> For the complete documentation index, see [llms.txt](https://nigelrogasch.gitbook.io/tesa-user-manual/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nigelrogasch.gitbook.io/tesa-user-manual/filter_data/butterworth_filter.md).

# Butterworth filter

This function filters the data using a zero-phase butterworth filter. Either a band pass, band stop, high pass, or low pass filter can be implemented. The filter order is defined by the user. This function uses the Matlab `butter` and `filtfilt` functions.

## EEGLAB user interface

![](/files/-LmJDKEbcWLPGJVscX-y)

**1.** Enter the frequencies for filtering.

**2.** Enter the order of the filter.

**3.** Select the filter type. For a band-pass filter, frequencies outside of the high and low pass cutoffs will be excluded, whereas for the band-stop filter, frequencies within the high and low pass cutoffs will be excluded. For a high-pass filter, frequencies below the cutoff will be excluded (note that low-pass field can be left blank), whereas for a low-pass filter, frequencies above the cuttoff will be excluded (note that high-pass field can be left blank).

## Scripts

### Base function

`EEG = tesa_filtbutter( EEG, high, low, ord, type );` Default use.

### Pop function

`EEG = pop_tesa_filtbutter( EEG );` Pop up window

`EEG = pop_tesa_filtbutter( EEG, high, low, ord, type );` Custom inputs

### Required inputs

| Input | Description                                                                                                                                                                              | Example    | Default |
| ----- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------- | ------- |
| EEG   | EEGLAB EEG structure                                                                                                                                                                     | EEG        | -       |
| high  | Integer setting the high pass frequency                                                                                                                                                  | 1          | -       |
| low   | Integer setting the low pass frequency                                                                                                                                                   | 100        | -       |
| ord   | Integer setting the filter order                                                                                                                                                         | 4          | -       |
| type  | String which sets the design of either a zero-phase band pass, band stop , high pass or low pass butterworth filter; string is either 'bandpass',  'bandstop', 'highpass', or 'lowpass'. | 'bandpass' | -       |

### Outputs

| Output | Description          |
| ------ | -------------------- |
| EEG    | EEGLAB EEG structure |

### Examples

`EEG = pop_tesa_filtbutter( EEG, 1, 100, 4, 'bandpass' );` Zero-phase, 4th-order band pass butterworth filter between 1-100 Hz.

`EEG = pop_tesa_filtbutter( EEG, 48, 52, 4, 'bandstop' );` Zero-phase, 4th-order band stop butterworth filter between 48-52 Hz.

`EEG = tesa_filtbutter( EEG, 1, [], 4, 'highpass' );` Zero-phase, 4th-order high pass butterworth filter allowing frequencies above 1 Hz

`EEG = tesa_filtbutter( EEG, [], 45, 4, 'lowpass' );` Zero-phase, 4th-order low pass butterworth filter allowing frequencies below 45 Hz


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