[aprssig] AFSK vs PSK
Stephen H. Smith
wa8lmf2 at aol.com
Sun Nov 24 15:16:18 CST 2013
On 11/24/2013 12:45 PM, Andrew Rich wrote:
> I often sit here listening to AX25 on AFSK and note the number of missed packets
> I think it is due to the SN ratio.
> Would something like PSK be a better option ?
> - Andrew -
The problem is audio tones over FM. The huge advantage is in using direct FSK
of the FM carrier rather than modulating it with AFSK/APSK audio tones.
Direct FSK is universally done in the commercial mobile data/land mobile field.
4-frequency direct FSK is used by P25 digital voice and by Yaesu's new
digital voice FM radios. (It's sometimes called "C4FM".) In the C4FM mode, the
carrier shifts between +1600 Hz, +800 Hz, -800 and -1600 from channel center,
allowing two bits to be sent per symbol or state change.
However for ham applications:
1) You have to cut into the radio to make direct DC logic-level connections to
the transmitter modulator and receiver discriminator -- something that most
appliance-operator hams are reluctant or unable to do. I.e. you lose the
"stuff it into the mic jack on transmit / take it off the speaker on receive"
simplicity of audio tone hookups.
2) Direct FSK is incompatible with the vast inventory of existing TNCs, both
external stand-alone units, and built into radios like the Kenwood APRS radios.
It's something IBM ran into years ago when they tried to alter the PC
standard with their PS/2 family of PCs. What they call the "tyranny of the
installed base". I.e. something may be superior but if it's incompatible
with the millions of existing devices, it has a tough time being accepted.
Stephen H. Smith wa8lmf (at) aol.com
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