The above walkthough is to give you a basic idea of the stages involved in
building a limited domain synthesizer. The quality of a limited domain
synthesizer will most likely be excellent in parts and very bad in
others which is typical of techniques like this. Each stage is, of
course, more complex than this and there are a number of things that can
be done to improve it.
For limited domain synthesize it should be possible to correct
the errors such that it is excellent always. To do so though requires
being able to diagnose where the problems are. The most
likely problems are listed here
Mis-labeling
Due to lipsmacks, and other reasons the labeling may not be correct.
The result may the wrong, extra or missing segments in the synthesized
utterance. Using emulabel you can check and hand correct the
labels.
Mis-spoken data
The speaker may have made a mistake in the content. This can often
happen even when the speaker is careful. Mistakes can be actual
content (it is easy to read a list of number wrongly), but also
hesitations and false starts can make the recording bad. Also note
that inconsistent prosodic variation can also affect the synthesis
quality. Re-recording can be considered for bad examples,
or you can delete them from the etc/LDOM.data list, assuming
there is enough variation in the rest of the examples to ensure
proper coverage of the domain.
Bad pitchmarking
Automatic pitchmarking is not really automatic. It is very worthwhile
checking to see if it is correct and re-running the pitchmarking with
better parameters until it is better. (We need better documentation
here on how to know what "correct" is.)
Looking at the data
There is never a substitute for actually looking at the data. Use
emulabel to actually look at the recorded utterances and see what
the labeling is. Ensure these match and files haven't got out of
order. Look at a random selection not just the first example.
Improving the unit clustering The clustering techniques and the
features used here are pretty generic and by no means optimal. Even
for the simple example given here it is not very good. See Chapter 12 on unit selection for more discussion on
this. Adding new features for use in cluster may help a lot.
The line between limited domain synthesis and unit selection is fuzzy.
The more complex and varied the phrases you synthesize are, the more
difficult it is to produce reliable synthesis.
