[aprssig] AFSK vs PSK

PE1RDW aprs at pe1rdw.demon.nl
Sun Nov 24 16:44:28 EST 2013

On 24-11-13 22:16, Stephen H. Smith wrote:
> 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.
PSK is done at high speed packet (1M2 and higher) but as stephen points 
out this is best done directly on the carier and offcourse these speeds 
are not realy usefull for the local access channels, only for the 
backbone (in case of APRS use), it is also done for sattelite telemetry.

Direct FSK is in use right now at 9k6 packet, this is a special variant 
called GMSK or MSK, most modern mobile radios have the DC connection 
Stephen mentioned in the form of a standarised 6 pin mini din, the same 
connector type used for PC keyboards and mice, the build in kenwood tnc 
can do this as wel, the UDR56K4 that is currently under development will 
have a software modem build in and will be able to do afsk gmsk psk 4fsk 
and even complex modulations like QAM.

73 Andre PE1RDW

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