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Signal Filtering

Petal Metrics includes real-time signal filtering to help you obtain cleaner EEG data by removing unwanted noise and artifacts.

Subscription Required

Signal filtering requires an Advanced subscription.

Available Filters​

Bandpass Filter​

The bandpass filter allows signals within a specified frequency range to pass through while attenuating frequencies outside that range. This is useful for isolating specific brainwave frequency bands.

Default settings:

  • Low cutoff: 1 Hz (removes slow drift)
  • High cutoff: 50 Hz (removes high-frequency noise)

Common frequency bands in EEG:

BandFrequency RangeAssociated With
Delta0.5 - 4 HzDeep sleep
Theta4 - 8 HzDrowsiness, meditation
Alpha8 - 13 HzRelaxed, eyes closed
Beta13 - 30 HzActive thinking, focus
Gamma30 - 50 HzHigh-level cognition

Notch Filter​

The notch filter (also called a band-stop filter) removes a specific frequency, typically used to eliminate power line interference.

Available frequencies:

  • 50 Hz: Standard in Europe, Asia, Africa, and most of the world
  • 60 Hz: Standard in North America, parts of South America, and Japan

Choose the frequency that matches your local power grid to remove electrical interference from your recordings.

Enabling Filters​

Accessing Filter Settings​

  1. Click Output in the left sidebar to open the Output Settings panel
  2. Scroll to the Signal Filtering section
  3. Toggle the main filter switch to enable filtering
Settings Persistence

Your filter configuration is automatically saved and will be restored the next time you launch the app.

Configuring the Bandpass Filter​

  1. Enable filtering using the main toggle
  2. Adjust the Low cutoff frequency (default: 1 Hz)
    • Higher values remove more slow drift but may cut off delta waves
  3. Adjust the High cutoff frequency (default: 50 Hz)
    • Lower values remove more high-frequency noise but may cut off gamma activity

Configuring the Notch Filter​

  1. Enable filtering using the main toggle
  2. Toggle the Notch Filter switch
  3. Select your power line frequency:
    • Click 50 Hz for most countries
    • Click 60 Hz for North America and parts of Asia

When to Use Filtering​

  • Research applications: Clean data is essential for analysis
  • Noisy environments: Electrical interference from nearby equipment
  • Real-time feedback: Cleaner signals improve visualization clarity
  • Artifact removal: Reduce impact of muscle movement and eye blinks

Consider disabling for:​

  • Raw data collection: When you need unprocessed signals
  • Post-processing pipelines: When you'll filter data later in MATLAB/Python
  • Debugging: To see the actual signal from the device

Filter Indicators​

When filtering is active:

  • The Signal Filtering card shows a green border and highlighted icon
  • Filter settings are displayed on the card
  • The main toggle shows the enabled state

When filtering is disabled:

  • The card returns to its default appearance
  • All filter controls are dimmed
  • Raw (unfiltered) data is displayed and recorded

Technical Details​

Filter Implementation​

  • Filter type: Butterworth (maximally flat passband)
  • Order: 4th order (24 dB/octave rolloff)
  • Processing: Real-time, applied per sample
  • Latency: Minimal (~4 samples at 256 Hz = ~16ms)

Interaction with Recording​

When CSV export is enabled:

  • Filtering ON: Recorded data is filtered
  • Filtering OFF: Recorded data is raw/unfiltered

The filter state at recording start determines what gets saved. Changing filter settings during recording will affect data from that point forward.

Troubleshooting​

Filters not available​

  • Verify you have an Advanced subscription
  • Check that you're signed in to the app
  • Restart the app if settings don't appear

Signal looks over-filtered​

  • Check your cutoff frequencies aren't too aggressive
  • Try widening the bandpass range (lower low cutoff, higher high cutoff)
  • Disable notch filter if not needed

Still seeing 50/60 Hz noise​

  • Ensure you selected the correct notch frequency for your region
  • Check that the notch filter toggle is enabled
  • Verify the main filter toggle is on

Availability​

Signal filtering is available exclusively on the Advanced subscription plan.

Upgrade to Advanced →