We now apply the same principle to the autocorrelation function for a stationary process. For
simplicity, we assume that the mean value is zero, i.e. we set μx = 0.
For the nth record, we form Xn(t)Xn(t + τ ) as shown in Figure 8.8, and average this
product over all records, i.e. an ensemble average.
Then, we can write the autocorrelation function as