6 ms·

Does this do any sort of FEC? One of the things that makes stuff like this tricky(and APRS in general) is that one flipped bit throws off the whole decode. Fro

Does this do any sort of FEC? One of the things that makes stuff like this tricky(and APRS in general) is that one flipped bit throws off the whole decode. From what I've seen out there would be a 40% improvement[1] in just base APRS if there was some form of basic FEC built into the protocol. Sadly most of "state of the art" is stuck back in the 80s. [1] http://eludium.stensat.org/mcguire/projects/FX-25/FX-25_performance.htm http://eludium.stensat.org/mcguire/projects/FX-25/FX-25_perf...

aidenn0 8y ago
Does AFSK imply the use more than two frequencies? Using 4 or 8 frequencies and trellis modulating should get significant gain (i.e. using 4 frequencies and trellis modulating 1 bit per chip should be more robust than using just 2 frequencies).
- vvanders 8y ago
  Nope, just two pairs of bell tones on the same frequency. Simple FEC just works pretty well with parity bits, just needs to be supported on all encoder/decoders.
- someguydave 8y ago
  AFSK, as used in amateur radio modems today, uses the Bell 202 modem standard, which is 2-ary FSK https://en.wikipedia.org/wiki/Bell_202_modem https://en.wikipedia.org/wiki/Bell_202_modem Unfortunately, most folks on the VHF/UHF bands use FM radios with pre-emphasis and de-emphasis circuitry, so you would need to design a waveform that deals with those challenges.
  - Evidlo 8y ago
    Also known as binary frequency shift keying (BFSK).
moftz 8y ago
AX.25 has no FEC but there is FX.25 which is an encapsulation of the AX.25 packet with a preamble at the front and reed soloman at the end. But the issue is the speed of AX.25, 1200baud is dreadfully slow to be adding much onto the packet. The FX.25 protocol is a neat idea that allows for old TNC hardware to still be on the air. However, TNCs can be built with the cheapest of the raspberry pi clones so there really is no good reason as to why things can't progress to something like FX.25. There are certainly limitation to using 1200 baud AFSK but moving outside of the audio band, you could get some higher data rate but maybe loose some dB.
- Steltek 8y ago
  The stagnation of packet radio is due to FCC legal restrictions on data rate that are divorced from the reality of bandwidth usage in modern protocols. You need to get to UHF before doing anything reasonable (if you call 56k reasonable) and by then, your range is toast unless your friend is on top of a mountain.
glockenspielen 8y ago
FEC is forward error correction. https://en.m.wikipedia.org/wiki/Forward_error_correction https://en.m.wikipedia.org/wiki/Forward_error_correction
mycall 8y ago
Could FEC be considered a form of encryption?
- myself248 8y ago
  No, just like speaking in phonetics isn't encryption. It doesn't hide the content of the message, it just adds extra data to make it easier to recover in the case of noise. FEC could be mistaken for encryption if the algorithm wasn't published, but the same could be said for pretty much any other digital mode. Even Morse...

aidenn0 8y ago

Does AFSK imply the use more than two frequencies? Using 4 or 8 frequencies and trellis modulating should get significant gain (i.e. using 4 frequencies and trellis modulating 1 bit per chip should be more robust than using just 2 frequencies).

vvanders 8y ago
Nope, just two pairs of bell tones on the same frequency. Simple FEC just works pretty well with parity bits, just needs to be supported on all encoder/decoders.
someguydave 8y ago
AFSK, as used in amateur radio modems today, uses the Bell 202 modem standard, which is 2-ary FSK https://en.wikipedia.org/wiki/Bell_202_modem https://en.wikipedia.org/wiki/Bell_202_modem Unfortunately, most folks on the VHF/UHF bands use FM radios with pre-emphasis and de-emphasis circuitry, so you would need to design a waveform that deals with those challenges.
- Evidlo 8y ago
  Also known as binary frequency shift keying (BFSK).

moftz 8y ago

AX.25 has no FEC but there is FX.25 which is an encapsulation of the AX.25 packet with a preamble at the front and reed soloman at the end. But the issue is the speed of AX.25, 1200baud is dreadfully slow to be adding much onto the packet. The FX.25 protocol is a neat idea that allows for old TNC hardware to still be on the air. However, TNCs can be built with the cheapest of the raspberry pi clones so there really is no good reason as to why things can't progress to something like FX.25. There are certainly limitation to using 1200 baud AFSK but moving outside of the audio band, you could get some higher data rate but maybe loose some dB.

Steltek 8y ago
The stagnation of packet radio is due to FCC legal restrictions on data rate that are divorced from the reality of bandwidth usage in modern protocols. You need to get to UHF before doing anything reasonable (if you call 56k reasonable) and by then, your range is toast unless your friend is on top of a mountain.

glockenspielen 8y ago

FEC is forward error correction. https://en.m.wikipedia.org/wiki/Forward_error_correction https://en.m.wikipedia.org/wiki/Forward_error_correction

mycall 8y ago

Could FEC be considered a form of encryption?

myself248 8y ago
No, just like speaking in phonetics isn't encryption. It doesn't hide the content of the message, it just adds extra data to make it easier to recover in the case of noise. FEC could be mistaken for encryption if the algorithm wasn't published, but the same could be said for pretty much any other digital mode. Even Morse...