[hts-users:03168] Questions from a novice about improving voice quality

[Matt Campbell <mattcampbell@xxxxxxxxx>]

Hello:

I've just started learning about HMM-based speech synthesis, and I find 
it quite interesting. It seems to me that HMM-based synthesis is more 
flexible than unit selection, and produces a more consistent quality of 
output. In particular, HMM-based synthesis seems to avoid the glitches 
that are common in unit selection when the input text doesn't overlap 
much with the pre-recorded units. So HMM-based synthesis seems like a 
return to the strengths of formant-based synthesis. But it seems to me 
that unit selection still does a much better job of reproducing the 
unique sound of the original speaker's voice.

I noticed that the CMU ARCTIC SLT demo voice is quite small, ~2 MB 
uncompressed. And as one might expect for such a small voice definition, 
it doesn't faithfully reproduce the sound of the woman's voice in the 
original recordings. Indeed, it sounds much like the small-footprint 
voices that I've heard on mobile devices, e.g. the compact US English 
female voice on the iPhone (Samantha) or the US English female voice on 
Android (SVOX Pico).

So, can a larger HMM voice be built, that more accurately reproduces 
the voice in the original recordings? For example, would it be possible 
to produce a larger mgc.pdf file to achieve this goal? In the case of 
the CMU ARCTIC SLT demo package, would increasing the MGCORDER option 
help? I ask about the MGC data because mgc.pdf is by far the largest 
file in the binary package, suggesting that it contains the most 
information about the sound of the voice.

If not the MGC order, then are there any other numbers that can be 
tuned to improve voice quality, potentially at the expense of a larger 
model or longer training time?

Or is this a fundamental limitation of HMM-based synthesis?

Even if the latter is true, I would probably accept the SLT voice in 
day-to-day use; after all, my current favorite synthesizer is a formant 
synthesizer. I just want to know more about what's possible with 
HMM-based synthesis.

Thanks,
Matt