[dsp] Problem with modem

[Joe Fitzgerald]

Mark wrote:

>	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.
>
>	
>  
>
Consider a variable decision threshold in your slicer.  The essence of 
this idea is to look at the outputs of your matched filters over a 
period of time (equivalent to, say, 10 or 20 bits), set slice level mid 
way between the highest output and the lowest output.  This scheme will 
allow you to decode a signal even if one of the tones has faded to zero.

See

http://www.wj.com/pdf/technotes/FSK_signals_demod.pdf

or

http://www.eecis.udel.edu/~mills/database/reports/dsp93/dsp93b.pdf

Good luck!

-Joe KM1P