[FieldTrip] Amplitude of power (ft_freqanalysis)

[Hoffman, Steven]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20191126/2b75518a/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: matlab_periodogram.png
Type: image/png
Size: 12509 bytes
Desc: matlab_periodogram.png
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20191126/2b75518a/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fieldtrip.png
Type: image/png
Size: 14037 bytes
Desc: fieldtrip.png
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20191126/2b75518a/attachment-0001.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Screen Shot 2019-11-22 at 3.50.05 PM.png
Type: image/png
Size: 66309 bytes
Desc: Screen Shot 2019-11-22 at 3.50.05 PM.png
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20191126/2b75518a/attachment-0002.png>