Effects Descriptions

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Effects Descriptions

http://www.eden.com/~keen

R.G. Keen keen@eden.com

AMPLITUDE BASED EFFECTS

Volume control

Manual level control. Twist the knob, the sound gets louder or softer. Examples : Morley and DeArmond Volume pedals ('70s)

Tremolo

Cyclical variation of volume by a low frequency oscillator of some sort; parameters are waveform of the LFO, LFO frequency, and depth of modulation; note that while the terms tremolo and vibrato are often used interchangeably, tremolo is actually variation in loudness, vibrato is variation in pitch or frequency. Examples : Diaz tremodillo (new), Boss PN-2 Pan tremolo (1989)

Auto tremolo

tremolo where the modulation frequency is varied by some feature of the input signal, generally amplitude.

Panning/ping-pong

generalization of tremolo to more than one channel; as one channel goes down in level, another goes up. With non-square LFO waveforms, gives the effect of the sound source moving from place to place in stereo or more channel setups. Examples : Boss PN-2 Pan tremolo (1989), Ibanez Flying Pan (1976?)

Gating/repeat percussion

tremolo with 100% modulation of the signal by a square wave. With exponentially decaying waveforms (guitar is a good one), gives the effect of striking the same note again at decreasing levels. Some Thomas Organ Vox amps have this as a built in effect. Examples : Vox Repeat percussion ('70), Walco "sound go-round" ('75?)

Compression

makes soft inputs louder, and loud ones softer, giving a one-level kind of sound with lessened dymanics. This is effectively volume control with the level determined by the negation of the averaged envelope of the input level. Early compressors were often called "Sustain" pedals. The Electro-Harmonix Black Finger was among the first compressors in the early 1970s, along with the Maestro sustainer. Examples : MXR Dynacomp ('74 - present), Boss CS-1 to 3 ('78 - present)

Expansion

Makes loud sounds louder and soft ones softer. Effectively volume control with the level determined by the averaged envelope of the input level. Compression and expansion can be complementary, as in >> com(pression/ex)panding for noise reduction.

Asymmetric compression/peak compression

Only the peaks of the input waveforms get compressed, not the overall level of the waveform envelope. Effectively, there is no averaging of the envelope and the instantaneous waveform level is compressed. This amounts to a much softer form of clipping, and is part of the tube sound, since tube with a soft B+ supply are prone to this.

Noise gating

modulates the output off when the input level is below a threshold. The modulation may be a square wave, or a variation of expansion where the low level inputs are "expanded" down into silence, which gives a less abrupt transition. Examples : Electro Harmonix silencer, MXR noise gate line driver ('70s)

Attack delay

A variation of noise gating where the transition to "on" from the "off" or no signal state is slowed. This gives an output which perceptibly rises in level with each new note envelope, reminiscent of a tape recording played backwards. Examples : Electro Harmonix Attack delay ('79), Boss SG-1 Slow gear ('80)

ADSR

Term borrowed from the synthesizer folks; stands for Attack Decay Sustain Release, which is the most general way to describe a musical envelope. It is possible to generate an artificial ADSR envelope for a musical note to help fool the ear as to which instrument generated the note. Example : Electro Harmonix Micro Synth (sort of) ('82)

Limiting

Like compression, but operates on signals over some threshold only. Well suited to keep an input from going over some level, but un-processed below that level, as in getting signals on tape without overloading the tape. Example : MXR Limiter ('80)

Auto swell

generally, a rise in level from some starting level to a final level when keyed manually or electronically. Can effectively add sustain to some notes and not others when keyed manually, or can add a "swell" in volume over a run of notes, or can help with presetting the level of a lead.

WAVEFORM DISTORTION EFFECTS

Symmetrical clipping

For a given input waveform, say a sine wave, the tops and bottoms of the waveform are clipped equally, symmetrically. Although the musical implications are more involved than this simplistic explanation, for a simple sine wave, symmetrical clipping generates only odd-order harmonics, giving a reedy, or raspy sound to the resultant waveform.
The hardness or softness of the clipping matters. Hard clipping results when the output wave equals the input up/down to a certain level, then stays at the clipping level until the input drops below the clipping level again, giving perfectly flat tops and bottoms to the clipped output. Soft clipping has no abrupt clipping level, but gently rounds the top/bottom of the output wave so the waveform is "softly" rounded on top/bottom, not flat-topped. Some solid state devices actually flat top, then invert, producing a hollow topped output waveform at hard clipping. There is a continuum of clipping hardness, depending on the circuitry used to clip. Soft clipping emphasizes the lower- order harmonics, the third and fifth, etc. Hard clipping has a mix slewed to the higher order seventh and up harmonics, which are harsher sounding.

