DSP lightning noise reduction for WEAK signals

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K1JJ:
Hola,

I am trying to determine the best approach to reduce static crashes when listening to weak ssb (or AM) signals on 75M.

I've been reading about the TimeWave 599ZX and JPS NIR-12 DSP boxes that process off the audio.  The reviews seem to indicate they don't do squat for lightning crashes, but some say they help.  The problem seems to be that the AGC is too slow on most receivers, so they recommend to turn OFF the AGC, ride the RF gain and put a pair of back-to-back diodes in the earphones for protection.  I am tempted to try one of these boxes, but they are older - five years old and older technology. Plus they come off the audio, which sounds rather hokey.

Then there's the software/computer route where we tap off the radio's 455 kc I.F. and process from there. This appears to be the better approach, but how GOOD is the latest software for processing LIGHTNING crashes and voice? 

What concerns me is that everyone who runs the newest $12K riceboxes with firmware I.F. derived DSP (or DSP software radios) still complains like hell about the static crashes in the DX window... and I can still hear as well or better than most of them when the static is brutal - without any DSP processing at all in my older FT-1000D.  I wonder if the trained brain and directional antenna system is still the only viable lightning "DSP" at this point in time? 

All I do now is put in 20db of front-end attenuation.  There must be a better way.

I would be interested in hearing of usable solutions or tests done that have proven lightning noise reduction other than antenna directivity.  What I am reading is when it comes to VERY WEAK signals at or near the noise floor, the latest DSP techniques simply mask the signal when lightning is involved and it's better without DSP.

It would be a minor job to put a computer in the shack and buy a $16 I.F. /software interface for the FT-1000D, but I would like to know if it's worth the effort for lightning first, before spinning my wheels with software that has not evolved there yet.

TNX.

T

WU2D:
JJ,

The difficulty in your question is that these are weak noise bursts, not strong ones that you can handle with conventional blankers and noise limiters. You need gain.

It may be worth investigating what you could do with a separate noise receiver attached to the same antenna, but tuned just out of the band to a quiet frequency. Got an extra receiver laying around?

This noise-only signal could be linearly phased against the primary signal at the IF.

This could be done pre-demodulation or in your case pre-sampling.

Better yet sample both signals and correct in the DSP.

Mike WU2D

WA1GFZ:
I2PHD's Ver .99 software gave a very slight advantage over the stock Racal running fast AGC. Fast AGC in the racal works best for static. Flex software isn't all that great. Actually the stock Racal blows it away in high static conditions. I noticed this when I changed over to HPSDR interface with Flex software. One night nothing but noise was coming out of the SDR while I could still copy the Racal audio. Read Sherwood's article on DSP. The best method is run a sense antenna and feed the noise into your RX out of phase to cancel it. I think Jay W1VD has a setup like that. You just have a null pot to adjust for best attenuation. Software will get better in time but it isn't there yet. I suspect it will be moe possible with new SDR configurations like QSR1 and HPSDR Mercury with the A/D conversion right after the preselector.
The problem with noise in an RX is the further in it goes the wider it gets due to high Q circuits. The wider it gets the harder it is to eliminate.  

Bacon, WA3WDR:
When I listen to a lightning crash, I hear a certain quality that must have something to do with the timing of the individual arc snaps.   I hear a fast sshhh that consists of a cascade of little snaps, and I hear multiple pops at fairly well defined intervals.

I think that examination of the mechanics of the lightning will show why some of these effects happen, and allow some degree of prediction of them.  DSP should be able to distinguish a lot of the louder lightning effects and block them, ecspecially if we tolerate some throughput delay for processing.

But it is not the typical application of DSP.  It is more of a time-domain issue than a frequency domain issue.  The thinking needs to change back to older time-domain thinking.

I would think that the correlator noise reduction technology from all the way back in the late 80s would help, if only because it reduces the initial burst of noise.  But intelligent time-domain muting of the frequency domain data seems to me to hold the most promise, because it would allow the fastest muting and unmuting, without the old intermodulation effects of the analog systems.  What I envision is a sensor that identifies a lightning artifact, and momentarily blocks the signal much as the old analog blanker systems did.  But instead of electrical hardware trying to clamp an analog voltage with a switching action, it should do this to numeric data in a spectrally smart way, using raised cosine or Gaussian curves, etc, and tailored to each frequency band so that periodic blanking does not 'beat' against the throughput audio any more than it has to.  The effect should be that the short snap never gets to the filter, the filter doesn't ring from it, the muting does not mangle the signals any more than it has to, and the remaining time slots blur together to the filter and to the ear, and we hear a quieter signal.

WQ9E:
For a conventional radio I doubt that DSP is going to do much for lightning and the DSP built into my 1000MP certainly doesn't help much.  If you go back to the 70's and 80's radios with blankers, they were generally designed to do their work before the pulse got wider due to narrow filters in the later IF stages.  I imagine once the pulse gets through the detector these add on audio dsp units will be of little value beyond their own ability to peak limit and protect your ears.  As suggested by another poster, it would be interesting to see how you could do with the noise input using a separate receiver source to generate the blanking pulse and apply that early in the receiver signal chain.  As I recall the optional KWM-2 noise blanker used this approach to grab ignition noise so that might provide some initial direction.

 

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