On Mar 30, 2012, at 9:56 AM, Ed Muns wrote:
> FSK requires a homebrew or commercial
> interface and AFSK (with respects to Uncle Bill's history lesson!) requires
> proper adjustments of drive level, compression off, etc.
I have clarified the AFSK nomenclature with Uncle Bill in private e-mail last
night.
What most of us today call "AFSK" can rightly be called SSB-AFSK.
Basically, if you take what Bill calls AM-AFSK, remove the AM carrier and one
of the AM sidebands, you get precisely what we call an RTTY signal (i.e., two
alternating FSK carries in the RF spectrum). The removal of the carrier and
one of the AM sidebands from the spectrum is what an SSB transmitter does.
When you plug an audio FSK signal (i.e., two tones in the audio range, for
example 2125 Hz for Mark and 2295 Hz for space) into an FM transmitter, you get
what Bill calls FM-AFSK (yes, that nomenclature is quite common). When you
plug the same audio signal into an AM transmitter and you get AM-AFSK. And
finally, when you plug the same audio signal into an SSB transmitter and you
get good old steam RTTY. (It is all in the Convolution Theorem.)
The FM-AFSK and AM-AFSK modes are sub-carrier systems (probably designated as
F2D emission mode; but I am not 100% certain and I don't *think* that they are
ham legal in the US below 28 MHz), but "SSB-AFSK" is nothing more or less than
an F1B emission mode signal.
So folks, at least in my opinion, it is OK to use the term AFSK to describe the
process of injecting an audio FSK ("AFSK") signal into an SSB transmitter to
produce RTTY. The pedant might want to call it SSB-AFSK, but it is
mathematically an AFSK process. I don't mind calling "FM-AFSK" as simply
"AFSK" either; because it is.
73
Chen, W7AY
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