Lowpass filter

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cbrnr
Posts: 2
Joined: Thu Jul 09, 2020 12:35 pm

Lowpass filter

Post by cbrnr »

Hello!

I'm a bit confused about the lowpass filter settings used during recording. I have an ActiveTwo amp, and I usually set the speed dial mode to 4 (sampling rate 2048Hz). In ActiView, I select a decimation factor of 1/4 to get signals with a sampling rate of 512Hz.

According to your website, the cutoff frequency of the hardware lowpass filter is 1/5 of the sampling rate, so 2048Hz / 5 = 409.6Hz for my setting. After decimation, this should be 409.6Hz / 4 = 102.4Hz. However, the BDF file headers contain a field with "LP: 104Hz" (which is also what ActiView shows for the bandwidth).

What is the correct cutoff frequency?

Thanks,
Clemens

Coen
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Re: Lowpass filter

Post by Coen »

1/5th is an approximation, 104 Hz is the correct -3dB frequency, for the response of the CIC filter see https://www.biosemi.com/faq/adjust_filter.htm

Best regards, Coen (BioSemi)

cbrnr
Posts: 2
Joined: Thu Jul 09, 2020 12:35 pm

Re: Lowpass filter

Post by cbrnr »

Thanks Coen, it's good to know that I can trust the BDF header!

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