The samples read from the machine can be presented in its raw form, or filtered using one of the built-in filtering modes:
| Mode | Description |
|---|---|
| Smooth | Heavily filtered to show the overall trend of the run |
| Steps | Filtered to remove unnecessary noise while keeping the step movements visible |
All modes are relying on low-pass Butterworth filters which provides a smooth, maximally flat frequency response with no ripples in the passband, ensuring minimal distortion.
A Butterworth low-pass filter is a type of signal filter designed for a smooth frequency response, meaning it allows low frequencies to pass while gradually attenuating higher frequencies without ripples. Unlike other filters that may cause distortions or sharp cutoffs, the Butterworth filter provides a maximally flat response in the passband, ensuring minimal signal distortion. It's widely used in audio processing, communications, and control systems where maintaining signal integrity is crucial.
Smooth Filter
The smooth filter is designed to remove the individual locomotive movements of the runner, instead focusing on the overall trend of the run.
Filter Settings:
| Parameter | Value | Comment |
|---|---|---|
| Order | 4 | 2-4 works ok. Higher order may lead to numeric issues when the bandwidth is low compared to the sampling frequency. |
| Bandwidth (Hz) | 1.3 | Should be less then the step frequency (about 1/3 of seems ok) because we want to filter out all steps |
Step Filtering
The step filtering is heavily tied to the Step Analysis function in the web app. It's designed to keep the step information while at the same time remove noise from line sway.
Filter Settings:
To isolate the steps from everything else, we want to look at approximately one full stride worth of samples, which while running is ~0.3-0.5s.
| Parameter | Value | Comment |
|---|---|---|
| Order | 4 | 2-4 works ok. Higher order may lead to numeric issues when the bandwidth is low compared to the sampling frequency. |
| Bandwidth (Hz) | 6 | This needs to be higher than the step frequency, but lower than the line sway frequency |