High-Pass Filters

High-pass filters are often used in the biomedical world for image processing, control systems, and filtering out of DC signals in measurements as well as in band pass filters. High-pass filters can make use of active circuit elements, passive circuit elements, or both to attenuate undesired lower noise frequencies and even amplify your desired signal. Watch the video below to learn how high-pass filters are designed, how they work, and how they can be used in biomedical instrumentation applications.

Self-Assessment Questions:

Click here to test your understanding of high-pass filters.

Discussion Questions:

1. The video discusses the application of ECG signal measurement for filtering of higher frequencies. The frequencies that make up ECG signals can range from 0 to 150 Hz. What could be some issues with using a high-pass filter to filter out 60 Hz noise from the ECG signal? How could these be combatted?
2. Although not as useful for low frequency biological signals like ECG and EMG signals, high-pass filters are very useful in medical imaging and control systems. Find two to three biological applications for high-pass filters. 
3. For the above applications you found, determine appropriate cutoff frequencies for noise above this signal range, and draw circuit diagrams with components that you would use to implement this filtration strategy. What would be the pros and cons of active versus passive filtration in these applications?
4. The video stated that in an ideal high-pass filter, all frequencies below the cutoff frequency are attenuated completely. Why is this not the case (both scientifically and practically) in the real world? Why is there a slow attenuation after the cutoff frequency is exceeded? What implications does this have for the cutoff frequencies you might choose?
5. Why is it so crucial that engineers use Fourier transforms when designing high-pass and bandpass filters? What can be determined about the success of certain filters based on these techniques? Use this article by Dogra et al. as a guide.

Continue Reading:

• A. Dogra and P. Bhalla, “Image sharpening by Gaussian and Butterworth high pass filter,” Biomed. Pharmacol. J., vol. 7, no. 2, pp. 707–713, 2014. Read the article here.
  • This article describes the principles of Gaussian and Butterworth high-pass filters used in image processing. These are helpful techniques in biomedical imaging. 
• Marchon NJS. Sharp transition FIR bandpass filter for processing bioelectric signals. J Phys Conf Ser. 2021;1921(1):012019. Read the article here.
  • Although not used on their own to filter EEG and ECG signals, high-pass filters are used as part of a bandpass filter in these applications. This article explains the use of linear FIR high-pass filters to achieve steep cutoffs of undesired frequencies in such signals.
• L. Keselbrener, M. Keselbrener, and S. Akselrod, “Nonlinear high pass filter for R-wave detection in ECG signal,” Med. Eng. Phys., vol. 19, no. 5, pp. 481–484, 1997. Read the article here.
  • For full ECG filters, high-pass filters are not used. That said, specific parts of the signal, such as the R wave, are detected using high-pass filter implementation. This article explains the limitations of linear filtering and how nonlinear filters are used to remove baseline drifts.

About the Creator:

This video was created by a student in the Rice Bioengineering Class of 2023 (used with permission).