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
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