def acovf_fft(x, demean=True):
    from scipy import signal
    x = np.asarray(x)

    if demean:
        x = x - x.mean()

    signal.fftconvolve(x,x[::-1])[len(x)-1:len(x)+10]/x.shape[0]
autocovariance function with call to fftconvolve, biased
dataset/ETHPy150Open statsmodels/statsmodels/statsmodels/sandbox/archive/tsa.py/acovf_fft