Distortion

FIELDS OF STUDY: Acoustics

ABSTRACT: Whenever an audio or visual signal is transmitted, as in an amplifier or camera lens, the process can alter the signal so that it is not reproduced accurately. This results in distortion. Many types of distortion exist, and it is sometimes used for creative effect.

PRINCIPAL TERMS

• amplifier distortion: an imperfect reproduction of the original signal when transmitting sound electronically.
• barrel distortion: the optical distortion caused by a wide-angle lens that makes an image seem inflated at the center, as though stretched across a barrel.
• interference: the distortion of a wanted signal by an unwanted one, such as an unwanted radio station "bleeding into" another because both are broadcasting on the same wavelength.
• pincushion distortion: the optical distortion caused by a telephoto lens that makes an image seem bunched up in the center.
• quantization distortion: a distortion common in digital signal processing, such as encoding music as audio files, that results from mapping an original signal of possibly infinite values to a signal with a smaller, countable set of values.
• signal-to-noise-and-distortion (SINAD) ratio: the ratio of total signal power received to noise and distortion received, indicating how well a signal or waveform has been reproduced.
• transverse wave: a wave that displaces its medium perpendicular to the direction of energy transfer.
• warping: the digital manipulation of an image that mimics distortion and can be used to correct for distortion or to introduce various creative effects.

What Is Distortion?

No method of transmitting or reproducing a signal is perfect. Distortion occurs when a signal is altered by the process of transmission or reproduction. There are many types of distortion that affect a variety of signals, from the visual signals one’s eyes receive through a telescope to the quality of audio signals reproduced from digital storage.

In general, the term "distortion" refers to the electronic transmission and reproduction of audio signals, often music. However, it also applies in a variety of other situations. All forms of distortion can be evaluated using a signal-to-noise-and-distortion (SINAD) ratio, which compares the total amount of information received—the signal, noise, and distortions combined—to just the noise and distortions. This ratio measures how well the signal was reproduced. The SINAD ratio is similar to the more familiar signal-to-noise ratio, but it accounts for distortion as well.

Optical and Image Distortions

Optical distortions occur as a result of the way lenses gather and bend light before transmitting it to the eye. One familiar type of distortion is the "fish-eye" effect, which occurs when looking outward through a heavily convex lens, such as a peephole in a door. This apparent "inflation" of the center of an image is a type of barrel distortion, so called because the image looks as though it has been wrapped around a barrel. In barrel distortion, light waves closer to the center of the lens are magnified more than those at the edges. The inverse effect, called pincushion distortion, is caused by an extreme telephoto lens. It results in an image that appears to be compressed toward the center. A pincushion distortion occurs because light waves farther from the center are magnified more than those closer in.

Some types of digital image distortion mimic these effects. The process of purposely distorting an image through digital means is called warping. It may be used to correct for unwanted distortion or to manipulate images for creative effect.

Interference and Audio Distortion

Sound may undergo many types of distortion, depending on how it is transmitted or reproduced. Sound is transmitted as a wave through air, water, or another medium. These sound waves may bounce off surfaces, causing echoes, or they may encounter interference from other sound waves of similar frequencies.

When music is being reproduced and transmitted electronically, an amplifier is often used to increase the voltage before the signal is sent to a speaker to produce sound. The signal must be scaled up linearly, and the voltage raised proportionally, to maintain the initial waveform. Nonlinearity results in amplifier distortions,such as harmonic distortion (the clipping of wave peaks) and intermodulation distortion (nonharmonic mixing of different frequencies). The greater the amplification, the more noticeable amplifier distortion may become.

As with image warping, distortion may be used purposely in music. The distortion of electric guitars that originally resulted from faulty amplifiers has become integral to many types of music, including rock and blues. Special equipment is used to create this distortion.

Radio waves and other forms of electromagnetic radiation are transverse waves. These types of waves oscillate up and down as they move. Radio waves are used to transmit information through the air, including audiovisual signals for analog television broadcasts. If they run into other radio waves, the amplitudes of the colliding waves may combine to form a higher-amplitude wave, or they may cancel each other out. This phenomenon, known as interference, results in what is commonly called "snow" or "static" on analog television sets. Interference can also result when unshielded electrical wires or other sources of electromagnetic fields are too close to wires transmitting an electrical signal.

Distortion via Encoding

When sound is stored digitally, as in the common MP3 file format, it is typically compressed. A sound recording produces a large amount of data, which translates to a very large data file. An algorithm can then be used to create a much smaller MP3 file, which contains significantly less data but is still capable of recreating a very close approximation of the original sound.

All audio-encoding systems are subject to quantization distortion. This results from the mapping of the original signal of possibly infinite (continuous) values to a smaller, countable set of values. The process frequently involves rounding and truncation, leading to an imperfect reproduction. A similar problem exists for storing photographic data.

Other Forms of Distortion

Various distortion subtypes exist in every engineering or physics subfield that deals with signal transmission, storage, or reproduction. The common thread in these cases is that no method of transmission or reproduction is perfect. All such methods will introduce some form of distortion that must be compensated for if the integrity of the signal is to be preserved.

Bibliography

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