<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div ocsi="x"><br></div></span></blockquote><div>Hi. In response to the question about filter order for a Butterworth filter, it is important for EVERYONE doing EEG/ERP research to realize that a steeper roll-off will mean that the filter will cause more temporal distortion of the waveform. This is true for all standard (FIR) filters, not just Butterworth filters. The most fundamental principle in filtering is that precision in the frequency domain is inversely related to precision in the time domain. That is, the narrower your passband and the sharper the roll-offs of a filter, the more the filter will spread the signal in the time domain. Unless you are primarily looking at frequency-domain phenomena, it is usually better to have a broader passband and a more gradual roll-off. So, unless you are looking at true EEG oscillations, you will probably want a low-order filter.</div><div><br></div><div>This is the opposite of the advice people usually give. But people are usually thinking about the frequency domain and forgetting about the time domain. If you don't think about what filters are doing in the time domain, you may end up creating artifactual peaks and oscillations in your waveforms. If in doubt, pass a known signal (like a single cycle of a square wave) through your filter and see how the filter distorts the waveform.</div><div><br></div><div>For a detailed overview, see Chapter 5 in An Introduction to the Event-Related Potential Technique.</div><div><br></div><div>Steve Luck</div><div><br></div><br><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div ocsi="x"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span style="font-family: Helvetica; font-size: medium; color: rgba(0, 0, 0, 0.498039); "><b>From:<span class="Apple-converted-space"> </span></b></span><span style="font-family: Helvetica; font-size: medium; ">"Wambua Kazi" <<a href="mailto:wambua.kazi@gmail.com">wambua.kazi@gmail.com</a>><br></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span style="font-family: Helvetica; font-size: medium; color: rgba(0, 0, 0, 0.498039); "><b>Date:<span class="Apple-converted-space"> </span></b></span><span style="font-family: Helvetica; font-size: medium; ">July 29, 2008 10:05:13 PM PDT<br></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span style="font-family: Helvetica; font-size: medium; color: rgba(0, 0, 0, 0.498039); "><b>To:<span class="Apple-converted-space"> </span></b></span><span style="font-family: Helvetica; font-size: medium; "><a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a><br></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span style="font-family: Helvetica; font-size: medium; color: rgba(0, 0, 0, 0.498039); "><b>Subject:<span class="Apple-converted-space"> </span></b></span><span style="font-family: Helvetica; font-size: medium; "><b>[Eeglablist] Pros & cons of Butterworth filter order</b><br></span></div><br><br><div dir="ltr">Hello,<br> I am trying to figure out how to bandpass filter an EEG data set. From skimming the literature it appears that Butterworth filters are commonly used for filtering EEG data and strike a good compromise between the steepness of the transition band and ripples in the pass and stop bands. It's also clear to me that the higher the order of the filter, the steeper the transition band. Are there any cons to using a higher order Butterworth filter?<br> thank you for your help,<br> -Wambua<br><br><a href="mailto:wambua.kazi@gmail.com">wambua.kazi@gmail.com</a><br></div><br></div></span></blockquote></div><br><div apple-content-edited="true"> <span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><div style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">--------------------------------------------------------------------</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Steven J. Luck, Ph.D.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Professor</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Center for Mind & Brain and Department of Psychology</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">University of California, Davis</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">267 Cousteau Place</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Davis, CA 95618</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">(530) 297-4424</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="mailto:sjluck@ucdavis.edu">sjluck@ucdavis.edu</a></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">--------------------------------------------------------------------</div><br class="Apple-interchange-newline"></span></div></span> </div><br></body></html>