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mel_features.py
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mel_features.py
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# MFCC Spectrogram conversion code from VGGish, Google Inc.
# https://github.com/tensorflow/models/tree/master/research/audioset
import numpy as np
def frame(data, window_length, hop_length):
num_samples = data.shape[0]
num_frames = 1 + int(np.floor((num_samples - window_length) / hop_length))
shape = (num_frames, window_length) + data.shape[1:]
strides = (data.strides[0] * hop_length,) + data.strides
return np.lib.stride_tricks.as_strided(data, shape=shape, strides=strides)
def periodic_hann(window_length):
return 0.5 - (0.5 * np.cos(2 * np.pi / window_length * np.arange(window_length)))
def stft_magnitude(signal, fft_length,
hop_length=None,
window_length=None):
frames = frame(signal, window_length, hop_length)
window = periodic_hann(window_length)
windowed_frames = frames * window
return np.abs(np.fft.rfft(windowed_frames, int(fft_length)))
# Mel spectrum constants and functions.
_MEL_BREAK_FREQUENCY_HERTZ = 700.0
_MEL_HIGH_FREQUENCY_Q = 1127.0
def hertz_to_mel(frequencies_hertz):
return _MEL_HIGH_FREQUENCY_Q * np.log(
1.0 + (frequencies_hertz / _MEL_BREAK_FREQUENCY_HERTZ))
def spectrogram_to_mel_matrix(num_mel_bins=20,
num_spectrogram_bins=129,
audio_sample_rate=8000,
lower_edge_hertz=125.0,
upper_edge_hertz=3800.0):
nyquist_hertz = audio_sample_rate / 2.
if lower_edge_hertz >= upper_edge_hertz:
raise ValueError("lower_edge_hertz %.1f >= upper_edge_hertz %.1f" %
(lower_edge_hertz, upper_edge_hertz))
spectrogram_bins_hertz = np.linspace(0.0, nyquist_hertz, num_spectrogram_bins)
spectrogram_bins_mel = hertz_to_mel(spectrogram_bins_hertz)
band_edges_mel = np.linspace(hertz_to_mel(lower_edge_hertz),
hertz_to_mel(upper_edge_hertz), num_mel_bins + 2)
# Matrix to post-multiply feature arrays whose rows are num_spectrogram_bins
# of spectrogram values.
mel_weights_matrix = np.empty((num_spectrogram_bins, num_mel_bins))
for i in range(num_mel_bins):
lower_edge_mel, center_mel, upper_edge_mel = band_edges_mel[i:i + 3]
lower_slope = ((spectrogram_bins_mel - lower_edge_mel) /
(center_mel - lower_edge_mel))
upper_slope = ((upper_edge_mel - spectrogram_bins_mel) /
(upper_edge_mel - center_mel))
# .. then intersect them with each other and zero.
mel_weights_matrix[:, i] = np.maximum(0.0, np.minimum(lower_slope,
upper_slope))
mel_weights_matrix[0, :] = 0.0
return mel_weights_matrix
def log_mel_spectrogram(data,
audio_sample_rate=8000,
log_offset=0.0,
window_length_secs=0.025,
hop_length_secs=0.010,
**kwargs):
window_length_samples = int(round(audio_sample_rate * window_length_secs))
hop_length_samples = int(round(audio_sample_rate * hop_length_secs))
fft_length = 2 ** int(np.ceil(np.log(window_length_samples) / np.log(2.0)))
spectrogram = stft_magnitude(
data,
fft_length=fft_length,
hop_length=hop_length_samples,
window_length=window_length_samples)
mel_spectrogram = np.dot(spectrogram, spectrogram_to_mel_matrix(
num_spectrogram_bins=spectrogram.shape[1],
audio_sample_rate=audio_sample_rate, **kwargs))
return np.log(mel_spectrogram + log_offset)
def log_mel_spectrogram_subtract_bg(data,
background=None,
audio_sample_rate=8000,
log_offset=0.0,
window_length_secs=0.025,
hop_length_secs=0.010,
**kwargs):
window_length_samples = int(round(audio_sample_rate * window_length_secs))
hop_length_samples = int(round(audio_sample_rate * hop_length_secs))
fft_length = 2 ** int(np.ceil(np.log(window_length_samples) / np.log(2.0)))
spectrogram = stft_magnitude(
data,
fft_length=fft_length,
hop_length=hop_length_samples,
window_length=window_length_samples)
if (background is not None):
spectrogram = spectrogram - background
zeros = np.where(spectrogram == 0)
negatives = np.where(spectrogram < 0)
spectrogram[zeros] = 0.001
spectrogram[negatives] = 0.001
mel_spectrogram = np.dot(spectrogram, spectrogram_to_mel_matrix(
num_spectrogram_bins=spectrogram.shape[1],
audio_sample_rate=audio_sample_rate, **kwargs))
return np.log(mel_spectrogram + log_offset)