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Analog EQ phase issues
Old 17th March 2005
  #1
Gear Maniac
 
papawhitehead's Avatar
Analog EQ phase issues

Hey, can someone explain to me what's up with the phase issues of analog eq. I keep hearing people talk about problems in the low end. What frequencies are affected by this? What Eqs are immune to this and why? Pultec?
Old 17th March 2005
  #2
Moderator
 
Tim Farrant's Avatar
 

What do you mean by "phase issues". All EQ introduces phase change, there is no way around it.
Old 17th March 2005
  #3
member no 666
 
Fletcher's Avatar
Actually, linear phase digital EQ doesn't introduce phase issues which is one of the reasons you have to apply so much to hear it work.

The way an analog equalizer works is that you have filters [fixed or variable] that carve out a little piece of the audio... that little piece of the audio is then added to the original signal in either positive or negative polarity ["boost" being the addition in positive polariy, "cut" being the addition in negative polarity].

Anytime you run any signal through an analog filter you're going to get a phase shift. The phase shift is a timing issue... a couple of examples... when you filter off the bottom you're not only cutting the low frequency information, you're also slowing down the low end component of the signal allowing the treble freqencies to pass first, when you roll off the top too low you slow down the treble frequencies... this is why things like Mackie desks have ****ty bottom and ****ty top but measure well between 20Hz and 20kHz... you're not measuring the timing of the arrival of the signal you're measuring the energy of the signal. That little supercomputer we call a brain does register even the smallest timing alteration within a signal... in other words we hear stuff quite clearly that isn't regularly measured.

What happens in the case of something like this kind of inexpensive desk is that the midrange [specifically upper midrange] comes out first which is why you get that kinda thin, "gank" component to the audio. This is why when you get behind something like an old Neve desk that has a frequency response of like 4-6Hz on the bottom to like 65-80kHz on the top the audio seems larger and smoother.

Now there are other components that fukk with phase... like transformers. The reason we find many transformers to sound "really musical" is because they slow down the upper frequency component of the audio allowing the bass to pass first [there are other things that go on with transformers but phase response is a biggie].

This is what we call "large" or "big" or "rich" or "full". Most of the reason Neve modules are as sought after as they are is because of this componenet of the sound of the transformers [specifically the output transformer which is why it's such a drag that companies like Vintech use an output transformer that exacerbates this phenomenon along with an added low end distortion characteristic... but I digress].

Any EQ that was designed without sufficient headroom and/or was designed so the bottom rolled off too high [the 3db down point should not be in double digits... the 3db down point on the top shouldn't be any lower than like 50kHz, and many of them will pass up to 150-200kHz... which is fondly exagerated with the saying "passes DC to light"].

There are some EQ's that do marvelously with both low and high end material... there are some that do miserably with low and high end material... there are some that spec mediocre and sound amazing, there are some that spec like they came from heaven above and sound like ass.

Much of it will be your sense of aesthetic... some of it will be you point of reference, some of it is definitely superstition.
Old 17th March 2005
  #4
Gear Maniac
 
papawhitehead's Avatar
thanks Fletcher

Thanks Fletcher, the reason I asked is because I'm getting an Analog board (Midas Venice) to track/mix through (HD2 system) and was concerned about using the EQ section. I really like that board and I don't remember hearing any serious phase problems when EQing. I'm not into doing radical EQ adjustments so maybe that's why..? Either way thanks for your time.
Old 17th March 2005
  #5
Gear Guru
 
Ethan Winer's Avatar
 

White,

> can someone explain to me what's up with the phase issues of analog eq. <

See THIS article which explains the role of phase shift in equalizers. For even more on the audibility of phase shift, also see the article linked at the bottom of that page.

--Ethan
Old 18th March 2005
  #6
Gear Maniac
 
papawhitehead's Avatar
Thanks for that info Ethan, I understand now and am no longer "afraid" to use my EQ. Take care -B
Old 18th March 2005
  #7
JTR
Lives for gear
 

Good question Papa - you might want to also ask Mr. F or perhaps Paul W / Dan K / Geoff T / George M, & etc. to explain why it's often noted that subtractive, rather than additive eq creates less phase shift artifacts...

And perhaps someone might like to provide some examples of analog Eq's known for their lack of phase shift, and the resultant sonic characteristic they are known for.
Old 18th March 2005
  #8
member no 666
 
Fletcher's Avatar
First off... subtractive EQ and additive EQ are the same in terms of phase shift... it's just that we tend to notice the effect of additive EQ more than subtractive EQ [so we actually apply less gain when going additive EQ than subtractive EQ so the result is generally less phase shift added to the signal when boosting than cutting].

The other "urban legend" is that you give the equalizer more headroom by using subtractive EQ than additive EQ... which is complete bull****. The equalizer is doing the same amount of work whether you're boosting or cutting... as you're adding to the original signal either way... the only question is whether you are adding the signal in a positive polarity orientation or a negative polarity orientation.

There really aren't any EQ's that are known for their "lack of phase shift" though it's a neat bull**** marketing term. A decade or so ago there was a company called "Nightpro" that made the EQ with the "Air" band... which was simply a 10kHz shelf that took like 3-4 octaves to ramp up to 10kHz... and the other 4 bands had like 3-4 octave band widths so they were all interactive and ****... while they claimed it was a "zero phase shift equalizer" the fact of the matter is that it wouldn't have worked if it didn't fukk with the phase relationship of the signal, so while things were rather broad and subtle, there still was "phase shift".

BTW... one of my favorite "phase shift" units that isn't an EQ in the classic sense of the word, but very much functions like an EQ in application is the Littlelabs "IBP" unit [which is a "variable all pass filter"]. A "variable all pass filter" will have different phase altering characteristics than a "hi" or "lo" pass filter... but can and does work incredibly well at what it does. A wonderful tool as it is quite fun to fukk with phase relationships for fun and profit.

Peace.
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