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[aprssig] APRS on all Radios!!!!

Stephen H. Smith wa8lmf2 at aol.com
Sun Aug 17 17:52:05 UTC 2008


Scott Miller wrote:
>> Could it simply be PSK-31 encoded? What is the lowest carrier tone that can carrry PSK-31? Some APRS dongles already are starting to decode PSK-31.  And with say a 120 Hz carrier, it would seem that it would remain sub-audible?  We would of course add CRC checks to make each info packet complete.
>>     
>
> I've decoded PSK-31 (sent by an OpenTracker) successfully at a carrier 
> frequency of 62.5 hz.  Sounds like war drums on the subwoofer.
>
>   
>> If we used PSK-31 (sub audible) we would look carefully at the hamming coding to make sure we used an optimum format and did not just try to stuff APRS as-is into that pipe.
>>     
>
> PSK-31 uses varicode to send printable characters.  No need to do that 
> here, you can just use BPSK to send straight binary data.
>
> Still, it sounds like there are some serious radio compatibility 
> questions to be addressed.  I'm afraid I don't even know how the radio 
> suppresses the PL tone - 100 hz is most certainly within the audible 
> range.  Is everything filtered below a certain cutoff frequency, and if 
> so, what's the cutoff?
>
> Scott
> N1VG
>
>
> _______________________________________________
>   

In standard land-mobile radios,  the combination of small coupling caps 
in the audio power amp and the limited low-end frequency response of the 
typical 3" or 4" mobile comm speaker are tailored to create a high-pass 
filter effect that cuts off everything below about 200-250 Hz.



--

Stephen H. Smith    wa8lmf (at) aol.com
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