There are many tutorials and guides explaining how to create a "robotic" voice by using a vocoder or a mix of filters that change pitch, tempo and maybe add a slight echoed delay. However, is it possible to do that opposite? How would you take a "robotic" sounding voice and make it sound more natural? I know certain effects, like adding emphasis, is still a research problem, but is it easier to remove the "tinny" sound found in a lot of synthetic voices?
I'm specifically interested in a code solution (preferably Python), because I'd like to wrap it around a text-to-speech synthesizer (like Festival or Flite) to work in real-time. Unfortunately, I'm not sure where to begin since Googling "how to make a robot voice sound more human" just returns tutorials on doing the exact opposite.
Here's an example voice of the Flite engine using an HTS voice. Especially near the end, the voice almost breaks up into a "buzz". Can that be removed, or would that actually require adding audio data?