Pitch-synchronous overlap and add

Perhaps the mostly widely used second generation signal processing techniques are the family called pitch synchronous overlap and add, (shortened to PSOLA and pronounced /p ax s ow 1 ax/). These techniques are used to modify the pitch and timing of speech but do so without performing any explicit source/filter separation. The basis of all the PSOLA techniques is to isolate individual pitch periods in the original speech, perform modification, and then resynthesise to create the final waveform. 

Time domain pitch synchronous overlap and add or TD-PSOLA is widely regarded as the most popular PSOLA technique and indeed may well be the most popular algorithm overall for pitch and timing adjustment [194], [322], [474]. 

The above technique bears some similarities to the TD-PSOLA technique in that it uses a pitch-synchronous analysis to isolate individual pitch periods, after which modification and resynihesis is performed. In fact in a technique called linear prediction pitch synchronous overlap and add or LP-PSOLA, we can use the PSOLA more or less directly on the residual rather than the waveform. As above, epoch detection is used to find the epochs. The residual is then separated into a number of symmetrical frames centred on the epoch. Pitch modification is performed by moving the residual frames closer or further away, and duration modification is performed by duplication or elimination of frames, in just the same way as in TD-PSOLA. The only difference is that these operations are performed on the residual which is then fed into the LP filter to produce speech. This technique differs only from the Hunt technique in the shape of the frames. Both techniques uses window functions with their highest point at the epoch in Hunt s technique the windows are asymmetrical with the idea that they are capturing a single impulse, in LP-PSOLA the windows are symmetrical. In listening tests, the two techniques produced virtually identical quality speech. 

The above technique bears some similarities to the TD-PSOLA technique in that it uses a pitch-synchronous analysis to isolate individual pitch periods, after which modification and resynthesis are performed. In fact, in a technique called linear-prediction pitch-synchronous overlap and add or LP-PSOLA, we can use PSOLA more or less directly on the residual rather than on the waveform. As above, epoch detection is used to find the epochs. The residual is then separated into a number of symmetrical frames centred on... 

The PSOLA method. The PSOLA (Pitch Synchronous OverLap-Add) method [Moulines and Charpentier, 1990] was designed mainly for the modification of speech signals. For time-scale modifications, the method is a slight variation of the technique described above, in which the length of the repeated/discarded segments is adjusted... 

Pitch-scale modification is performed 1 recombining the frames on epochs that are set at different distances apart from the analysis epochs, as shown in Figure 14.3. All other things being equal, if we take for example a section of speech with an average pitch of 100 Hz, the epochs will lie 10 ms apart. From these epochs we perform the analysis and separate the speech into the pitch-synchronous frames. We can now create a new set of epochs that are closer together, say 9 ms apart. If we now recombine the frames by the overlap-add method, we find that we have created a signal that now has a pitch of 1.0/0.009 = 111 Hz. Conversely, if we create a synthetic set of epochs that... 

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