Bug in IO module for BrainVision: Wrong conversion from Seconds to Hertz

In BrainVision Recorder *.vhdr files, the highpass filter settings are often listed as “low-cutoff in seconds”. The relevant mne.io function translates this into Hz as such:

https://github.com/mne-tools/mne-python/blob/239c83dfe0686084d1c96c945579dc8cc10e895c/mne/io/brainvision/brainvision.py#L597-L598

With this method, the standard setting of low-cutoff=10s is translated to highpass=0.1Hz. However, the BrainProducts website specifies 0.016Hz (see under “Lower cutoff frequency (high pass) / time constant”).

The conversion from seconds to Hz is probably not implemented correctly in the mne.io function for brainvision. Or am I missing something?

Some references:

• book excerpt with brief explanation and equation

• handy calculator and equations

[dmedine]

Hello. My name is David Medine and I am a developer at Brain Products. The reason they use a time constant to represent the nature of a filter is for strange, convoluted, historical reasons.

What is important is that sappelhoff is correct. The denominator should have a factor of 2pi in it.

[larsoner]

Thanks for the confirmation @dmedine. Looks like #5000 should be correct, then.