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[dsp] Problem with modem

Mark mark at cpovo.net
Tue Nov 2 12:54:54 UTC 2004


	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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