Pre-emphasize the entire signal at once by self.emphasis_factor,
        overwriting ``self.data``.

def _pre_emphasis(self):
        """
        Pre-emphasize the entire signal at once by self.emphasis_factor,
        overwriting ``self.data``.
        """
        self.data = numpy.append(self.data[0], self.data[1:] - self.emphasis_factor * self.data[:-1])