[RTTY] Shift 170 -vs- 200

[Ktfrog007 at aol.com]

Hi,
 
Back to an old thread from a couple weeks ago:
 
I run MMTTY with AFSK, almost all S&P both in and out of  contests.  I keep 
the AFC OFF and pop it ON only if I need  it for some reason.  I have the 
SHIFT set to FREE and if I'm having trouble  tuning in a station (for 
whatever reason) I click the AFC ON and see  if it will fine tune for me.  The 
instant I'm happy,  back OFF it goes.  This will handle unusual shifts, although 
if the  signals are very weak it may not work well.  
 
When the QSO is over, I click HAM to put things back to normal again.
 
Do not run with AFC ON and the shift freewheeling.  It may  stay reasonably 
close to 170 Hz by locking to those you are working, but  you can end up 
with a random shift and people will say your mother  dresses you funny and no 
one will like you.
 
73,
Kermit, AB1J
 
 
 
 
In a message dated 6/9/2011 11:59:14 A.M. GMT Standard Time,  
psussman at pactor.com writes:

I've  noticed over the years that some manufacturers (for whatever reason)
have  moved to use a 200Hz shift instead of the standard 170Hz shift. The
reason  for the move is unclear, perhaps it is merely for simplicity.

Whatever  the case, it is claimed that tuning between two stations, one
using 170Hz  shift and the other using 200Hz, is not all that great. When
the tuning  between the two is centered, there is an offset of only 15Hz.
(200-170 =  30; 30/2 = 15) In the days when filters were a lot broader,
this offset was  insignificant. I think Chen will bear me out that under
strong signal  conditions this offset doesn't matter.

The only time when such an  offset is important is when very sharp filters
with tight skirts are used  AND the signal strength is weak or fluctuating.
That would lead to more  errors.

I've been using an SCS modem for years with 200Hz shift on AFSK  and I don't
seem to have a problem decoding weak RTTY signals.

Any  thoughts or explanation?

Thanks,

73 de Phil -  N8PS



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