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Author Topic: "Subractive EQ" vs. "Additive EQ"?  (Read 6591 times)

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Offline Church-Audio

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #15 on: April 03, 2008, 11:20:47 AM »
Chris, no it's not automatic gain compensation.  It might not be intuitive, but it's a physical reality.  While music might not supposed to be a square wave, a lot of modern music is mixed like it is.  And thus the effect can be demonstrated both on test signals and on real audio tracks.

It is probably a non-issue for live sound, because you'd never notice a small increase in peak level, and if you did, the mixer should have plenty of headroom.

But if you take nearly any loud modern rock CD with peak at 0dBFS, and apply a high cut, you can see an increase in peak level.  This is not a psychoacoustic effect, it is easily measurable.  I have attached a picture, this is probably an extreme example, a 220Hz square wave with a 360Hz low pass applied resulting in a 0.75dB increase in peak level, even though RMS drops.  But take any loud rock track, say something that is -11dBFS RMS or more, and try some high cuts.  It's not too hard to increase peak level by 0.2dB.

Again, not a big deal if you have 2 or 3dB or headroom . . . but I get a lot of tracks sent to me that peak at 0dBFS . . . were they just magically mixed that way?  Or did somebody normalize, or worse, limit, or really bad, clip the master bus?  I mean, do they want the thing mastered, or do they want to master it themselves?

Live recordings are different because (thankfully) concerts aren't as heavily limited as CDs are these days.  So if that's all you do, you might never see this.  But please don't accept a rule of thumb as a physical law, because one day you might try to apply that technique to a different situation and encounter a problem.

dactylus, the A above middle C is 440Hz, an octave above, 880Hz, below, 220Hz, etc.  The range of fundamental tones of music generally is about 30Hz to 4kHz.  But you can't just think in terms of the fundamental, because every instrument has its series of overtones, which fill out the rest of the audible spectrum.

I talked to one of my mad scientist friends and he helped explain it to me..
What your talking about is a phase shift that can be introduced by using an eq... This phase shift can cause an alignment of frequency's / harmonics in such a way as to increase the overall peek frequency.. This is true but the chances of this happening in a complex source such as music are about 1billion to one..

 I would say you have a better chance of winning the lottery then you do introducing this phase shift that just happens to align harmonics in the right way to boost peek levels..

 When your talking about millions of peeks in a song.. This peek would only be at one center frequency and only for a short duration..

Now the huge problem with your theory is this.. Lets say a phase shift happens and the frequencies all magically align them selves.. The net increase will be very small compared to the 4db cut I just did at 150hz.. So it makes your theory moot. It could not really happen in real life with out some really bad filter design.. And other issues with poorly designed plugins or a poorly designed eq section on a digital console. In other words in the real world this does not happen. I guess theories are nice but 20 years of mixing live and in the studio tell me that its not something that will likely ever be an issue for me, and even less likely for anyone else doing concert recording..

Further more trying to keep your 3-6 db of headroom when your only applying eq cuts to a track only serves one purpose to increase your noise floor. I guess we will have to agree to disagree. The other main issue is nobody I does compression first then eq second.... So if you were to eq the song then apply compression the net result would be a further smoothing of the peeks and would nail anything sticking out that would cause a increase in your peek level to begin with. When you use a simple signal like a square wave its easy to see the phase shift your talking about but when your using your theory on a track with real music its much harder. And remember when cutting frequencies with an eq your going to be reducing the overall gain so much that a slight increase in your peek level is not going to amount to anything.. And you will have to normalize or compress again anyway to get your levels back up.. So your .2db increase in peek level will not be of any concern.
I just wanted to set the record straight here.  :P
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Offline SparkE!

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #16 on: April 03, 2008, 12:58:47 PM »
There's a pretty good description of this on Wikipedia's site.  Just search for square wave.  Here's an animated gif from that site:



As it turns out, the fundamental Fourier component has a peak level that is nearly 2.1 dB above the peak level of the square wave.  The exact ratio is 4/π.

