[Eeglablist] Differences between pre and post downsampled

Arnaud Delorme arno at salk.edu
Fri Feb 14 20:44:40 PST 2014


Dear Steven,

what method did you use to compute the spectrum? 
You probably used the 'fft' method. Try removing the 'specmode', 'fft' parameters when computing the spectrum. Then the study function will use the standard pwelch method to compute the spectrum and the problem might not appear. We will investigate further.

Thanks,

Arno

On Feb 7, 2014, at 3:46 PM, Steven Pillen <stevendpillen at gmail.com> wrote:

> Hello, EEGLABlist
> 
> When downsampling the data in a study, we noticed a strange phenomenon.
> 
> Originally, the chart for a specific participant with a specific electrode in a specific condition looked like this with 2048 Hz:
> 
> http://imgur.com/2ZrgfjB
> 
> 
> Then, after we downsampled all the data to 512 Hz, it was consistently the case that the magnitude of the graphs changed, yet the shape remained the same, as seen below:
> 
> http://imgur.com/1y2Hi9S
> 
> Notice the Y axis in each chart.  It shifts about 12 decibels downwards.
> 
> Checking the .datspec files, we noticed there was a difference of about 12 in the values for those specific indices, as well.
> 
> Is this difference in magnitude something we should expect?  What does the downsampling function cause this kind of change?
> 
> 
> 
> 
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