Spectral features

The Spectral features block extracts frequency and power characteristics of a signal. Low-pass and high-pass filters can also be applied to filter out unwanted frequencies. It is great for analyzing repetitive patterns in a signal, such as movements or vibrations from an accelerometer.

Filtering out high frequencies from an accelerometer for movement detectionFiltering out high frequencies from an accelerometer for movement detection

Filtering out high frequencies from an accelerometer for movement detection

Spectral analysis parameters

Scaling

  • Scale axes: Multiplies axes by this number to scale data from the sensor

Filter

  • Type: Type of filter to apply to the raw data (low-pass, high-pass, or none)
  • Cut-off frequency: Cut-off frequency of the filter in hertz
  • Order: Order of the Butterworth filter

Spectral power

  • FFT length: The FFT size
  • No. of peaks: Number of spectral power peaks to extract
  • Peaks threshold: Minimum threshold to extract a peak (frequency domain normalized to [0, 1])
  • Power edges: Splits the power spectral density in various buckets (V**2/Hz unit)

How does the spectral features block work?

The spectral analysis block generates 3 types of features per axis:

  • The root mean square of the filter output (1 scalar value)
  • The frequency and height of spectral power peaks
  • The average power spectral density for each bucket

The raw signal is first scaled up or down based on the Scale axes value and offset by its mean value. A Butterworth filter is then applied (except if None selected); the order of the filter indicates how steep the slope is at the cut-off frequency.

At this point the root mean square of the filter output is added to the features' list.

The filter output is then used to extract:

  • Spectral power peaks: after performing the FFT, the No. of peaks peaks with the highest magnitude are stored in the features' list (frequency and height). Peaks threshold can be tuned to define a minimum value to extract a peak.
  • PSD for each bucket: after computing the power spectral density, the signal is divided in power buckets, defined by the Power edges parameter. Each sample frequency of the PSD is added to a power bucket. The power buckets are then averaged and added to the features' list. This process allows to extract a power representation of the signal.

Number of generated features

Let's consider an input signal with 3 axis and the following parameters:

  • No. of peaks = 3
  • Power edges = 0.1, 0.5, 1.0, 2.0, 5.0

The number of generated features per axis is:

  • 1 value for RMS of the filter output
  • 6 values for power peaks (frequency & height for each peak)
  • 4 values for power buckets (number of Power edges - 1)

33 features are generated in total for the input signal.


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