Time domain PSOLA

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], [473]. 

---

Given the parameters of the model, we can reconstruct a time domain waveform for each frame by use of the synthesis Equation 14.3. Figure 14.6 shows a real and resynthesised frame of speech. An entire waveform can be resynthesised by overlapping and adding the frames just as with the PSOLA method (in fact the use of overlap add techniques was first developed for conjunction with sinusoidal models). 

PSOLA which operates in the time domain. It separates the original speech into fi-ames pitch-s5mchronousfy and performs modification by overlapping and adding these fi ames onto a new set of epochs, created to match the synthesis specification. 

The idea behind an EDA is to locate a single instant in each pitch period that serves as an anchor for further analysis. These positions are often known as pitch marks, pitch epochs or simply epochs. In general, they can refer to any reference point, but are often described in terms of salient positions on the glottal-flow signal, such as the peak of the flow for each pitch period. For many types of analysis (such as TD-PSOLA, which will be described in Section 14.2) it doesn t really matter where the anchor point is chosen, so long as it is consistent from period to period. Often a time-domain reference point such as the peak of the highest excursion in each period is used, or the trough of the lowest excursion. That said, many analysis techniques focus on one particular point known as... 

Figure 14.5 Explanation of how it is possible to change pitch in PSOLA without changing the spectral-envelope characteristics, (a) The time-domain pattern of a single impulse fed through an HR filter. The shape of the waveform is entirely dependent on the filter and not dependent on the impulse, (b) A series of impulse responses, clearly separated in time. As before, the pattern of the response for each period is dependent only on the filter characteristics. The period of this waveform is determined separately, by the distance between the impulses, (c) The same series but the impulse responses are positioned so close together that they overlap. The only difference between the perception of this and that of case (b) is that here the pitch will he higher because the separation is so much less. The spectral envelope will be the same because the impulse response itself is hardly altered.

The caveat of the PSOLA analysis and resynthesis method is that it only works satisfactorily on sounds containing regular cycles displaying a single energy point, referred to as the local maxima the human voice fulfils this requirement, but not all sounds do. The good news is that PSOLA is fast, as it deals directly with the sound samples in the time-domain. 

---