Re: [Discuss-gnuradio] Correct method for "compressing" a power spectrum

[Bob McGwier]

I suggest that the best algorithm for this would be the rank order mean 
alternated with the max so long as you are going to insert "heuristic 
grass".  So it would be max, ROM, max, ROM, .....


Let [B1, B2, B3, B4, ... BN]  be powers of five adjacent bins.

Put them in rank order

[R1, R2, R3, R4,  ... RN]

If N is even,  the rank order mean is (R_(N/2) + R_/(N/2 +1)*0.5.
If N is odd,  the rank order mean is R_(N/2 +0.5)

But I still see a problem:

I suggest that in order to prevent "scalloping" of a swept tone across 
this algorithm or any other like it,  that some "bin"  in the lossy 
compression set of bins must ALWAYS be forced to take on the large of 
the power spectrum otherwise your alternative min/max/min/max might jump 
up and down as you sweep a tone through it.  Or did I miss something?

Bob

John Ackermann N8UR wrote:
> One thing you might look at is the "Rosenfell" algorithm used in some 
> HP spectrum analyzers (like the 8566).  As I recall, they alternately 
> take the maximum and minimum values in the bins -- ie, if you're 
> collapsing three bins into one and the original values are
>
> 010 050 010|010 050 010|120 130 120|120 130 120|010 050 010|010 050 010
>
> You'd end up with
>
> 050 010 130 120 050 010
>
> The idea is that this looks more like the "grass" that you would see 
> on a non-digital analyzer and accentuates the difference between noise 
> and stable signal, where averaging the bins would lose that 
> distinction -- it has the effect of removing indicators that you're 
> looking at noise.
>
> Not sure if this might be applicable to the application, and not sure 
> I got the details just right from memory, but I think searching 
> "Rosenfell" would find more details.
>
> John
>
>
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