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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 Differential coding ： ウィキペディア英語版 Differential coding In digital communications, differential coding is a technique used to provide ''unambiguous'' signal reception when using some types of modulation. It makes data to be transmitted to depend not only on the current bit (or symbol), but also on the previous one. The common types of modulation that require differential coding include phase shift keying and quadrature amplitude modulation. ==Purposes of differential coding== To demodulate BPSK one needs to make a local oscillator ''synchronous'' with the remote one. This is accomplished by a carrier recovery circuit. However, a carrier can be recovered in different ways, depending upon a valid phases count (2 for BPSK). For this coding, if a carrier is recovered incorrectly, the received data is inverted. Assuming that $x\_i$ is a bit intended for transmission, and $y\_i$ is a bit actually transmitted (differentially encoded), if :$y\_i=y\_\; \backslash oplus\; x\_i,\; \backslash qquad\; (1)$ is transmitted, then on the decoding side :$x\_i=y\_i\; \backslash oplus\; y\_.\; \backslash qquad\; (2)$ can be reconstructed, where $\backslash oplus\{\}$ indicates binary or modulo-2 addition. Now $x\_i$ depends only on a difference between $y\_i$ and $y\_\{i-1\}$ and not on their values. So, whether the data stream is inverted or not, the decoded data will always be correct. When data is transmitted over twisted-pair wires, it is easy to accidentally insert an extra half-twist in the cable between the transmitter and the receiver. When this happens, the received data is inverted. There are several different line codes designed to be polarity insensitive〔("Spread spectrum direct sequence" ) by Daniel Kraus〕 -- whether the data stream is inverted or not, the decoded data will always be correct. The line codes with this property include differential Manchester encoding, bipolar encoding, NRZI, biphase mark code, coded mark inversion, and MLT-3 encoding. 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「Differential coding」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.