Formant-synthesis quality

Synthesis techniques based on vocal-tract models 

Rgure 13.5 Formant patterns for different speech rates (a) schematic target and transition formant positions for three vowek and (b) as in (a), but here vowel 2 is so short that the steady-state target positions are not reached. 

Given that one of the main problems hes with generating natural dynamic trajectories of the formants, we now turn to a series of techniques designed to do just that by means of determining these dynamics from natural speech. 

An alternative to using formants as the primary means of control is to use the parameters of the vocal-tract transfer function directly. The key here is that, if we assume the all-pole tube model, we can in fact determine these parameters automatically ly means of LP, performed by the covariance or autocorrelation technique described in Chapter 12. In the following section we will explain in detail the coimnonality between LP and formant synthesis, where the two techniques diverge and how LP can be used to generate speech. 

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Before going into this, we should ask - how good does the speech sound if we give the formant synthesiser perfect input The specification-to-parameter component may produce errors and if we are interested in assessing the quality of the formant synthesis itself, it may be diffieult to do this from the specification directly. Instead we can use the technique of copy synthesis, where we forget about automatic text-to-speech conversion, and instead artificially generate the best possible parameters for the synthesiser. This test is in fact one of the comer stones of speech synthesis research it allows us to work on one part of the system in a modular fashion, but more importantly it acts as a proof of concept as to the synthesiser s eventual suitability for inelusion in the full TTS system. The key point is that if the synthesis sounds bad with the best possible input, then it will only sound worse when potentially error-full input is given instead. In effect copy synthesis sets the upper limit on expeeted quality from any system. 

We have presented LP synthesis from one particular viewpoint specifically one where we show the similarity between this and formant and articulatory synthesis. It is quite common to see another type of explanation of the same system. This alternative explanation is based on the principle that in general we wish to record speech and play it back untouched in doing so we have of course exactly recreated the original signal and hence the quality is perfect. The problem is of course that we can t collect an example of everything we wish to say in fact we can only... 

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