[RTTY] FSK vs AFSK

[Kok Chen]

On Dec 10, 2004, at 4:58 AM, John Fleming wrote:
> It doesn't matter which you use if you know what you're doing.  If 
> you're not sure, use FSK.  Properly operated, the guy on the other end 
> can't tell the difference.

I completely agree with John.

If you have no means of monitoring the spectrum of your own signals, it 
is safer to just use FSK.  That said, using the rule of thumb "no ALC, 
no audio processing" is mostly adequate in being able to produce a 
clean RTTY signal -- you have to constantly monitor the ALC, but it is 
part of being a good operator.

The use of AFSK can actually produce a more friendly RTTY signal than 
with FSK.  When using AFSK, I generate a signal that has been passed 
through a sharp 450 Hz audio bandpass filter before it goes to the rig. 
  The AFSK signal can be generated so that keying sidebands are more 
than 35 dB down outside this 450 Hz slot -- something that is just 
about impossible with rigs in FSK mode.  And it does not reduce the 
printability of the signal at the other end by more than a smidgen 
(i.e., statistically insignificant).

There is one more compelling reason to use AFSK: in these days, you 
probably have to include PSK31 as one of the modes that you use, and 
you will have to set up your station to produce clean AFSK (well APSK) 
anyway.

On the other hand, if you are using high power and the sound 
card/computer is not well shielded, FSK might be a much easier way to 
avoid RFI.

73
Chen, W7AY