What is positive phase shift in filters?

DSP, Plug-in and Host development discussion.
mystran
KVRAF
5381 posts since 12 Feb, 2006 from Helsinki, Finland

Post Thu Oct 10, 2019 6:32 pm

JCJR wrote:
Thu Oct 10, 2019 10:33 am
In fact, in my ignorance this caused a woeful misunderstanding only resolved a few years ago. I had read about processes claiming to "un-do" the time-delay of previous layers of phase shifting, and I couldn't understand how one could possibly un-do several additive layers of "negative slope" phase shift curves, if the only stable causal tools available also have the same direction "negative slope" phase shift curves.

Until someone kindly pointed out that filters such as shelving and peaking filters have "positive bump" or "negative bump" phase shift curves, not the continuous negative slope curve. So it was indeed possible to un-do some kinds of "positive bump" phase curves with complementary "negative bump" phase curves, and vice-versa. Doh! The light finally shines, however feebly. :)
If and only if the original filter is (strictly) minimum-phase, you can cancel out the response (both magnitude and phase) simply using an inverse filter obtained by swapping the poles and zeroes. This is usually the case for peaking and shelving filters.

This doesn't work for LP/BP/HP because the zeroes are on the unit circle and it doesn't work for AP because the zeroes are outside the unit circle; in either case the poles would not be stable.

As for equating phase-shift with time-delays, it just doesn't work. The fact that we have multiple concepts such as phase-delay and group-delay should make it pretty obvious that the "time delay" really depends on what aspect of the signal you are looking at.
If you'd like Signaldust to return, please ask Katinka Tuisku to resign.

Z1202
KVRian
1072 posts since 12 Apr, 2002

Re: What is positive phase shift in filters?

Post Fri Oct 11, 2019 5:42 am

Just as a followup to my original post, I realized that (of course), given two phase-shifted sine signals (whatever the phase shift is) you can construct a sine signal with any other given phase shift simply by mixing them.

mystran
KVRAF
5381 posts since 12 Feb, 2006 from Helsinki, Finland

Re: What is positive phase shift in filters?

Post Fri Oct 11, 2019 6:03 am

Z1202 wrote:
Fri Oct 11, 2019 5:42 am
Just as a followup to my original post, I realized that (of course), given two phase-shifted sine signals (whatever the phase shift is) you can construct a sine signal with any other given phase shift simply by mixing them.
Any n linearly independent vectors in R^n form a basis. ;)
If you'd like Signaldust to return, please ask Katinka Tuisku to resign.

Z1202
KVRian
1072 posts since 12 Apr, 2002

Re: What is positive phase shift in filters?

Post Fri Oct 11, 2019 6:46 am

mystran wrote:
Fri Oct 11, 2019 6:03 am
Z1202 wrote:
Fri Oct 11, 2019 5:42 am
Just as a followup to my original post, I realized that (of course), given two phase-shifted sine signals (whatever the phase shift is) you can construct a sine signal with any other given phase shift simply by mixing them.
Any n linearly independent vectors in R^n form a basis. ;)
Yup, that was it :D

Return to “DSP and Plug-in Development”