[Eeglablist] [eeglablist] aliasing in pop_spectopo():

[Michael Plöchl]

hi anders,

we recently encountered a similar problem. let me guess: your data is  
recorded with a sampling frequency of 512 Hz and you are zero padding  
them to twice the length, i.e. for example from 512 to 1024 samples.  
if this is the case there is nothing wrong with your data. the reason  
why the data look strange is illustrated in the attachment. the  
recorded samples are depicted by black squares and the underlying  
function that one would obtain by continuous fourier transformation is  
shown by the gray trace. when using discrete fourier transformation  
all samples except the one for the center frequency fall on the zero  
line and thus are not visible when plotting the non-zero-padded data.  
when you now zero pad the data to twice its length, the "artificially  
added" samples will fall exactly between the original samples and thus  
on the peaks of the side lobes. plotting this in matlab (i.e. directly  
connecting each sample with a line) will then look like a sawtooth  
pattern as it was the case in your data.
to see if this really is the reason, just change the amount of zero  
padding to a non-integer multiple of the sampling frequency (e.g. from  
a default of 1024 to 1000) or disable the zero padding completely and  
check whether your data look better then.

i hope i could help,

michael


On Oct 3, 2011, at 3:21 PM, Anders Sand wrote:

> Hi all,
>
> when I apply pop_spectopo(): (using the GUI) on any .bdf data I get  
> what looks like aliasing (see http://www.mypicx.com/10032011/Aliasing/) 
> . I have tried importing the files via the biosignal toolbox and via  
> converting (via polyrex) to .cnt. It looks the same for any .bdf  
> file I open but looks normal for all example data attached with  
> EEGLab. All frequency parameters are default.
>
> Could somebody help me troubleshoot this problem?
>
> Thanks
> Anders
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