Saturday, June 23, 2012

SNR 101 - EQ

I've been wanting to do a series of posts on some of the basics of sound. As more are added you can find them on the Topics page under Sound 101

EQ. It stands for equalization but it's really anything but. Originally designed to even out frequency response of early recordings it's now used in highly specialized ways in every step of the mixing process. At it's most basic it's as simple as a tone knob on an instrument, amp, or old radio. Turn one way and you get more bass, the other way is the treble. Slightly more advanced are the bass and treble knobs on modern radios that allow you to boost or cut. It's really that simple, right on up to thirty-one band graphic EQs and parametrics. But we'll get to those later.

An EQ is just a filter. The most basic design is a fixed one that uses a resistor and capacitor. Depending on their values and what arrangement you hook them up in, you can create a filter that cuts out either the lows or the highs. Like a lot of things in audio they are labeled the opposite of how you might think. A filter that cuts out the lows is called a "high pass" (it blocks lows and allows highs to pass) and one that cuts the highs is called a "low pass". If you design one of each carefully it becomes a crossover. That's a device you use to send lower frequencies to a woofer and the higher ones to a tweeter. Things get complicated pretty fast so we'll leave it at that for now.

If you go a step further and make one of the elements of a filter adjustable you start to get something more like the tone controls and EQs that we're used to seeing. In fact, it's actually quite a bit more complicated. Passive devices like capacitors and resistors can only cut, it takes an active circuit to provide boost. We'll save that for another lesson too. But the point to get here is that no matter how big and complicated an EQ is, it's just an adjustable filter.

Before we go much farther I want to touch on phase issues. Some engineers bag on one model or another and say that it's a "phase nightmare" or will warn against using too much EQ on something because it creates "phase issues".  EQs do create phase issues, that's how they work, but they don't create phase issues at the speaker unless they are otherwise poorly designed.  The signal going into an EQ is split up into sections or bands of frequencies and each one is acted on by one or multiple controls. Those controls set up a phase difference in each frequency range that when combined with unaltered signal at the output cause that frequency to be partially canceled out. The signal leaving the box doesn't have any phase issues though and if there are any in the listening area it's much more likely that they come from cable or speaker issues, or aren't phase issues at all but something else that's hurting the sound.

So that's a basic look at how EQs work, here's a list of some different types of EQ and some of their uses:
  • Graphic - This is the type you see in racks by a mixer or maybe in an amp rack. Each band of frequencies is fixed and the level is run up and down by a little fader. When you're all done adjusting you get a "graphic" representation (sometimes called a "curve") that shows you approximately what you've done to the sound. I say approximate because the filters are usually notch shaped and may vary in width. A traditional graphic EQ with 31 bands would be called a 1/3 octave unit because each fader represents that much sound. But the only position that the filter is actually that wide is when it's all the way down, the curve is much wider when a shallower setting is used. Constant Q units maintain the notch shape and are better suited to cutting out frequencies that are feeding back in a system. They are more common these days but you may see a traditional design in a front of house rack for more gentle shaping of the sound.
  • Parametric - This type is named so because it gives you control over more parameters of the filter. These are what you see on the channel strips of a console although they also exist in rack mount units. The controls include a level knob to control cut or boost, a frequency knob (sometimes called a sweep because you can sweep through the frequencies), and a Q or width knob that controls how wide the filter is. Some mixers have just the level and frequency controls and these are not fully parametric so they are often called "sweep" EQs. You have very precise control with a parametric and a lot of consoles will actually overlap the frequencies that adjacent sections have. Parametric plugins and digital consoles will often show you the curve as you are working on it. Sitting with an EQ plugin is a great way to learn.
  • Notch - Just like it sounds, this term applies to any EQ with a narrow filter width. Think of this as a surgical tool to cut out frequencies that are feeding back and leave the rest intact. 
  • Shelf - Often the very low and very high frequencies on a parametric EQ are this type. Instead of having a width, they extend from the frequency you set them at, all the way up or down (depending) right out of hearing range. For example, a low shelf might be set at 80 Hz or have a sweep control that lets you set it where you want, but it affects the sound from that frequency right on down to zero. Some consoles and many plugins let you switch your low and high shelves to the peaking type so they will act the same as the mid bands on a parametric EQ.
  • Low Cut - A common feature on many consoles, this will often be a fixed filter but may be sweepable. When active it cuts out all low frequencies. It's useful to take stage rumble and popped Ps out of a vocal channel. It's a good idea to activate it on any channel that really doesn't need the lows coming through. Often the kick drum and bass will be the only channels of a mix with the low cut not active.
  • High Cut - Not as common but acts just the same as the low cut but at the upper end of the range.
  • Band Pass - This is a lot like a crossover, but instead of separating one signal into two ranges, it engages a high and low cut together to let just the mids through. Usually fully adjustable it allows to you zero in on a particular range of frequencies. It's useful for inserting dynamic processing or effects on just one range of frequencies on a channel. It can also be used to clean up something like an old tape recording, removing the hiss and the rumble.
EQ is a big topic and I'm not out to write the book on it here, just give sort of an introduction and hopefully get you interested enough to go off and do some more reading. So get to it Brethren of the Knob and Fader. Feel free to add on to the topic in the comments section.

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