annotate src/mfcc_analyzer.cpp @ 0:add35537fdbb tip

Initial import

//  Copyright 2011, Ian Hobson.
//
//  This file is part of gpsynth.
//
//  gpsynth is free software: you can redistribute it and/or modify
//  it under the terms of the GNU General Public License as published by
//  the Free Software Foundation, either version 3 of the License, or
//  (at your option) any later version.
//
//  gpsynth is distributed in the hope that it will be useful,
//  but WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//  GNU General Public License for more details.
//
//  You should have received a copy of the GNU General Public License
//  along with gpsynth in the file COPYING. 
//  If not, see http://www.gnu.org/licenses/.

#include "mfcc_analyzer.hpp"

#include "boost_ex.hpp"

#include <algorithm>
#include <cmath>
#include <iostream>
#include <iterator>
#include <limits>
#include <numeric>

namespace {
  
// using the scale described in the HTK book
double MelToHz(double mel) {
  return 700.0 * (std::pow(10.0, mel / 2595.0) - 1.0);
}

double HzToMel(double hz) {
  return 2595.0 * std::log10(1.0 + (hz / 700.0));
}
  
} // namespace

namespace dsp {

MFCCAnalyzer::MFCCAnalyzer(int frame_size,
                           int mel_bands,
                           int number_of_mfccs,
                           double frequency_min,
                           double frequency_max)
: frame_size_(frame_size),
  mel_bands_(mel_bands),
  number_of_mfccs_(number_of_mfccs),
  sample_rate_(0),
  frequency_min_(frequency_min),
  frequency_max_(frequency_max)
{
  InitializeMelDCT();
}
  
MFCCAnalyzer::~MFCCAnalyzer() {
  fftw_free(mel_bands_dct_in_);
  fftw_free(mel_bands_dct_out_);
  fftw_destroy_plan(dct_plan_);
}

void MFCCAnalyzer::SetSampleRate(double sample_rate) {
  if (sample_rate_ != sample_rate) {
    sample_rate_ = sample_rate;
    InitializeFilters();
  }
}

// extracts mfccs from a magnitude spectrum
const std::vector<double>&
MFCCAnalyzer::ExtractMFCCS(const std::vector<double>& spectrum) {
  // filter spectrum
  for (std::size_t i = 0; i < mel_bands_; i++) {
    mel_bands_dct_in_[i] = std::inner_product(spectrum.begin(),
                                              spectrum.end(),
                                              mel_filters_[i].begin(),
                                              0.0);
  }
  // take the logs of the filter outputs (avoiding log(0) == -inf)
  std::replace(mel_bands_dct_in_, mel_bands_dct_in_ + mel_bands_,
               0.0, std::numeric_limits<double>::min());
  std::transform(mel_bands_dct_in_, mel_bands_dct_in_ + mel_bands_,
                 mel_bands_dct_in_,
                 static_cast<double(*)(double)>(std::log10));
  // take the DCT of the mel bands
  fftw_execute(dct_plan_);
  // store result
  mfccs_.assign(mel_bands_dct_out_, mel_bands_dct_out_ + number_of_mfccs_);
  return mfccs_;
}

void MFCCAnalyzer::InitializeMelDCT() {
  int mem_size = sizeof(double) * mel_bands_;
  mel_bands_dct_in_ = (double*)fftw_malloc(mem_size);
  mel_bands_dct_out_ = (double*)fftw_malloc(mem_size);
  dct_plan_ = fftw_plan_r2r_1d(number_of_mfccs_,
                               mel_bands_dct_in_,
                               mel_bands_dct_out_,
                               FFTW_REDFT10,
                               FFTW_ESTIMATE);
}

void MFCCAnalyzer::InitializeFilters() {
  // create a series of triangular filters mapped linearly on the mel-frequency
  // scale.
  double bin_frequency = sample_rate_ / (2 * frame_size_);
  double mel_min = HzToMel(frequency_min_);
  double mel_max = HzToMel(frequency_max_);
  double mel_width = (mel_max - mel_min) / (mel_bands_ + 1.0);
  double mel_left = mel_min;
  mel_filters_.resize(mel_bands_);
  for (int i = 0; i < mel_bands_; i++) {
    double mel_centre = mel_left + mel_width;
    double mel_right = mel_centre + mel_width;
    double left_hz = MelToHz(mel_left);
    double right_hz = MelToHz(mel_right);
    int bin_left = left_hz / bin_frequency + 0.5;
    int bin_right = right_hz / bin_frequency + 0.5;
    int bin_centre = (bin_left + bin_right) / 2.0 + 0.5;
    double filter_height = 2.0 / (bin_right - bin_left);
    mel_filters_[i].assign(frame_size_, 0.0);
    // generate the up ramp
    double bin_delta = filter_height / (bin_centre - bin_left);
    double ramp = 0.0;
    for (int bin = bin_left; bin < bin_centre; bin++) {
      mel_filters_[i][bin] = ramp;
      ramp += bin_delta;
    }
    // down ramp
    for (int bin = bin_centre; bin < bin_right; bin++) {
      mel_filters_[i][bin] = ramp;
      ramp -= bin_delta;
    }
    // move mel bin along
    mel_left += mel_width;
  }
}
  
} // dsp namespace