Full frequency directed transfer function.

        .. math:: \mathrm{ffDTF}_{ij}(f) =
                  \\frac{H_{ij}(f)}{\sqrt{\sum_f H_{i:}(f) H_{i:}'(f)}}

        References
        ----------
        A. Korzeniewska, M. Mańczak, M. Kaminski, K. J. Blinowska, S. Kasicki.
        Determination of information flow direction among brain structures by a
        modified directed transfer function (dDTF) method. J. Neurosci. Meth.
        125(1-2): 195-207, 2003.

def ffDTF(self):
        """Full frequency directed transfer function.

        .. math:: \mathrm{ffDTF}_{ij}(f) =
                  \\frac{H_{ij}(f)}{\sqrt{\sum_f H_{i:}(f) H_{i:}'(f)}}

        References
        ----------
        A. Korzeniewska, M. Mańczak, M. Kaminski, K. J. Blinowska, S. Kasicki.
        Determination of information flow direction among brain structures by a
        modified directed transfer function (dDTF) method. J. Neurosci. Meth.
        125(1-2): 195-207, 2003.
        """
        H = self.H()
        return np.abs(H * self.nfft / np.sqrt(np.sum(H * H.conj(), axis=(1, 2),
                                                     keepdims=True)))