Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!purdue!gatech!gitpyr!prism!ce1zzes
From: ce1zzes@prism.gatech.EDU (Eric Sheppard)
Newsgroups: sci.electronics
Subject: Re: MODEM SPEED INCREASE - GOT IT
Keywords: 300-1200+ BAUD
Message-ID: <1580@hydra.gatech.EDU>
Date: 23 Aug 89 21:35:29 GMT
References: <1447@esunix.UUCP>
Organization: Georgia Institute of Technology
Lines: 39

In article <1447@esunix.UUCP>, hcooper@esunix.UUCP (Harrison Cooper) writes:
> Thanks to all which answered my question (now relizing it may have been
> more stupid than I thought) relating to a modem conversion.  I suppose
> if it were possible, manufactures would have done it in the first place.
> Of course, this also answers why 300 baud units are so much cheaper.  I
> did take the one unit apart and found it using a Exar FSK chip.
> 
> However, I did find out more than I knew before about modems, so all is
> not lost due to my request.  OK, now if I decide to build a 1200+ modem
> (why not just buy one? Because I like to build things for the fun of it)
> who has a good inexpensive chip set (kinda like the one chip phones)
> that doesn't take much glue to get up and running.  Again, I do not
> really need a smart modem, since I am running only a dumb terminal.

Are you in luck!  I just happen to be building a 300/1200 baud FSK
modem as a project for Digital Communications.  As you saw, the modem uses
the Exar 2211 FSK Demodulator chip to convert the tones into data pulses.
From my progress thus far, FSK modulation is achieved through a Voltage-
Controlled-Oscillator (VCO), which is used to generate two tones (one tone
for mark, the other tone for space).  To communicate with another modem, your
transmitted tones must not interfere with the other modem.  To communicate,
the two modems merely use different frequency pairs.  The originating modem
uses the 2205/2225 Hz frequency pair, and the answering modem uses the
1205/1275 Hz frequency pair (as I recall; they may be slightly different).

Once I get this thingy up and running (I'm working on the filter circuits now),
I would like to try to modify it for PSK modulation, and experiment with
2400 baud systems.


Eric Sheppard
Georgia Institute of Technology, Atlanta Georgia, 30332
uucp: ...!{allegra,amd,hplabs,seismo,ut-ngp}!gatech!prism!ce1zzes
ARPA: ce1zzes@prism.gatech.edu
-- 
Eric Sheppard
Georgia Institute of Technology, Atlanta Georgia, 30332
uucp: ...!{allegra,amd,hplabs,seismo,ut-ngp}!gatech!prism!ce1zzes
ARPA: ce1zzes@prism.gatech.edu