8840Re: [wsjtgroup] JT65HF Quit Decoding
- Jan 2, 2012On 1/1/2012 8:11 PM, Jim Brown wrote:
> In attempting to fix the problem, I've uninstalled and reinstalled theThanks for the responses. After tearing my hair for most of the evening
> software and rebooted Windows.
trying to figure out what cockpit error might have caused this, I
finally found it. I'm using a K3, and the button used to switch digital
modes with a "long push" is used with a "tap" to turn AFX on and off.
When setting the mode to AFSK to work that DX spot, I had accidentally
turned AFX on. I turned it off and the decoder started working again.
One other VERY important thing I've learned is that with a lot of QRN
(this was on 160M), narrowing the IF bandwidth and centering it on my TX
offset can help decode weaker signals. This is not surprising --
computer sound cards are notoriously non-linear and most have far less
dynamic range than the better ham rigs, so impulse noise can easily push
them well into clip. When I asked Joe Taylor about narrowing the IF to
improve S/N, he emphasized issues with phase shift in the IF filters as
potentially hurting decode, and that the filtering in the sound card was
doing that with his decode algorithms. At the time (in Q&A at his
lecture in Visalia), I didn't think to raise the sound card
non-linearity issue, and my 50 years in pro audio causes me to strongly
agree with him on the issue of phase shift. But taking the shortcomings
of sound cards into account, I'm suggesting that since most phase shift
with good filters occurs at the skirts, narrowing the bandwidth to
something like 500 Hz for JT65 could be a good thing for a 200 Hz
bandwidth JT65 signal, both to improve S/N and to minimize QRM from a
strong signal with a different offset.
73, Jim K9YC
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