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55-445: Distortion is usually unwanted, and so engineers strive to eliminate or minimize it. In some situations, however, distortion may be desirable. For example, in noise reduction systems like the Dolby system , an audio signal is deliberately distorted in ways that emphasize aspects of the signal that are subject to electrical noise , then it is symmetrically "undistorted" after passing through

110-408: A filter . For example, the non-uniform frequency response curve of AC-coupled cascade amplifier is an example of frequency distortion. In the audio case, this is mainly caused by room acoustics, poor loudspeakers and microphones, long loudspeaker cables in combination with frequency dependent loudspeaker impedance , etc. This form of distortion mostly occurs due to electrical reactance . Here, all

165-476: 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 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. Harmonic distortion adds overtones that are whole number multiples of

220-663: A de-emphasis process applied at playback. Systems include the professional systems Dolby A and Dolby SR by Dolby Laboratories , dbx Professional and dbx Type I by dbx , Donald Aldous' EMT NoiseBX, Burwen Noise Eliminator  [ it ] , Telefunken 's telcom c4  [ de ] and MXR Innovations' MXR as well as the consumer systems Dolby NR , Dolby B , Dolby C and Dolby S , dbx Type II , Telefunken's High Com and Nakamichi 's High-Com II , Toshiba 's (Aurex AD-4) adres  [ ja ] , JVC 's ANRS  [ ja ] and Super ANRS , Fisher / Sanyo 's Super D , SNRS , and

275-400: A de-emphasis process applied during playback. Modern digital sound recordings no longer need to worry about tape hiss so analog-style noise reduction systems are not necessary. However, an interesting twist is that dither systems actually add noise to a signal to improve its quality. Dual-ended compander noise reduction systems have a pre-emphasis process applied during recording and then

330-516: A decoder. The Telefunken High Com integrated circuit U401BR could be utilized to work as a mostly Dolby B –compatible compander as well. In various late-generation High Com tape decks the Dolby-B emulating D NR Expander functionality worked not only for playback, but, as an undocumented feature, also during recording. dbx was a competing analog noise reduction system developed by David E. Blackmer , founder of Dbx, Inc. It used

385-418: A device's mechanism or signal processing algorithms . In electronic systems , a major type of noise is hiss created by random electron motion due to thermal agitation. These agitated electrons rapidly add and subtract from the output signal and thus create detectable noise . In the case of photographic film and magnetic tape , noise (both visible and audible) is introduced due to the grain structure of

440-456: A feeling, or enhance visual impact. Such distortions or "abstractions" primarily refer to purposeful deviations from photorealistic perspective or from realistic proportionality. Examples include " The Weeping Woman " by Picasso and " The Adoration of the Shepherds " by El Greco, whose human subject matters are irregularly and (as is often with physical distortions) asymmetrically proportioned in

495-518: A greater or lesser degree. The local signal-and-noise orthogonalization algorithm can be used to avoid changes to the signals. Noise reduction techniques in Digital Signal Processing (DSP) are essential for improving the quality of signals in various applications, including audio processing, telecommunications, and biomedical engineering. Noise, which is unwanted random variation in signals, can degrade signal clarity and accuracy. DSP offers

550-409: A noisy communication channel, reducing the noise in the received signal. Distortion is also used as a musical effect , particularly with electric guitars . The addition of noise or other outside signals ( hum , interference ) is not considered distortion, though the effects of quantization distortion are sometimes included in noise. Quality measures that reflect both noise and distortion include

605-533: A percentage. The level at which harmonic distortion becomes audible depends on the exact nature of the distortion. Different types of distortion (like crossover distortion ) are more audible than others (like soft clipping ) even if the THD measurements are identical. Harmonic distortion in radio frequency applications is rarely expressed as THD. Non-flat frequency response is a form of distortion that occurs when different frequencies are amplified by different amounts in

