[FieldTrip] Amplitude of power (ft_freqanalysis)

[STEPHAN MORATTI]

Dear Steven

I guess it is because the spectra is mirrored at the nyquist frequency and
you stay with half the spectra....

Best
Stephan

El mar., 26 nov. 2019 22:03, Hoffman, Steven <steven.hoffman5 at montana.edu>
escribió:

>
> To whom may be able to help me out,
>
> We were trying to recover the actual amplitude units for our power
> spectra, and I noticed something interesting in the output from FieldTrip.
> I ran a simulation where I take a 20Hz sine wave of +/- 5 "Volts", and
> calculate the power on it (Using the mtfft method). What we expected the
> amplitude to be was 25 (since power is the V^2 /Hz). However FieldTrip gave
> a value closer to about 12 V for the amplitude.
>
> I then calculated the power spectra of the simulated +/- 5 "Volt" sine
> wave using the Matlab built in "periodogram.m" function. This too yielded
> an amplitude not of 25, but of 12.5 .
>
> After some digging I found some documentation in the matlab that said that
> the units of the power spectra were actually (Amp ^2 / 2) . So in this case
> that makes sense as (5^2/2 = 12.5).
>
> My question is why the divide by two? I don't think I've ever seen
> amplitude expressed this way, its always V^2 / Hz ...
>
> Attached are figures of my matlab simulations (One for Field Trip, and for
> matlab's periodogram.m), and the snippet from the Matlab documentation for
> periodogram.m where it talks about amplitude being Amp^2 / 2.
>
>
> Thanks much for any help
>
> -Steven Hoffman
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> https://doi.org/10.1371/journal.pcbi.1002202
>
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