n blocks.
    mode : str, default: 'psd'
        What sort of spectrum to use:
            'psd'
                Returns the power spectral density.
            'complex'
                Returns the complex-valued frequency spectrum.
            'magnitude'
                Returns the magnitude spectrum.
            'angle'
                Returns the phase spectrum without unwrapping.
            'phase'
                Returns the phase spectrum with unwrapping.

    Returns
    -------
    spectrum : array-like
        2D array, columns are the periodograms of successive segments.

    freqs : array-like
        1-D array, frequencies corresponding to the rows in *spectrum*.

    t : array-like
        1-D array, the times corresponding to midpoints of segments
        (i.e the columns in *spectrum*).

    See Also
    --------
    psd : differs in the overlap and in the return values.
    complex_spectrum : similar, but with complex valued frequencies.
    magnitude_spectrum : similar single segment when *mode* is 'magnitude'.
    angle_spectrum : similar to single segment when *mode* is 'angle'.
    phase_spectrum : similar to single segment when *mode* is 'phase'.

    Notes
    -----
    *detrend* and *scale_by_freq* only apply when *mode* is set to 'psd'.

    N