'power spectral density using fftshift
currently returns two-sided according to fft frequencies, use first half'

def spdshift(self, n):

    mapadded = self.padarr(self.ma, n)
    arpadded = self.padarr(self.ar, n)
    hw = (fft.fft(fft.fftshift(mapadded)) / fft.fft(fft.fftshift(arpadded)))
    w = ((fft.fftfreq(n) * 2) * np.pi)
    wslice = slice(((n // 2) - 1), None, None)
    return ((hw * hw.conj()).real, w)
