Distortion

A distortion is the alteration of the original shape (or other characteristic) of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted. In some fields, distortion is desirable, such as electric guitar (where distortion is often induced purposely with the amplifier for an apparently louder sound). The slight distortion of analog tapes and vacuum tubes is considered pleasing in certain situations. The addition of noise or other extraneous signals (hum, interference) is not considered to be distortion, though the effects of distortion are sometimes considered noise.

Electronic signals

In telecommunication and signal processing, a noise-free "system" can be characterised by a transfer function, such that the output $y(t)$ can be written as a function of the input $x$ as

y(t) = F(x(t))

When the transfer function comprises only a perfect gain constant A and perfect delay T

y(t) = A\cdot x(t-T)

the output is undistorted. Distortion occurs when the transfer function F is more complicated than this. If F is a linear function, for instance a filter whose gain and/or delay varies with frequency, then the signal will experience linear distortion. Linear distortion will not change the shape of a single sinuosoid, but will usually change the shape of a multi-tone signal.

This diagram shows the behaviour of a signal (made up of a square wave followed by a sine wave) as it is passed through various distorting functions.

The first trace (in black) shows the input. It also shows the output from a non-distorting transfer function (straight line).
A high-pass filter (green trace) will distort the shape of a square wave by reducing its low frequency components. This is the cause of the "droop" seen on the top of the pulses. This "pulse distortion" can be very significant when a train of pulses must pass through an AC-coupled (high-pass filtered) amplifier. As the sine wave contains only one frequency, its shape is unaltered.
A low-pass filter (blue trace) will round the pulses by removing the high frequency components. All systems are low pass to some extent. Note that the phase of the sine wave is different for the lowpass and the highpass cases, due to the phase distortion of the filters.
A slightly non-linear transfer function (purple), this one is gently compressing as may be typical of a tube audio amplifier, will compress the peaks of the sine wave. This will cause small amounts of low order harmonics to be generated.
A hard-clipping transfer function (red) will generate high order harmonics. Parts of the transfer function are flat, which indicates that all information about the input signal has been lost in this region.

The transfer function of an ideal amplifier, with perfect gain and delay, is only an approximation. The true behavior of the system is usually different. Nonlinearities in the transfer function of an active device (such as vacuum tubes, transistors, and operational amplifiers) are a common source of non-linear distortion; in passive components (such as a coaxial cable or optical fiber), linear distortion can be caused by inhomogeneities, reflections, and so on in the propagation path.

Amplitude distortion

Amplitude distortion is distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions.

Crossover distortion

Crossover distortion is a type of distortion in class-B push-pull amplifiers where, due to the forward voltage of the B-E junction of the output BJTs, the output signal does not follow the input until the input exceeds this forward voltage.

Frequency distortion

This form of distortion occurs when different frequencies are amplified by different amounts, mainly caused by combination of active device and components. For example, the non-uniform frequency response curve of RC-coupled cascade amplifier is an example of frequency distortion.

Phase distortion

This form of distortion mostly occurs due to the reactive component, such as capacitive reactance or inductor capacitance. Here, all the components of the input signal are not amplified with the same phase shift, hence causing some parts of the output signal to be out of phase with the rest of the output.

Group delay distortion

Can be found only in dispersive media. In a waveguide, propagation velocity varies with frequency In a filter, group delay tends to peak near the cut-off frequency, resulting in pulse distortion

Correction of distortion

As the system output is given by y(t) = F(x(t)), then if the inverse function F^-1 can be found, and used intentionally to distort either the input or the output of the system, then the distortion will be corrected.

An example of such correction is where LP/Vinyl recordings or FM audio transmissions are deliberately pre-emphasised by a linear filter, the reproducing system applies an inverse filter to make the overall system undistorted.

Correction is not possible if the inverse does not exist, for instance if the transfer function has flat spots (the inverse would map a single input point to more than one output point). Such a situation can occur when an amplifier is overdriven, resulting in clipping or slew rate distortion, when for a moment the output is determined by the characteristics of the amplifier alone, and not by the input signal.

Teletypewriter or modem signaling

In binary signaling such as FSK, distortion is the shifting of the significant instants of the signal pulses from their proper positions relative to the beginning of the start pulse. The magnitude of the distortion is expressed in percent of an ideal unit pulse length. This is sometimes called 'bias' distortion.

Audio distortion

In this context, distortion refers to any kind of deformation of a waveform, compared to an input. clipping, compression, non-linear behavior of electronic components, modulation and mixing phenomena or power supply inefficiencies can cause distortion.

Intentional distortion

In most fields, distortion is characterized as unwanted change to a signal.

Guitar sound

In the world of guitar music and guitar amplification, distortion is actively sought. In many types of music, distortion is applied to guitars and other instruments, particularly within rock and heavy metal. Guitar distortion can provide a sustaining tone for playing solos or leads, or a rough, crunchy tone suitable for rhythm guitar. This is a specific application of the above definition. There are four popular methods used to achieve intentional distortion:

from the amplifier (sometimes from the preamplifier),
from a stomp box,
an other outboard signal processor or
from realtime working software (Digital signal processing on a computer).

The earliest uses of intentional distortion that have been recorded were achieved through "doctoring" amplifiers and speakers, intentionally misusing them by removing some of their vacuum tubes or punching holes in their speaker cones. Later distortion and fuzz effects were achieved through electronics.

In fuzzboxes and solid state distortions, the signal is boosted, and the tops of the waveform clipped off. In vacuum tube distortion, or tube modelling distortion, the top of the wave form is compressed, thus giving a smoother distorted signal, that retains more of the original waveform. This is generally considered more pleasing to the ear (see tube sound). This is commonly referred to as overdrive, as it was originally (and still is) attained by driving the tubes in an amplifier a little harder than they can handle without affecting the signal.

Many solid state distortion devices attempt to emulate the valve sound of overdriven vacuum tubes.

Optics

In optics, distortion is a divergence from rectilinear projection caused by a change in magnification with increasing distance from the optical axis of an optical system. If the magnification increases with distance, it produces pincushion distortion; if it decreases with distance the effect is barrel distortion.

Map projections

In cartography, a distortion is a misrepresentation of the area or shape of a feature. The Mercator projection, for example, distorts Greenland because of its high latitude, in the sense that its shape and size are not the same as those on a globe.

References

External links

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