Add MFCCExtractor class

Author: vitalsong

include/dsplib.h

@@ -25,6 +25,7 @@
 #include <dsplib/spectrum.h>
 #include <dsplib/stft.h>
 #include <dsplib/assert.h>
+#include <dsplib/mfcc.h>
 
 #include <dsplib/audio/noise-gate.h>
 #include <dsplib/audio/compressor.h>

include/dsplib/mfcc.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <dsplib/array.h>
+
+namespace dsplib {
+
+template<typename T>
+T hz2mel(const T& freq) noexcept {
+    return 2595 * log10(1 + freq / 700);
+}
+
+template<typename T>
+T mel2hz(const T& mel) noexcept {
+    return 700 * (power(10, (mel / 2595)) - 1);
+}
+
+class MFCCExtractor
+{
+public:
+    explicit MFCCExtractor(int fs, int nfft, int overlap, int nmels, int nbands, const arr_real& win);
+
+    arr_real process(const arr_real& x);
+
+    static arr_real design_band_edges(int min_freq, int max_freq, int nbands);
+
+    static std::vector<arr_real> design_filterbank(int fs, int min_freq, int max_freq, int nbands, int nfft);
+
+private:
+    const int winlen_;
+    const arr_real win_;
+    const int hop_size_;
+    const int nmels_;
+    const int nfft_;
+    const std::vector<arr_real> fbank_;
+    const std::vector<arr_real> dctmat_;
+    arr_real buf_;
+};
+
+}   // namespace dsplib

lib/mfcc.cpp

@@ -0,0 +1,120 @@
+#include <dsplib/mfcc.h>
+
+#include <dsplib/window.h>
+#include <dsplib/array.h>
+#include <dsplib/fft.h>
+#include <dsplib/utils.h>
+#include <dsplib/math.h>
+
+namespace dsplib {
+
+namespace {
+
+arr_int _freq2bin(const arr_real& freqs, int nfft, int fs) {
+    arr_int bins(freqs.size());
+    for (int i = 0; i < freqs.size(); ++i) {
+        bins[i] = std::round((freqs[i] / fs) * nfft);
+    }
+    return bins;
+}
+
+//TODO: add to utils
+arr_real _linspace(real_t start, real_t end, int count) {
+    arr_real out(count);
+    const real_t step = (count == 1) ? 0 : (end - start) / real_t(count - 1);
+    for (int i = 0; i < count; ++i) {
+        out[i] = start + (step * i);
+    }
+    return out;
+}
+
+std::vector<arr_real> _gen_dcttable(int nmels, int nbands) {
+    // The DCT-II unitary algorithm
+    const real_t A = std::sqrt(1.0 / nbands);
+    const real_t B = std::sqrt(2.0 / nbands);
+    const real_t piv = pi / nbands;
+    std::vector<arr_real> mat(nmels, ones(nbands) * A);
+    for (int k = 0; k < nbands; ++k) {
+        for (int n = 1; n < nmels; ++n) {
+            mat[n][k] = B * std::cos(piv * n * (k + 0.5));
+        }
+    }
+    return mat;
+}
+
+}   // namespace
+
+arr_real MFCCExtractor::design_band_edges(int min_freq, int max_freq, int nbands) {
+    const auto min_mel = hz2mel(real_t(min_freq));
+    const auto max_mel = hz2mel(real_t(max_freq));
+    const auto mels = _linspace(min_mel, max_mel, nbands + 2);
+    const auto freqs = mel2hz(mels);
+    return freqs;
+}
+
+std::vector<arr_real> MFCCExtractor::design_filterbank(int fs, int min_freq, int max_freq, int nbands, int nfft) {
+    const auto band_edges = design_band_edges(min_freq, max_freq, nbands);
+    const auto bins = _freq2bin(band_edges, nfft, fs) + 1;   //matlab-style indexies
+    const int nb = band_edges.size();
+    const auto band_width = *band_edges.slice(2, nb) - *band_edges.slice(0, nb - 2);
+    const auto weight_bands = band_width / 2;
+
+    std::vector<arr_real> filterbank;
+    for (int en = 0; en < nbands; ++en) {
+        arr_real flt(nfft);
+
+        // rising side of triangle
+        for (int i = bins[en]; i <= bins[en + 1]; ++i) {
+            flt[i - 1] = real_t(i - bins[en]) / (bins[en + 1] - bins[en]);
+        }
+
+        // falling side of triangle
+        for (int i = bins[en + 1] + 1; i <= bins[en + 2]; ++i) {
+            flt[i - 1] = 1.0 - real_t(i - bins[en + 1]) / (bins[en + 2] - bins[en + 1]);
+        }
+
+        //normalization
+        flt /= weight_bands(en);
+
+        filterbank.push_back(flt);
+    }
+
+    return filterbank;
+}
+
+MFCCExtractor::MFCCExtractor(int fs, int nfft, int overlap, int nmels, int nbands, const arr_real& win)
+  : winlen_{win.size()}
+  , win_{win}
+  , hop_size_{winlen_ - overlap}
+  , nmels_{nmels}
+  , nfft_{nfft}
+  , fbank_{design_filterbank(fs, 0, fs / 2, nbands, nfft)}
+  , dctmat_{_gen_dcttable(nmels, nbands)}
+  , buf_(winlen_) {
+}
+
+arr_real MFCCExtractor::process(const arr_real& x) {
+    DSPLIB_ASSERT(x.size() == hop_size_, "input size must be equal hop_size");
+
+    buf_.slice(0, winlen_ - hop_size_) = buf_.slice(hop_size_, winlen_);
+    buf_.slice(winlen_ - hop_size_, winlen_) = x;
+    const arr_real xq = abs2(fft(buf_ * win_, nfft_));
+
+    //filterbank
+    //TODO: use only nfft/2+1 points for real signal
+    //TODO: store and multiply only small non-zero part of filterbank - (fftpos, fbank)
+    arr_real z(fbank_.size());
+    for (int i = 0; i < fbank_.size(); ++i) {
+        z[i] = dot(fbank_[i], xq);
+    }
+
+    //cepstral
+    arr_real y(nmels_);
+    for (int i = 0; i < nmels_; ++i) {
+        y[i] = dot(z, dctmat_[i]);
+    }
+
+    return y;
+}
+
+}   // namespace dsplib