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PostPosted: Wed Feb 15, 2012 1:29 pm 
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Joined: Wed Feb 15, 2012 12:48 pm
Posts: 4
I am designing for EEG analysis and want to use the Spectral Analysis box. I see it has amplitude output, as well as phase, real and imaginary outputs. I want to make use of the power in the spectrum, which is amplitude squared. But I am confused by the terminology used in the Spectral Analysis box and associated boxes. Using information from the tutorials/examples for these boxes...

Spectral Analysis box says:
possible outputs include the spectrum amplitude (the power of the signal in a number of frequency bands).

Power Spectrum Display says:
We connect the 'Amplitude' output connector of the Spectral Analysis box to the input connector of a Power Spectrum Display box. The player may now be launched to visualize the power spectrum of the signal.

One question is... if the Spectral Analysis box outputs spectrum amplitude... then does the Power Spectrum Display box square the spectrum (amplitude) input before display?

I want to use the Frequency Band Selector box... it says its input is 'Input Spectrum'... is this the spectrum amplitude from the Spectral Analysis box?

I also want to use the Spectrum Average box... its input just says 'Spectrum'... does this take the spectrum amplitudes and average them?

As I said at the beginning, I want to use the power in the spectrum, but am not sure when to square the spectrum amplitudes... my guess is that I use a Spectral Analysis box, followed by a Frequency Band Selector box, followed by a Spectrum Average box, which should give me the average of spectral amplitudes over my chosen band of frequencies, and then square this averaged amplitude to provide the spectral power indicator I need.

Sorry for the lengthy discussion, just trying to make as clear as possible what is it I need to know about the boxes associated with spectral analysis so that my design theory will be correct.
Any help appreciated
Thanks
Robert


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PostPosted: Wed Feb 15, 2012 2:18 pm 
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Joined: Tue Nov 02, 2010 8:51 am
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Location: Mensia Technologies Paris FR
Hello Robert!

In OpenViBE all boxes concerned with spectra are using only amplitudes, the values are not squared at any point.

The Frequency Band Selector indeed uses the amplitude output from the spectral analysis.

As for your last question, is the spectral power the average of amplitudes squared or the average of squared amplitudes? Depending on your definition you will have to square the spectrum either before or after the average calculation.

Best
Jozef

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PostPosted: Wed Feb 15, 2012 2:37 pm 
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Thanks Jozef
That answers all my questions for me... and it means that the Power Spectrum Display box is really a Spectrum Amplitude Display box.

I think to be more precise I will square all my spectrum amplitude values and then use the Spectrum Average box to give me the spectrum power indicator that I need.

I appreciate the prompt and informative reply
Regards
Robert


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PostPosted: Wed Feb 15, 2012 3:13 pm 
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Joined: Wed Feb 15, 2012 12:48 pm
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Sorry to continue with this thread again... but I am having a further difficulty with spectral analysis and amplitudes.

I want to square the spectral amplitudes to make them spectral powers before averaging them... but cannot find a suitable mathematical box for this. I tried using a Simple DSP box after the Frequency Band Selector box so that I could square the spectral amplitude values output from the Selector box, but the streams do no match. The Frequency Band Selector box has a spectrum output but the Simple DSP box needs a signal input... and the respective output and input connectors are colour coded differently.

Is there a different maths box, suitable for spectral values, that I cannot find?
Is there a conversion box that will change the spectrum output to an array or matrix output that can be treated mathematically?

Again... any help appreciated
Robert


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PostPosted: Wed Feb 15, 2012 3:45 pm 
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Apologies... upon further investigation I have found that the Simple DSP box can be configured for different streams, including spectrum.

:D


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PostPosted: Fri May 12, 2017 6:02 am 
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Joined: Mon Dec 12, 2016 2:35 pm
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Hi ,
If I will square all my spectrum amplitude values and then use the Spectrum Average box to give me the spectrum power indicator that I need.Is it true to square the spectrum amplitude to find the power and if so then what will be the unit of it ?kindly tell me about its graphical representation as well
Thanks
Adeel


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