[Speex-dev] Steganographic extension to Speex

[Björn Thalheim]

Hello kind developers,

I am planning on creating a steganographic extension for speex/CELP.
Since speex/CELP is lossy, there should be quite many bits that one
can use for hiding data.

I'm familiar with the principles of CELP (two filter loops, one for
pitch, the other for formants, let the raw speech frame run through
those filter loops, take residue signal, map it on a codebook entry,
transmit the the two filters and the codebook key). Please correct
me if I understood something wrong.

As if right now, my questions are more from an engineers point of
view (which I could not find out/understand by reading the source
code) and totally newby-code-related (sorry if they're too easy!). I
use speex 1.1.6, for the reason than encoding and decoding actually
returned the "original" speech with that version, unfortunately not
with 1.1.10 or 1.0.5 (after encoding and decoding I only heard noise).

So my questions are:
- where are the two filter loops applied to the raw signal?
- where is the residue (innovation) signal calculated?
- where is the innovation mapped on a codebook entry (key)?
- which bits of a transmitted frame are the two filters, which are
codebook keys?
- is the codebook from the beginning on static (so already known by
sender and receiver) or transmitted somewhere at the beginning of a
VOIP connection?

In order to do that, I probably need to document CELP as used in
speex a little closer, which could maybe be used for the speex manual.

Regards,

Björn


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