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/*
* Copyright (c) 2015 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_THREE_BAND_FILTER_BANK_H_
#define MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_
#include <array>
#include <cstring>
#include <memory>
#include <vector>
#include "api/array_view.h"
namespace webrtc {
constexpr int kSparsity = 4;
constexpr int kStrideLog2 = 2;
constexpr int kStride = 1 << kStrideLog2;
constexpr int kNumZeroFilters = 2;
constexpr int kFilterSize = 4;
constexpr int kMemorySize = kFilterSize * kStride - 1;
static_assert(kMemorySize == 15,
"The memory size must be sufficient to provide memory for the "
"shifted filters");
// An implementation of a 3-band FIR filter-bank with DCT modulation, similar to
// the proposed in "Multirate Signal Processing for Communication Systems" by
// Fredric J Harris.
// The low-pass filter prototype has these characteristics:
// * Pass-band ripple = 0.3dB
// * Pass-band frequency = 0.147 (7kHz at 48kHz)
// * Stop-band attenuation = 40dB
// * Stop-band frequency = 0.192 (9.2kHz at 48kHz)
// * Delay = 24 samples (500us at 48kHz)
// * Linear phase
// This filter bank does not satisfy perfect reconstruction. The SNR after
// analysis and synthesis (with no processing in between) is approximately 9.5dB
// depending on the input signal after compensating for the delay.
class ThreeBandFilterBank final {
public:
static const int kNumBands = 3;
static const int kFullBandSize = 480;
static const int kSplitBandSize =
ThreeBandFilterBank::kFullBandSize / ThreeBandFilterBank::kNumBands;
static const int kNumNonZeroFilters =
kSparsity * ThreeBandFilterBank::kNumBands - kNumZeroFilters;
ThreeBandFilterBank();
~ThreeBandFilterBank();
// Splits |in| of size kFullBandSize into 3 downsampled frequency bands in
// |out|, each of size 160.
void Analysis(rtc::ArrayView<const float, kFullBandSize> in,
rtc::ArrayView<const rtc::ArrayView<float>, kNumBands> out);
// Merges the 3 downsampled frequency bands in |in|, each of size 160, into
// |out|, which is of size kFullBandSize.
void Synthesis(rtc::ArrayView<const rtc::ArrayView<float>, kNumBands> in,
rtc::ArrayView<float, kFullBandSize> out);
private:
std::array<std::array<float, kMemorySize>, kNumNonZeroFilters>
state_analysis_;
std::array<std::array<float, kMemorySize>, kNumNonZeroFilters>
state_synthesis_;
};
} // namespace webrtc
#endif // MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_
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