Poor Sound Quality with Rockwell Chipset

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I have a 14,400 bps modem that uses the Rockwell chipset, and I've
noticed that the quality of sound is not very good. I use the PVF
tools to convert a sound file to a .rmd file using the following:

$ basictopvf file | pvfspeed -s 7200 | pvftormd Rockwell 4 > file.rmd

If I then play it back with this:

$ rmdtopvf file.rmd | pvfspeed -s 8000 | pvftobasic | cat > /dev/audio

I notice the sound quality has deteriorated. If I call in and listen
to the message over the phone, it has very low volume. If I leave a
message and use the above to play it back, it sounds very bad, and has
a low rumble in the background, sort of like static slowed way down.
In other words, it's sort of an erratic sound. Is there anything
I can do to get better results, or is this typical of the Rockwell
chipset? If so, What kind of modems get the best sound quality for
voice? I'm thinking about upgrading to a 28,800 or 33,600 bps modem
(or better), and I definitely want voice on it, but I'd like better
sound than the one I have. I also want an external modem.

Chuck

-- 
Chuck Martin       <cmartin@bigfoot.com>
========================================================================
"Our Constitution was made for a moral and religious people. It is
wholly inadequate for the government of any other." - John Adams, 1798
.