In order to have a more useful waveform of the short-time autocorrelation function, the nonlinear processing is usually used in pitch tracking: Y(n)=C[x(n)]
One of the nonlinear technique is center-clipping. The relation between input x(n) and y(n) is:
where CL is the clipping threshold. Generally CL is about 30% of the maximum magnitude of the signal. To get the high CL, the peak value of the first 1/3 and the last 1/3 of the signal are measured. The lowest one is set to be the maximum magnitude and the 60-80% of this maximum magnitude to be CL.
The effects and implementation of center-clipping can be observed in the following image.