Distortion may bean undesired change in an audio waveform that creates unpleasant sounds or it may be the intentional alteration of a sound signal through electronic amplification, "overdriving" it beyond the system's ability to reproduce the waveform faithfully. Distortion is applied usually and most notably to guitars.
In a typical sustained musical tone, most of the component frequencies are harmonic (related to each other by simple ratios, and thus mutually reinforcing), while others are inharmonic (especially in sounds produced by plucking or striking).
Extreme amplification can lead to clipping of the original signal, which makes the inharmonic partials stronger in proportion to the harmonic ones. Furthermore, if two or more notes are played simultaneously, they give rise to an audible difference tones that further complicate the waveform (e.g., notes at C = 262 Hz and F# = 370 Hz will create a third note at (370 - 262) = 108 Hz = slightly flat A, inharmonic in relation to both original frequencies). High levels of overdrive thus interestingly blur the boundary between timbre and harmony.
Creating distortion effect
An electric guitar is capable of a sustained output of 50 millivolts or more: a signal of this level may overload the amplifier. Guitar amplifiers are specially designed to cope with sustained overload conditions applied to their inputs. Nevertheless, this overload can cause distortion in the driver or power stages of the amplifier when the guitar is played at high volumes. The sound quality of an overdriven valve amplifier is the prime reason why guitarists over the years have chosen to use them.
If the first valve cannot be overdriven by the guitar alone, a pre-amplifier can be used to increase the voltage of the guitar output. This can be built into the guitar, or be in the form of foot pedals referred to as "overdrivers" or "boosters". The amount of gain varies; some can achieve such high levels of gain that part of the unit itself is driven into distortion. This is also the basis on which many fuzz boxes and other distortion devices work. The majority of fuzz boxes are activated by an electronic trigger, usually a Schmtt trigger. The generates a square wave output at the same frequency as the input from the guitar. The output of the fuzz box is a mix of amplified guitar and the square wave. The balance of undistorted signal to square wave is set by a control usually labelled "distortion" or "overdrive".
Throughout the years, manufacturers have made strenuous attempts to evolve fuzz boxes or, more recently, digital sound processors, that simulate an overdriven valve amplifier. However, invariably that is exactly what they sound like: simulations and not quite the real thing.
When solid state circuitry is overloaded, the waveform "clips" at a certain signal voltage depending on design criteria. Once the voltage of the input signal causes clipping to occur, any further increase of input signal will not cause a change in the level of output signal. This phenomenon is utilized when producing sustain units. The unit gives its maximum output as soon as the guitar output rises above a very few millivolts, thus completely eliminating the normal attack ad decay characteristics of a guitar note. Electronic trigger circuits producing square waves have the same characteristic since the output level of the square wave from the trigger is virtually independent of the input signal level.
A simple and economic solid state pre-amp can be used to boost the level of the signal from a guitar so that it drives a valve amplifier's input stage into smooth distortion and then, if the gain factor of the pre-amp is increased, to create heavy "crunch" and "thrash" distortion sounds. More expensive valve powered pre-amps can offer a variety of subtle, smooth valve sounds.