def setPrefilter(self, edfsignal, prefilter):
        """
        Sets the prefilter of signal edfsignal ("HP:0.1Hz", "LP:75Hz N:50Hz", etc.)

        :param edfsignal: int
        :param prefilter: str

        Notes
        -----
        This function is optional for every signal and can be called only after opening a file in writemode and before the first sample write action.
        """
        if edfsignal < 0 or edfsignal > self.n_channels:
            raise ChannelDoesNotExist(edfsignal)
        self.channels[edfsignal]['prefilter'] = prefilter
        self.update_header()