Expert: Paul Soderman - 4/29/2008

Question
Hello Paul:

I am not sure you can address my question but I am using a Butterworth Low Pass filter to smooth water quality data being influenced by the tides and am using MatLab to do this. My question is that I really don't understand the filter or how it works and why use this kind of filter and not some other kind. I saw the formula that MatLab uses but it's very hard to try to grab my hands on the specifics. I am reading a lot but not understanding much and I am at a stand-still currently. I hope you are able to assist or point me in the right direction...Thanks

Answer
Hi  Steve
I think you have made a good choice of filter with Butterworth.  It is the cleanest low pass filter with a smooth, linear response and well defined roll off.  Check Wikipedia butterworth filter for comparison with other filters.  I don't think you need to worry much about the mathematics, just run your signal through the filter and compare with an unfiltered signal.  You should see a clean response with no aliasing.  I  am not familiar with the MatLab version, but it is no doubt a standard model.  I think you are on the right track.
Paul