Pitch-synchronous overlap and add PSOLA

One last major difficulty is that, while our phonotactic grammar may give us all the possible phoneme pairs which exist in a language, in reality a significant minority may never occur in any known word. In English this is particularly problematic with the /zh/ phoneme, which is nearly always followed by /er/ (e.g. measure). For cases like this, imaginative nse of phoneme pairs across word boundaries or the creation of nonsense words will be required. 

Experience has shown that, with a suitably recorded and analysed diphone set, it is usually possible to concatenate the diphones without any interpolation or smoothing at the concatenation point. This is to be expected if the steady-state/transition model is correct (see Section 13.2.6). As we shall see in Chapter 16, however, this assumption is really possible only because the diphones have been well articulated, come from neutral contexts and have been recorded well. It is not safe to assume that other units can always be successfully concatenated in phone middles. 

Once we have a sequence of diphone units that match the phonetic part of the specification, we need to modify the pitch and timing of each to match the prosodic part of the specification. The rest of the chapter focuses on techniques for performing this task. 

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 

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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... 

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