How do you correct phase shift? You can only do so much to an EQ before you lose all your power so how do you correct it correctly(?)?

Mike G.

Reply posted by Tom Young on December 10, 2001
Any filter (equalizer, crossover, high/low pass) exhibits phase shift that is increased as the gain of the filter is boosted or cut. Zero boost or cut equals zero phase shift. Despite some aggressive marketing hype, there are no filters or equalizers that do not cause phase shift. Digital filters emulate analog filters and they therefore also have phase shift.

So, unfortunately the answer to your question is: "reduce the degree that you are eq-ing and the numbers of filters you have implemented".

But you may be describing a phenomena that is also or mostly caused by the attenuation that has resulted from over-EQ'ing. The more you EQ out (cut, specifically), the more sucked-out the overall sound becomes. Phase shift is also pretty bad in this situation, but it is primarily a case of your having removed too much of the signal.

Yes, this may be deemed 'necessary' due to poor equipment mixed with horrible acoustics and/or hopeless mic techniques/requirements... but there is no magical way to reduce the suck-out other than minimizing the EQ.

Tom Young
Electroacoustic Design Services

Reply posted by Paul Henderson on December 10, 2001
Something worth noting here...as long as the system is behaving in a minimum-phase manner (at some physical point of measurement), the phase characteristic of the system is directly related to the magnitude response.

By applying minimum phase corrective equalization filters to the system (analog or digital), you are then effectively correcting both the magnitude and phase response of the system simultaneously (at that specific acoustic position). Without getting into the details of what makes up a minimum phase system and how this may vary over space in electroacoustic systems, suffice it to say that the nonlinear phase response in corrective equalization is a necessary and useful product of the application of filters.

Without it, you would be dealing with a highly nonminimum phase electrical response, which brings with it drastic time-domain and acoustical difficulties.

As Mr. Young pointed out, over-EQ'ing will be the cause of problems, specifically when the filter is not matched to the minimum-phase anomaly it is attempting to correct. This brings with it both problems in magnitude response, but also increased group delay and hence time smear near the filter resonance. This all goes back to the ability to accurately measure a system and intelligently interpret results using a transfer function measurement platform like SmaartLive or SIM.

Paul Henderson

Reply posted by Mike G. on December 11, 2001
EQ does come into the picture though. CD horn EQ is radical. This causes lots of phase shift. IF you look at Servodrive subs, they are not acoustically flat from 30-160hz therefore they need EQ. Now we've introduced phase shift. How do you correct for this problem outside of your EQ?

Mike G.

Reply posted by Andy Peters on December 11, 2001
-"CD horn EQ is radical. This causes lots of phase shift."

CD Horn EQ is a 6 dB/8ve rise above its cutoff. Not very radical. And, I believe the CD horn's roll-off is minimum phase, which means it CAN be corrected by electronic EQ.

Reply posted by Thomas Danley on December 11, 2001 at 02:44:04:
-"EQ does come into the picture though. CDhorn EQ is radical. This causes lots of phase shift. IF you look at Servodrive subs, they are not acoustically flat from 30-160hz therefore they need EQ. Now we've introduced phase shift. How do you correct for this problem outside of your EQ?"

A side point, once you have 2 or more BT-7's (the horns) together, the response is reasonably flat across the band, a group of 6 is + - 1.5 dB 28 -110 Hz with an acoustic phase near zero degrees over much of that band. (see TEF measurements at web site if interested) Even 2 have less frequency response deviation than one would see on most conventional VC sub boxes going from a cold to hot voice coil.

Your real issue, however, is with phase shift, it seems to me.

An important concept, a "minimum phase" thing like a speaker or most electrical networks (not including all pass filters) has a fixed relationship between amplitude and phase. One cannot change the amplitude without also changing the phase a corresponding amount.

If one had a minimum phase speaker which had a non flat response, one could use a normal filter or EQ to make both the amplitude AND phase flat at the same time as the filter correction has the right amplitude and phase to counter act the speakers response .

In the case of your CD horn, many drivers do have some non-minimum phase response (due to the mechanical bits) but for the most part, the horn and driver response can be EQ'd reasonably flat leaving only the driver's non minimum phase part.

Where it becomes much more complicated is when you include more than one source (or a reflection), now the problem is in 3D. Now, any separation between the two sources is the same as a phase shift between them, if the phase shift is significant (more than 1/4 wavelength) then the output of the array is directed according to the pattern of cancellation and addition. As music is a time variant signal, one can also look at the difference as one in time.

An EQ cannot fix a problem in Time, which was causing a response problem (a reflection for example usually seen as a dip at some frequency). An attempt to correct a time problem WILL put a large amount of phase shift where there wasn't any needed.

While one can improve the look of a response curve, adding a lot of phase shift may also have a negative effect on the sound quality.

Bottom line, so far as I see it, under the right circumstances a filter correction or EQ can be totally seamless and transparent when used to correct an inverse problem at the source.

On the other hand, when used to compensate a time related interference, then EQ can make a flatter response but is also more likely to screw up the sound by adding phase shifts where they don't belong.

Cheers,
TD

Reply posted by Mikael Holm on December 11, 2001
- "How do you correct for this problem outside of your EQ?"

By stacking more boxes.

Miffe

Reply posted by Mike G on December 11, 2001
This is more a system question then a filter question. If you have a 180-degrees phase shift at 45hz, what do you do about it?

If you are 40 degrees out between 130-160 how do you correct for this. It is correctable, at least to a certain extent. I don't want to do this with my EQ. How else is this accomplished?

Mike G.