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Hello Vladimir and Hamid,

I would rather disagree with Vladimir's answer and say that, the order does matter.
In theory because of the aliasing effect of downsampling, you could well ends up with "spurious" signal in the lower part of your downsampled signal spectrum.

Example: your original signal is sampled at 800Hz, and you want to downsample it at 400Hz, and low pass filter at 150Hz
Take a signal at 300Hz in your original signal, it's below the 400Hz Nyquist frequency, so no problem.
Keep one sample out of 2, i.e. downsample at 400Hz, then 300Hz>200Hz, the new Nyquist frequency. So the orginally 300Hz signal will appear as a 100Hz (*) signal. Low pass filtering at 150Hz will not remove this spurious signal...

You should thus low pass filter, at least at the new Nyquist frequency (half the new sampling rate), your signal before actually downsampling it.

Best,
Chris

(*) 200Hz - (300Hz-200Hz)

Vladimir Litvak a écrit :
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Dear Amir,

In theory there should be no difference. Since after downsampling
filtering will work faster I'd suggest you downsample first, just
because of processing speed.

Best,

Vladimir

On Sun, Jul 26, 2009 at 5:18 PM, Amir H Javadi<[log in to unmask]> wrote:
  
Hello everyone

I want to down-sample and filter my data. Which sequence does result a
better data, down-sampling and then filtering, or filtering and then
down-sampling?

Have a good time
Amir