Line data    Source code

       1             : /*
       2             :  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
       3             :  *
       4             :  *  Use of this source code is governed by a BSD-style license
       5             :  *  that can be found in the LICENSE file in the root of the source
       6             :  *  tree. An additional intellectual property rights grant can be found
       7             :  *  in the file PATENTS.  All contributing project authors may
       8             :  *  be found in the AUTHORS file in the root of the source tree.
       9             :  */
      10             : 
      11             : #ifndef WEBRTC_MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_
      12             : #define WEBRTC_MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_
      13             : 
      14             : #include <cstring>
      15             : #include <memory>
      16             : #include <vector>
      17             : 
      18             : #include "webrtc/common_audio/sparse_fir_filter.h"
      19             : 
      20             : namespace webrtc {
      21             : 
      22             : // An implementation of a 3-band FIR filter-bank with DCT modulation, similar to
      23             : // the proposed in "Multirate Signal Processing for Communication Systems" by
      24             : // Fredric J Harris.
      25             : // The low-pass filter prototype has these characteristics:
      26             : // * Pass-band ripple = 0.3dB
      27             : // * Pass-band frequency = 0.147 (7kHz at 48kHz)
      28             : // * Stop-band attenuation = 40dB
      29             : // * Stop-band frequency = 0.192 (9.2kHz at 48kHz)
      30             : // * Delay = 24 samples (500us at 48kHz)
      31             : // * Linear phase
      32             : // This filter bank does not satisfy perfect reconstruction. The SNR after
      33             : // analysis and synthesis (with no processing in between) is approximately 9.5dB
      34             : // depending on the input signal after compensating for the delay.
      35           0 : class ThreeBandFilterBank final {
      36             :  public:
      37             :   explicit ThreeBandFilterBank(size_t length);
      38             :   ~ThreeBandFilterBank();
      39             : 
      40             :   // Splits |in| into 3 downsampled frequency bands in |out|.
      41             :   // |length| is the |in| length. Each of the 3 bands of |out| has to have a
      42             :   // length of |length| / 3.
      43             :   void Analysis(const float* in, size_t length, float* const* out);
      44             : 
      45             :   // Merges the 3 downsampled frequency bands in |in| into |out|.
      46             :   // |split_length| is the length of each band of |in|. |out| has to have at
      47             :   // least a length of 3 * |split_length|.
      48             :   void Synthesis(const float* const* in, size_t split_length, float* out);
      49             : 
      50             :  private:
      51             :   void DownModulate(const float* in,
      52             :                     size_t split_length,
      53             :                     size_t offset,
      54             :                     float* const* out);
      55             :   void UpModulate(const float* const* in,
      56             :                   size_t split_length,
      57             :                   size_t offset,
      58             :                   float* out);
      59             : 
      60             :   std::vector<float> in_buffer_;
      61             :   std::vector<float> out_buffer_;
      62             :   std::vector<std::unique_ptr<SparseFIRFilter>> analysis_filters_;
      63             :   std::vector<std::unique_ptr<SparseFIRFilter>> synthesis_filters_;
      64             :   std::vector<std::vector<float>> dct_modulation_;
      65             : };
      66             : 
      67             : }  // namespace webrtc
      68             : 
      69             : #endif  // WEBRTC_MODULES_AUDIO_PROCESSING_THREE_BAND_FILTER_BANK_H_