On 11/4/2011 2:20 PM, Leif Asbrink wrote:
I don't think the FLEX system is any better or worse in this regard, as
it is a fundamental limit to signal processing.
For SSB I think you are right, but not for CW, AM and FM.
CW we'll get to in a moment, but I don't consider AM or FM to be weak
signal modes. They certainly present challenges for detection in the
presence of noise, but even though I use both of them I really don't
care too much. They are just for local chatting.
A way of approaching it is to match the detection method with the known
characteristics of the signal. For CW that means selecting the proper
bandwidth, and the PSDR does a really good job with it's narrow filters.
??????????????
OK. I've snipped a bunch of your following comments as basically I am
aggressively agreeing with you.
PSDR has some very good filter topologies, that run rings around my
conventional radios. Is it the the best possible? No, of course not, but
it is very good. I don't have Linrad on my F3K or SDR1K boxes but I have
experimented with Winrad and have found exactly what you state.
Everything here is locked to my Z3801 GPSDO, so I am within a fraction
of a Hz at 10Gigs. Piping the audio from PSDR via VAC to Winrad I can
get a significant improvement in CW copyability, at the expense of a
significant delay. If I had control over the sampling rates it would be
even better. PSDR, and the whole FLEX system, is intended for a mass
audience. The thing could be modified to get much better performance but
would there be a market for it?
This filter will do an excellent job on crowded HF bands
but it will not improve the detection of really weak CW
signals in a background of white noise for the simple reason
that the complete signal path also involves the human
brain that has a filtering that is similar or even a bit
narrower (for a well trained weak signal operator.)
I agree, but cutting out the superfluous hiss can be good to reduce
fatigue. 48 hours of listening to noise puts me into an odd mindset.
(cf: William Hurt/ Altered States).
The usual HF operator does not understand the VHF weak-signal attraction.
Such a narrow filter must be exactly centered on the carrier
and therefore an AFC (automatic frequency control) is needed.
You have to find the signal first, before AFC can be used.
On HF where the frequency response of antennas and of the
interference source become important Linrad is not so good.
But that is not it's intended application. The HF crud is very different
than that we find on the VHF+ bands. Unfortunately, it seems to be
creeping upwards. There will come a time that we have to move into the
countryside to be able to hear anything.
73 Alf NU8I
Scottsdale AZ DM43an
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