[RTTY] RTTY Shifts

[Kok Chen]

In an email to me, Russ WA3FRP brought up something interesting -- of  
switching to wider shifts on the lower bands.

When shifts are too narrow, the mark and space can be so close to one  
another that they no longer fade independently.  When that happens,  
any ATC circuit that tries to work around selective fading no longer  
functions, since there is _no_ selective fading.  When both tones  
vanish below the noise at the same time and the decoder can no longer  
decide if mark or space was transmitted.

The question is therefore (1) is selective fading on the low band  
replaced mostly by flat fading, and (2) whether the use of greater  
frequency diversity (larger shift) will bring back selective fading so  
that at least one of the tones survives through a fade.

If all else is equal, selective fading is preferable to flat fading  
for RTTY since that is what ATC circuits are designed to overcome.  
Flat fading is better solved with the use of temporal interleavers and  
error correction codes, such as used in MFSK16, Olivia and DominoEX --  
the modern digital modes tend to spread errors in the time dimension  
to overcome fading.

It is easy enough to generate a signal that has 4 tones that are  
separated by 200 Hz each, for example, to try to objectively measure  
selective fading at different frequency diversity.  The problem is  
that such a signal might not pass FCC 97.309.  Short of that, it might  
be sufficient to just watch a 500 Hz wide MFSK signal on a spectrum  
analyzer.  For starters, I think I will try to look for fast Olivia or  
DominoEX on 80m to watch carefully.  The slower baud rates don't work  
as well since selective fades could have come and gone by the time the  
tone you want to watch is transmitted.

73
Chen, W7AY