Filtering Noise from a signal
in [DSP]
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From: bos1234 on 1 Jun 2010 19:34 In class we analysed a signal and filtered the noise out. However the noise was in a different bandwidth to the original signal hence it was easy to filter. If noise and the signal were to overlap in freq. spectrum, are there any techniques to filter out the noise??
From: John on 1 Jun 2010 19:58 On Jun 1, 7:34 pm, "bos1234" <suren130(a)n_o_s_p_a_m.gmail.com> wrote: > In class we analysed a signal and filtered the noise out. However the noise > was in a different bandwidth to the original signal hence it was easy to > filter. > > If noise and the signal were to overlap in freq. spectrum, are there any > techniques to filter out the noise?? No, but there are techniques to reduce the noise provided that you have a sample of it that does not include signal, like you might have in speech. Search for "spectral subtraction". John
From: Tim Wescott on 1 Jun 2010 20:40 On 06/01/2010 04:34 PM, bos1234 wrote: > In class we analysed a signal and filtered the noise out. However the noise > was in a different bandwidth to the original signal hence it was easy to > filter. > > If noise and the signal were to overlap in freq. spectrum, are there any > techniques to filter out the noise?? No. The word "filter" is used as a close analogy to a filter that you might use to filter liquids in the kitchen, or in a chemistry class. When you use a coffee filter filter, for example, you use a filter that has pores that are smaller than the coffee grounds. This lets the water and anything dissolved in it (like than nice coffee -- mmm!) through, but it blocks the coffee grounds because they won't fit. A filter in signal processing terms is much the same, except instead of filtering by size, you're filtering by position in the spectrum. Unless there is some characteristic of the signal that distinguishes it from the noise, you can't filter it out. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Fred Marshall on 1 Jun 2010 22:47 Tim Wescott wrote: > On 06/01/2010 04:34 PM, bos1234 wrote: >> In class we analysed a signal and filtered the noise out. However the >> noise >> was in a different bandwidth to the original signal hence it was easy to >> filter. >> >> If noise and the signal were to overlap in freq. spectrum, are there any >> techniques to filter out the noise?? > > No. The word "filter" is used as a close analogy to a filter that you > might use to filter liquids in the kitchen, or in a chemistry class. > When you use a coffee filter filter, for example, you use a filter that > has pores that are smaller than the coffee grounds. This lets the water > and anything dissolved in it (like than nice coffee -- mmm!) through, > but it blocks the coffee grounds because they won't fit. > > A filter in signal processing terms is much the same, except instead of > filtering by size, you're filtering by position in the spectrum. > > Unless there is some characteristic of the signal that distinguishes it > from the noise, you can't filter it out. > Well .... what about a line canceller? It uses a reference signal, which is suitably scaled and delayed, to be subtracted from a composite. To the extent that the reference is stable and the desired signal isn't, then only the reference will be subtracted. Any consituent of the composite that departs from the "reference model" will get through. And, to your point, any constituent of the composite that is stable and matches the reference, will be attentuated. [many words about dyamics, etc......] We say "spectrum" which is defined over all time or over some temporal epoch. But the actual signal may not even come close to having the stability implied by our model. I think sometimes we forget that for convenience. Fred
From: maury on 2 Jun 2010 12:38
On Jun 1, 6:34 pm, "bos1234" <suren130(a)n_o_s_p_a_m.gmail.com> wrote: > In class we analysed a signal and filtered the noise out. However the noise > was in a different bandwidth to the original signal hence it was easy to > filter. > > If noise and the signal were to overlap in freq. spectrum, are there any > techniques to filter out the noise?? Yes there are. Go to http://www.danvillesignal.com/comp.dsp-conference/comp.dsp-conference-presentations.html and look at the paper on noise reduction techniques. For example, let's say the noise is 6 dB below the signal, then you can use coefficient shirkage: take FFT, do coefficient shirkage, take IFFT. Voila, no noise. |