more precisely `rolling`) an `mfft`-point FFT input by
`phase_shift` samples results in a multiplication of the output by
``np.exp(2j*np.pi*q*phase_shift/mfft)`` at the frequency q * `delta_f`.

The default value 0 ensures that there is no phase shift on the
zeroth slice (in which t=0 is centered).
No phase shift (``phase_shift is None``) is equivalent to
``phase_shift = -mfft//2``. In this case slices are not shifted
before calculating the FFT.

The absolute value of `phase_shift` is limited to be less than `mfft`.

See Also
--------
delta_f: Width of the frequency bins of the STFT.
f: Frequencies values of the STFT.
mfft: Length of input for the FFT used
ShortTimeFFT: Class this property belongs to.
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