // Empty frame.
+    std::fill(lpc_coeffs.begin(), lpc_coeffs.end(), 0);
+    return;
+  }
+  DenoiseAutoCorrelation(auto_corr);
+  std::array<float, kNumLpcCoefficients - 1> lpc_coeffs_pre{};
+  ComputeInitialInverseFilterCoefficients(auto_corr, lpc_coeffs_pre);
+  // LPC coefficients post-processing.
+  // TODO(bugs.webrtc.org/9076): Consider removing these steps.
+  lpc_coeffs_pre[0] *= 0.9f;
+  lpc_coeffs_pre[1] *= 0.9f * 0.9f;
+  lpc_coeffs_pre[2] *= 0.9f * 0.9f * 0.9f;
+  lpc_coeffs_pre[3] *= 0.9f * 0.9f * 0.9f * 0.9f;
+  constexpr float kC = 0.8f;
+  lpc_coeffs[0] = lpc_coeffs_pre[0] + kC;
+  lpc_coeffs[1] = lpc_coeffs_pre[1] + kC * lpc_coeffs_pre[0];
+  lpc_coeffs[2] = lpc_coeffs_pre[2] + kC * lpc_coeffs_pre[1];
+  lpc_coeffs[3] = lpc_coeffs_pre[3] + kC * lpc_coeffs_pre[2];
+  lpc_coeffs[4] = kC * lpc_coeffs_pre[3];
+  static_assert(kNumLpcCoefficients == 5, "Update `lpc_coeffs(_pre)`.");
+}
+
+void ComputeLpResidual(
+    rtc::ArrayView<const float, kNumLpcCoefficients> lpc_coeffs,
+    rtc::ArrayView<const float> x,
+    rtc::ArrayView<float> y) {
+  RTC_DCHECK_GT(x.size(), kNumLpcCoefficients);
+  RTC_DCHECK_EQ(x.size(), y.size());
+  // The code below implements the following operation:
+  // y[i] = x[i] + dot_product({x[i], ..., x[i - kNumLpcCoefficients + 1]},
+  //                           lpc_coeffs)
+  // Edge case: i < kNumLpcCoefficients.
+  y[0] = x[0];
+  for (int i = 1; i < kNumLpcCoefficients; ++i) {
+    y[i] =
+        std::inner_product(x.crend() - i, x.crend(), lpc_coeffs.cbegin(), x[i]);
+  }
+  // Regular case.
+  auto last = x.crend();
+  for (int i = kNumLpcCoefficients; rtc::SafeLt(i, y.size()); ++i, --last) {
+    y[i] = std::inner_product(last - kNumLpcCoefficients, last,
+                              lpc_coeffs.cbegin(), x[i]);
+  }
+}
+
+}  // namespace rnn_vad
+}  // namespace webrtc