The number of FFT data


#1

[B]/muse/dsp/bandpower/raw_fft0 [/B]

As the explanation, there are 130 decimal values showing the absolute power on a log scale(dB) of each frequency from 0hz-110Hz. But when I checked it, not 130 values, but 129 values. is there anything wrong? another question is how to map these decimal values to 0hz-110hz.

Thanks.


#2

nobody found it?


#3

Hi tsubasa,

Actually 129 is the correct number of coefficients for the FFTs. The documentation was incorrect, sorry about that. It’s been updated.

The FFTs are calculated using a 256 sample window, which gives a transform that has 256 components and is symmetric (i.e. mirrored) around an additional component at 0Hz. In other words, you have 128 components, followed by one for 0Hz, and then the mirror image of the same components. This means you need only consider half of them (because the other half are the same, only reflected) plus the one for 0Hz at the centre, which gives you 129 in total.

To get the frequency resolution for the bins, you can divide the sampling rate by the FFT length, so in the case of Muse: 220/256 ~ 0.86Hz/bin

So, the zeroth index of the FFT array represents 0Hz, the next index represents 0-0.86Hz, and so on up to 128*0.86 = 110Hz, which is the maximum frequency that our FFT with its 220Hz sampling rate can detect.

Hopefully that helps. Here are a couple of potentially useful links:

Obligatory wikipedia: http://en.wikipedia.org/wiki/Fast_Fourier_transform
Quite a good stack exchange post on why FFTs are mirrored: http://dsp.stackexchange.com/questions/4825/why-is-the-fft-mirrored


#4

Hi, tom,
thanks. because we are doing research by using the MUSE, so we should be very care about these kinda things. Previously, we also found the locations (A1 FP1 FP2 A2) were incorrect in document. I hope the document would be more clear and accurate. thanks for your work.


#5

Ah, good catch. I’ve corrected the location information as well now. The left and right ear electrodes can be considered to be at TP9 and TP10, respectively. The forehead electrodes are at FP1 and FP2 as previously documented.