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'OT: Another modem question.'
1998\09\10@172211 by ogerio Odriozola

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Thanks for all the answers, I guess I found out that's not what I was
looking for. What I need is to connect some sort of modem to an audio input
on end A, "hear it" 5 miles away at end B and do the same back from B to A.
I've seen stand alone modem chips but too slow (300 bps and 1200 bps). I
need 9600.
Is there a way to bypass the phone part of the modem?
Maybe two different tone generators and two tone decoders (one = 0 the
other = 1)? Would that be fast enough?
Thankyou for the answers once again!

Rogerio

1998\09\10@191804 by Sean Breheny

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Hi Rogerio,

I think you need to specify this "audio input" a bit more: what kind of
bandwidth does it have? what kind of noise can we expect on it? is it
wireless or wired? Do you need full duplex transmission?

5 miles is a long distance. A wired link is probably impractical because of
the need to lay cable or fiber over that whole distance.

Tell us some more about what you are doing and what exactly this "audio
input" is.

Sean

At 04:17 PM 9/10/98 -0500, you wrote:
{Quote hidden}

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
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1998\09\11@112727 by ogerio Odriozola

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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





.....shb7KILLspamspam@spam@CORNELL.EDU on 10/09/98 05:36:28 PM

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To:   .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
cc:    (bcc: Rogerio Odriozola/MTY/TVA/Dataflux)
Subject:  Re: OT: Another modem question.




Hi Rogerio,
I think you need to specify this "audio input" a bit more: what kind of
bandwidth does it have? what kind of noise can we expect on it? is it
wireless or wired? Do you need full duplex transmission?
5 miles is a long distance. A wired link is probably impractical because of
the need to lay cable or fiber over that whole distance.
Tell us some more about what you are doing and what exactly this "audio
input" is.
Sean
At 04:17 PM 9/10/98 -0500, you wrote:
>Thanks for all the answers, I guess I found out that's not what I was
>looking for. What I need is to connect some sort of modem to an audio
input
>on end A, "hear it" 5 miles away at end B and do the same back from B to
A.
>I've seen stand alone modem chips but too slow (300 bps and 1200 bps). I
>need 9600.
>Is there a way to bypass the phone part of the modem?
>Maybe two different tone generators and two tone decoders (one = 0 the
>other = 1)? Would that be fast enough?
>Thankyou for the answers once again!
>
>Rogerio
>
+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
EraseMEshb7spam_OUTspamTakeThisOuTcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1998\09\11@155831 by Sean Breheny

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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:
{Quote hidden}

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
shb7spamspam_OUTcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1998\09\11@171457 by paulb

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Rogerio Odriozola wrote:

> 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).

 It's just a pity you can't simply tap into the digital stream in the
codec.  You should really only need to swap modules or re-program it.
Still, you work with what you have.

 In this case, you have *no* requirement whatsoever for 'phone line
compliance.  What you really want to do is tap the direct audio input
and output of the inside modem chips to your link.  On older modems,
this should be possible; more recent ones have the "bridge" integrated
into the chipset.  You are still going to find it easier to bring the
isolation transformer connections directly out (jumper them direct to
the RJ connector).

 If the modem, as has been discussed, uses opto-couplers instead of an
isolation transformer and you can figure the circuit, you should have
your 2-wire to 4-wire circuit split done for you.  Split out the send
and receive optos separately, arrange bias to match the original and you
should be away (though there is another trick here).

 Hope this helps in some way.
--
 Cheers,
       Paul B.

1998\09\11@222430 by Sean Breheny

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Hi Rogerio, Paul, and others,

I agree with Paul, his is a better suggestion than mine (sometimes the
simplest of things elude me!). It would be much better to get an old 9600
baud modem, tap into the direct audio of the main modem chip (if
available), and feed it to the codecs. The only remaining question would be
if this audio is pre-emphasized in some way to equalize the frequency
response of the telephone interface circuit. Probably this won't be a problem.

Good luck,

Sean

At 07:12 AM 9/12/98 +1000, you wrote:
{Quote hidden}

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
@spam@shb7KILLspamspamcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1998\09\14@064749 by ogerio Odriozola

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Thanks to all, I do have a couple of 9600 modems which I'm willing to let
die in the test. I'l let you know if something good comes out.
Thanks!

Rogerio





KILLspamshb7KILLspamspamCORNELL.EDU on 11/09/98 09:22:08 PM

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To:   spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU
cc:    (bcc: Rogerio Odriozola/MTY/TVA/Dataflux)
Subject:  Re: OT: Another modem question.




Hi Rogerio, Paul, and others,
I agree with Paul, his is a better suggestion than mine (sometimes the
simplest of things elude me!). It would be much better to get an old 9600
baud modem, tap into the direct audio of the main modem chip (if
available), and feed it to the codecs. The only remaining question would be
if this audio is pre-emphasized in some way to equalize the frequency
response of the telephone interface circuit. Probably this won't be a
problem.
Good luck,
Sean
At 07:12 AM 9/12/98 +1000, you wrote:
{Quote hidden}

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
TakeThisOuTshb7EraseMEspamspam_OUTcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1998\09\15@040654 by paulb

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Rogerio Odriozola wrote:

> Thanks to all, I do have a couple of 9600 modems which I'm willing to
> let die in the test. I'l let you know if something good comes out.

>> Split out the send and receive optos separately, arrange bias to
>> match the original and you should be away (though there is another
>> trick here).

 Let me just mention the "trick".  Modems generally contain a "hybrid"
to at least reduce, if not eliminate "sidetone", that is, transmitted
signal included in the received.  On a "two wire" circuit, this must be
done, while Rogerio is obviously considering "four wire" links with
separate transmit and receive circuits.

 Where a transformer is used for coupling, it is (nearly) always bi-
directional, and the hybrid (two-wire to 4-wire split) is performed on
the "modem" side of the transformer.  In older modems this is a separate
circuit using an op-amp or discrete components, but on more recent ones
it may well be integrated (as in a common Telecom SLIC device).

 The situation where optocouplers are used can be either way.  The DC-
requiring line circuit may provide a "hybrid" function to maximise the
noise and linearity performance of the receive optocoupler, in which
case you have a 4-wire interface provided cleanly at the optocouplers.

 The alternative is that receive and transmit optocouplers are simply
in series (or indeed, parallel) and the receive circuit cops 100%
sidetone.  In this case, the modem-side circuit is balanced to expect,
and cancel the sidetone, so that if you connect to it pure incoming
signal without sidetone, it will actually re-introduce it (hybrid out of
balance)!

 Disabling the hybrid for 4-wire feed in a modem *not* designed for it
is the real challenge in this application.
--
 Cheers,
       Paul B.

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