Re: Voice compression

very poor, quite unusable.

You are not seriously expecting me to debug your code ?
CVSD 16kBit was used in the 70ies for military secure communication.
The then SpaceShuttle ADM ( = CVSD ) is a simple digital implementation, 16kBit i guess.
Therefore at 16kBit CVSD is usable, but not for public phone system.
Initial circuits were analog:
https://get.hidrive.com/5gdAmSyB   cvsd-ptarmigan.pdf
The CML FX209 is an early integrated analog version:
https://get.hidrive.com/HhS2FWU4   cvsd-steele.pdf
The Harris HC55564 is a simple digital IC.
The CML FX609 is the next and final generation with PCM-like
filter that reduces high frequency noise.
We did use the Harris. On switching to the FX609 had a test with
all the employees in the company with handset what they liked
better: 90:10 for the FX609. The problem with "better" is that
everyone is accustomed to PCM-filtered speech.
All these ICs one can get via ebay.com from China.
Using 2 on breadboards one can build a simple channel that
"distorts" speech for testing. Reference would be an old
PCM-chip.
As for quality: the 64kBit PCM may be the gold standard,
but the cordless DECT phones use ADPCM at 32kBit
with hardly any loss of quality. This is not the
original CCITT-ADPCM that was very complex. But i still
doubt implementation on an AVR is easy.
CVSDs i did years/decades ago on PICs/68HC05.
MfG  JRD