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660-500: A range of algorithms to reduce noise while preserving the integrity of the original signal. Spectral subtraction is one of the simplest and most widely used noise reduction techniques, especially in speech processing. It works by estimating the power spectrum of the noise during silent periods and subtracting this noise spectrum from the noisy signal. This technique assumes that noise is additive and relatively stationary. While effective, spectral subtraction can introduce "musical noise,"

715-456: A root-mean-squared (RMS) encode/decode algorithm with the noise-prone high frequencies boosted, and the entire signal fed through a 2:1 compander. dbx operated across the entire audible bandwidth and unlike Dolby B was unusable without a decoder. However, it could achieve up to 30 dB of noise reduction. Since analog video recordings use frequency modulation for the luminance part (composite video signal in direct color systems), which keeps

770-503: A signal but does alter the balance of existing ones. 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 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

825-412: A sound wave's frequencies. Nonlinearities that give rise to amplitude distortion in audio systems are most often measured in terms of the harmonics (overtones) added to a pure sinewave fed to the system. Harmonic distortion may be expressed in terms of the relative strength of individual components, in decibels , or the root mean square of all harmonic components: Total harmonic distortion (THD), as

880-419: A type of artificial noise, if the noise spectrum estimate is inaccurate. Applications: Primarily used in audio signal processing, including mobile telephony and hearing aids. Advantages: Simple to implement and computationally efficient. Limitations: Tends to perform poorly in the presence of non-stationary noise, and can introduce artifacts. Adaptive filters are highly effective in situations where noise

935-442: A typical class-B emitter-follower complementary output stage. Under no signal conditions, the output is exactly midway between the supplies (i.e., at 0 V). When this is the case, the base-emitter bias of both the transistors is zero, so they are in the cut-off region where the transistors are not conducting. Consider a positive-going swing: As long as the input is less than the required forward V BE drop (≈ 0.65 V) of

990-421: A variety of sources. Further use of these images will often require that the noise be reduced either for aesthetic purposes, or for practical purposes such as computer vision . Crossover distortion Crossover distortion is a type of distortion which is caused by switching between devices driving a load. It is most commonly seen in complementary, or "push-pull", class-B amplifier stages, although it

1045-533: A voltage gain of just under 1. In the circuit shown, the NPN emitter-follower and the PNP emitter-follower will generally have very slightly different voltage gains, leading to slightly different gains above and below ground. Other more subtle forms of crossover distortion, stemming from slight differences between the PNP and NPN devices, exist. As with most kinds of distortion, another way in which crossover distortion can be reduced

1100-406: A way that is not possible in standard perspective . In optics , image/optical distortion is a divergence from rectilinear projection caused by a change in magnification with increasing distance from the optical axis of an optical system. In cartography , a distortion is the misrepresentation of the area or shape of a feature. The Mercator projection , for example, distorts by exaggerating

1155-497: Is a statistical approach to noise reduction that minimizes the mean square error between the desired signal and the actual output. This technique relies on knowledge of both the signal and noise power spectra, and it can provide optimal noise reduction if these spectra are accurately estimated. Applications: Frequently applied in image processing, audio restoration, and radar. Advantages: Provides optimal noise reduction for stationary noise. Limitations: Requires accurate estimates of

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1210-575: Is an advanced noise reduction technique that uses redundancy in the signal by averaging similar patches across the signal or image. While computationally more demanding, NLM is highly effective in removing noise from images and audio signals without blurring. Applications: Applied primarily in image denoising, especially in medical imaging and photography. Advantages: Preserves details and edges in images. Limitations: Computationally intensive, often requiring hardware acceleration or approximations for real-time applications. Boosting signals in seismic data

1265-530: Is concentrated about it. Yet another approach is the automatic noise limiter and noise blanker commonly found on HAM radio transceivers, CB radio transceivers, etc. Both of the aforementioned filters can be used separately, or in conjunction with each other at the same time, depending on the transceiver itself. Most digital audio workstations (DAWs) and audio editing software have one or more noise reduction functions. Images taken with digital cameras or conventional film cameras will pick up noise from

