Frequency filters
Two types of filters are implemented in Letswave :
- Butterworth filters (bandpass, low pass, high pass, notch)
- FFT filters (bandpass, low pass, high pass, notch). See also the specific FFT notch filter which can be used to apply a notch filter at a given frequency and its harmonics.
Apply a Butterworth filter to the dataset. If possible, apply the filter to your datasets before segmentation into epochs. Except for specific applications, it is recommended to use this filter for preprocessing.
- Filter type. Choose between a bandpass filter, a low pass filter, a high pass filter, or a notch filter.
- Low cutoff frequency (Hz). The low frequency cutoff of the filter.
- Low cutoff width (Hz). The width of the low cutoff frequency. A Hanning window is used to design the cutoff transition.
- High cutoff frequency (Hz). The high frequency cutoff of the filter.
- Filter order. The order of the filter. Higher order will produce sharper filters, but may also introduce distortions in the signals.
Apply an FFT filter to the dataset(s).
- Filter type. Choose between a bandpass filter, a low pass filter, a high pass filter, or a notch filter.
- Low cutoff frequency (Hz). The low frequency cutoff of the filter.
- Low cutoff width (Hz). The width of the low cutoff frequency. A Hanning window is used to design the cutoff transition.
- High cutoff frequency (Hz). The high frequency cutoff of the filter.
- High cutoff width (Hz). The width of the high cutoff frequency. A Hanning window is used to design the cutoff transition.
In this example, we apply a bandpass filter which will remove frequencies below 0.5 Hz and frequencies above 30 Hz. The width of the transition is 0.25 Hz for the low cutoff frequency and 1 Hz for the high cutoff frequency.
###FFT notch filter
Apply a notch FFT filter to the dataset(s). In addition to removing the main frequency, the filter can also be used to remove N harmonics of that frequency.
- Notch frequency. The frequency to remove from the dataset (e.g. 50 Hz).
- Notch width. The width of the notch filter. (e.g. with a Notch Frequency = 50 Hz and a notch width = 2, the filter will remove all frequencies between 49-51 Hz.
- Slope cutoff width. The width of the transition cutoff, designed using a Hanning function.
- Number of harmonics. The number of harmonics to remove. (e.g. with a Notch Frequency = 2 and Number of Harmonics = 2, the filter will remove two frequencies: 50 Hz and 100 Hz).