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PowerPedia:Amplitude distortion

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Amplitude distortion is There was an error working with the wiki: Code[12] occurring in a There was an error working with the wiki: Code[13], subsystem, or device when the There was an error working with the wiki: Code[14] amplitude is not a linear function of the There was an error working with the wiki: Code[15] amplitude under specified conditions. Generally, output is a There was an error working with the wiki: Code[16] function of input only for a fixed portion of the transfer characteristics. In this region, Ic=ßIb where Ic is collector current and Ib is base current, following There was an error working with the wiki: Code[16] relation y=mx\,.

Distortion

A There was an error working with the wiki: Code[2] to achieve the desired sound). The slight distortion of analog There was an error working with the wiki: Code[3] and There was an error working with the wiki: Code[4] or other extraneous signals (There was an error working with the wiki: Code[5], There was an error working with the wiki: Code[6]) is not considered to be distortion, though the effects of distortion are sometimes considered noise.

When output is not in this portion, two forms of amplitude distortion might arise

# harmonic distortion

# intermodulation distortion

Harmonic distortion

Harmonic distortion : The creation of There was an error working with the wiki: Code[18] of the There was an error working with the wiki: Code[19] of a There was an error working with the wiki: Code[20] input to a system.

The total harmonic distortion, or THD, of a There was an error working with the wiki: Code[7] is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental.

In most cases, the transfer function of a system is There was an error working with the wiki: Code[8]. When a signal passes through a non-linear device, additional content is added at the harmonics of the original frequencies. THD is a measurement of the extent of that distortion.

The measurement is most commonly the ratio of the sum of the Power (physics) of all There was an error working with the wiki: Code[9] Frequency to the power of the fundamental:

:

\mbox{THD} = {\sum{\mbox{harmonic powers}} \over \mbox{fundamental frequency power}} = {{P_2 + P_3 + P_4 + \cdots + P_n} \over P_1}

Other calculations for amplitudes, voltages, currents, and so forth are equivalent. For a voltage signal, for instance, the ratio of There was an error working with the wiki: Code[21] voltages is equivalent to the power ratio:

:\mbox{THD} = {\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots + V_n^2} \over V_1}

In this calculation, Vn means the RMS voltage of harmonic n.

Other definitions may be used. A measurement must specify how it was measured. Measurements for calculating the THD are made at the There was an error working with the wiki: Code[10] as distortion attenuation. A meaningful measurement must include the number of harmonics included (and should include other information about the test conditions).

THD+N means total harmonic distortion plus noise. This measurement is much more common and more comparable between devices. This is usually measured by inputting a There was an error working with the wiki: Code[22], There was an error working with the wiki: Code[23]ing it, and measuring the ratio between the signal with and without the sine wave:

:

\mbox{THD+N} = {\sum{\mbox{harmonic powers}} + \mbox{noise power} \over \mbox{total output power}}

A meaningful measurement must include the There was an error working with the wiki: Code[24] of measurement. This measurement includes effects from Intermodulation distortion, There was an error working with the wiki: Code[25], and so on, instead of just harmonic distortion.

Intermodulation distortion

Intermodulation distortion : This form of distortion occurs when two sine waves of frequencies X and Y are present at the input, resulting in the creation of several other frequency components, whose frequencies include (X+Y), (X-Y), (2X-Y), (2Y-X), and generally (mX ± nY) for integer m and n. Generally the size of the unwanted output falls rapidly as m and n increase.

Intermodulation distortion is There was an error working with the wiki: Code[26] characterized by the appearance, in the There was an error working with the wiki: Code[27] of a device, of frequencies that are linear combinations of the There was an error working with the wiki: Code[28] frequencies and all harmonics present in the There was an error working with the wiki: Code[29] signals.

Due to the additional outputs, this form of distortion is definitely unwanted in audio, radio and telecommunication amplifiers, and it occurs for more than two waves as well.

In a narrowband system such as a radio communication system, unwanted outputs such as X-Y and 2X+Y will be remote from the wanted band and so be ignored by the system. In contrast, 2X-Y and 2Y-X will be close to the wanted signals. These so-called third order distortion products (third order as m+n = 3) tend to dominante the non-linear distortion of narrowband systems.

Amplitude distortion is measured with the system operating under steady-state conditions with a sinusoidal input There was an error working with the wiki: Code[11]. When other frequencies are present, the term "amplitude" refers to that of the There was an error working with the wiki: Code[30] only.

There was an error working with the wiki: Code[31] components themselves are not usually considered to characterize There was an error working with the wiki: Code[32] distortion. When the harmonics are included as part of the distortion, a statement to that effect should be made. This is usually considered total harmonic distortion.

IMD in its most basic and most testable form shows up as presence of frequencies not in the input signal. If the sum of two pure tones is the input to the system, IMD shows up as the presence of new tones in the output whose frequencies are the sum and difference of the input tone frequencies.

In modern record production, it is a commonplace technique to exploit the intermodulation distortion characteristics produced by vacuum tube electronics and audio tape. For example once a recording engineer has mixed the various tracks that make up a song into the stereo format, he may send the mix to a vacuum tube based stereo compressor and overload the vacuum tube electrical components. The resulting output will sound fuller and smoother due to the creation of second and third order harmonics.

This technique applies mostly to vacuum tube based equipment though some use electro-optical based compressors to similar effect. Solid-state or integrated-circuit based equipment is rarely used for this effect as its harmonic distortion character is not favorable.

A recording engineer may also record the mix to an audio tape format called reel to reel. In this technique, the engineer will increase the level at which the mix is recorded to audio tape far past the level recommended by the tape's manufacturer. This will result in a slight compressing of the dynamic (volume) range and the production of several second and third order harmonics.

RF technicians and audio engineers often experience problems with intermodulation distortion when setting up wireless equipment for live performances and events. Often times, wireless equipment for performer’s in-ear monitors or wireless microphones operate on similar frequencies to digital televisions signals, creating harmonic frequencies that interfere with other equipment. With security, technical crew, performance and other wireless signals in use at larger live sporting or concert events, it has become common for hundreds of individual frequencies operating in the same area. Audio engineers have to rely on complex software to calculate all of the possible overlapping and distorted frequencies when setting up such a large live event.

Related

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References and external articles

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Explanation of THD measurements

Rane audio's definition of both THD and THD+N

Conversion: Distortion attenuation in dB to distortion factor THD in %

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.

A Musical-Distortion Primer

Distortion 101

Multiband Distortion ensemble for Reaktor

See also

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