Getting back to the bad LPC run that initilized this discussion:
The error I have had from bad LPC pitch tracking
(in the cmix implementation mostly, I'll confess) was very much
describable as a "burble"...it was caused by excessive
"octave switching" and other
instability in the Pitch analysis...
when I have used Csound (or other) LPC
succesfully, I have used tools
(P.Lansky's, from old cmix dist.) to
examine the pitch info & edit it....
try cutting out the lpreson/freson side
of the instrument, and listen to the pitch
of the source you're actually using.
(Dunno your instrument, so I'm just guessin')

Also try not using the pitch info from lpread...
just "compose" the source pitch.
This will give a less "life-like" speech effect
(if that's what you're doing),
but it will clearly show if the pitch data is
"wobbling" all over...which it often will do in LPC analysis.


Just trying to help.
CharlieB