# Filtering and detrending questions
**URL:** https://mne.discourse.group/t/filtering-and-detrending-questions/8860
**Category:** Support & Discussions
**Tags:** preprocessing, eeg, epochs
**Created:** [May 28, 2024, 12:22am UTC](https://mne.discourse.group/t/filtering-and-detrending-questions/8860 "2024-05-28T00:22:56Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Chris-Winnard](https://mne.discourse.group/u/Chris-Winnard)
#### Post date: [May 28, 2024, 12:22am UTC](https://mne.discourse.group/t/filtering-and-detrending-questions/8860/1 "2024-05-28T00:22:56Z")
Hi,
I had a couple of closely related queries.
I’m currently using PyPrep’s NoisyChannels class to autoremove bad channels, and I’ve read a couple of slightly different things about the included filtering/detrending. Originally I was applying a 1-45Hz bp filter, a 50Hz notch filter, and then using NoisyChannels and the related functions, assuming these wouldn’t include any additional filtering or detrending. In one iterative workflow I actually run NoisyChannels() twice.
However I have been getting a message about a 1Hz highpass filter being applied and I was a little confused by the docs/the code on GitHub. Obviously I don’t want to be filtering repeatedly if I don’t need to, and if any other steps are included in detrending (it’s a little hard to tell from the GitHub code), I’d want to at least know what these are.
If I run the following, will it simply find the noisy channels without any other filtering/DSP steps?
```python
chans = NoisyChannels(eeg_fF, do_detrend=False)
chans.find_bad_by_correlation()
chans.find_bad_by_deviation()
chans.find_bad_by_ransac()
```
Similarly, I was also curious about what _exactly_ linear detrending in the MNE epochs function entails(?) I currently have it set to “linear\_detrend=1”.
Any help would be really appreciated.
This is for PyPrep 0.4.3/MNE-Python 1.6.1 (Win10).
---
### Author:  [@mscheltienne](https://mne.discourse.group/u/mscheltienne)
#### Post date: [May 28, 2024, 8:27am UTC](https://mne.discourse.group/t/filtering-and-detrending-questions/8860/2 "2024-05-28T08:27:11Z")
@sappelhoff could tell you more than I, but here is what I found: `find_bad_by_correlation` starts with [those 2 lines](https://github.com/sappelhoff/pyprep/blob/e4840d9c31ccd880e427448edd8317ff9f6f7909/pyprep/find_noisy_channels.py#L401-L402C37):
```python
if self.EEGFiltered is None:
self.EEGFiltered = self._get_filtered_data()
```
Which runs this method: [pyprep/pyprep/find\_noisy\_channels.py at e4840d9c31ccd880e427448edd8317ff9f6f7909 · sappelhoff/pyprep · GitHub](https://github.com/sappelhoff/pyprep/blob/e4840d9c31ccd880e427448edd8317ff9f6f7909/pyprep/find_noisy_channels.py#L108-L127)
Which **copies** the data and applies a (1, 50) Hz filter on the copy.
Mathieu
---
### Author:  [@richard](https://mne.discourse.group/u/richard)
#### Post date: [May 28, 2024, 8:40am UTC](https://mne.discourse.group/t/filtering-and-detrending-questions/8860/3 "2024-05-28T08:40:49Z")
I can confirm that PyPREP doesn’t alter the input data.
---
### Author:  [@Chris-Winnard](https://mne.discourse.group/u/Chris-Winnard)
#### Post date: [May 28, 2024, 5:49pm UTC](https://mne.discourse.group/t/filtering-and-detrending-questions/8860/4 "2024-05-28T17:49:50Z")
My mistake- I’d actually looked at the docs and misread it as possibly meaning there was a filter on the data itself. Thanks 😄
---
### Author:  [@Chris-Winnard](https://mne.discourse.group/u/Chris-Winnard)
#### Post date: [May 28, 2024, 5:51pm UTC](https://mne.discourse.group/t/filtering-and-detrending-questions/8860/5 "2024-05-28T17:51:21Z")
Thanks. 🙂 Do you know if the linear detrend (i.e detrend=1) method used in epoching includes filtering? Apologies if it’s a silly question.