I should also point out that a square wave is the worst case waveform for these types of effects and that you'd only see a 2.1 dB increase in peak level if you used a brick wall filter.  Of course, no such thing exists, so that's not a big concern.  However, if you use a low pass filter with high Q poles, you can get a lot of ringing at the rising and falling edges of the filtered square wave and that can cause a huge increase in peak level, especially if the fundamental frequency is harmonically related to the cuttoff frequency of your low pass filter.  On the other hand, that's not the type of filter you'd be using for equalization of audio sources.  More likely you'd be using a parametric equalizer with relatively low Q stages, so ringing would not be a concern.

So yes, it's definitely possible to increase peak levels while actually reducing the rms power in the signal by reducing the level of a certain portion of the signals spectrum, but it's not at all likely to happen in the real world when equalizing raw, uncompressed, unclipped, unlimited live recordings like we do here.
« Last Edit: April 03, 2008, 03:22:57 PM by SparkE! »
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Offline morst

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #17 on: April 21, 2008, 01:27:32 PM »
I find that having 5 bands per side of a stereo mix is more then enough to do any type of eqing that might be needed. Anything more then that and you have a real problem with your source. And you need to rethink your approach.
Good point. EQ introduces phase shift, and should probably be used mostly as a last resort. Dan Healy of Grateful Dead fame evidently uses carefully tuned high-pass filters to do most of his EQ work. My friend Brad Sarno spoke to him at length during his recent stint filling in for DSO's injured FoH man. . .

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Short answer to amazing sound: EQ sucks, only use it when you have to. But do hi-pass-filter everything everything everything. No loose mud or sub frequencies at all. This lets drums have way more impact and the groove felt in the belly is much tighter and defined.
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RebelRebel

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #19 on: April 21, 2008, 04:18:05 PM »
http://www.independentrecording.net/irn/resources/freqchart/main_display.htm



http://img.photobucket.com/albums/v280/steveydevey/forum/eqinstrumentrange10.gif

http://www.cco.caltech.edu/~boyk/spectra/spectra.htm


I use a EQ technique I like to call "moving my microphones"  :P

This is an awesome piece of advice when you're not stealthing. I'm a stealth taper.
Hey, EQ is not a bad word if you know what you're doing. ;)

a good thing to do (if you are using omnidirectional mics) to determine where you will get the best sound pickup is to cover one ear and walk around the venue to see where it sounds best to the one ear..when you find the spot, put the mics where your head was when you found it. same thing with cardiods, but dont cover the ear completely , but cup one ear so that you are blocking sound from the rear(sort of like cardiods do)


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Offline Ben Turnbull

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #21 on: May 06, 2008, 07:09:03 PM »
Great coverage guys, thanks. +Ts all round.  I try not to ever get too carried away with the EQ that I've done.  Rarely get above 3db with gentle bumps via parametric EQ tools in Audition. 

Why I do it is probably more a matter of what I like in my music and a function of my mics inability or deficiency to reproduce the frequencies I find needing attention.  Some of it could also be a function of the age of my ears, but for the most part my mics need a slight lift in the high end.  I also bump the bottom in an attempt to bring out the double bass in some small jazz settings.  I do this for me mostly and if what I've done "gets out" it's at least documented. 

The nugget I pulled from the above is that EQ should be applied post compression.  I don't do a lot of compression, but I have found that it helps firm up a small jazz trio when I run 6' from the kit.  The dynamic range can run from -40 to the odd clip so I've found that a little soft knee tends to bring up the low levels and dampen the sharp spike of the drums.  I'm learning.   :P  I just wish I knew if what I'm doing instinctively is at all valid technically.
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Offline morst

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #22 on: May 08, 2008, 04:28:40 AM »
The nugget I pulled from the above is that EQ should be applied post compression.  I don't do a lot of compression, but I have found that it helps firm up a small jazz trio when I run 6' from the kit.  The dynamic range can run from -40 to the odd clip so I've found that a little soft knee tends to bring up the low levels and dampen the sharp spike of the drums.  I'm learning.   :P  I just wish I knew if what I'm doing instinctively is at all valid technically.
If it improves the sound then it's valid! People don't listen with meters, they listen with ears!

