Thanks very much for your reply, super helpful!! Sorry if that was difficult to answer. I guess I'm interested in how far we've gone from TTS engines like the LPC [1] engines we had in the 80s, or what you get from festival [2]. Maybe there isn't as clear a separation between their methods and the modern Google-scale deep-learning approaches as I thought.
[1] https://en.wikipedia.org/wiki/Linear_predictive_coding
(e.g. as seen in https://en.wikipedia.org/wiki/Texas_Instruments_TI-99/4A)
[2] http://www.festvox.org/festival/