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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Cross modulation ： ウィキペディア英語版 Intermodulation Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities in a system. The intermodulation between each frequency component will form additional signals at frequencies that are not just at harmonic frequencies (integer multiples) of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at multiples of those sum and difference frequencies. Intermodulation is caused by non-linear behaviour of the signal processing (physical equipment or even algorithms) being used. The theoretical outcome of these non-linearities can be calculated by generating a Volterra series of the characteristic, while the usual approximation of those non-linearities is obtained by generating a Taylor series. Practically all audio equipment has some non-linearity, so it will exhibit some amount of IMD, which however may be low enough to be imperceptible by humans. Due to the characteristics of the human auditory system, the same percentage of IMD is perceived as more bothersome than the same amount of harmonic distortion. Intermodulation is also rarely desirable in radio, as it creates unwanted spurious emissions, often in the form of sidebands. For radio transmissions this increases the occupied bandwidth, leading to adjacent channel interference, which can reduce audio clarity or increase spectrum usage. IMD is only distinct from harmonic distortion in that the stimulus signal is different. The same nonlinear system will produce both THD (with a solitary sine wave input) and IMD (with more complex tones). In music, for instance, IMD is intentionally applied to electric guitars using overdriven amplifiers or effects pedals to produce new tones at ''sub''harmonics of the tones being played on the instrument. See Power_chord#Analysis. IMD is also distinct from intentional modulation (such as a frequency mixer in superheterodyne receivers) where signals to be modulated are presented to an intentional nonlinear element (multiplied). See non-linear mixers such as mixer diodes and even single-transistor oscillator-mixer circuits. However, while the intermodulation products of the received signal with the local oscillator signal are intended, superheterodyne mixers can, at the same time, also produce unwanted intermodulation effects from strong signals near in frequency to the desired signal that fall within the passband of the receiver. ==Causes of intermodulation== A linear system cannot produce intermodulation. If the input of a linear time-invariant system is a signal of a single frequency, then the output is a signal of the same frequency; only the amplitude and phase can differ from the input signal. Non-linear systems generate harmonics in response to sinusoidal input, meaning that if the input of a non-linear system is a signal of a single frequency, $~f\_a,$ then the output is a signal which includes a number of integer multiples of the input frequency; (i.e. some of $~\; f\_a,\; 2f\_a,\; 3f\_a,\; 4f\_a,\; \backslash ldots$). Intermodulation occurs when the input to a non-linear system is composed of two or more frequencies. Consider an input signal that contains three frequency components at$~f\_a$, $~\; f\_b$, and $~f\_c$; which may be expressed as :$\backslash \; x(t)\; =\; M\_a\; \backslash sin(2\; \backslash pi\; f\_a\; t\; +\; \backslash phi\_a)\; +\; M\_b\; \backslash sin(2\; \backslash pi\; f\_b\; t\; +\; \backslash phi\_b)\; +\; M\_c\; \backslash sin(2\; \backslash pi\; f\_c\; t\; +\; \backslash phi\_c)$ where the $\backslash \; M$ and $\backslash \; \backslash phi$ are the amplitudes and phases of the three components, respectively. We obtain our output signal, $\backslash \; y(t)$, by passing our input through a non-linear function $G$: :$\backslash \; y(t)\; =\; G\backslash left(x(t)\backslash right)\backslash ,$ $\backslash \; y(t)$ will contain the three frequencies of the input signal, $~f\_a$, $~\; f\_b$, and $~f\_c$ (which are known as the ''fundamental'' frequencies), as well as a number of linear combinations of the fundamental frequencies, each of the form :$\backslash \; k\_af\_a\; +\; k\_bf\_b\; +\; k\_cf\_c$ where $~k\_a$, $~\; k\_b$, and $~k\_c$ are arbitrary integers which can assume positive or negative values. These are the intermodulation products (or IMPs). In general, each of these frequency components will have a different amplitude and phase, which depends on the specific non-linear function being used, and also on the amplitudes and phases of the original input components. More generally, given an input signal containing an arbitrary number $N$ of frequency components $f\_a,\; f\_b,\; \backslash ldots,\; f\_N$, the output signal will contain a number of frequency components, each of which may be described by :$k\_a\; f\_a\; +\; k\_b\; f\_b\; +\; \backslash cdots\; +\; k\_N\; f\_N,\backslash ,$ where the coefficients $k\_a,\; k\_b,\; \backslash ldots,\; k\_N$ are arbitrary integer values. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Intermodulation」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.