docs: 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)))