This MATLAB exercise designs and implements a pitch period detector based on detecting and tracking peaks in the autocorrelation of the LPC error signal during regions of voiced speech. The pitch detection procedure is called the SIFT (Simple Inverse Filtering Tracking) method. The SIFT pitch period detector uses a secondary autocorrelation peak in order to detect and correct pitch period detection errors due to effects such as pitch period doubling and related phenomena. The file '4.6 LPC Sift Pitch Detector.pdf' provides a User's Guide for this exercise.