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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 WEBRTC_MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_
#define WEBRTC_MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_
#include <cstring>
#include <vector>
#include "webrtc/common_audio/sparse_fir_filter.h"
#include "webrtc/system_wrappers/include/scoped_vector.h"
namespace webrtc {
// 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:
explicit ThreeBandFilterBank(size_t length);
// Splits |in| into 3 downsampled frequency bands in |out|.
// |length| is the |in| length. Each of the 3 bands of |out| has to have a
// length of |length| / 3.
void Analysis(const float* in, size_t length, float* const* out);
// Merges the 3 downsampled frequency bands in |in| into |out|.
// |split_length| is the length of each band of |in|. |out| has to have at
// least a length of 3 * |split_length|.
void Synthesis(const float* const* in, size_t split_length, float* out);
private:
void DownModulate(const float* in,
size_t split_length,
size_t offset,
float* const* out);
void UpModulate(const float* const* in,
size_t split_length,
size_t offset,
float* out);
std::vector<float> in_buffer_;
std::vector<float> out_buffer_;
ScopedVector<SparseFIRFilter> analysis_filters_;
ScopedVector<SparseFIRFilter> synthesis_filters_;
std::vector<std::vector<float>> dct_modulation_;
};
} // namespace webrtc
#endif // WEBRTC_MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_
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