Re: Recovering data below a high pass cutoff frequency

Marc wrote: 
Is it possible to recover any data and/or information (e.g. trends)
about a signal below the HP cutoff frequency of a system?   System:
Data is band limited by hardware, at the low end by an AC coupled
amplifier (around 1 Hz) and at the high end by a hardware anti-aliasing
filter (around 64 Hz).   Is it possible in software to ascertain any
information about the signal at frequencies below 1 Hz?

Sure. A simple R-C rolloff is rather gradual. A time constant of .159 sec/radian will be 3 dB down (half power) at 1 Hz, 5 dB down at .5 Hz, and 20 dB down at .1 Hz. Those attenuations should be easy to recover from/ 40 dB down at .01 Hz is more problematic.

Filter theory tells me that there is attenuated signal on either side
of the passband and that depending on the slope of the filter responses
I can process signal a certain distance from the Nyquist frequency.


Not after you sample it. The filter is there to prevent aliasing.

But are there any actual advanced methods to analyze frequency response
out side the passband? I've been hearing some buzz about this sort of
thing that seems like it contradicts basic filter theory. I'm just
checking with you guys to make sure I'm not missing something
counter-intuitive that may be out there.

All you need is gain and freedom from noise. Freedom from noise is the hard part, but as Stephan Bernsee once wrote, papier ist duldig (paper is patient-- read "compliant").

Jerry
--
Engineering is the art of making what you want from things you can get.
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.

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