[dsp] Problem with modem

[John B. Stephensen]

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.

73,

John
KD6OZH

----- 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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