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.