CVSD uses a bit-serial data stream. Harris datasheets for obsolete
Codecs are HC55516, HC55532. The "recording"-circuit can be an analog
hack ( Kop, flipflop, 4 Bit shiftregister ) that sends data via SPI.
The "playback" would have to emulate this circuit in software and output
via a 8 bit D/A ( R2R resistor network, but serial ICs may be easier in
SMD ).
16kBit/sec is very moderate quality, 24kBit/sec more reasonable.
We used these in the 80ies for digital answering machines in cars for
the analog telephone system via radio that predated GSM in Germany.
24kBit was for incoming messages in RAM, 16 kBit for the fixed messages
from EPROM. CVSD was ok, as the analog radio was a bit noisy
anyway.
At 32kBit/sec ADPCM is better, but you probably do not intend to use a
64kBit PCM codec as a frontend. If you use a handset or a digital
PCM-link, the quality of CVSD may be not competitive. For playback via
a loudspeaker sufficient, there is usually enough background noise.

MfG JRD

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pozz <pozzugno@gmail.com> writes:

I tried to reduce sampling frequency to 4kHz, but the quality is
drastically reduced.

Try 6.5 khz. I'll write a little more later but I've dealt with this
problem and there are some reasonable approaches.

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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

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