Analogue EQ and phase

[morls]

I'm studying to gain a deeper understanding of phase in relation to audio frequencies. At the moment I'm trying to work out whether ALL eq has an effect on phase, or whether it depends on the implementation. In particular, I'm struggling to understand whether phase changes are introduced by running a complete stereo mix through 2 Pultec EQs.

Looking at the circuit, the entire waveform passes through the passive EQ circuit before amplification. AFAIK the EQ circuit doesn't split the signal and then recombine before the amplification stage, so it's hard for me to imagine how phase shift can be introduced here if the complete signal is always intact.

Is it useful to think of phase as a dimension that exists between two separate signal paths? So that when phase of one signal is considered, it is always in relation to another signal which is independent in time?

Stephen

 

[JohnRoberts]

It's useful to not overthink phase.

Phase mainly becomes an issue when two similar audio streams combine with different phase response.

JR

 

[morls]

Thanks JR,

So, for a stereo processing chain I'd consider L and R as 2 similar audio streams, and look out for any phase differences introduced by processing?

Unless using a stereo/linked processor (analog processing only), in which case they're processed as one 2-channel audio stream?

 

[Bo Deadly]

I'm studying to gain a deeper understanding of phase in relation to audio frequencies. At the moment I'm trying to work out whether ALL eq has an effect on phase, or whether it depends on the implementation. In particular, I'm struggling to understand whether phase changes are introduced by running a complete stereo mix through 2 Pultec EQs.

Yes. All filtering results in a phase shift at the affected frequencies. Filtering and phase shifting is really the same thing.

But if you perform the same filtering on two independent signals, the phase shifting will be the same and therefore the two signals will be in phase.

 

[abbey road d enfer]

At the moment I'm trying to work out whether ALL eq has an effect on phase, or whether it depends on the implementation.

ALL analog EQ's introduce phase-shift. The amount depends on the slope of the frequency response curve. It may be counter-intuitive, but the maximum phase-shift is not at the top or bottom of the response curve; actually it's zero there, and the max phase-shift is on the slope.



Dotted line is phase (RH scale), plain line is amplitude (LH scale)

In particular, I'm struggling to understand whether phase changes are introduced by running a complete stereo mix through 2 Pultec EQs.

Yes, but as others said, if the EQ is the same on both channels, the phase-shift will be the same and there will be no difference betweeen channels.

Looking at the circuit, the entire waveform passes through the passive EQ circuit before amplification. AFAIK the EQ circuit doesn't split the signal and then recombine before the amplification stage, so it's hard for me to imagine how phase shift can be introduced here if the complete signal is always intact.

It seems you are confusing different concepts. Running parallel paths and mixing them MAY result in a response that is not minimum-phase. I won't expand on the MP concept.
But even simply passing signal through a capacitor results in phase-shift, because it reacts with the load.

Is it useful to think of phase as a dimension that exists between two separate signal paths?

Not necessarily. But yes, phase is always relative. We usually consider it as being a temporal shift between the output and input of a quadripole (a black box where signal goes in and out). But the first occurences of phase in EE is how current and voltage are displaced in capacitors and inductors.

So that when phase of one signal is considered, it is always in relation to another signal

Yes. Most commonly (but not always) in reference to an input voltage

which is independent in time?

I think you mean by that that being the reference, it's generally considered as starting at time zero.