ZCR

  autocov computes the autocovariance between two column vectors X and Y with same length N using the Fast Fourier Transform algorithm from 0 to N-2. The resulting autocovariance column vector acv is given by the formula: acv(p,1) = 1/(N-p) * sum_{i=1}^{N}(X_{i} - X_bar) * (Y_{i+p} - Y_bar) where X_bar and Y_bar are the mean estimates: X_bar = 1/N * sum_{i=1}^{N} X_{i} Y_bar = 1/N * sum_{i=1}^{N} Y_{i} It satisfies the following identities: 1. variance consistency: if acv = autocov(X,X), then acv(1,1) = var(X) 2. covariance consistence: if acv = autocov(X,Y), then acv(1,1) = cov(X,Y)