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

[Marcus D. Leech]

Brian Padalino wrote:
> On Mon, Mar 9, 2009 at 8:47 AM, Marcus D. Leech <address@hidden> wrote:
>   
>> The original {1,2,4,8,16}M-bin data are used for SETI analysis, while
>> the "compressed" version
>>  is used for a quick visual, "conventional" spectral display.
>>     
>
> In SETI analysis, is it more interesting to see a wider bandwidth
> signal, or single frequency bins of larger magnitudes?
>   
Ah, the very crux of the signal-detection arguments in SETI!   The most
popular hypothesis is
  that our ET friends will use as narrow a signal as possible, since
that concentrates all the transmitter
  power over a single frequency.  But there are other theories around
about what else ET might
  do to let us know that they're there.  Modulating the microwave output
of the stellar corona, for example
  (See SASER project).   Something like that would be wideband, and not
terribly coherent.
> If you would rather see wider bandwidths, it might be interesting to
> set each of N bins to an opacity of 1/N and draw them on top of each
> other.  That way the more that are drawn on top of each other appear
> darker in appearance (wider bandwidth) as opposed to a single outlier
> skewing all results.  Moreover, for zooming, I think this would be the
> most dynamic while maintaining full fidelity of the signal
> representation.
>
> I think there are a lot of good visualizations that can be done
> without compromising the frequency resolution you have obtained with
> such large FFTs.
>
> I'd be interested to hear what method you end up using, and what is
> important in your SETI analysis.
>   
I have two different "visualizations" available.   The "compressed"
spectrum is all about the "usual"
  fairly-coarse spectrum you might want for mapping radial velocities of
neutral hydrogen, for example.
  The full-resolution spectrum is used both for automated analysis, with
a companion visualizer using
  a completely-conventional "waterfall" display, in which you can
display any given segment with
  full resolution.

At very high resolutions, these waterfall plots produce some fairly
spectacular-looking stuff particularly
  when the low-noise feed is sitting in an office full of computers,
pointing at the ceiling.

-- 
Marcus Leech
Principal Investigator, Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org