The following is an example of filter delay. In the attached image you will see a 2nd order elliptic filter (purple), and also a 6th order butterworth filter (green). You will notice that the 2nd order filter is a little behind the raw trace (white), whereas the 6th order filter is far behind the raw trace. This is a great example of why we use faster 2nd or 3rd order filters for feedback. We want to be as fast as possible. While 6th order filters are nice for later analysis that can be slower and more precise, for feedback we want to be as fast as possible.
Bioexplorer has done a great job of making this example possible.



This also gives me a great opportunity to dispel a couple rumors that I get asked about quite often:

First, there is no such thing as "Real Time". Even if your processor is really really fast, you still have to wait until after something happens before you can say what it was that happened. It's just one of those things. You can be pretty darn fast, and the computer can make an educated guess in order to be even faster, but "real time" is not possible, and that has nothing to do with the speed of your computer. There is always a delay in ALL systems.

Also, sampling rate is a separate question from "real time". If you have a really high sampling rate, you get a better picture on the screen, just like your digital camera, but this does not effect the speed of the feedback that is given to the client. You filter just as fast from a detailed picture as you do from a low resolution one.

Check out the attached image, and if you would like to try this experiment yourself, and you own a copy of Bioexplorer, download the attached Bioexplorer screen to try this at home.

Also, I talk a lot more in depth about filter order, and why filter order matters to your feedback in my on-line Bioexplorer web classes held every other Saturday. Sign up on-line at www.eegcourses.com

This message has been edited. Last edited by: Kurt,

Filter_order_demo.bxd (44 Kb, 262 downloads) Filter Order Demo