Butterworth Filter help text missing something?

[gstanczak]

Hello, is the help text for the Butterworth Filter vi missing something?  It never mentions that the low frequency cutoff terminal is also the high frequency cutoff input when lowpass filter is selected.  When I first started using this vi, it took me a while to debug what I was working on because I would wire my high cutoff value to the high frequency terminal when I had lowpass filter selected, and my low cutoff value to the low frequency terminal when I had highpass filter selected, only to learn later that the low frqeuncy cutoff terminal is the high AND low cutoff input depending on the filter selected?  Is this right?  Here is the help text...

	high cutoff freq: fh is the high cutoff frequency. The VI ignores this parameter when filter type is 0 (Lowpass) or 1 (Highpass). When filter type is 2 (Bandpass) or 3 (Bandstop), high cutoff freq: fh must be greater than low cutoff freq: fl and observe the Nyquist criterion.
	low cutoff freq: fl is the low cutoff frequency and must observe the Nyquist criterion. The default is 0.125. If low cutoff freq: fl is less than or equal to zero or greater than half the value of sampling freq: fs, the VI sets Filtered X to an empty array and returns an error. When filter type is 2 (Bandpass) or 3 (Bandstop), low cutoff freq: fl must be less than high cutoff freq: fh.

On a side note, I just discovered the initial condition terminal on this vi and I am extremely excited about it!

Thanks,

Greg

[Davidek]

Hi Greg

In the helpfile you pasted it says:

high cutoff freq: fh is the high cutoff frequency. The VI ignores this parameter when filter type is 0 (Lowpass) or 1 (Highpass)

Isn't that the information you are looking for?

Best Regards

David

NISW

[gstanczak]

David,

My confusion is why the high cutoff freq: fh is ignored when the selected filter type is lowpass.  That is one of the filter types that requires a high cutoff frequency.

filter type         required inputs:

lowpass filter -  high cutoff              (the help says in this case high cutoff is ignored?)

highpass filter-  low cutoff

bandpass filter- high and low cutoffs

bandstop filter- high and low cutoffs

One reading the help text is left wondering where to wire their high frequency cutoff when they have lowpass filter selected.  In actuality, the low cutoff freq: lh acts as the high cutoff input when lowpass filter is selected, but you would only learn this after experimenting with the VI.

Regards,

Greg

[Darin.K]

I would also be confused if I just started wiring up this VI.  Reading the context help it would be very easy to misread or misinterpret "high cutoff freq" as "high freq cutoff".  These are two very different things.  For a lowpass filter, there is only a single cutoff frequency, so the low cutoff freq is equal to the high freq cutoff.  You could argue that it is equally valid to consider the single cutoff as both high and low, just like a single person in line is both first and last.  Certainly NI made a semantic choice (I happen to agree with it), but the detailed help seems to clearly show that choice.

All of that said, I think that there is room for improvement in the help file description.   At least you'll never make that mistake again.

[gstanczak]

Alrighty, I suppose.  I feel better at least that you see where I'm coming from, and no I won't make that mistake again.

Cheers,

Greg

[Davidek]

Hi

I think Darin summed it up well, but I see that it can be confusing. Any suggestions to how it should be written to make it clearer?

Best Regards

David