Editor’s Note: Dennis originally wrote this article in 1986 and was last revised in 1998, yet the information is as relevant now as it was then.
John Roberts is one of my heroes. John wrote a regular column for the now defunct magazine Recording Engineer/Producer (RE/P) entitled “Exposing Audio Mythology.”
“Laying to Rest…or at least exposing the false premises upon which they are based…some of the pro audio industry’s more obvious ‘Old Wives Tales’ “—such was the opening for John’s first column. Great stuff, you could almost hear the theme music and see the masked rider off in the distance.
He originally intended to do a few columns on the most flagrant abuses, that was in early 1983. He continued until mid-1986. Every issue, without fail, he waged war on the myth-sayers. John is resting now.
Myth exposing is too much for one person. I’m arrogant enough, and angry enough, to help out. So I thought I would expose some of the most popular myths regarding equalizers.
MYTH #1: There exists such a thing as a combining filter.
Many contractors are very confused over just what a combining filter is. So am I. Filter designers have many names for different types of filters: Butterworth, Chebyshev, Bessel, etc., but combining isn’t one of them.
The problem here is with the use of the word filter. We must distinguish between what is being thought and what is being said. Within the context of using this phrase lies the real intent, i.e., how much ripple exists in the output.
The outputs of filter banks combine (or actually, re-combine) to form a resultant curve characterized by an overall shape and a ripple content with associated phase shift. How this combining takes place and the bandwidth of the individual filters dictates the amount of ripple. The type of filter used has nothing to do with it.
Combining is done by electronically summing together all of the filter outputs. It is not a filter at all: it is a means of summing individual filter’s outputs. All equalizers combine their filter outputs. It is wrong to say an equalizer is non-combining. The only examples of non-combining filters are real time analyzers and crossovers.
An example of the misuse of this term concerns comparison between constant-Q and conventional graphic equalizers. (Conventional, as used here, refers to any graphic equalizer that is not constant-Q.)
The popular, albeit false, belief is that conventional equalizers use combining filters, while constant-Q designs use non-combining filters. Both designs sum their outputs together. The difference lies in the smoothness of the combined curves. The fallacy lies in taking the answer out of context.
Setting a conventional equalizer to have the same bandwidth as a constant-Q design produces a combined result exactly the same if the number of summers is the same. However, the only condition where this occurs is either full boost or full cut.
Most users do not understand this is the only position where the affected bandwidth is one-third octave wide (for one-third designs). At all other boost/cut settings, the bandwidth degrades to over one octave wide.
There is no doubt that if two adjacent filters located one-third octave apart degrade to where each is one octave wide, then the summed result will be very smooth. There is also no doubt that this is no longer a one-third octave equalizer. It now acts as an octave equalizer.
If that is what is required, then a conventional equalizer is the correct choice; however, if one-third octave control is required, then only a constant-Q design will do.
MYTH #2: Minimum Phase behavior is an important criteria when buying an equalizer.
Minimum phase is one of the few things you don’t have to worry about when buying an equalizer. It’s not that it isn’t important, it is. It’s just that no known examples of commercial equalizers that are not minimum phase even exist. None. Forget all the marketing hype to the contrary.
A precise definition of minimum phase is a detailed mathematical concept involving positive real transfer functions, i.e., transfer functions with all zeros restricted to the left half s-plane. If the last sentence produced a zero in the middle of your brain, don’t worry. All you need to know is minimum phase behavior is not a problem in any equalizer you may consider purchasing.
Here again is an example of sloppy rhetoric. A failure to communicate clearly what is being thought. Somewhere years ago some marketing type needed a term, a buzz word if you will, for distinguishing his company’s equalizer from everybody else’s.
Some engineer dropped the term minimum phase and the marketing guy went nuts. That’s it, thought he; never mind that it doesn’t fit what is trying to be said, it sounds good. Nice and high-tech, so he used it to try to build a smoke screen between comparable products.
What they wanted to say was their product could create boost/cut curves with less phase shift than their competitors, and that this was a good thing. Problem was, here comes the engineer again to say this simply wasn’t true. Any two equalizers producing the same curve do so with exactly the same phase shift. Same universe, same physics, same results—much to marketing’s chagrin.
So they compromised on claiming their product had MINIMUM PHASE characteristics. Never mind that all the competition also had minimum phase behavior. The customer wouldn’t know that. The promotion implied that the other products didn’t. Let the buying public figure out otherwise.
Okay, now you know otherwise. Don’t be hoodwinked by this buzz word.
MYTH #3: Only one brand of equalizer exhibits complementary phase performance.
Speaking of buzz words, here’s a beaut: complementary phase shift. Somebody worked overtime on this campaign. I guess what gets me so angry about this issue is the arrogance of the manufacturer. The underlying premise is that the pro audio public is so gullible they will believe anything if presented profoundly.
Well, they are wrong. All of you are a whole lot smarter than they give you credit for. Street smarts go a long way in solving problems.
Complementary phase shift means nothing more than the equalizer displays symmetrical boost/cut curves (and is minimum phase). The boost curves are mirror images of the cut curves. That means the phase shift of the boost curves are also mirror images of the cut curves. If two things are mirror images of each other, they are complementary. Nothing too profound here.
Now, it is not true all equalizers exhibit symmetrical boost/cut curves. Therefore, not all equalizers have complementary phase shift. At least two of the more popular brands do not.
So, if you perceive this to be an important parameter when buying an equalizer, you are correct in asking whether the unit has symmetrical boost/cut curves. I can give you a list of a dozen manufacturers whose equalizers do. In truth, every example of graphic equalizer I’m familiar with has symmetrical boost/cut curves, as well as most of the parametrics on the market.
In fact, you have to look long and hard to find examples of equalizers that are not complementary phase performers. As I said, I know of two, there may be more.