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From: Afinko on 23 Jan 2010 09:37
Hi,

my question is very simple:
How to compute RMS in frequency domain?
I do NOT want to transform signal to the time domain by inverse DFT(FFT)
and compute then the RMS.

Is there a way, how to compute RMS directly in frequency domain?
The signal is general (random), not only sine waves.

Thanks in advance.
Afi
From: Rune Allnor on 23 Jan 2010 10:22
On 23 Jan, 15:37, "Afinko" <afi...(a)gmail.com> wrote:
> Hi,
>
> my question is very simple:
> How to compute RMS in frequency domain?

The same way that you compute RMS values in time domain?
The only problem would be to keep track of the odd scaling
coeffcient through the DFT, but you would have to consult
the docs for your FFT imp,ementation for that.

Rune
From: Greg Berchin on 23 Jan 2010 10:39
Parseval's Theorem.

See http://www.mathworks.nl/matlabcentral/newsreader/view_thread/137712,
message 6.

Greg
From: Jerry Avins on 23 Jan 2010 14:42
Afinko wrote:
> Hi,
>
> my question is very simple:
> How to compute RMS in frequency domain?
> I do NOT want to transform signal to the time domain by inverse DFT(FFT)
> and compute then the RMS.
>
> Is there a way, how to compute RMS directly in frequency domain?
> The signal is general (random), not only sine waves.

RMS applies to a short-time mean; on the order of the longest period in
the (time) waveform. Define carefully what RMS means in the frequency
domain and the kind of calculation you want will probably be clear. (I
suspect that the meaning will always remain murky except in certain
special cases.)

Jerry
--
Engineering is the art of making what you want from things you can get.
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From: glen herrmannsfeldt on 23 Jan 2010 15:10
Jerry Avins <jya(a)ieee.org> wrote:
(snip)

> RMS applies to a short-time mean; on the order of the longest period in
> the (time) waveform. Define carefully what RMS means in the frequency
> domain and the kind of calculation you want will probably be clear. (I
> suspect that the meaning will always remain murky except in certain
> special cases.)

Well, you can calculate RMS for any time duration you want,
including infinite for theoretical signals.

In many cases you want the shorter term version, though, I agree.

I have a program that will compute the peak and RMS for a WAV
file, such as recorded by a digital audio recorder. I sometimes
try to use the peak and RMS to equalize the level between tracks,
but it isn't quite as easy as I might like.

-- glen
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