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Re: 38k4 Modem Standard



Hi George,

you wrote:
>Can someone point me to where I can find 
>info on the modulation format for the 
>38k4 modem?  

38k4 is just regular G3RUH 9600 baud at four
times the speed. You'll want to read some
of the G3RUH 9600 baud articles at

http://www.amsat.org/amsat/articles/g3ruh/g3ruh-index.html

in particular, check out

"9600 Baud Packet Radio Modem Design" available at

http://www.amsat.org/amsat/articles/g3ruh/109.txt

>I gather it is FSK, but I can't find things 
>like Mark/Space frequencies, bandwidth 
>(19.2 kHz?), etc.  Thanks.

38k4 is frequency shift keying, not audio
frequency shift keying, so you won't receive mark 
and space frequencies.  In an FM receiver, the 
signal will sound like static or white noise.  
Each bit "pulse" overlaps with several of the
preceding and subsequent bits.

To vastly oversimplify, in a wide enough FM 
receiver you want to get the audio from the 
receiver's discriminator and you want to look 
at the signal 38,400 times a second to see if 
the received signal voltage is above or below 
"zero." This determines if the "raw" received 
bit is a one or a zero.  Take those "raw" bits, 
undo the NRZI encoding and the bit scrambling, 
search for HDLC flags, frame up the AX.25 
packets and check the CRC to see if you got 
a valid packet. Somewhere in all this you will
need to recover the clock and also remove 
intersymbol interference.  (Possibly I've 
missed a step here...)

Mark, N8MH, has just posted some sample
recordings if you want to listen to what 38k4
actually sounds like. Remember, it sounds
like noise.  His test files are at

http://files.f8s.com/other/38k4_test1.zip
http://files.f8s.com/other/38k4_test2.zip

Douglas KA2UPW
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