Topband: dsp afterthought

[jljarvis]

all...

Privately, W8JI observed that there IS a technique to 
drive a noise gate with a single pulse.  In this scenario,
a delay line is created, through which the main signal path travels.  
I thought it might be worth repeating.

The single pulse can be detected ahead of the delay...the gate
drive signal can be derived..and then applied when the single impulse
arrives at the gate.  

However, as far as I know, there is no commercial implementation of
this noise blanker technique presently implemented in DSP.  Please
tell me if that is incorrect.  

The technique is commonly used in high power laser related research, where
only a single pulse exists, and it is necessary to gate a detector on 
and off, with precision, to avoid interfering signals or background.  
The delay line in that case is created by  mirrors on an optical 
bench @ 1nS/ft in air.  

My comments about DSP Noise Reduction still stand.
Unless some manufacturer is willing to engage in public discussion about
their noise reduction algorithms, I will continue to believe that they're
doing nothing more than walking-window averaging.   

The NR knob determines how many sample-intervals are included in the average.  
The greater the number of samples averaged together, the more the signal of 
interest is  distorted.  Similarly, gaussian noise is reduced proportional to 
the square root of the number of samples.  i.e. turn the knob right, white 
noise goes down, signal gets muddy.  

The value of this thread lies not in a couple of folks demonstrating 
knowledge of instrumentation techniques.....rather, it's a fertile field
for additional development by radio manufacturers.   We need somehow to
engage them in the discussion.

Food for thought:  Could we develop a pre-receiver front-end, which 
incorporates a noise amp, delay line, and noise gate, which could 
be shaped to respond to lightning-type non-repetitive impulses.  
There would need to be both pulse width and level selection tuning,
to make this optimizable.  


Herewith, my earlier offending comment:

> 2)  Computer DSP can only impact Gaussian noise, not
> impulse noise.  So it doesn't work well with receiver
> NR schemes.  In general, the reason for this is, DSP
> samples signals over some sample period, and determines 
> how to treat the NEXT signal event when it arrives.
>  If that NEXT signal is significantly different
> from the preceeding events, the treatment will be inappropriate.

Jim/N2EA