digitalについて、調べてみた件

デ ジ タ ル ( 英 語 : digital, 英 語 発 音 : 。 デ ィ ジ タ ル ) 量 と は 、 離 散 量 ( と び と び の 値 し か な い 量 ) の こ と 。 連 続 量 を 表 す ア ナ ロ グ と 反 対 の 概 念 で あ る 。 工 業 的 に は 、 状 態 を 示 す 量 を 量 子 化 ・ 離 散 化 し て 処 理 ( 取 得 、 蓄 積 、 加 工 、 伝 送 な ど ) を 行 う 方 式 の こ と で あ る 。 

計 数 ( け い す う ) と い う 訳 語 も あ る 。 古 い 学 術 文 献 や 通 商 産 業 省 の 文 書 な ど で 使 わ れ て い る 。 digitalの 語 源 は ラ テ ン 語 の 「 指  (digitus)」 で あ り 、 数 を 指 で 数 え る と こ ろ か ら 離 散 的 な 数 を 意 味 す る よ う に な っ た 。 

概 要 

デ ー タ の 数 値 化 に あ た っ て は 量 子 化 を 行 い 、 整 数 値 ( す な わ ち digit) で 表 現 す る の が 一 般 的 で あ る 。 例 え ば 、 上 昇 中 の 位 置 で は 、 階 段 の 何 段 目 か が デ ジ タ ル で 、 坂 道 中 の 位 置 が ア ナ ロ グ で あ る 。 整 数 で 表 現 す る か 、 実 数 で 表 現 す る か の 違 い が あ る 。 デ ジ タ ル で は 、 デ ー タ 量 を 離 散 的 な 値 ( 離 散 量 ) と し て 表 現 す る こ と に な り 、 そ れ ら の 中 間 の 量 は 誤 差 を 含 ん だ 隣 の 離 散 量 で 表 現 す る 。 こ の 誤 差 は 適 切 な 量 子 化 を 行 う こ と で 実 用 上 影 響 の な い 範 囲 に す る こ と が で き 、 デ ー タ 量 に 比 例 し た ア ナ ロ グ 量 を 用 い る の と ほ ぼ 等 価 な 処 理 が 提 供 で き る 。 

今 日 の コ ン ピ ュ ー タ の 主 流 で あ る デ ジ タ ル コ ン ピ ュ ー タ で は 、 0と 1だ け か ら な る 2進 数 を 物 理 的 な 表 現 形 式 ( 電 圧 の 高 ・ 低 な ど ) と し て 用 い る の で 、 デ ジ タ ル は 0と 1か ら 成 る と い う 説 明 が よ く な さ れ る 。 し か し 、 は っ き り と 区 別 で き る 2以 上 の 状 態 で 表 現 さ れ て い る デ ー タ ( 例 :  そ ろ ば ん の 玉 な ど ) は 、 ど れ も デ ジ タ ル と 呼 ぶ こ と が で き る 。 

表 記 

一 般 的 に は 「 デ ジ タ ル 」 と 表 記 さ れ る 。 し か し 、 電 気 ・ 電 子 ・ 情 報 工 学 の 分 野 で は 「 デ ィ ジ タ ル 」 と 表 記 さ れ る 。 日 本 工 業 規 格  (JIS X 0001, JIS X 0005) で も 、 「 デ ィ ジ タ ル 」 ( デ ィ ジ タ ル 計 算 機 、 デ ィ ジ タ ル 化 す る 、 デ ィ ジ タ ル デ ー タ な ど ) に な っ て い る 。 こ れ は 、 “digital” の つ づ り  “di” を 意 識 し て の こ と で あ る ( disk= デ ィ ス ク 、 display= デ ィ ス プ レ イ な ど と 同 様 ) 。 逆 に 日 本 語 表 記 に つ ら れ て “degital”と 誤 表 記 さ れ る こ と も 少 な く な い 。 