1320-400: Is especially crucial for seismic imaging , inversion, and interpretation, thereby greatly improving the success rate in oil & gas exploration. The useful signal that is smeared in the ambient random noise is often neglected and thus may cause fake discontinuity of seismic events and artifacts in the final migrated image. Enhancing the useful signal while preserving edge properties of

1375-592: Is frequently confused with the far more common Dolby noise-reduction system . Unlike Dolby and dbx Type I and Type II noise reduction systems, DNL and DNR are playback-only signal processing systems that do not require the source material to first be encoded. They can be used to remove background noise from any audio signal, including magnetic tape recordings and FM radio broadcasts, reducing noise by as much as 10 dB. They can also be used in conjunction with other noise reduction systems, provided that they are used prior to applying DNR to prevent DNR from causing

1430-414: Is occasionally seen in other types of circuits as well. The term crossover signifies the "crossing over" of the signal between devices, in this case, from the upper transistor to the lower and vice versa. The term is not related to the audio loudspeaker crossover filter —a filtering circuit which divides an audio signal into frequency bands to drive separate drivers in multiway speakers. The image shows

1485-1101: 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 . Telegraphic distortion is a similar and older problem, distorting the ratio between mark and space intervals. With respect to audio, distortion refers to any kind of deformation of an output waveform compared to its input, usually clipping , harmonic distortion , or intermodulation distortion ( mixing phenomena) caused by non-linear behavior of electronic components and power supply limitations. Terms for specific types of nonlinear audio distortion include: crossover distortion and slew-induced distortion (SID). Other forms of audio distortion are non-flat frequency response , compression , modulation , aliasing , quantization noise , wow and flutter from analog media such as vinyl records and magnetic tape . The human ear cannot hear phase distortion , except that it may affect

1540-504: Is through the use of feedback . By comparing the output to the desired output, and adjusting the input to correct for any error, we can significantly reduce distortion. This may be done with an operational amplifier , as shown below, or with a discrete circuit. In the example shown, the operational amplifier is used to reduce the distortion of a push-pull pair. Operational amplifiers are differential voltage amplifiers with very high gain (sometimes modeled as infinite gain). In an ideal model,

1595-424: Is to define a dynamic threshold for filtering noise, that is derived from the local signal, again with respect to a local time-frequency region. Everything below the threshold will be filtered, everything above the threshold, like partials of a voice or wanted noise , will be untouched. The region is typically defined by the location of the signal's instantaneous frequency, as most of the signal energy to be preserved

1650-653: Is unpredictable or non-stationary. In adaptive filtering, the filter's parameters are continuously adjusted to minimize the difference between the desired signal and the actual output. The Least Mean Squares (LMS) and Recursive Least Squares (RLS) algorithms are commonly used for adaptive noise cancellation. Applications: Used in active noise-canceling headphones, biomedical devices (e.g., EEG and ECG processing), and communications. Advantages: Can adapt to changing noise environments in real-time. Limitations: Higher computational requirements, which may be challenging for real-time applications on low-power devices. Wiener filtering

1705-405: The signal-to-noise and distortion (SINAD) ratio and total harmonic distortion plus noise (THD+N). In telecommunications and signal processing , a noise-free system can be characterised by a transfer function , such that the output y ( t ) {\displaystyle y(t)} can be written as a function of the input x {\displaystyle x} as When

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1760-426: The stereo imaging . In most fields, distortion is characterized as unwanted change to a signal. Distortion in music is often intentionally used as an effect when applied to an electric guitar signal in styles of rock music such as heavy metal and punk rock . In the visual arts a distortion is any change made by an artist to the size, shape or visual character of a form in order to express an idea, convey

