Hello Dick
I am on the road at the moment and unable to post to the RTTY list,
but wanted to let you know my findings on this subject
The link below dates from 2009, when I was doing a lot of single-band
RTTY contesting on 40m, where decoding can be limited almost entirely
by QRM - in other words, interfering carriers as opposed to Gaussian
noise. In intense QRM, optimum overall accuracy of decoding is best
achieved by using a narrower roofing filter to keep the nearby
carriers from activating the receiver AGC, while accepting that there
will be a partial degradation in decoding accuracy.
It is a fine balance, and bandwidth on its own is not a good
indicator. Group delay characteristics are also extremely important,
but it's much harder to predict how these affect performance.
http://lists.contesting.com/archives//html/RTTY/2009-08/msg00150.html
That narrower filter was an absolute lifesaver, allowing me to work
Asian and Oceania multipliers through gaps in the wall of Central
European QRM, and it gave me a clear run frequency at times when the
400Hz filter was letting through too much QRM. The stock 400Hz filter
works better on the higher bands where QRM is less intense, but not
when the situation becomes limited by QRM and AGC capture.
I should be posting live again from home in Scotland by the end of
the week, but would be grateful if you could quote the link above to
the list.
73 from Ian GM3SEK
(somewhere in England)
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