Hi Stu, Hi Tim,

I hope you are doing well.

The observations you have made about the changing phase shifts in dependence on the frequency and resonance value can be explain by Fourier-transformations which is the mathematical theory behind every kind of frequency filtering.

It says something like that every manipulation of a periodic function in the frequency space results in a manipulation of the signal in the phase space and vice versa.

That's why a signal filtered with different filter slopes, has different phase shifts along the slop. Because of the different gradients among the slopes, the phase shifts between the different frequencies alter as well.
If you change the resonance of a filter, the slope is changing again and you will get again changing phases between the different frequencies of your audio signal.

This is perfectly consistent with the phase shift behavior you have measured, Tim!
The phase shift "... increases with frequency... " due to the heavier manipulation of higher frequencies to obtain low pass filtering.

I'm not sure, but I would say that the reason why you don't here a phase shift at 1 Khz with a 30dB slope is, that the phase shift is coincidentally exactly 2*pi or 360 degrees with these settings. So that one could think that there is no phase shift, but the phase is already shifted with 2 periods.

I hope I could help with my knowledge of my physics studies.

Ollie

--- In Doepfer_a100@yahoogroups.com , "grimshaw_stuart" <grimshaw@...> wrote:
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> wow.
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> thanks tim, that's really helpful. of course, like all the best answers, it raises more questions :)
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> like this one: what's so special about a cut off of 1kz and a slope of 30db that the filtered signal stays in phase with the original up the cut off frequency
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> stu
>