[aprssig] AFSK vs PSK

Andrew Rich vk4tec at tech-software.net
Sun Nov 24 15:36:53 CST 2013


Very interesting

That might be why 

Balloon projects use FSK

Where the carrier Freq is shifted between two distinct F1 and F2

Reception is done on SSB ? correct me if I am wrong.

Is this what they cal true RTTY ?

Like CW but jump high and low 

- Andrew - 

-----Original Message-----
From: aprssig-bounces at tapr.org [mailto:aprssig-bounces at tapr.org] On Behalf
Of Stephen H. Smith
Sent: Monday, 25 November 2013 7:16 AM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] AFSK vs PSK

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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