[RTTY] Updated AA5AU RTTY Contest Notes - XE and WPX contests

[Bob Wanderer]

Regarding lowering the RF Gain for best RTTY reception: Gee,
back in the early 1960's it was suggested by the ARRL to
lower the RF Gain and "ride" the AF Gain for best CW
reception.  Don't know about RTTY, as didn't even know about
that mode back then (Novice in the days of HF CW with 75W
input, crystal control, and
one-year-and-upgrade-or-lose-it-forever).  I think this did
not apply to voice operation (a good amount of SSB, but AM
was still used extensively).

73,
Bob AA0CY

-----Original Message-----
From: rtty-bounces at contesting.com
[mailto:rtty-bounces at contesting.com]On
Behalf Of Kok Chen
Sent: Wednesday, February 16, 2005 11:35 AM
To: 'RTTY'
Cc: 7L4IOU; Don Hill AA5AU
Subject: Re: [RTTY] Updated AA5AU RTTY Contest Notes - XE
and WPX
contests


On Feb 16, 2005, at 6:28 AM, 7L4IOU wrote:

> I discussed with Hal JF1PJK and Terry JA7IC
> about 756PRO Twin Peak Filter.

By the way, I wasn't sure if Don got his "turn the RF gain
down" hint
from Hisami-san, but that is a very good general advice for
everybody
using internal DSP on _any_ rig.

The reason is that when the A/D converter of a DSP unit is
saturated,
you will no longer get the frequency response of the filter
that you
had originally designed it for.

The same thing occurs when using an external A/D such as a
sound card.
But with sound card software, you can include a "VU meter"
that gives
you a clue that the A/D is clipping.

Another comment on dual-peak input filters.  If you are
already using a
"matched filter" in the demodulator, using a dual peak
filter at the
input could degrade copy.  RITTY for example, uses matched
filters.
The better modems that appeared on the analysis which Alex
VE3NEA
posted earlier are also probably using matched filters.

Using a double peak filter is not the same as using a
matched filter.
It kinda gets you half-way there, i.e., it is better than a
flat
filter.

With a real matched filter, the input of the slicer sees is
triangular
waveform.  It is the peak of the triangles that gives the
slicer that
fractional better dB of SNR which gives you copy on the
marginal
signals.  I suspect that if you were to look at the slicer
of a
non-matched filter modem, but using a twin peak filter ahead
of it, you
will see some preemphasis at the peaks too, just that it is
not a pure
triangular waveform.

73
Chen, W7AY

P.S.  If you are using PSK31, it is also a good idea to turn
the RF
gain down (or apply attenuation) when there is a loud
station in the
passband.  You will be able to notice that the louder
stations suddenly
have better IMD and are clobbering the weaker stations less.
Basically, IMD from the front-end of your rig.  This is also
a problem
with RTTY if there are more than one loud station under the
roofing
filter of the rig -- so the comment applies even if you're
using a
narrow IF filter.

Since the mark and space do not overlap in an RTTY station,
there can
be no IMD from a single station.  But during WPX, I had
captured the
spectrum of a loud station that had a small IMD component
precisely 170
Hz away from his mark at a little over 20 dB down.  It is
possible that
multipath was causing a mark/space overlap (I have no other
explanation
- a front end filter that has some group delay would also do
that but
mine should, I think, not).  Unfortunately, I only saw this
spectrum
afterwards, or I would have recorded the signal itself for a
better
look at the phenomenon.



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