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[dsp] Problem with modem

John B. Stephensen kd6ozh at verizon.net
Tue Nov 2 21:21:55 UTC 2004

The demodulator is designed to measure the difference in amplitude between
signals at 2200 Hz and 1200 Hz so it is working normally. FSK modems like this
normally have a limiter ahead of the demodulator so that amplitude variations
are removed from the input.



----- Original Message -----
From: "Mark" <mark at cpovo.net>
To: <dsp at lists.tapr.org>
Sent: Tuesday, 02 November, 2004 12:54 UTC
Subject: [dsp] Problem with modem

> Hi,
> I used the article below to design a 1200bps modem using
> an ATMEL ATMEGA8 processor.
> <http://www.baycom.org/~tom/ham/da95/d_dspmod.pdf>
> On that article there is a C routine example of the demodulator
> that I have adapted to the ATMEGA8 processor using 16bit fixed point
> math and the internal 10bit ADC.
> It works very well decoding 1200/2200Hz AFSK Packet signals.
> But it gets in trouble decoding the signal when the amplitude
> of the two frequencies becomes different.
> I don't know exactly what I shoud do to overcome the amplitude
> difference, maybe some sort of AGC, but I'm not sure on how to do it.
> Would like to hear the comments of the experts on this list.
> Any idea is wellcome.
> Thanks!
> Mark Jordan - PY3SS
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