Hi Rogerio,

Ah, I see what's going on! Well, your best bet for a quick solution would
be to use a standard 9600 baud modem and use SLIC (subscriber loop
interface circuit) chips to interface it to the CODEC. Motorola and Lucent
technologies are two prominent companies that make SLIC chips. You can just
modify their suggested circuit slightly, I would think, and get a pretty
much off the shelf solution. These are the kind of chips used in PBXs.

You could implement a modem yourself, but to achieve 9600 baud over a 20khz
BW, I don't think that you could use FSK. IIRC (thanks, BTW, for letting me
know what this is!) FSK has a BW efficiency of about .25 bits/sec/Hz, so
9600 baud would take up about 38kHz. So, you'd need something similar to
QPSK, and that's not very easy to implement, requires PLLs and quadrature
sine generators, etc. Unless you could find a chip to do this, or have lots
of free time to get something like this to work, I'd go with the SLIC
circuit. Many of these are designed specifically to interface to CODECs.

Good luck,

Sean


At 10:26 AM 9/11/98 -0500, you wrote:
>Sorry Sean, here's the whole story. This is an existing fiber link, 5 miles
>long, it has digital codec's for video and audio, the line level audio
>input bandwidth is 20hz-20khz with a SN ratio of 85 db. and 600 ohms
>impedance, it's really clean (broadcast quality audio). I do need full
>duplex. The links are independant each way, each using it's own fiber,
>codec's, etc.
>What I was thinking was that since modems use "chirps" or tones to
>comunicate thru phone lines which have restricted bandwidth there should be
>a way to make them work thru audio channels which cover the whole audible
>spectrum. Implementing a phone line is not posible.
>Thanks!
>
>Rogerio

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
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