Effects units are devices that affect the sound of an electric instrument when plugged in to the electrical signal path the instrument sends, most often an electric guitar, or bass guitar. They can also be used on other instruments or sound sources, like the Rhodes Piano or standard MIDI keyboards, synths or even the human voice. While some effect units transform the sound completely, others just color the sound picture in a minor way.
An effects unit consists of one or more electronic devices which typically contain analog circuitry for processing audio signals, similar to that found in music synthesizers, for example active and passive filters, envelope followers, voltage-controlled oscillators, or digital delays.
Effects units are packaged by their manufacturers, and used by musicians, in various sizes, the most common of which are the stomp-box and the rack-mount unit. A "Stomp box" is a metal box, containing the circuitry, which is placed on the floor in front of the musician and connected in line with, say, the guitar cord. The box is typically controlled by one or more foot-pedal on-off switches and typically contains only one or two effects. A second type of effects unit may contain the identical electronic circuit, but is mounted in a standard 19" equipment rack. Usually, however, rack-mount effects units contain several different types of effects. They are typically controlled by knobs or switches on the front panel, and often by a MIDI digital control interface. Musicians who prefer multiple stomp-boxes use “Off-boards”; these may be simply pieces of plywood with several stomp-box units fastened to the plywood and connected in series. Rack Mounted effects or off-boards can combine several effects in one unit, and can include analog controls such as pedals or knobs.
Modern desktop and notebook computers often have sound processing capabilities that rival commercially available effects boxes. Some can process sound through VST-plugins. With a decent sound card, you could play any instrument through the computer, emulating any effects unit or even an amplifier in a convincing way. Many VST-plugins are freely downloadable from the World Wide Web.
Types of Effects
The gain of the amplifier is varied to reduce the dynamic range of the signal.
Tremolo produces a periodic variation in the amplitude (volume) of the note. A sine wave applied as input to a voltage-controlled amplifier produces this effect.
Overdrive and Distortion
The signal is cranked up past the limits of the amplifier, resulting in clipping. Example: Guitar on Spirit in the Sky by Norman Greenbaum.
An effect that gives the guitar an almost vocal effect. Example: "White Room" by Cream, used by Eric Clapton. Primarily invented for the Organ Music, but then somehow found by Jimi Hendrix and Eric Clapton and used for their personal gain.
"Organic" effect. Makes weird noises.
Adjusts the frequency response in a number of different bands of EQ. Variants include the Parametric EQ, which instead of flatly boosting and cutting frequencies, curves the frequency response to include changes in adjacent frequencies. Examples: Boss PN-2 and GE-2
Clean Boost or any other "booster"
Takes your guitar signal, kicks it up a notch, and then sends it on its merry way. Generally used for preventing signal loss through long chains of effects units (pedals) and getting overdrive tones out of a tube amp. On stage, used for volume boosts for solos. Examples: Zachary Vex's Super Hard On, catalinbread's Super Chile Picoso.
Heil Talk Box
A vowel-tuned wah that actually takes your voice as the wah control.Time-based
First used by Les Paul, e.g. I'm Forever Blowing Bubbles. (Modern digital delay units, the first of which was the Eventide Harmonizer, involve sound waves being converted from analog to digital signals, and clocked through large banks of RAM memory. Paul achieved time delay by stretching audiotape between two reel-to-reel tape decks spaced several feet apart.)
Uses delays to simulate an echo
Usually short delays to simulate more than one person playing at a time
Uses very short variable delays to cause a changing comb filter effect
Simulates echoes in stadiums, halls, other performance areas. Even actual surfaces, such as plate metal and metal springs, are sometimes simulated.
Also introduced by the Harmonizer, which has a knob on the front to "change your pitch up." First used on Itchycoo Park by Small Faces.
Vibrato refers to a variation in frequency of a note, for example as an opera singer holding one note for a long time will vary the frequency up and down. A sine wave applied as input to a voltage-controlled oscillator produces this effect.
Guitarists often use the terms "vibrato" and "tremolo" inconsistently. A so-called vibrato unit in a guitar amplifier actually produces tremolo, while a tremolo arm on a guitar produces vibrato. However, finger vibrato is genuine vibrato. See Electric guitar, tremolo, vibrato.
Other specific effects
It simulates a fretless guitar
Acoustic Guitar Simulator
Simulates an acoustic guitar. Example: Boss AC-2
A Leslie speaker simulation effect. One particular effect of this type was made famous by Jimi Hendrix.
Simulates either a single coil pickup if the musician has a humbucker orr vice-versa.
Creates an ambience through some amalgam of effects.
Models your tone to act like its coming out of a set of old greenbacks in a vintage AC30 cabinet, or most other examples you can think of.
Guitar amplifier Modeling
Models your tone to sound like its going through a 5150 or some other ridiculously expensive amplifier.
These types of effects are usually digital, and can therefore be found as features of effect processors such as the Boss ME series and Vox multieffects.