[TowerTalk] Idealized low band system designs

[Jim Lux]

At 11:20 AM 2/17/2005, Michael Tope wrote:
>I think folks like N4HY and SM5BSZ have been working on
>the rudiminents of what you describe, Jim. I recall N4HY
>describing a DSP noise blanker whereby you would look
>at the time samples and then essentially erase and then
>interpolate across large impulses that didn't fit with the pattern.
>With the very fast DSPs on the market today it would seem like
>there should be a whole host of things that could be done
>where signals are sorted out on the basis on spectral signature.
>It probably won't ever get so good that my urban QTH can
>be made as quiet as the rural countryside, but there is
>certainly a lot of room for improvement for us city dwellers.
>
>Switching power supply and motor impulse noise for instance
>probably have enough repetition and pattern in them that  it can
>be exploited for noise reduction purposes. Noise reduction
>that actually improves SNR - wouldn't that be a hoot :)
>
>Mike, W4EF.........................................................


You can do much, much better than the noise blanker strategy, in 
theory.  The problem really isn't the DSP speed, it's the intellectual 
resources to develop and apply the algorithms. It's a narrow band problem 
with a <8 kilosample per second data stream.  Even the grungiest PC can do 
a LOT of operations in 125 microseconds.

I was just involved in coming up with some cost estimates for a similar 
sort of system (complexity, algorithm and DSP wise), and it's a several 
"full time equivalent" kind of project for the better part of a year.  I 
can't see it being done by any company (not enough return on investment for 
something that will probably cost >$1-2M to do well).  Our best bet is some 
PhD candidate takes it on as their dissertation project.

It "might" turn out to be easier than that, but you'd have to take what are 
basically raw algorithms, implement them for the special ham modulations 
case, iterate through the various approaches to find ones that worked, 
etc.  This morning, while driving to work, I was thinking about how you'd 
even go about creating a realistic set of "synthetic pile-ups" to test 
with... You've got urban noise, distant lightning crashes, all the various 
signals, etc.

Actually, I'll bet that one of those three-letter-government agencies has 
already solved the problem, since they have the resources and the desire to 
do so.  All we have to do is wait for them to publish in the open 
literature. Sort of like the marine biologists using the Navy sonar 
algorithms lately.


Jim, W6RMK