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diff --git a/third_party/libwebrtc/modules/audio_processing/agc2/biquad_filter.h b/third_party/libwebrtc/modules/audio_processing/agc2/biquad_filter.h
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+/*
+ *  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.
+ */
+
+#ifndef MODULES_AUDIO_PROCESSING_AGC2_BIQUAD_FILTER_H_
+#define MODULES_AUDIO_PROCESSING_AGC2_BIQUAD_FILTER_H_
+
+#include "api/array_view.h"
+
+namespace webrtc {
+
+// Transposed direct form I implementation of a bi-quad filter.
+//        b[0] + b[1] • z^(-1) + b[2] • z^(-2)
+// H(z) = ------------------------------------
+//          1 + a[1] • z^(-1) + a[2] • z^(-2)
+class BiQuadFilter {
+ public:
+  // Normalized filter coefficients.
+  // Computed as `[b, a] = scipy.signal.butter(N=2, Wn, btype)`.
+  struct Config {
+    float b[3];  // b[0], b[1], b[2].
+    float a[2];  // a[1], a[2].
+  };
+
+  explicit BiQuadFilter(const Config& config);
+  BiQuadFilter(const BiQuadFilter&) = delete;
+  BiQuadFilter& operator=(const BiQuadFilter&) = delete;
+  ~BiQuadFilter();
+
+  // Sets the filter configuration and resets the internal state.
+  void SetConfig(const Config& config);
+
+  // Zeroes the filter state.
+  void Reset();
+
+  // Filters `x` and writes the output in `y`, which must have the same length
+  // of `x`. In-place processing is supported.
+  void Process(rtc::ArrayView<const float> x, rtc::ArrayView<float> y);
+
+ private:
+  Config config_;
+  struct State {
+    float b[2];
+    float a[2];
+  } state_;
+};
+
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AGC2_BIQUAD_FILTER_H_