[aprssig] APRS on all Radios!!!!
Stephen H. Smith
wa8lmf2 at aol.com
Sun Aug 17 13:52:05 EDT 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?
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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