Re: [Discuss-gnuradio] White Noise detection and elimination

[Martin Braun (CEL)]

On Mon, Nov 18, 2013 at 08:29:02AM -0500, Robert James wrote:
> I'm working on a series of blocks to measure the level of white noise
> and attentuate it.  (Yes, I know there are better ways of doing this,
> like a Wiener filter - my goal here is to experiment and learn.)
> 
> Model: Assume white noise is present in equal power at all frequencies.
> 
> Plan:
> Signal --> FFT --> [Find min power val over entire freq vector] -->
> [Subtract that min val from all freq] --> IFFT --> Output
> 
> Two questions:
> 
> 1. Will this work?

I doubt it will do what you want it to.
What do you mean, 'subtract that min val from all freq'? How do you
subtract a power value from a complex amplitude?

> 2. What format does the FFT output vector use? I imagine to find the
> min power at any freq I need to write my own block - hopefully I can
> do this in Python.  Code to do that in Python is trivial, but I can't
> find the FFT output vector format documented.  Likewise for "Subtract"
> - I mean this as a mathematical subtraction, *not* attenuation
> (multiply), which would attentuate the signal just as much.  So if the
> FFT output vector is in a logarithmic format, I need to first turn it
> into a linear format before subtracting.

I'm not sure you understand what the FFT does. Output is complex values.

A power spectrum density could be displayed logarithmically, but that's
*not* an FFT.

I'm not sure what you're attempting, but it looks like you want to

- find frequencies with power
- design a filter that attenuates the rest
- filter your (time-domain) signal.

MB

-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

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KIT -- University of the State of Baden-Württemberg and
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