Different vowel sounds can be produced by changing the amplitudes of the individual delays. The real magic is obtaining those values — my early experiments were plagued with howling feedback, and situations where the DC offset increased exponentially ;)

Ultimately, I looked at the source of an app called rt_lpc (http://soundlab.cs.princeton.edu/software/rt_lpc/) and this was very useful. I have a vague idea of how the encoding works now (their code could stand more comments IMHO ;), the decoding & playback is pretty straight-forward.

Currently I’m not using any blending or transitions between phonemes (tho I am synthesizing the sound at a 10khz sample rate and using anti-aliasing during playback, because I like how it sounds ;) When the feedback amplitudes change, the playback buffer retains its old state, and the result is a smooth-sounding transition between frames. Pretty neat.

Really, I’m amazed that LPC works AT ALL. It’s very clever.

I’d really like to share the code (the dictionary and speech-to-phoneme aspects are interesting, too). Hopefully I’ll get the go-ahead from the team soon… I’ll write another post when that’s ready.