Other data-driven synthesis techniques

While the HMM techniques described in this chapter constitute the leading approach, other data-driven techniques have been developed. AU in a sense share the same basic philosophy, namely that it is inherently desirable to use a model to generate speech since this enables compact representations and manipulation of the model parameters, and all are attempts at solving the problems of specilying the model parameters by hand. 

HMM synthesis is an effective solution to the problem of how to map from the specification to the parameters. While most approaches aim to generate cepstral parameters, some generate formants and in this sense the HMM approach can be seen as a direct replacement for the provision of these rules by hand as described in Chapter 13. Issues still remain regarding the naturalness of the parameter-to-speech part of HMM synthesis, but as confidence in the abihty to solve the specification-to-parameter part is gained, new techniques will be developed to solve these naturalness problems. 

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Ease of data acquisition Whether the system is rule-driven or data-driven , some data has to be acquired, even if this is just to help the rule-writer determine appropriate values for the rules. Here linear prediction clearly wins, because its parameters can easily be determined from any real speech w aveform. When formant synthesisers were mainly being developed, no fully reliable formant trackers existed, so the formant values had to be determined either manually or semi-manually. While better formant traekers now exist, many other parameters required in formant S5mthesis (e.g. zero loeations or bandwidth values) are still somewhat difficult to determine. Articulatory synthesis is partieularly interesting in that in the past it was next to impossible to acquire data. Now, various techniques such as EMA and MRI have made this much easier, and so it should be possible to collect much bigger databases for this purpose. The inability to collect accurate articulatory data is certainly one of the main reasons why articulatory synthesis never really took off. 

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