Intermodulation distortion, the production of sum and difference frequencies from frequencies in the input waveform, varies with the amount and hardness of clipping. Intermodulation sounds harsh and ugly. The amount of intermodulation is a characteristic of the circuit that produces the distortion.

Asymmetrical clipping

The top(or bottom) of the waveform is clipped more than the bottom (top) half. This causes the generation of both even and odd harmonics, in contrast to symmetrical clipping's odd-order only. The even harmonics are smoother and more musical sounding, not as harsh as the odd ones. The hardness of the clipping and the degree of asymmetry affect the sound. The more asymmetrical, the more pronounced the even-order harmonics; the harsher the clipping, the more the harmonics are slewed toward higher order. See Half Wave Rectification.
Tubes in general produce asymmetrical distortion unless the circuitry is set up to remove them, as happens in push-pull.

The comments on intermodulation apply here.

Infinite limiting

In essence, the waveform is amplified "infinitely" and hard and symmetrically clipped, producing a rectangular output waveform which shares only the zero crossings with the input waveform. Sounds buzzy and synthesizer-ish.
Examples : Mosrite Fuzz-rite ('68), Early Maestro Fuzz tones ('64-'70s)

Half wave rectification

Half wave rectification represents the logical conclusion of asymmetrical clipping. One half of the waveform is flat, the other half is unchanged. This produces a prominent second harmonic, heard as an octave. There are analog devices which produce an octave effect injust this way; I have heard that the "Octavia" effect is based on this.
Examples : Foxx tone machine, Tycobrahe and R.M. octavias, Super Fuzz. ('70s)

Full wave rectification

In full wave rectification, one half of the input waveform is "folded" to the opposite polarity, producing an output with a net DC component, none of the original fundamental frequency of the input waveform, and only the second and higher harmonics of the original input frequency. Produces very strong octave of the input waveform, as well as a slew of even-, odd-, and intermod- distortion products when more than a single frequency is the input ( as is the case for all musical instruments).

Arbitrary waveform generation

This effect generates a completely new waveform of arbitrary shape which shares the same frequency as the input waveform. Guitar synthesizers do a version of this.

FILTER/FREQUENCY RESPONSE EFFECTS

EQ/tone controls

Allow you to cut or boost the highs, lows, mids etc. Tend to be broad-brush kinds of controls - all the "high's" get raised or cut. Range is typically +/- 12 to 20 db boost/cut.
Example : the tone control on your guitar.

Treble/mid/bass boost

like an additional eq control, but tends to be narrower in frequency range, and perhaps more boost range, no cut. Electro Harmonix made a range of boosters starting in the early 70's to boost treble (screaming tree), Bass (Mole) or everything (linear power booster). Very simple, similar circuits were used.
Examples : Dan Armstrong purple peaker, Vox treble booster. ('72)

Cabinet simulation

A filter network designed to mimic the two- or four-pole low frequency rolloff of a guitar speaker cabinet, usually to get that "miked cabinet" sound into a PA without really miking a cabinet.
Examples : Tech 21 Sans Amp (new)

Resonator

a filter with a boost in frequency at a narrow range of frequencies. This sounds like a wah pedal when the pedal is not being moved.
Example : Boss SP-1 Spectrum ('79)

Wah

A resonator that can have its center frequency moved up or down in frequency by moving a pedal. The "wah" name comes from the way it mimics the moving resonance of the human vocal tract in speech as the sound "wah" is made. The first wah wah was the Vox Clyde McCoy, which was originally designed to emulate the sound of trumpet players using a mute(!).
Example : Crybaby ('60s to present)

Auto wah or "Envelope Follower"

A wah filter where the center frequency is determined by the loudness of the input signal, making a moving resonance on every note. The Mutron-III was about the first envelope effect. It was made in the mid 1970s, and is still thought to be the best ever made by many players.
Example : Electro Harmonix Doctor Q ('76), MXR envelope filter ('76)

Tremolo-wah

Wah where the center frequency is moved back and forth cyclically, as though the pedal was connected to a motor or some such. This can generate effects similar to a rotating speaker or phasing.
Example : Boss Auto Wah AW-2 (80s to present)

"Vibrato"

a cyclical variation in the basic frequency of the input signal, similar to the effect of moving the whammy bar on a suitably equipped guitar. True vibrato as an add on effect requires some kind of time delay, and was hard to do until analog (and now digital) delays came to be. The Uni-vibe, (trictly speaking a phase shifter) dsigned in about 1967, had a vibrato setting. However it is more famous fo it's "chorus" setting.
Examples : Boss CE-1 Chorus Ensemble ('77)

Phase shifting

This effect is a filter response generated by using long phase delays and mixing with the original signal to cause a number of deep notches and/or peaks in the overal filter response. This mimics the larger number of notches and peaks caused by true time delayed flanging. Most simple phase shifters or phasers do this by generating two notches, although some pedals make four notches. Flangers may make many notches. Phasers may also incorporate feedback to sharpen up the effect of the notches.
About the first phaser was the Maestro PS-1 designed by Oberheim in about 1970. The MXR Phase-90 and very inexpensive but good sounding Electro Harmonix "Small Stone" were used on too many pop songs in the 1970s.

