Jerry Avins <jya@ieee.org> wrote in 
news:ObOdne7FpJwEP57anZ2dnUVZ_smnnZ2d@rcn.net:

>> Do you mean to just use V^2 to find the relative power and not dividing
>> this by R??
> 

Often, these things are expressed as a ratio of powers between the output 
and input, so the "R" cancels out.  

For EEG, one would assume you're interested in ratios anyway, as unless 
you're modeling the whole bioelectric phenomena of the head, the scalp 
measurements will always have unknown scale to some extent

-- 
Scott
Reverse name to reply

cherrie wrote:
>>>> Use decibel measurements to provide relative fictitious power. For
> such
>> an application, using magnitude or RMS merely changes an additive
>> constant.
>>
>> Yes... or in other words: the reference to "relative fictious power"
>> becomes a scaling factor c, as in c*V^2, hence: Simply use V^2.
>>
>> It might be R or it might be an empirically found (constant) fudge
> factor,
>> as long as you don't require absolute powers.
>>
>> -mn
>>
> 
> Do you mean to just use V^2 to find the relative power and not dividing
> this by R??

There's nothing better than that when the overall system gain isn't 
known. There are many imponderables here. How much of the neural signal 
is captured by the electrodes? What is the gain of the amplifier-ADC 
chain? The appropriate value of R is only one of the missing pieces.

It is easy to compute relative power, but it is also easy to be misled 
by subtleties. When comparing the power in two leads, the assumption 
that they have equal sensitivity really ought to be checked. When 
comparing powers at different frequencies in the same lead (by means of 
a Fourier transform or by any other frequency-separation technique) the 
actual frequency response of the channel must be accounted for.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On 2 Okt, 12:12, "cherrie" <cherriegel...@hotmail.co.uk> wrote:
> Hi all,
>
> I am working on some EEG data and I would like to compute the power vs
> time. I know that i can perform an FFT to obtain the power, however, this
> would be for specific frequencies. I read that the power = magnitude
> square of the voltage. Is it valid then to compute the power at each time
> point by squaring the voltage magnitude and then plotting these against
> time to obtain the EEG power vs time plot?

That's generally the case, yes. I don't know if this is
valid for EEG signals, but for the types of signals
I usually work with I prefer to use an envelope detector
before squaring the amplitudes. The power curve becomes
somewhat smoother that way.

Rune

That's what I would do, without more specific requirements.

-mn

>>> Use decibel measurements to provide relative fictitious power. For
such
>
>an application, using magnitude or RMS merely changes an additive
>constant.
>
>Yes... or in other words: the reference to "relative fictious power"
>becomes a scaling factor c, as in c*V^2, hence: Simply use V^2.
>
>It might be R or it might be an empirically found (constant) fudge
factor,
>as long as you don't require absolute powers.
>
>-mn
>

Do you mean to just use V^2 to find the relative power and not dividing
this by R??

Thanks,cherrie

>> Use decibel measurements to provide relative fictitious power. For such

an application, using magnitude or RMS merely changes an additive
constant.

Yes... or in other words: the reference to "relative fictious power"
becomes a scaling factor c, as in c*V^2, hence: Simply use V^2.

It might be R or it might be an empirically found (constant) fudge factor,
as long as you don't require absolute powers.

-mn

Jerry Avins wrote:
> John Sampson wrote:
>> cherrie wrote:
>>> Hi all,
>>>
>>> I am working on some EEG data and I would like to compute the power vs
>>> time. I know that i can perform an FFT to obtain the power, however, 
>>> this
>>> would be for specific frequencies. I read that the power = magnitude
>>> square of the voltage. Is it valid then to compute the power at each 
>>> time
>>> point by squaring the voltage magnitude and then plotting these against
>>> time to obtain the EEG power vs time plot?
>>>
>>> Thank you for your help in advance.
>>>
>>> cherrie
>>>
>>>
>>
>> It is valid if you divide V^2 by the impedance.
> 
> Provided the impedance has no imaginary part.

cherriegeller@hotmail.co.uk wrote:
 > Hi, Thank you for your reply to my post \"compute EEG power vs time\".
 > You mentioned that its ok to do V^2 provided i divide by impedance. I
 > think this is from P=V^2/R ?? But how do i obtain R? This is EEG
 > recorded from the brain not from a circuitry?? Sorry i am really new
 > to this. Thanks

Please correspond via the newsgroup unless the message is personal. 
Power is V^2/R where R is a real resistance and V is an RMS quantity. A 
decibel is (strictly) a measure of relative power, but it is often used 
to relate voltages *as if* there were real power involved.

I imagine you aren't investigating how many watts the brain actually 
produces Isolated EEG signals wouldn't by themselves reveal that anyway. 
Use decibel measurements to provide relative fictitious power. For such 
an application, using magnitude or RMS merely changes an additive constant.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

As long as you don't need absolute power, V^2 will do.

-mn

On Oct 2, 9:11 am, Jerry Avins <j...@ieee.org> wrote:
> John Sampson wrote:
> > cherrie wrote:
> >> Hi all,
>
> >> I am working on some EEG data and I would like to compute the power vs
> >> time. I know that i can perform an FFT to obtain the power, however, t=
his
> >> would be for specific frequencies. I read that the power =3D magnitude
> >> square of the voltage. Is it valid then to compute the power at each t=
ime
> >> point by squaring the voltage magnitude and then plotting these against
> >> time to obtain the EEG power vs time plot?
>
> >> Thank you for your help in advance.
>
> >> cherrie
>
> > It is valid if you divide V^2 by the impedance.
>
> Provided the impedance has no imaginary part.
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF


Well if Z is complex then you can still talk about complex power,
divide Vrms^2 by conj(Z). But I don't think the OP cares about that.

John

John Sampson wrote:
> cherrie wrote:
>> Hi all,
>>
>> I am working on some EEG data and I would like to compute the power vs
>> time. I know that i can perform an FFT to obtain the power, however, this
>> would be for specific frequencies. I read that the power = magnitude
>> square of the voltage. Is it valid then to compute the power at each time
>> point by squaring the voltage magnitude and then plotting these against
>> time to obtain the EEG power vs time plot?
>>
>> Thank you for your help in advance.
>>
>> cherrie
>>
>>
> 
> It is valid if you divide V^2 by the impedance.

Provided the impedance has no imaginary part.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