Filter Overview

Low-Pass Filter

A device or algorithm that only allows the low-frequency component of the signal to pass through and attenuates the high-frequency component of the signal.

E.g., RC LP Filter: a capacitor blocks DC and passes AC voltage.

Usage:

1. reduce electric noise from muscle movement (>30 Hz)
2. remove powerline noise
   • AC power lines in walls radiate a strong, oscillating 50/60 Hz electric field.
   • This electric field induces a small, oscillating voltage on the conductive human body by the capacitive coupling effect.
   • Salty sweat on the scalp efficiently conducts this 50/60 Hz voltage to the exact location where EEG electrodes are placed.
   • The electrodes measure the sum of the brain’s microvolt signals and the much stronger induced 50/60 Hz voltage, causing the noise to overwhelm the biological signal.
3. prevent frequency aliasing

High-Pass Filter:

A device or algorithm that only allows the high-frequency component of the signal to pass through and attenuates the low-frequency component of the signal.

Usage:

1. Baseline drift
   • Baseline potential changes from the original position due to
     • Mental state changes: focus
     • Neuron Background activity
     • Body Processes: metabolism / blood flow

Bandpass Filter

Combination of high-pass and low-pass filter

Comparison Between Analogue and Digital Filters

Key Difference:

Analogue Filter

• Process continuous electric signals
• Can process wide range of frequencies
• Done through hardware

Digital Filter

• Process discrete samples of electrics signals
• limited by the sample rate → Nyquist Frequency
• Done through software

Nyquist frequency is the the highest frequency that a digital system can accurately record; it is half of the sampling rate

Frequency Aliasing

• Frequency aliasing is a distortion that occurs in digital signal processing when a signal contains frequencies above the Nyquist frequency (half the sampling rate).
• It causes high-frequency components to be incorrectly represented as lower frequencies in the sampled data.
• Analogue Low-pass filters are used to prevent this by removing frequencies above the Nyquist limit before sampling.