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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, 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. "Off-boards" are used by musicians who prefer multiple stomp-boxes; these may be simply pieces of plywood with several stomp-box units fastened to the plywood and connected in series. Rackmounted 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, musicians can play any instrument through a 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

Dynamics

Compressor: The gain of the amplifier is varied to reduce the dynamic range of the signal.
Tremolo: Tremolo produces a periodic variation in the amplitude (volume) of the note. i.e. A sine wave applied as input to a voltage-controlled amplifier produces this effect.

Tone

Overdrive and distortion: The signal is amplified past the limits of the amplifier, resulting in clipping. Example: Guitar on "Spirit in the Sky" by Norman Greenbaum
Wah-wah pedal: An effect that gives the instrument an almost vocal effect, familiar as the wah-wah pedal. Example: "White Room" by Cream, used by Eric Clapton. Popular in funk and psychedelic rock (i.e. Jimi Hendrix).
Ring modulation: "Organic" effect.
Equalizer: 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": Amplifies some aspect of the instrument's signal output. 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, Big Block Effects NOS Boost.
Talk Box: A vowel-tuned wah that actually takes a human voice as the wah control. Used in many Bon Jovi songs

Time-based

Delay: 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.)
Echo: Uses delays to simulate an echo.
Chorus: Usually short delays to simulate more than one person playing at a time
Flanging: Uses very short variable delays to cause a changing comb filter effect
Reverb: Simulates echoes in stadiums, halls, other performance areas. Even actual surfaces, such as plate metal and metal springs, are sometimes simulated.

Frequency

Pitch shifter: 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: Vibrato refers to a variation in frequency of a note, for example as an opera singer holding one note for a long time will varies 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

Defretter: It simulates a fretless guitar
Acoustic guitar simulator: Simulates an acoustic guitar. Example: Boss AC-2
Rotary speaker: A Leslie speaker simulation effect. One particular effect of this type (the Uni-Vibe) was made famous by Jimi Hendrix.
Envelope Follower: Uses the signal amplitude envelope to control one or more effects. Example: ToadWorks Enveloope
Pickup simulation: Simulates either a single-coil pickup if the musician has a humbucker or vice-versa.
Ambiance modelling: Creates an ambiance through an amalgam of effects.
Guitar amplifier modelling: Models instrument tone to imitate the tone produced by various amplifiers, especially to attain the valve sound with solid-state equipment.

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.

Notable manufacturers

Behringer (homepage)
BOSS (homepage)
Digitech (homepage)
DOD (homepage)
Jim Dunlop (homepage)
Electro Harmonix (homepage)
Korg (homepage)
Lexicon(homepage)
Line6 (homepage)
Roland (homepage)
TC Electronic (homepage)
Vox (homepage)
Zoom (homepage)

Boutique Pedal Manufacturers

AnalogMan (homepage)
Big Block Effects (homepage)
catalinbread (homepage)
Effector 13 (homepage)
Empress Effects (homepage)
Frantone (homepage)
Fulltone (homepage)
Metasonix (homepage)
Robert Keeley (homepage)
Smart People Factory (homepage)
ToadWorks USA (homepage)
ZVEX Effects (homepage)

Boutique Manufacturers: Homemade and designed pedals.

External links

Big Muff & Guitar Effects Museum.

Music Sound

Effects unit

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