def bandpass(self, frequency, q=1.0):
        """bandpass takes 2 parameters: filter center frequency in Hz and "q"
        or band-width (default=1.0).

        It gradually removes frequencies outside the band specified.
        """
        self.command.append('bandpass')
        self.command.append(frequency)
        self.command.append(str(q) + 'q')
        return self