[aprssig] AFSK vs PSK

Stephen H. Smith wa8lmf2 at aol.com
Sun Nov 24 16:16:18 EST 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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