def computeMonoX(self, start=0, stop=None):
        """Computes and return SX, the mono channel or mean over the
        channels of the power spectrum of the signal
        """
        fs, data = wav.read(self.files['inputAudioFilename'])
        data = np.double(data) / self.scaleData
        if len(data.shape)>1 and data.shape[1]>1:
            data = data.mean(axis=1)