Hi Chris, Matlab's resample function that we use filters the data before downsampling to prevent aliasing so taking that into account my answer is correct. Best, Vladimir On Mon, Jul 27, 2009 at 9:57 AM, Christophe Phillips<[log in to unmask]> wrote: > > 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 : > > 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 > > > > > >