This function converts the input signal
    (stored in a numpy array) to MONO (if it is STEREO)

def stereo2mono(x):
    '''
    This function converts the input signal
    (stored in a numpy array) to MONO (if it is STEREO)
    '''
    if isinstance(x, int):
        return -1
    if x.ndim==1:
        return x
    elif x.ndim==2:
        if x.shape[1]==1:
            return x.flatten()
        else:
            if x.shape[1]==2:
                return ( (x[:,1] / 2) + (x[:,0] / 2) )
            else:
                return -1