When setting up data acquisition parameters, one area of befuddlement often involves the use of filters. Which ones should I use, and how do I set them up? What are they really, and why do I need them? In a nutshell, filters are configurable controls for removing certain frequencies from a signal, while leaving others unaffected, and are useful for removing noise and improving signal quality. Filters can be set up in the software as “online” (applied in real time during acquisition) or “offline” (applied post- acquisition). Some BIOPAC amplifiers, such as the 100C series, include a bank of filter switches relevant to the type of signal being acquired. As far as which filter to use, that is dependent on the type of physiological signal being acquired and BIOPAC amplifiers and software presets will help steer you toward the correct type of filter to use. Subsequent blog entries will delve more into specifics about which types are appropriate for which signals.
As in our earlier blog about frequencies, we’ll again be using the “cars” analogy to bring filtering concepts into sharper focus. Say we want to look only at cars on a freeway that are traveling at or above the speed limit. To effectively do this, we’ll need to find a way to remove the slower vehicles from view. To isolate the “fastest” cars, we would apply a “High Pass” filter. By “High Pass,” we mean this filter eliminates all frequencies that fall below a specified cutoff point, and allows the higher frequencies to “pass through,” or remain present in the signal. Using the “cars/freeway” analogy, setting a High Pass filter to target cars traveling at 60 mph would eliminate all cars traveling at or below 60 mph and leave all faster cars unaffected.
Conversely, we can apply a “Low Pass” filter, which would eliminate all cars traveling faster than a specified speed and leave only the slower cars intact. Another “combination” filter that can be applied is a “Band Pass” filter, which eliminates the fastest and slowest moving “cars” and retains all cars that are traveling at a specified speed. Additional common filters are:
Band Stop isolates and removes a specific frequency or frequencies (all cars traveling at one particular speed, or range of speed, leaving all the others). This filter is most commonly used to eliminate 50 Hz or 60 Hz electrical noise frequencies from a signal.
Comb Band Stop is another powerful filter commonly used for removing electrical noise. The Comb Band Stop filter removes not only the 50 Hz or 60 Hz frequency, but also all overharmonics, or “echoes” of that frequency. For example, if you are in an area of the world that uses a 50 Hz line frequency, the Comb Band Stop filter will eliminate from the signal the 50 Hz frequency, plus 100 Hz, 150 Hz, 200 Hz, etc. If you live in the U.S. or other part of the world that uses a 60 Hz line frequency, this filter will remove 60 Hz, 120 Hz, 180 Hz, 240 Hz, etc. (A better analogy for this type of filter might be musical notes and their octaves rather than cars traveling at speed.)
For further reading on filters, please see the following resources:
BIOPAC offers a wide array of wired and wireless equipment that can be used in your research. To find more information on solutions for recording and analyzing signals such as ECG, heart rate, respiration and more using any platforms mentioned in this blog post, you can visit the individual application pages on the BIOPAC website.