Added first layer of the echo canceller 3 functionality

webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.h

Index: webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.h
diff --git a/webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.h b/webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.h
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index 0000000000000000000000000000000000000000..0f587f5913b5f01eeaa92473c7dc8ffa26f7f397
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+++ b/webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.h
@@ -0,0 +1,54 @@
+/*
+ *  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 WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_CASCADED_BIQUAD_FILTER_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_CASCADED_BIQUAD_FILTER_H_
+
+#include <vector>
+
+#include "webrtc/base/array_view.h"
+#include "webrtc/base/constructormagic.h"
+
+namespace webrtc {
+
+class CascadedBiQuadFilter {

[hlundin-webrtc, 2016/12/16 10:04:46]

Short comment for the class. Cascades a number of identical (?) biquads.

[ivoc, 2016/12/19 11:30:22]

It would also be nice to make a note of which form this class implements, i.e.
direct form 1, 2 or something else.

[peah-webrtc, 2016/12/20 10:10:25]

Done.

[peah-webrtc, 2016/12/20 10:10:25]

Done.

+ public:
+  struct BiQuadState {

[ivoc, 2016/12/19 11:30:22]

I think you can consider restructuring this a little bit. I think it would be
nice to have one class called BiQuadFilter that implements a single filter. It
could have it's own state and coefficients as members and it should have a
process function. WDYT?

[peah-webrtc, 2016/12/20 10:10:24]

It would be nicer code-wise but I prefer to have it as one class as it will be
faster (due to the same coefficients being used for all states) and have a lower
memory footprint, as long as not biquads of different coefficients are used (I
think the far most common thing is to have biquads of the same coefficients).

Even though only one biquad is used in cascade in the current code, another part
of the aec3 code that is yet to be landed is using this functionality with more
than one biquad in cascade.

WDYT?

[ivoc, 2016/12/21 13:04:04]

Ok, that makes sense, let's leave it like this.

[peah-webrtc, 2016/12/21 23:13:48]

Acknowledged.

+    float b[2];

[hlundin-webrtc, 2016/12/16 10:04:46]

Slightly confusing that both the state and the coefficients have components 'a'
and 'b'. I think the state should have x and y instead.

[peah-webrtc, 2016/12/20 10:10:25]

Agree.
Done.

+    float a[2];
+  };
+
+  struct BiQuadCoefficients {
+    float b[3];
+    float a[2];
+  };
+
+  CascadedBiQuadFilter(
+      const CascadedBiQuadFilter::BiQuadCoefficients& coefficients,
+      size_t num_biquads);
+  ~CascadedBiQuadFilter();
+  void Process(rtc::ArrayView<const float> x, rtc::ArrayView<float> y);

[hlundin-webrtc, 2016/12/16 10:04:46]

Comment on the Process methods, and how they differ.

[peah-webrtc, 2016/12/20 10:10:24]

Done.

+  void Process(rtc::ArrayView<float> y);
+
+ private:
+  void ApplyBiQuad(rtc::ArrayView<const float> x,
+                   rtc::ArrayView<float> y,
+                   CascadedBiQuadFilter::BiQuadState* biquad_state);
+
+  const size_t num_biquads_;

[ivoc, 2016/12/19 11:30:22]

I don't think this is used anywhere. Also, the size is available from the vector
if you need it later. So, I think it should be removed.

[peah-webrtc, 2016/12/20 10:10:25]

Great find!
Done.

+  std::vector<BiQuadState> biquad_states_;
+  BiQuadCoefficients coefficients_;
+
+  RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(CascadedBiQuadFilter);
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_AEC3_CASCADED_BIQUAD_FILTER_H_