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

[Kok Chen <chen@mac.com>]

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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