Quest Marketing - Lighting

T E C H T A L K

Spectral Processors:
Equalizers, Filters, Tone Controls
These processors alter the spectral content of a signal. Again, this can be a single instrument, a group of sounds, or an entire mix -- either recorded or through a PA system.

The spectral content of a signal concerns its tonal balance. In an individual note this is the relationship between the fundamental and its harmonics. To properly understand this requires a knowledge of musical tones, also known as frequencies. Human hearing ranges from about 20 hz to about 20 khz. The significant tones of all instruments lay within this range.

Tone controls alter that relationship. They allow anything from a subtle change in tone character to a dramatic alteration making it almost unrecognizable. What parameters do they change?

Gain -- amplification or reduction of certain frequencies

Center Frequency -- where the maximum alteration occurs

Bandwidth -- over how great a range is the alteration, sometimes measured in octaves, divisions of octaves (1/3, 1/6), or in "Q"

Definitions:

Tone Control -- Simple bass and treble, found on "lo-fi" systems.
Filters -- circuits that roll off either the LF or HF range, usually called shelving filters
Equalizers -- more sophisticated controls found on individual channel strips in a console. They can be 3 to 4 to 5 bands and may be fixed or variable --Always allow gain adjustments, sometimes center frequencies. Now popular are broad band "vintage" style units with large bandwidth and gentle slopes.
Graphic Equalizers -- incorrectly known as a "room EQ". Ten or more bands that alter the tonal balance with a graphic representation of faders as opposed to pots. Usually only gain is adjustable per band. Well known are 1/3 oct units, also there are 1 octave, 1/6 and rarely 1/12.
Parametric Equalizer -- gives control over all three parameters. Usually has less bands than a graphic but can give greater control. Very useful in the studio.
Notch Filter -- a type of parametric that only cuts with a very narrow bandwidth. Used for removing the "ring" of feedback modes, can sometimes be replaced with a "feedback eliminator" -- an active implementation of the same idea.
Phase Coherent EQ -- Almost all quality equalizers claim to be phase coherent or some similar wording. All analog EQs introduce phase shift -- the more alteration the more phase problems. For that reason use of EQ should be minimized where possible. An "over-EQ'd" signal loses clarity and punch. EQ should not be used in lieu of proper mike techniques. Only digital EQs can be truly phase accurate, but not all digital EQs are. And you may not want "phase accurate" digital EQ -- many people think it sounds less musical. In digital designs, a FIR (finite impulse response) EQ is a non-phase shift design, and an IIR (infinite impulse response) introduces analog style phase characteristics.

Applications?

Used almost everywhere, frequently too much.

EQ can be used instead of level adjustments. For example in a live mix it may be preferable to change the EQ of two channels to differentiate them and allow both to be intelligible without interfering as much. In other words instead of always turning a signal "up" to make it heard, it can be more useful to change its EQ to give it some "space" in a mix. This is particularly important with vocals; choosing the right mic and accentuating the proper frequencies will make them stand out instead of just trying to make it louder.

Systems can be equalized to give a sound character. Sometimes referred to as "room EQ" though that term is very misleading. You can't EQ a room, only speakers. EQ cannot correct for large acoustic deficiencies, though it can assist in optimizing a system. Useful for removing feedback or "ring" modes in lives PA.

Similar applications to dynamic processors.

John Saviano