[RTTY] Fw: [CQ-Contest] CQWW, the 1st part... sooner than you think!

[Kok Chen]

On Aug 24, 2008, at 11:35 AM, Bill, W6WRT wrote:

> That is true when AFSK is done properly. Done improperly, you can tell
> the difference easily.

The same can be said about poorly generated FSK.

When not waveshaped, an FSK (with instantaneous rise and fall times)  
is very wide.  It is only about 20 dB down from the carrier at 220 Hz  
away from the mark and space tones, and barely 40 dB down 2 kc away  
from the RTTY signal.  The quality of the signal from an FSK  
transmitter depends on the attention paid to FSK keying waveforms.

On the other hand, good AFSK generators can apply waveshaping in  
software and you are limited only by the IMD performance of the SSB  
transmitter.

See for example, Alex VE3NEA's article "CW Waveshaping in DSP  
Software" in the May/June 2006 issue of QEX.  Alex, by the way is the  
author of the "CW Skimmer" and some of us might remember his  
comparative demodulation curves for some RTTY software on this  
reflector a few years ago.

RTTY behaves the same way as described by Alex in his QEX article.   
You are turning on and off two different carriers, instead of a single  
one.

As someone who writes my own software to work digital modes, I prefer  
SSB modulation ("AFSK") since I can more precisely control the signal  
which I transmit.  With FSK, I am at the mercy of the rig manufacturer  
as to how they control the keying waveforms, and in many cases such as  
FSK done in DDS chips, they are opaque to the experimenter.

73
Chen, W7AY

P.S. I highly recommend Alex' QEX article mentioned above for anyone  
even remotely interested in digital modes.

More recently, another very interesting article pertaining to digital  
modes is "Signal Resilience to Ionospheric Distortion of HF Digital  
Chat Modes" by HB9TPL in the November/December 2007 QEX.  The latter  
article shows very clearly how PSK31 beats the pants out of RTTY when  
there is no multipath and flutter,  but RTTY with sufficiently large  
power will still punch through flutter while not a single character  
will get through multipath no matter how much power you use on PSK31  
(which is sort of obvious since the phase error is a constant no  
matter how much power you use).