It seems that opening and closing the low pass filter introduces an offset to the signal.
Top display is the signal in, bottom display is the output of the highpass node.
This is the patch in the gif: High Pass Bug.audulus (11.4 KB)
It seems that opening and closing the low pass filter introduces an offset to the signal.
Top display is the signal in, bottom display is the output of the highpass node.
This is the patch in the gif: High Pass Bug.audulus (11.4 KB)
At first I thought you were talking about a phase shift which is unavoidable, but now I see that you are talking about a DC offset in the signal. I believe that this is a consequence of the filter algorithm used for the built in nodes. I believe they are exponentially weighted, moving average, first order filters. When you rapidly change the filter cutoff frequency, the filter takes a while to settle because of the delay inherent in the moving average. This effect is more pronounced the closer you get to an alpha of 1. Assuming that alpha is <1 the offset will eventually decay. I don’t believe that this is something that can be fixed without changing the type of filter. Bi-quads have some similar problems which is why they often don’t modulate well.
Thanks for the insight. I added some slew to the input signal to the high pass node and the problem self-corrects more quickly, but I had to make sure it was also VERY little slew because the of asymptotic decay of the signal going through the low pass filter was attenuating the LFO signal.
Lots of filters end up having DC offsets, including analog ones (my SEM one had a -3v dc offset when in HP mode even with nothing going through it).
Well! Today I learned…
The offset in this one is only short term. I doubt you would be able to detect it at audio frequencies unless you were modulating the cutout frequency at audio rates as well. In that case I guess you would be introducing an additional low frequency sub-harmonic. The offset is only noticeable when the modulation frequency is similar to the cutoff frequency. I did get some interesting waveforms while I was playing with @robertsyrett’s setup: