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From:  Krishna <krishna.pillai@gmail.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Question about SNR and noise
Date: Sun, 09 Sep 2007 07:56:33 -0700
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On Sep 9, 2:26 am, "Hugo Human" <s24093...@hotmail.com> wrote:

> I have never seen this :
> > noise = 10^(-snrdB/20)*randn(size(sig));
>
> As far as I know it's
> noise = 10^(-snrdB/10)*randn(size(sig));
> because
> SNR = 10*log(1/var);

I do not disagree to the SNR = 10*log(1/var);. However, note that am
scaling a noise voltage signal  (and not the
power signal). Hence the 10^(-snrdB/20) for scaling.

> But what I want to know is, because your signal's power is
> 0.5, how does this influence your equation....

No. Note that I have used a scaling factor of sqrt(2). Hence the
signal power approx=1 (0dB).

> Is this why you use
> SNR = 20*log(1/var);
> ????
No. I have never said SNR = 20*log(1/var).

Does that clarify?

Krishna
http://dsplog.blogspot.com