Time domain PSOLA in Software Creation QR Code in Software Time domain PSOLA

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14.2.1 Time domain PSOLA using software toencode qr code with web,windows application NW-7 Time domain pitch qrcode for None 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 technique works pitch-synchronously, which means that there is one analysis frames per pitch period. A pre-requisite for this is that we need to be able to identify the epochs in the speech signal, and with PSOLA it is vital that this is done with very high accuracy.

To perform this step, an algorithm of the the type discussed in Section 12.7 is used. The epoch positions are often taken to be at the instant of glottal closure for each period (Sections 12.

6.2, 12.7.

2) but so long as the epoch lies in the same relative position for every frame PSOLA should work well. The signal is then separated into frames with a hanning window which extends one pitch period before and one pitch period after the epoch, as shown in Figure 14.1.

These windowed frames can then be recombined by placing their centres back on the original epoch positions, and adding the overlapping regions (hence the name, overlap and add). When this is done, the result is a speech waveform perceptually indistinguishable from the original. The waveform is not exactly the same, as the sinusoid multiplication carried out during analysis is not exactly reversed during synthesis, but the overlap add procedure comes close enough that the difference is not noticeable.

Time-scale modi cation is achieved by elimination or duplication of frames, shown in Figure 14.2. For a given set of frames, if we duplicate one of these frames and insert it again into the sequence and then perform overlap and add, we will create a speech waveform that is nearly identical to the original except that it is one frame longer.

In general, listeners can t detect that this operation has been performed and the desired effect of a longer stretch of natural speech is percieved. Importantly, the listeners can t detect that in the new signal two consecutive frames are identical, rather than slowing evolving which is what we see in real speech. By eliminating a frame we can achieve the converse effect, and again, listeners normally do not detect that a frame is missing.

A rule of thumb is often quoted that these processes can be used to lengthen or shorten a section of speech by a factor of about 2 without any or much noticeable degradation. In reality, it is fairer to say that the more modi cation one performs the more likely it is that the listener will notice. Pitch-scale modi cation is performed by recombining the frames on epochs which are set at.

Synthesis by Concatenation and Signal Processing Modi cation different distanc qrcode for None es apart from the analysis epochs, shown in Figure 14.3. All other things being equal, if we take for example a section of speech with an average pitch of 100Hz, the epochs will lie 10ms 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 which are closer together, say 9ms apart. If we now recombine the frames by the overlap add method, we nd that we have created a signal which now has a pitch of 1.

0/0.009 = 111Hz. Conversely, if we create a synthetic set of epochs which are further apart, and overlap and add the frames on those, we nd that we generate a synthetic waveform of lower pitch.

This lowering process partly explains why we need frames which are twice the local pitch period; this is to ensure that up to a factor of 0.5, when we move the frames apart we always have some speech to add at the frame edges. Just as with timing modi cation in general listeners can not detect any unnaturalness in slight modi cations.

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