1815-547: The Hungarian/East-German Ex-Ko system. In some compander systems, the compression is applied during professional media production and only the expansion is applied by the listener; for example, systems like dbx disc , High-Com II , CX 20 and UC used for vinyl recordings and Dolby FM , High Com FM and FMX used in FM radio broadcasting. The first widely used audio noise reduction technique

1870-520: The components of the input signal are not amplified with the same phase shift, hence making some parts of the output signal out of phase with the rest of the output. Can be found only in dispersive media . In a waveguide , phase velocity varies with frequency. In a filter, group delay tends to peak near the cut-off frequency , resulting in pulse distortion. When analog long distance trunks were commonplace, for example in 12 channel carrier , group delay distortion had to be corrected in repeaters . As

1925-441: The input signal determine the output. Many symmetrical electronic circuits reduce the magnitude of even harmonics generated by the non-linearities of the amplifier's components, by combining two signals from opposite halves of the circuit where distortion components that are roughly the same magnitude but out of phase. Examples include push-pull amplifiers and long-tailed pairs . In binary signaling such as FSK , distortion

1980-468: The medium. In photographic film, the size of the grains in the film determines the film's sensitivity, more sensitive film having larger-sized grains. In magnetic tape, the larger the grains of the magnetic particles (usually ferric oxide or magnetite ), the more prone the medium is to noise. To compensate for this, larger areas of film or magnetic tape may be used to lower the noise to an acceptable level. Noise reduction algorithms tend to alter signals to

2035-626: The other noise reduction system to mistrack. One of DNR's first widespread applications was in the GM Delco car stereo systems in US GM cars introduced in 1984. It was also used in factory car stereos in Jeep vehicles in the 1980s, such as the Cherokee XJ . Today, DNR, DNL, and similar systems are most commonly encountered as a noise reduction system in microphone systems. A second class of algorithms work in

2090-407: The output of the op amp is held such that both inputs of the op amp must be at exactly the same voltage. In this case, since the inverting input is directly connected to the output, the voltage at the non-inverting input is always equal to the voltage at the output and inverting input, hence eliminating distortion. With a more precise model of an operational amplifier (with non-infinite gain) distortion

2145-467: The output voltage is not a true replica or amplified version of the input, and we can see that as a "kink" in the output waveform near 0 V (or where one transistor stops conducting and the other starts). This kink is the most pronounced form of crossover distortion, and it becomes more evident and intrusive when the output voltage swing is reduced. Less pronounced forms of distortion may be observed in this circuit as well. An emitter-follower will have

2200-437: 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 multiple input points to a single output point). This produces an uncorrectable loss of information. Such a situation can occur when an amplifier is overdriven—causing clipping or slew rate distortion when, for a moment, the amplifier characteristics alone and not

2255-506: The playback of phonograph records to address scratches, pops, and surface non-linearities. Single-ended dynamic range expanders like the Phase Linear Autocorrelator Noise Reduction and Dynamic Range Recovery System (Models 1000 and 4000) can reduce various noise from old recordings. Dual-ended systems (such as Dolby noise-reduction system or dbx ) have a pre-emphasis process applied during recording and then

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2310-399: The recording medium at the time of recording. Single-ended hiss reduction systems (such as DNL or DNR ) work to reduce noise as it occurs, including both before and after the recording process as well as for live broadcast applications. Single-ended surface noise reduction (such as CEDAR and the earlier SAE 5000A, Burwen TNE 7000, and Packburn 101/323/323A/323AA and 325 ) is applied to

2365-642: The seismic profiles by attenuating random noise can help reduce interpretation difficulties and misleading risks for oil and gas detection. Tape hiss is a performance-limiting issue in analog tape recording . This is related to the particle size and texture used in the magnetic emulsion that is sprayed on the recording media, and also to the relative tape velocity across the tape heads . Four types of noise reduction exist: single-ended pre-recording, single-ended hiss reduction, single-ended surface noise reduction, and codec or dual-ended systems. Single-ended pre-recording systems (such as Dolby HX Pro ), work to affect

