/**
* Copyright (c) 2022 Xiaomi Corporation (authors: Fangjun Kuang)
*
* See LICENSE for clarification regarding multiple authors
*
* 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.
*/
// This file is copied/modified from kaldi/src/feat/mel-computations.cc
#include "frontend/mel-computations.h"
#include <algorithm>
#include <sstream>
#include "frontend/feature-window.h"
namespace knf {
std::ostream &operator<<(std::ostream &os, const MelBanksOptions &opts) {
os << opts.ToString();
return os;
}
float MelBanks::VtlnWarpFreq(
float vtln_low_cutoff, // upper+lower frequency cutoffs for VTLN.
float vtln_high_cutoff,
float low_freq, // upper+lower frequency cutoffs in mel computation
float high_freq,
float vtln_warp_factor,
float freq) {
/// This computes a VTLN warping function that is not the same as HTK's one,
/// but has similar inputs (this function has the advantage of never
/// producing
/// empty bins).
/// This function computes a warp function F(freq), defined between low_freq
/// and high_freq inclusive, with the following properties:
/// F(low_freq) == low_freq
/// F(high_freq) == high_freq
/// The function is continuous and piecewise linear with two inflection
/// points.
/// The lower inflection point (measured in terms of the unwarped
/// frequency) is at frequency l, determined as described below.
/// The higher inflection point is at a frequency h, determined as
/// described below.
/// If l <= f <= h, then F(f) = f/vtln_warp_factor.
/// If the higher inflection point (measured in terms of the unwarped
/// frequency) is at h, then max(h, F(h)) == vtln_high_cutoff.
/// Since (by the last point) F(h) == h/vtln_warp_factor, then
/// max(h, h/vtln_warp_factor) == vtln_high_cutoff, so
/// h = vtln_high_cutoff / max(1, 1/vtln_warp_factor).
/// = vtln_high_cutoff * min(1, vtln_warp_factor).
/// If the lower inflection point (measured in terms of the unwarped
/// frequency) is at l, then min(l, F(l)) == vtln_low_cutoff
/// This implies that l = vtln_low_cutoff / min(1, 1/vtln_warp_factor)
/// = vtln_low_cutoff * max(1, vtln_warp_factor)
if (freq < low_freq || freq > high_freq)
return freq; // in case this gets called
// for out-of-range frequencies, just return the freq.
CHECK_GT(vtln_low_cutoff, low_freq);
CHECK_LT(vtln_high_cutoff, high_freq);
float one = 1.0f;
float l = vtln_low_cutoff * std::max(one, vtln_warp_factor);
float h = vtln_high_cutoff * std::min(one, vtln_warp_factor);
float scale = 1.0f / vtln_warp_factor;
float Fl = scale * l; // F(l);
float Fh = scale * h; // F(h);
CHECK(l > low_freq && h < high_freq);
// slope of left part of the 3-piece linear function
float scale_left = (Fl - low_freq) / (l - low_freq);
// [slope of center part is just "scale"]
// slope of right part of the 3-piece linear function
float scale_right = (high_freq - Fh) / (high_freq - h);
if (freq < l) {
return low_freq + scale_left * (freq - low_freq);
} else if (freq < h) {
return scale * freq;
} else { // freq >= h
return high_freq + scale_right * (freq - high_freq);
}
}
float MelBanks::VtlnWarpMelFreq(
float vtln_low_cutoff, // upper+lower frequency cutoffs for VTLN.
float vtln_high_cutoff,
float low_freq, // upper+lower frequency cutoffs in mel computation
float high_freq,
float vtln_warp_factor,
float mel_freq) {
return MelScale(VtlnWarpFreq(vtln_low_cutoff,
vtln_high_cutoff,
low_freq,
high_freq,
vtln_warp_factor,
InverseMelScale(mel_freq)));
}
MelBanks::MelBanks(const MelBanksOptions &opts,
const FrameExtractionOptions &frame_opts,
float vtln_warp_factor)
: htk_mode_(opts.htk_mode) {
int32_t num_bins = opts.num_bins;
if (num_bins < 3) LOG(FATAL) << "Must have at least 3 mel bins";
float sample_freq = frame_opts.samp_freq;
int32_t window_length_padded = frame_opts.PaddedWindowSize();
CHECK_EQ(window_length_padded % 2, 0);
int32_t num_fft_bins = window_length_padded / 2;
float nyquist = 0.5f * sample_freq;
float low_freq = opts.low_freq, high_freq;
if (opts.high_freq > 0.0f)
high_freq = opts.high_freq;
else
high_freq = nyquist + opts.high_freq;
if (low_freq < 0.0f || low_freq >= nyquist || high_freq <= 0.0f ||
high_freq > nyquist || high_freq <= low_freq) {
LOG(FATAL) << "Bad values in options: low-freq " << low_freq
<< " and high-freq " << high_freq << " vs. nyquist "
<< nyquist;
}
float fft_bin_width = sample_freq / window_length_padded;
// fft-bin width [think of it as Nyquist-freq / half-window-length]
float mel_low_freq = MelScale(low_freq);
float mel_high_freq = MelScale(high_freq);
debug_ = opts.debug_mel;
// divide by num_bins+1 in next line because of end-effects where the bins
// spread out to the sides.
