Source code for diffsptk.core.c2acr

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# Copyright 2022 SPTK Working Group                                        #
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import torch
import torch.nn as nn


class CepstrumToAutocorrelation(nn.Module):
    """See `this page <https://sp-nitech.github.io/sptk/latest/main/c2acr.html>`_
    for details.

    Parameters
    ----------
    acr_order : int >= 0 [scalar]
        Order of autocorrelation, :math:`M_2`.

    fft_length : int >= 2 [scalar]
        Number of FFT bins, :math:`L`.

    """

    def __init__(self, acr_order, fft_length):
        super(CepstrumToAutocorrelation, self).__init__()

        self.acr_order = acr_order
        self.fft_length = fft_length

        assert 0 <= self.acr_order
        assert 2 <= self.fft_length
        assert self.acr_order <= self.fft_length // 2

    def forward(self, c):
        """Convert cepstrum to autocorrelation.

        Parameters
        ----------
        c : Tensor [shape=(..., M1+1)]
            Cepstrum.

        Returns
        -------
        r : Tensor [shape=(..., M2+1)]
            Autocorrelation.

        Examples
        --------
        >>> c = diffsptk.nrand(4)
        >>> c
        tensor([-0.1751,  0.1950, -0.3211,  0.3523, -0.5453])
        >>> c2acr = diffsptk.CepstrumToAutocorrelation(4, 16)
        >>> r = c2acr(c)
        >>> r
        tensor([ 1.0672, -0.0485, -0.1564,  0.2666, -0.4551])

        """
        x = torch.fft.rfft(c, n=self.fft_length).real
        x = torch.exp(2 * x)
        r = torch.fft.hfft(x, norm="forward")[..., : self.acr_order + 1]
        return r