eq phase question

[genericperson]

i had a question on stereo eq' s versus two identical mono eq's.

if you put an analog mono eq (outboard gear) on the left side of the stereo buss, and then an identical mono eq on the right side, would it mess up the stereo signal because they won't be identical?

i know that all eq's exept digital FIRs exhibit phase shift, so i'm wondering if two separate mono eq's will make the stereo field all whacked out even if you try hard to match the controls up.

[Ethan Winer]

GP,

> i'm wondering if two separate mono eq's will make the stereo field all whacked out <

No, and in fact a stereo EQ is two mono EQs with the two sets of controls ganged together so they apply the same settings to both channels.

Also, I wouldn't worry about phase shift because it's benign. Go to my Articles page:

www.ethanwiner.com/articles.html

Then look for the two articles, 6th in the list, that explain this in detail.

--Ethan

[genericperson]

hi Ethan,

I read your article on your website about eq's and phase shift.

I don't think that is how analog eq's work. My understanding is that analog eq's put a frequency filter very similar to a crossover in a multi-driver loudspeaker.

Then you can apply boost/cut to a specific band because the filter makes the controls focus on that spot and ignores the spectrum falling outside of the band. Wideneing or narrowing the affected band using the filters gives you the "q" control.

Their *is* some phase shift going on, because the components in the filter have a small amount of electrical delay. This causes a slight time discrepancy compared to the adjcacent band, hence the slight phase shift involved going between bands. It's the same situation with drivers on loudspeakers employing crossovers.

But the equalization is not *based* on the phase shift. Your article makes it sound like the eq is intentionally putting things out of phase with itself to provide the eq curves. But how could you provide "boost" to a frequency band that is already in phase with itself using your phase-principal of gain/loss? You can't, unless you're doubling the signal upon itself. And I really don't think that is what's going on inside most analog eq's.

Now I could very well be mistaken here, but I don't think I am.

[Berolzheimer]

Heh .....what about those filters that seperate the bands that you are then boosting? Subtractive analog filtering......phase shift.

[genericperson]

so when you have something like a 12db per octave slope, it's *phase shift* making the slope?

[Ethan Winer]

Gennie,

I'll be gentle.

> I could very well be mistaken here, but I don't think I am. <

Yes, you are very mistaken. Here are a few points:

> My understanding is that analog eq's put a frequency filter very similar to a crossover in a multi-driver loudspeaker. <

A simple "frequency filter" is created from passive components that have reactive properties. This means their impedance changes with frequency. A capacitor passes less as the frequency gets lower, and an inductor passes less as the frequency gets higher. Each has a falloff of 6 dB per octave. If you put a capacitor in series followed by an inductor as a shunt to ground, you can get a low frequency rolloff of 12 dB per octave. Reverse the components and you instead get a high frequency rolloff of 12 dB per octave. Add more stages and you can get many dB per octave, all from purely passive components.

> But the equalization is not *based* on the phase shift. Your article makes it sound like the eq is intentionally putting things out of phase with itself to provide the eq curves. <

Yes, the article is exactly correct. That is how it works.

> how could you provide "boost" to a frequency band that is already in phase with itself using your phase-principal of gain/loss? <

To get boost requires an active circuit. Or a passive circuit with active makeup gain.

Besides the articles you already saw, there are several other, more technical articles on my site that show a variety of active filters. They don't include a detailed blow-by-blow explanation of each circuit component, but they do show how EQ and other filter circuits are constructed.

--Ethan

[genericperson]

thanks Ethan,

I'll have to check that out. The idea that phase is the "it" behind an eq circuit is a major revelation. I'm still trying to take it all in actually. Completely mind-blowing.

[jbuntz]

The amount of phase shift in an eq shouldn't be enough to significantly screw up the stereo image.

[Ethan Winer]

JB,

> The amount of phase shift in an eq shouldn't be enough to significantly screw up the stereo image. <

More to the point, you should always EQ both channels identically anyway, and that definitely won't change imaging at all.

--Ethan