[RTTY] The Problem with AFSK ...

[Carter]

On 1/19/2013 12:00 PM, Joe Subich, W4TV wrote:
>
 > With all the recent talk that AFSK is cleaner than FSK, this morning
 > on 12 meters I watched a classic example of the problems with using
 > AFSK and the reason I would rather put up with the "hard" keying of
 > some FSK implementations rather than supposedly "cleaner" AFSK.

[snip]

Let me go where angels fear to tread  --and put on my asbestos suit -- 
before I put in my 2 cents worth...

1) AFIK, there are two types of "FSK":

a) FSK that is generated internal to the rig where the manufacturer 
'keys' an audio tone -- an 'internal' AFSK if you will, that the rig 
manufacturer calls 'FSK'. This can be OK as hopefully the rig maker is 
controlling many of the variables such as level, mark/space switching, 
etc versus the operator jamming his sound card into the mic jack.

b) What I call 'true' FSK as implemented in my old Drake T-4XB and 
TenTec Omni 6. This true FSK switches a varicap diode off and on, 
changing the frequency of the VFO by adding capacitance. IMHO, this is 
the best/cleanest way of getting on RTTY.

2) AFSK:

There are two things to be careful of here; first and most obvious is 
the issue of levels. We have all heard the "ghost" signals from someone 
over driving their rig.

The second -- and more subtle -- issue is that "zero crossing" switching 
between mark and space should be implemented as Irv Hoff W6FFC (SK) used 
when he created his AK-1 (?) AFSK unit that went along with his ST-6 
demodulator.

That's my story and I'm stickin' to it.

Carter  K8VT