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[dsp] Problem with modem

Joe Fitzgerald jfitzgerald at alum.wpi.edu
Wed Nov 3 15:20:38 UTC 2004

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.





Good luck!

-Joe KM1P

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