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Topic: Filter Range auto-adjusted silently
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EMebane



Joined: Apr 15, 2019
Posts: 98

PostPosted: Jul 24, 2021 5:00 PM    Post subject: Filter Range auto-adjusted silently

I noticed a few of these in the console. I don't usually run the console, so I don't know if these just started appearing:

Filter warning: high frequency 17.5 is above 16.0, changed to 16.0

I switched the filters from Butterworth to something else and the problem stopped. I switched back to Butterworth and the problem still did not return.

Any idea what this was? Do some filters have limits that are enforced at runtime? The design always ran without any warnings appearing in the BioEra interface and I hope my filter ranges haven't been being auto-adjusted all along.
jarek



Joined: Oct 22, 2007
Posts: 910

PostPosted: Jul 24, 2021 5:10 PM    Post subject:

This warning is because of Nyquist limit, when the upper filter frequency can't be higher than half of the sample rate.

Not sure why it shows only with the Butterworth option, but this is also the most typically used option, so may have been fine tuned more than other options.

This warning has always been there, so probably your SPS has changed.
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