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From: paulr@syma.sussex.ac.uk (Paul T Russell)
Newsgroups: comp.dsp
Subject: Re: Practical DSP problem
Message-ID: <3594@syma.sussex.ac.uk>
Date: 9 Oct 90 11:33:15 GMT
References: <5169@hemuli.tik.vtt.fi>
Organization: University of Sussex
Lines: 28

From article <5169@hemuli.tik.vtt.fi>, by mattila@hemuli.tik.vtt.fi (Sakari Mattila):
> 
> On finding a sine wave of known frequency buried in noise ...
>  
> If you know the phase of the sine, how about summing it for some
> tens or even thousands of cycles ?

I think this would only work if the noise had a nice amplitude
distribution. In our case, it's probably not nicely distributed.
Also, I think it would take too long (ie. too many samples) to
get a reasonable result. We are sampling at 44.1 kHz and need
results within around 1 second.

> If you do not know the phase,
> how about programming a PLL (Phase Locked Loop) ?  PLL is good
> at finding the signal, if the signal is stable in frequency and
> phase. If your signal is variable, try to filter it out of the noise.

We don't know the absolute phase of the signal but we do know the
exact frequency. We are generating two sine waves at frequencies
f1 and f2 and trying to measure the distortion product at 2f1 - f2.
The signal is swamped by the two much larger components at f1 and
f2, and is close to the noise level. 
-- 
           Paul Russell, Department of Experimental Psychology
         University of Sussex, Falmer, Brighton BN1 9QG, England
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