Filter phase shift/delay

[Veracohr]

Hi there.

Depending on the source, information about filters will refer to phase shift and/or group delay. When viewing filters from a theoretical standpoint, with periodic sine wave inputs, "phase shift" and "delay" basically amount to the same thing don't they? What about when the input isn't periodic or sinusoidal? In that case, it seems to me the distinction becomes important. Is it actually a time delay or actually a phase shift? Is this even a valid question?

The reason I ask is because I think it will make a difference on the crossover circuit I'm making. I've got filters with theoretical 180 degree phase shift at the 3dB point - 4-pole Butterworth. I want to take the unfiltered signal and align it with the filtered. I could do this with an all-pass filter, but I could also do it with a unity gain inverting amp, right? That's where my question comes from - if the "phase shift" a filter produces is actually a time delay, I would need an all-pass filter to delay the unfiltered signal by the same amount. If it actually is a phase shift with no time delay, it could be accomplished by inverting the signal.

Hopefully I haven't made any completely dumb errors in thinking. It's entirely possible.

 

[RCinFLA]

Delay is a phase shift versus frequency, or the rate of change of phase shift vs. frequency. A fixed constant delay is a linear phase shift versus frequency. All filters have some delay. In filter theory, delay within a given range of frequencies is often referred as group delay.

A Butterworth filter does not have a constant delay across its passband.

To do what you want to do is best done with DSP's.

 

[Veracohr]

To do what you want to do is best done with DSP's.

I'm sure that's true, but it's not what I'm doing.

Let me simplify my question:

If I want to align the unfiltered signal with the filtered signal (within the passband), and there's supposed to be a 180 degree phase shift, can I do it with an inverting amp or does it have to be an all-pass filter?

 

[crutschow]

The phase shift of a filter varies with frequency, so to correct for that you will need some type of all-phase network that matches that phase-shift of the particular type and order of filter you are using.