[Speex-dev] interpolation of LSFs and bandwidth expansion

[tunkeat]

hi,

thanks for answers to my previous qns. have some
more of them. hope it's ok to ask questions on 
the basics of CELP here. let me know if it's not.

1) synthesized filter stability after interpolation of LSFs

I read from some resources that if LSF representation
of LPCs is used for interpolation, stability of the
synthesis filter is guaranteed.  from another source,
i read that the stability of the interpolated
LSF synthesis filter is satisfied only by preserving
the intra-model interlacing theorem (the LSFs are interlaced
with each other for a given LP analysis order)

from speex codes, i see that stability of LSFs is enforced
each time using the function lsp_enforce_margin.  So, 
the synthesized filter after interpolation of filters may be 
unstable? will it be stable under certain conditions as
mentioned in the previous paragraph?

2) bandwidth expansion

lag windowing is applied to the autocorrelated signal and
from what I understand, it's used for bandwidth expansion.

in speex codes, there is a function bw_lpc for bandwidth expansion
for reconstructed LPCs as well.  what's the difference between these 2
(the initial
lag windowing and this function) and why the need 
for the second bandwidth expansion (when lag windowing is already
performed initially)?

tia,
tk