Hi all,

First of all I want to thank everyone who answered my mail concerning
audio compression algorithms. They were really helpful and now I have
a general idea of how to proceed.

For those of you who didn't read the mail: I'm thinking of building a
digital telephone answering machine and needed some help in deciding
what type of memory and software compression to use for storing the
speech.

I'm going the hard way without the use of ISD's chips. I'm
thinking of using a 2M x 4 DRAM chip for storing 4-bit ADPCM codes at
6kHz sampling frequency. This gives me approximately 6 minutes of
speech with reasonable quality which ought to be enough.

I've written ADPCM decoding and encoding routines and found that I
need about 110 program cycles per sample to encode and about 80
program cycles to decode. With overhead I'll probably need a crystal
running at 8MHz.

Now to the problem. I need to use DRAM which I have very little
experience in using (I'm really a software guy, not hardware). How
does one proceed to design a battery backed-up system with a DRAM and
a PIC processor? I understand that the DRAM must go into self-refresh
mode when the power is cut. Also, how do I get the memory to refresh
during normal operation? I understand that I probably need some kind
of external counter and perhaps something else to run the refreshing.
I'd really appreciate ideas and schematics for solving some of these
problems or all of them :-)

Thanks in advance,
Danjel McGougan.