# Filter cHPI

After determining the head position using cHPI coils, the next step is to remove any noise generated by these coils. This is accomplished with the [`filter_chpi`](https://mne.tools/stable/generated/mne.chpi.filter_chpi.html) command from the MNE library. Users can adjust the parameters related to this step in the `config_pipeline.cfg` file, allowing for customization based on the specific needs of their dataset. If the data does not contain cHPI coil information, this procedure will be skipped automatically.

```json
"filter_chpi": {
	"include_line": false,
	"t_step": 0.001,
	"t_window": 0.2,
	"ext_order": 1,
	"allow_line_only": false
},
```

<figure><img src="https://136130075-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FST6HjOO2v7zr80O4PyU3%2Fuploads%2FW49bHKc8MIzTe9EuuTty%2Fsusb-CC120208.png?alt=media&#x26;token=ac7fa177-1c2d-4594-8551-1fb15a224329" alt=""><figcaption><p>After filtering cHPI coil noises</p></figcaption></figure>

{% hint style="info" %}
By default, `filter_chpi` command also includes a notch filter for removing line noises. However, we disable this feature and postpone line noise removal to the next step in MEGAP.
{% endhint %}

In CTF devices, the contribution from each cHPI coil is extracted through [spectral line extraction](https://www.sciencedirect.com/science/article/pii/S1053811912011597?via%3Dihub), which eliminates the need for running the `filter_chpi` command. This is because the CTF system does not exhibit any distinct peaks in PSDs for cHPI signals, as it uses a different method to monitor and track head positions. Therefore, the procedure for filtering noise from cHPI coils is not required for CTF data.

