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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 Frequency modulator ： ウィキペディア英語版 Frequency modulation In telecommunications and signal processing, frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. This contrasts with amplitude modulation, in which the amplitude of the carrier wave varies, while the frequency remains constant. In analog frequency modulation, such as FM radio broadcasting of an audio signal representing voice or music, the instantaneous frequency deviation, the difference between the frequency of the carrier and its center frequency, is proportional to the modulating signal. Digital data can be encoded and transmitted via FM by shifting the carrier's frequency among a predefined set of frequencies representing digits - for example one frequency can represent a binary 1 and a second can represent binary 0. This modulation technique is known as frequency-shift keying (FSK). FSK is widely used in modems and fax modems, and can also be used to send Morse code. Radioteletype also uses FSK. Frequency modulation is widely used for FM radio broadcasting. It is also used in telemetry, radar, seismic prospecting, and monitoring newborns for seizures via EEG,〔B. Boashash, editor, "Time-Frequency Signal Analysis and Processing – A Comprehensive Reference", Elsevier Science, Oxford, 2003; ISBN 0-08-044335-4〕 two-way radio systems, music synthesis, magnetic tape-recording systems and some video-transmission systems. In radio transmission, an advantage of frequency modulation is that it has a larger signal-to-noise ratio and therefore rejects radio frequency interference better than an equal power amplitude modulation (AM) signal. For this reason, most music is broadcast over FM radio. Frequency modulation has a close relationship with phase modulation; phase modulation is often used as an intermediate step to achieve frequency modulation. ==Theory== If the information to be transmitted (i.e., the baseband signal) is $x\_m(t)$ and the sinusoidal carrier is $x\_c(t)\; =\; A\_c\; \backslash cos\; (2\; \backslash pi\; f\_c\; t)\backslash ,$, where ''fc'' is the carrier's base frequency, and ''Ac'' is the carrier's amplitude, the modulator combines the carrier with the baseband data signal to get the transmitted signal: :$y(t)\; =\; A\_c\; \backslash cos\; \backslash left(\; 2\; \backslash pi\; \backslash int\_^\; f(\backslash tau)\; d\; \backslash tau\; \backslash right)$ ::$=\; A\_\; \backslash cos\; \backslash left(\; 2\; \backslash pi\; \backslash int\_^\; \backslash left(f\_\; +\; f\_\; x\_(\backslash tau)\; \backslash right)\; d\; \backslash tau\; \backslash right)$ ::$=\; A\_\; \backslash cos\; \backslash left(\; 2\; \backslash pi\; f\_\; t\; +\; 2\; \backslash pi\; f\_\; \backslash int\_^x\_(\backslash tau)\; d\; \backslash tau\; \backslash right)$ In this equation, $f(\backslash tau)\backslash ,$ is the ''instantaneous frequency'' of the oscillator and $f\_\backslash ,$ is the ''frequency deviation'', which represents the maximum shift away from ''fc'' in one direction, assuming ''x''''m''(''t'') is limited to the range ±1. While most of the energy of the signal is contained within ''fc'' ± ''f''Δ, it can be shown by Fourier analysis that a wider range of frequencies is required to precisely represent an FM signal. The frequency spectrum of an actual FM signal has components extending infinitely, although their amplitude decreases and higher-order components are often neglected in practical design problems.〔 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「Frequency modulation」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.