This is Part 2 of how many effects are they. We described 6 possible different effect types from Amplitude,Time Delayed,Wavefrom Distortion effects and more. Now let's get to the descriptions of each type of effect.
AMPLITUDE BASED EFFECTS
- Volume control Manual level control. Twist the knob, the sound gets louder or softer.
- 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.
- 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.
- 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.
- 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.
- 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/exp)anding 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 theinput ( 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.
- Treble/mid/bass boost like an additional eq control, but tends to be narrower in frequency range, and perhaps more boost range, no cut.
- 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.
- 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.
- 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.
- 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
- 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.
- "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.
- 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.
TIME DELAY EFFECTS
- True vibrato
- Reverse echo/reverb
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.
- 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.
- 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.
- 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".
- Voice tracking (vocoder) Ring modulation (Double Side Band Suppressed Carrier generation) Single Side Band Suppressed Carrier generation
- Leslie vibrato, tremolo, varying filtering generated by rotating speaker
- Aphex filtering, selective frequency band distortion
- Enhancers Split the signal into a few bands, slightly distort some, remix.
Credits go to the following:
Mark Hammer, David Mourning, Murray Traue, Jamie Heilman, Scott Lehman