hardware filters

[maysanb]

Hey

I'm using one of the analog channels connected via analog box to record triggers as they are played together with the sound:

Screenshot 2024-09-30 155447.png (17.22 KiB) Viewed 2595 times

The upper plot is the trigger I send and the lower plot is the trigger recorded.
Why does the recorded trigger look like it passed through high-pass filter? (loss of steady state)

In the specs it says that only low-pass is applied (ActiveTwo) and low pass doesn't cause this shape. I also checked using an oscilloscopethe signal before it enters the eyelink that connect to the biosemi and it was still a step.

Can you please help elaborate exactly the filters that are done to this signal? I'm getting a delayed signal in the recording and I want to know if it's due to the filters.

thanks!

[Coen]

The BioSemi system has DC amplifiers, there is no high-pass filter. Check the signal in ActiView with the (display) filters switched off. You have either a high-pass in your file reader, or you have capacitively coupled the trigger line to the Analog Input Box.

Best regards, Coen (BioSemi)

[maysanb]

I did use analog box as I mentioned in the beginning, but I checked the output of the analog box using a statoscope and it was still a step func (no steady state loss), so the "filter" that changes this signal is after.
The file reader I use is mne read bdf and it doesn't apply filters.

[Coen]

Which "analog box" exactly are you talking about ?

It must be something else than a BioSemi AIB (https://www.biosemi.com/aib.htm) because the BioSemi box only has a digital optical output (and it is unlikely that you can measure anything meaningful from this optical data stream).