Examples : Boss PH-2 Super phaser (new), MXR phase-45 90 and 100 ('70s)

TIME DELAY EFFECTS

Echo

Analog (MXR Analog Delay, EH Memory Man) or Digital delays (Boss DD-3)

Reverb

Spring reverebs (Fender) or Digital/Analog reverb pedals (Boss RV-3)

True vibrato

Boss VB-2 ('80s), Electro Harmonix clone Theory ('78)

Flanging

A/DA flanger ('76), MXR Flanger ('78), EH Electric Mistress ('76)

Chorus/ADT

Boss Chorus Ensemble CE-1 ('75), MXR stereo Chorus ('78)

Slapback

EH full double tracking ('80), any delay pedal set to short delay time.

Reverse echo/reverb

?

Sampling

Boss DD-3 (new), DOD PDS-8000 (8 secs sampling, '89) and DFX91 (1 sec, new)

OTHER MISCELLANEOUS EFFECTS

Octave division

Takes the fundamental frequency of an input signal, divides it by two, and creates an octave-lower, sometimes a two-octave lower signal, which are usually mixed back with the original signal. This is most often done with digital logic flipflops to divide the signal by two/four after squaring up the input to drive the flipflops. This provides outputs that are substantially square waves, sounds like fuzz bass.
Some kind of filtering is usually provided to tame the sharp buzz of the square waves. The simple dividers like this get very confused when fed more than one tone at once, so single note runs are all that is really practical - unless you like confused effects.

About the first lower octaver was the Maestro OB-1 octaver, which tracked quite poorly, but the note was not too fuzzy. The mid 1970s Electro Harmonix "octave multiplexor" (1 octave down) and MXR "Blue box" (2 octaves down) were more successful but the lower octave is pure fuzz.

Harmony generation

Generation of other notes at musically-interesting intervals along with your notes. The classic device to do this is the Eventide Harmonizer. It is very difficult to do this electronically so that the effect produces musically-useful sounds consistently, hence Eventide's high price.
There is a new (1996) Boss pedal harmonizer now available which allows selecting the key in which you are playing, and calculates and plays the desired harmonic (3rd, 5th, etc). It is not cheap but several times cheaper than an Eventide.

Phase lock tracking

An electronic circuit called a "phase locked loop" can produce an output signal that is exactly an integer multiple or small-numbers fractions of a reference signal in frequency. You can generate: a signal that follows your notes, perhaps lagging a little with a glide onto the note an octave or two above a third/fifth/seventh, etc. above or below your notes.
Sounds kind of like a computer playing harmony with you.

The outputs are usually square wave or filtered square wave, and sound kind of synthesizer-y. Modern all-digital MIDI-fied effects do something like this in their computer processors, and may not be as limited in output waveform.

Noise addition

Noise (hiss, rumble, etc) is deliberately added to the input signal. If this is done with restraint and matching the input signal envelope, it can add a breathing effect like the hiss of air in a flute.
Filtered low frequencies can add a growling quality.

Talk box

This effect is produced by using a small amp to produce sound that is conducted into your mouth by a tube, so you can mouth the words to a song, using your vocal tract resonances to shape the instrument sound, which is then picked up by a microphone. This is the archetypical "talking guitar".
One of the first talk boxes was "The Bag" by Kustom Electronics (not the amp company). This came in a psychodelic upholstered wine flask shaped bag, and was used by Jeff Beck. Peter Frampton popularized the talk box in the mid 1970s with "Do you feel like I do".

Examples : Heil sound talk box (new), Electro Harmonix Golden Throat ('70s)

Voice tracking (vocoder)

Ring modulation (Double Side Band Suppressed Carrier generation) Single Side Band Suppressed Carrier generation
Example : Roland vocoder

COMMON COMBINATIONS

Leslie

vibrato, tremolo, varying filtering generated by rotating speaker. There are also several "leslie simulator" effects such as the new Korg unit, and 1970's Multivox "Full Rotor" and "Little David".
Example : Maestro Rover ('75), RMI Roto Phaser ('76)

Aphex

filtering, selective frequency band distortion

Enhancers

Split the signal into a few bands, slightly distort some, remix.

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