I would agree that using EQ to boost a recording should probably done after dynamics adjustments like compression or limiting. I believe someone mentioned that boosting frequencies before compression can cause the comp to trigger, which can be a great boon when mixing live sound, but can interfere with the natural sound of recordings. If you are cutting frequencies on a recording, it's fine to do that before the dynamics processing.

Another thing I have noticed with EQ is that cheap equalizers don't sound so good when you boost, but some of the really nice (usually $$$) ones sound just fine. I recall from live mixing that the eq units which induce less phase shift are more prone to sound good when you need them to boost frequencies.

Also, perhaps someone can refresh my memory regarding steepness of curves and phase inversion. I think I remember that rolloff curves of +-12 or 24 dB/octave do not invert, but curves of +-6 or 18 do? Been too long since I sat down with
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Offline Ben Turnbull

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #23 on: May 08, 2008, 06:34:10 PM »
+T Morst, I'll do some testing next time I find myself running at the Clown. ;D
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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #24 on: May 08, 2008, 06:36:18 PM »
The nugget I pulled from the above is that EQ should be applied post compression.  I don't do a lot of compression, but I have found that it helps firm up a small jazz trio when I run 6' from the kit.  The dynamic range can run from -40 to the odd clip so I've found that a little soft knee tends to bring up the low levels and dampen the sharp spike of the drums.  I'm learning.   :P  I just wish I knew if what I'm doing instinctively is at all valid technically.
If it improves the sound then it's valid! People don't listen with meters, they listen with ears!

I would agree that using EQ to boost a recording should probably done after dynamics adjustments like compression or limiting. I believe someone mentioned that boosting frequencies before compression can cause the comp to trigger, which can be a great boon when mixing live sound, but can interfere with the natural sound of recordings. If you are cutting frequencies on a recording, it's fine to do that before the dynamics processing.

Another thing I have noticed with EQ is that cheap equalizers don't sound so good when you boost, but some of the really nice (usually $$$) ones sound just fine. I recall from live mixing that the eq units which induce less phase shift are more prone to sound good when you need them to boost frequencies.

Also, perhaps someone can refresh my memory regarding steepness of curves and phase inversion. I think I remember that rolloff curves of +-12 or 24 dB/octave do not invert, but curves of +-6 or 18 do? Been too long since I sat down with
The Yamaha Sound Reinforcement Handbook!


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Offline morst

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Re: "Subtractive EQ" vs. "Additive EQ"?
« Reply #25 on: May 08, 2008, 10:40:59 PM »
« Last Edit: May 08, 2008, 11:08:26 PM by morst »
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Re: "Subtractive EQ" vs. "Additive EQ"?
« Reply #26 on: May 09, 2008, 12:15:18 AM »
http://taperssection.com/index.php/topic,58384.msg893445.html#msg893445
Holy Crap Teddy! Thanks for the map to the research department!!! +t!!!  :o

you didnt know that was there//? man, that things been there for like a year.  ;D

Offline morst

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Re: "Subtractive EQ" vs. "Additive EQ"?
« Reply #27 on: May 09, 2008, 01:06:46 AM »
http://taperssection.com/index.php/topic,58384.msg893445.html#msg893445
Holy Crap Teddy! Thanks for the map to the research department!!! +t!!!  :o

you didnt know that was there//? man, that things been there for like a year.  ;D
Naw, this site is rather comprehensive, what with the million plus posts or whatever!!   :yikes:

Hard to find everything without the occasional tour guide!!   8)
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Offline Jammin72

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Re: "Subractive EQ" vs. "Additive EQ"?
« Reply #28 on: May 14, 2008, 10:40:56 AM »
I used Subtractive EQ on this one...

In this case for most systems it's definitely an improvement.

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