fbank.h

// Copyright (c) 2017 Personal (Binbin Zhang)
// Created on 2017-07-26
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef FBANK_H_
#define FBANK_H_

#include <assert.h>

#include <limits>
#include <random>
#include <utility>
#include <vector>

#include "fft.h"

#ifndef M_PI
#define M_PI 3.1415926535897932384626433832795
#endif
#ifndef M_2PI
#define M_2PI 6.283185307179586476925286766559005
#endif

class Fbank {
 public:
  Fbank(int num_bins, int sample_rate, int frame_length, int frame_shift)
      : num_bins_(num_bins),
        sample_rate_(sample_rate),
        frame_length_(frame_length),
        frame_shift_(frame_shift),
        use_log_(true),
        remove_dc_offset_(true),
        generator_(0),
        distribution_(0, 1.0),
        dither_(0.0) {
    fft_points_ = UpperPowerOfTwo(frame_length_);
    int num_fft_bins = fft_points_ / 2;
    float fft_bin_width = static_cast<float>(sample_rate_) / fft_points_;
    int low_freq = 20, high_freq = sample_rate_ / 2;
    float mel_low_freq = MelScale(low_freq);
    float mel_high_freq = MelScale(high_freq);
    float mel_freq_delta = (mel_high_freq - mel_low_freq) / (num_bins + 1);
    bins_.resize(num_bins_);
    center_freqs_.resize(num_bins_);
    for (int bin = 0; bin < num_bins; bin++) {
      float left_mel = mel_low_freq + bin * mel_freq_delta,
            center_mel = mel_low_freq + (bin + 1) * mel_freq_delta,
            right_mel = mel_low_freq + (bin + 2) * mel_freq_delta;
      center_freqs_[bin] = InverseMelScale(center_mel);
      std::vector<float> this_bin(num_fft_bins);
      int first_index = -1, last_index = -1;
      for (int i = 0; i < num_fft_bins; i++) {
        float freq = (fft_bin_width * i);  // Center frequency of this fft
        // bin.
        float mel = MelScale(freq);
        if (mel > left_mel && mel < right_mel) {
          float weight;
          if (mel <= center_mel)
            weight = (mel - left_mel) / (center_mel - left_mel);
          else
            weight = (right_mel - mel) / (right_mel - center_mel);
          this_bin[i] = weight;
          if (first_index == -1) first_index = i;
          last_index = i;
        }
      }
      assert(first_index != -1 && last_index >= first_index);
      bins_[bin].first = first_index;
      int size = last_index + 1 - first_index;
      bins_[bin].second.resize(size);
      for (int i = 0; i < size; i++) {
        bins_[bin].second[i] = this_bin[first_index + i];
      }
    }

    // hamming window
    hamming_window_.resize(frame_length_);
    double a = M_2PI / (frame_length - 1);
    for (int i = 0; i < frame_length; i++) {
      hamming_window_[i] = 0.54 - 0.46 * cos(a * i);
    }
  }

  void SetUseLog(bool use_log) { use_log_ = use_log; }

  void SetRemoveDcOffset(bool remove_dc_offset) {
    remove_dc_offset_ = remove_dc_offset;
  }

  void SetDither(float dither) { dither_ = dither; }

  int NumBins() const { return num_bins_; }

  static inline float InverseMelScale(float mel_freq) {
    return 700.0f * (expf(mel_freq / 1127.0f) - 1.0f);
  }

  static inline float MelScale(float freq) {
    return 1127.0f * logf(1.0f + freq / 700.0f);
  }

  int UpperPowerOfTwo(int n) const {
    return static_cast<int>(pow(2, ceil(log(n) / log(2))));
  }

  // preemphasis
  void PreEmphasis(float coeff, std::vector<float>* data) const {
    if (coeff == 0.0) return;
    for (int i = data->size() - 1; i > 0; i--)
      (*data)[i] -= coeff * (*data)[i - 1];
    (*data)[0] -= coeff * (*data)[0];
  }

  // Apply hamming window on data in place
  void Hamming(std::vector<float>* data) const {
    assert(data->size() >= hamming_window_.size());
    for (int i = 0; i < hamming_window_.size(); i++) {
      (*data)[i] *= hamming_window_[i];
    }
  }

  // Compute fbank feat, return num frames
  int Compute(const std::vector<float>& wave, std::vector<float>* feat) {
    int num_samples = wave.size();
    if (num_samples < frame_length_) return 0;
    int num_frames = 1 + ((num_samples - frame_length_) / frame_shift_);
    feat->resize(num_frames * num_bins_);
    std::vector<float> fft_real(fft_points_, 0), fft_img(fft_points_, 0);
    std::vector<float> power(fft_points_ / 2);
    for (int i = 0; i < num_frames; i++) {
      std::vector<float> data(wave.data() + i * frame_shift_,
                              wave.data() + i * frame_shift_ + frame_length_);
      // optional add noise
      if (dither_ != 0.0) {
        for (int j = 0; j < data.size(); j++)
          data[j] += dither_ * distribution_(generator_);
      }
      // optinal remove dc offset
      if (remove_dc_offset_) {
        float mean = 0.0;
        for (int j = 0; j < data.size(); j++) mean += data[j];
        mean /= data.size();
        for (int j = 0; j < data.size(); j++) data[j] -= mean;
      }

      PreEmphasis(0.97, &data);
      Hamming(&data);
      // copy data to fft_real
      memset(fft_real.data(), 0, sizeof(float) * fft_points_);
      memset(fft_img.data(), 0, sizeof(float) * fft_points_);
      memcpy(fft_real.data(), data.data(), sizeof(float) * frame_length_);
      fft(fft_real.data(), fft_img.data(), fft_points_);
      // power
      for (int j = 0; j < fft_points_ / 2; j++) {
        power[j] = fft_real[j] * fft_real[j] + fft_img[j] * fft_img[j];
      }

      // cepstral coefficients, triangle filter array
      for (int j = 0; j < num_bins_; j++) {
        float mel_energy = 0.0;
        int s = bins_[j].first;
        for (int k = 0; k < bins_[j].second.size(); k++) {
          mel_energy += bins_[j].second[k] * power[s + k];
        }
        // optional use log
        if (use_log_) {
          if (mel_energy < std::numeric_limits<float>::epsilon())
            mel_energy = std::numeric_limits<float>::epsilon();
          mel_energy = logf(mel_energy);
        }

        (*feat)[i * num_bins_ + j] = mel_energy;
        // printf("%f ", mel_energy);
      }
      // printf("\n");
    }
    return num_frames;
  }

 private:
  int num_bins_;
  int sample_rate_;
  int frame_length_, frame_shift_;
  int fft_points_;
  bool use_log_;
  bool remove_dc_offset_;
  std::vector<float> center_freqs_;
  std::vector<std::pair<int, std::vector<float> > > bins_;
  std::vector<float> hamming_window_;
  std::default_random_engine generator_;
  std::normal_distribution<float> distribution_;
  float dither_;
};

#endif  // FBANK_H_