Limited-domain synthesis systems

Faced with the choice between fully natural but inflexible canned speech, and somewhat unnatural but fully flexible TTS, some researchers have proposed limited domain synthesis systems which aim to combine the benefits of both. There are as many different approaches to this. Some systems attempt to mix canned speech and TTS. Black and Lenzo [52] proposed a system for cleverly joining words and carrier phrases, for use in applications such as a talking clock. The phrase splicing approach of Donovan et al [139] used recorded carrier phrases and cleverly spliced with unit selection synthesis from recordings of the same speaker. A somewhat different approach is to use what is basically the same system as for normal unit selection synthesis but to load the database of recordings with words and phrases fi-om the required domain [440], [436], [394], [9]. 

The wide distribution of PKSs in the microbial world and the extreme chemical diversity of their products do in fact result from a varied use of the well-known catalytic domains described above for the canonical PKS systems. Taking a theoretic view of polyketide diversity, Gonzalez-Lergier et al. (41) have suggested that even if the starter and extender units are fixed, over 100,000 linear heptaketide structures are possible using only the 5 common reductive outcomes at the P-carbon position (ketone, (R- or S-) alcohol, trans-double bond, or alkane). Recently, it has become apparent that even this does not represent the upper limit for polyketide diversification. To create chemical functionalities beyond those mentioned above, nature has recruited some enzymes from sources other than fatty acid synthesis (the mevalonate pathway in primary metabolism is one example) not typically thought of as type I PKS domains. Next, we explore the ways PKS-containing systems have modified these domains for the catalysis of some unique chemistries observed in natural products. 

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