[RTTY] keyed AFSK? - was: Best RTTY rig?

[Kok Chen]

On 3/21/02 6:40 PM, "WA9ALS - John" <wa9als@starband.net> wrote:

>> Actually, there is a fourth mode which is probably used
>> more than all the others put together.  That is to take
>> the two audio tones and use them to MODULATE a carrier, either
>> AM or FM, as opposed to using them to create a frequency-shifted
>> carrier.  This method is what was originally called AFSK, and is
>> used for VHF FM packet even now.  As far as I know, this
>> mode has disappeared from HF - there may be some hard-core types
>> using it on 10, but I have never heard it myself.
> 
> Bill, why was this done?  I suppose it was more copyable than voice under
> certain conditions?

It is the same as connecting an AFSK generator to a sideband
rig.

Except that with AM, you not only send the LSB, but you also send
the USB and the carrier.

(Which is not *that* different from some of the RTTY signals you
hear during a contest, HI HI)

And, if you are using high tones, you are using up more than 4 kc
of the spectrum!  (Again, not that different from some stations :-)

I suspect that with modulated FSK, tuning a signal in is really
easy.  Same as MCW; the tone you hear is always what the sender
intended, even if you are off tuned.

Hmmm, by independently decoding both sidebands, selective fading
could be less of a problem, too.  Maybe it is a very nice mode,
afterall :-).

In the US, isn't it only allowed on 51 MHz and above now?

73
Chen, AA6TY