2420-438: The signal and noise statistics, which may not always be feasible in real-world applications. Kalman filtering is a recursive algorithm that estimates the state of a dynamic system from a series of noisy measurements. While typically used for tracking and prediction, it is also applicable to noise reduction, especially for signals that can be modeled as time-varying. Kalman filtering is particularly effective in applications where

2475-666: The signal into different frequency components using a wavelet transform and then removes the noise by thresholding the wavelet coefficients. This method is effective for signals with sharp transients, like biomedical signals, because wavelet transforms can provide both time and frequency information. Applications: Commonly used in image processing, ECG and EEG signal denoising, and audio processing. Advantages: Preserves sharp signal features and offers flexibility in handling non-stationary noise. Limitations: The choice of wavelet basis and thresholding parameters significantly impacts performance, requiring careful tuning. Non-local means (NLM)

2530-427: The signal is dynamic and the noise characteristics vary over time. Applications: Used in speech enhancement, radar, and control systems. Advantages: Provides excellent performance for time-varying signals with non-stationary noise. Limitations: Requires a mathematical model of the system dynamics, which may be complex to design for certain applications. Wavelet -based denoising (or wavelet thresholding) decomposes

2585-416: The signal to some degree. Noise rejection is the ability of a circuit to isolate an undesired signal component from the desired signal component, as with common-mode rejection ratio . All signal processing devices, both analog and digital , have traits that make them susceptible to noise. Noise can be random with an even frequency distribution ( white noise ), or frequency-dependent noise introduced by

2640-455: The signal-to-noise ratio on tape up to 10 dB depending on the initial signal volume. When it was played back, the decoder reversed the process, in effect reducing the noise level by up to 10 dB. The Dolby B system (developed in conjunction with Henry Kloss ) was a single-band system designed for consumer products. The Dolby B system, while not as effective as Dolby A, had the advantage of remaining listenable on playback systems without

2695-467: The size of regions at high latitude . [REDACTED]  This article incorporates public domain material from Federal Standard 1037C . General Services Administration . Archived from the original on 2022-01-22.  (in support of MIL-STD-188 ). Noise reduction Noise reduction is the process of removing noise from a signal . Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort

2750-411: The system output is given by y(t) = F(x(t)), then if the inverse function F can be found, and used intentionally to distort either the input or the output of the system, then the distortion is corrected. An example of a similar 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

2805-438: The tape at saturation level, audio-style noise reduction is unnecessary. Dynamic noise limiter ( DNL ) is an audio noise reduction system originally introduced by Philips in 1971 for use on cassette decks . Its circuitry is also based on a single chip . It was further developed into dynamic noise reduction ( DNR ) by National Semiconductor to reduce noise levels on long-distance telephony . First sold in 1981, DNR

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2860-415: The time-frequency domain using some linear or nonlinear filters that have local characteristics and are often called time-frequency filters . Noise can therefore be also removed by use of spectral editing tools, which work in this time-frequency domain, allowing local modifications without affecting nearby signal energy. This can be done manually much like in a paint program drawing pictures. Another way

2915-402: The transfer function comprises only a perfect gain constant A and perfect delay 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, the signal suffers linear distortion. Linear distortion does not introduce new frequency components to

2970-442: The upper NPN transistor , it will remain off or conduct very little. This is the same as a diode operation as far as the base circuit is concerned, and the output voltage does not follow the input (the lower PNP transistor is still off because its base-emitter diode is being reverse biased by the positive-going input). The same applies to the lower transistor but for a negative-going input. Thus, between about ±0.65 V of input,

3025-410: Was developed by Ray Dolby in 1966. Intended for professional use, Dolby Type A was an encode/decode system in which the amplitude of frequencies in four bands was increased during recording (encoding), then decreased proportionately during playback (decoding). In particular, when recording quiet parts of an audio signal, the frequencies above 1 kHz would be boosted. This had the effect of increasing

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