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[dsp] Problem with modem

Veenstra, Lester (N-IntelSat) lester.veenstra at lmco.com
Wed Nov 3 16:10:49 UTC 2004


Frederick Electronics Corp lives again !

-----Original Message-----
From: dsp-bounces at lists.tapr.org [mailto:dsp-bounces at lists.tapr.org] On
Behalf Of Joe Fitzgerald
Sent: Wednesday, November 03, 2004 10:21 AM
To: TAPR DSP Mailing List
Subject: Re: [dsp] Problem with modem


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

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