[RTTY] Mark or Space only?

Kok Chen chen at mac.com
Fri Mar 9 19:45:07 EST 2007


On Mar 9, 2007, at 3:37 PM, DJ3IW Götz wrote:
> I am not aware that any of the soundcard type software
> we are using now has such a feature. MMTTY does not have it.
> And yes, MARK ONLY would improve print if for instance the
> space signal was much distorted even being absent completely.

Yes, there is at least one software modem that explicitly implements  
Mark-only and Space-only decoding.

Even when the modem does not implement it explicitly, you can still  
get a mark-only (or space-only) behavior if the modem uses some kind  
of automatic threshold correction (ATC) algorithm in the slicer.  ATC  
are schemes which changes the decision threshold between the mark and  
space signals so that when one of the tones undergoes selective  
fading, the threshold or decision level would move to allow you to  
sustain print from the remaining tone.

With a modem that slices with an ATC, you can apply an I.F. filter  
that lets through only one of the tones (or using a notch filter to  
notch away the signal near the unwanted tone) and you will still be  
able to copy without the modem explicitly having a mark only or space  
only setting.

In fact, the way cocoaModem currently implements mark-only or space- 
only is to feed the ATC with just one of the demodulated tones.

Mark-only and Space-only are not "cure all" solutions.  You do need a  
few dB better signal to noise ratio to achieve the same print  
performance compared to when you have both tones available.  In  
addition, if selective fading takes out the only tone that you are  
decoding from, poof goes the print -- there is a reason we RTTY nuts  
use two tones instead of a single tone, HI HI.

73
Chen, W7AY



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