float mel_freq_delta = (mel_high_freq - mel_low_freq) / (num_bins + 1);
float vtln_low = opts.vtln_low, vtln_high = opts.vtln_high;
if (vtln_high < 0.0f) {
vtln_high += nyquist;
}
if (vtln_warp_factor != 1.0f &&
(vtln_low < 0.0f || vtln_low <= low_freq || vtln_low >= high_freq ||
vtln_high <= 0.0f || vtln_high >= high_freq ||
vtln_high <= vtln_low)) {
LOG(FATAL) << "Bad values in options: vtln-low " << vtln_low
<< " and vtln-high " << vtln_high << ", versus "
<< "low-freq " << low_freq << " and high-freq " << high_freq;
}
bins_.resize(num_bins);
center_freqs_.resize(num_bins);
for (int32_t 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;
if (vtln_warp_factor != 1.0f) {
left_mel = VtlnWarpMelFreq(vtln_low,
vtln_high,
low_freq,
high_freq,
vtln_warp_factor,
left_mel);
center_mel = VtlnWarpMelFreq(vtln_low,
vtln_high,
low_freq,
high_freq,
vtln_warp_factor,
center_mel);
right_mel = VtlnWarpMelFreq(vtln_low,
vtln_high,
low_freq,
high_freq,
vtln_warp_factor,
right_mel);
}
center_freqs_[bin] = InverseMelScale(center_mel);
// this_bin will be a vector of coefficients that is only
// nonzero where this mel bin is active.
std::vector<float> this_bin(num_fft_bins);
int32_t first_index = -1, last_index = -1;
for (int32_t 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;
}
}
CHECK(first_index != -1 && last_index >= first_index &&
"You may have set num_mel_bins too large.");
bins_[bin].first = first_index;
int32_t size = last_index + 1 - first_index;
bins_[bin].second.insert(bins_[bin].second.end(),
this_bin.begin() + first_index,
this_bin.begin() + first_index + size);
// Replicate a bug in HTK, for testing purposes.
if (opts.htk_mode && bin == 0 && mel_low_freq != 0.0f) {
bins_[bin].second[0] = 0.0;
}
} // for (int32_t bin = 0; bin < num_bins; ++bin) {
if (debug_) {
std::ostringstream os;
for (size_t i = 0; i < bins_.size(); i++) {
os << "bin " << i << ", offset = " << bins_[i].first << ", vec = ";
for (auto k : bins_[i].second) os << k << ", ";
os << "\n";
}
LOG(INFO) << os.str();
}
}
// "power_spectrum" contains fft energies.
void MelBanks::Compute(const float *power_spectrum,
float *mel_energies_out) const {
int32_t num_bins = bins_.size();
for (int32_t i = 0; i < num_bins; i++) {
int32_t offset = bins_[i].first;
const auto &v = bins_[i].second;
float energy = 0;
for (int32_t k = 0; k != v.size(); ++k) {
energy += v[k] * power_spectrum[k + offset];
}
// HTK-like flooring- for testing purposes (we prefer dither)
if (htk_mode_ && energy < 1.0) {
energy = 1.0;
}
mel_energies_out[i] = energy;
// The following assert was added due to a problem with OpenBlas that
// we had at one point (it was a bug in that library). Just to detect
// it early.
CHECK_EQ(energy, energy); // check that energy is not nan
}
if (debug_) {
fprintf(stderr, "MEL BANKS:\n");
for (int32_t i = 0; i < num_bins; i++)
fprintf(stderr, " %f", mel_energies_out[i]);
fprintf(stderr, "\n");
}
}
} // namespace knf