| /* |
| * 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_ |