def mc2e(mc, alpha=0.35, irlen=256):
"""Compute energy from mel-cepstrum
Inspired from hts_engine
Parameters
----------
mc : array
Mel-spectrum
alpha : float
All-pass constant.
irlen : int
IIR filter length
Returns
-------
energy : floating point, scalar
frame energy
"""
# back to linear frequency domain
c = freqt(mc, irlen - 1, -alpha)
# compute impule response from cepsturm
ir = c2ir(c, irlen)
return np.sum(np.abs(ir * ir))
def sp2mc(powerspec, order, alpha):
"""Convert spectrum envelope to mel-cepstrum
This is a simplified implementation of ``mcep`` for input type
is 4.
Parameters
----------
powerspec : array
Power spectrum
order : int
Order of mel-cepstrum
alpha : float
All-pass constant.
Returns
-------
mc : array, shape(``order+1``)
mel-cepstrum
See Also
--------
pysptk.sptk.mcep
pysptk.conversion.mc2sp
"""
# |X(ω)|² -> log(|X(ω)²|)
logperiodogram = np.log(powerspec)
# transform log-periodogram to real cepstrum
# log(|X(ω)|²) -> c(m)
c = np.fft.irfft(logperiodogram)
c[0] /= 2.0
# c(m) -> cₐ(m)
return freqt(c, order, alpha)
def mc2sp(mc, alpha, fftlen):
"""Convert mel-cepstrum back to power spectrum
Parameters
----------
mc : array
Mel-spectrum
alpha : float
All-pass constant.
fftlen : int
FFT length
Returns
-------
powerspec : array, shape(``fftlen//2 +1``)
Power spectrum
See Also
--------
pysptk.sptk.mcep
pysptk.conversion.sp2mc
"""
# back to cepstrum from mel-cesptrum
# cₐ(m) -> c(m)
c = freqt(mc, int(fftlen // 2), -alpha)
c[0] *= 2.0
symc = np.zeros(fftlen)
symc[0] = c[0]
for i in range(1, len(c)):
symc[i] = c[i]
symc[-i] = c[i]
# back to power spectrum
# c(m) -> log(|X(ω)|²) -> |X(ω)|²
return np.exp(np.fft.rfft(symc).real)