'power spectral density using padding to length n done by fft
currently returns two-sided according to fft frequencies, use first half'

def spddirect(self, n):

    hw = (fft.fft(self.ma, n) / fft.fft(self.ar, n))
    w = ((fft.fftfreq(n) * 2) * np.pi)
    wslice = slice(None, (n // 2), None)
    return ((((np.abs(hw) ** 2) * 0.5) / np.pi), w)
