PHONETICS: The PSOLA algorithm (Pitch-Synchronous-Overlap-Add) is used for the manipulation (F0 and duration) of the speech signal as well as for speech synthesis. The PSOLA manipulations are applied directly to the waveform. First, the fundamental frequency is detected by a separate algorithm and a window is placed around every pulse. This window is normally twice as large as the fundamental frequency. In order to increase the fundamental frequency, the windows are shifted more closely to each other such that they overlap. To decrease the fundamental frequency, the windows are shifted apart. The algorithm can also be used to manipulate duration: to increase duration, windows are copied and pasted into the original signal. To decrease duration, a number of windows are cut out of the signal.
Tutorial on PSOLA