特 徴 

デ ジ タ ル デ ー タ は 、 離 散 値 と し て 数 値 化 し て い る の で 、 ア ナ ロ グ デ ー タ と 比 べ て 短 期 的 に は 劣 化 し に く い 特 性 を も つ 。 伝 送 ・ 記 録 再 生 な ど を 行 う 場 合 、 デ ジ タ ル 量 も ア ナ ロ グ 量 と 同 様 に 電 圧 ・ 電 流 な ど の 電 気 信 号 に 置 き 換 え て 取 り 扱 わ れ る 。 外 乱 が 生 じ て 信 号 に ノ イ ズ が 混 入 し た 場 合 、 ア ナ ロ グ 処 理 で は 特 別 な 処 理 を 行 わ な い 限 り 信 号 に 混 じ っ た ノ イ ズ を 取 り 除 く こ と が 困 難 で あ る 。 こ れ に 対 し デ ジ タ ル 処 理 で は 、 数 値 は 離 散 化 し て あ っ て 中 間 値 を も た な い の で 、 ノ イ ズ に よ っ て 生 じ た 誤 差 が 一 定 量 以 下 な ら ば そ れ を 無 視 で き 、 数 値 デ ー タ を 劣 化 す る 前 の 値 に 復 元 す る こ と が で き る ( 例 え ば 、 デ ー タ が 整 数 表 現 の 場 合 、 1が ノ イ ズ に よ っ て 0.8や 1.2に 変 化 し て も 1と 認 識 さ せ ら れ る ) 。 

実 際 の 記 録 ・ 伝 送 な ど で は ノ イ ズ な ど の 影 響 が 無 視 で き ず 、 元 の デ ー タ と 異 な る デ ー タ が 再 生 さ れ て し ま う こ と も あ る ( 1が 0.4ま た は 1.6に 変 化 す る と 、 異 な る 値 0ま た は 2と し て 再 生 さ れ る ) 。 し か し 、 デ ー タ を あ ら か じ め 誤 り 訂 正 符 号 な ど を 使 っ て 冗 長 化 し て お く と 、 途 中 で 劣 化 し て も 自 動 的 に 修 復 し た り 、 誤 り の 発 生 を 検 出 し て 再 送 を 要 求 し た り す る こ と が で き 、 信 頼 性 の 高 い 処 理 を 提 供 す る こ と が 可 能 に な る 。 

デ ジ タ ル 処 理 

デ ジ タ ル 化 処 理 

ア ナ ロ グ デ ー タ を デ ジ タ ル デ ー タ に 変 換 す る こ と を 「 デ ジ タ ル 化 す る 」 、 「 デ ジ タ イ ズ す る 」 な ど と い う 。 

デ ジ タ ル 処 理 の 適 用 

デ ジ タ ル デ ー タ を そ の ま ま 扱 う 場 合 ( 単 純 な リ ニ ア サ ン プ リ ン グ ) に つ い て 述 べ る 。 

実 際 の デ ジ タ ル 処 理 で は 、 2進 数 1桁 を ビ ッ ト と し 、 8ビ ッ ト な ど の ま と ま っ た 単 位 を オ ク テ ッ ト ま た は バ イ ト と し て 取 り 扱 い 、 さ ら に そ の ま と ま り を ワ ー ド と い う 単 位 と し て 取 り 扱 う こ と が 多 い 。 こ れ は 処 理 装 置 や 記 憶 装 置 の 語 長 に 合 わ せ て 効 率 よ く 使 え る よ う に す る た め で あ る 。 

デ ジ タ ル デ ー タ に お い て は 、 表 現 可 能 な 数 値 範 囲 を 超 え た り 、 最 小 値 に 近 い 数 値 を 扱 っ た り す る 際 に は 注 意 が 必 要 で あ る 。 

ア ナ ロ グ 処 理 で は 、 多 少 入 力 電 圧 が 規 定 よ り 超 過 し て も 影 響 が な い か 、 わ ず か な 影 響 で 済 む 場 合 も あ る 。 し か し デ ジ タ ル 処 理 で は 、 定 義 さ れ た 最 大 値 を 超 え た 場 合 に は 桁 あ ふ れ ( オ ー バ ー フ ロ ー ) と な り 、 以 後 の 演 算 処 理 の 結 果 は 保 証 さ れ な い 。 ま た 、 最 小 値 に 近 い 数 値 で は 量 子 化 誤 差 が 無 視 で き ず 、 S/N比 の 劣 化 と し て 現 れ る こ と が あ る 。 さ ら に 、 数 値 計 算 の 際 に 不 用 意 な 処 理 手 順 に よ る 桁 落 ち が 生 じ 、 著 し い 有 効 桁 数 の 減 少 を 招 く こ と も あ る の で 、 注 意 を 要 す る 。 

符 号 化 

様 々 な 分 野 で そ れ ぞ れ 適 切 な 表 現 形 式 を 用 い て デ ー タ を 符 号 化 し て い る 。 

  • 数 値 は 、 整 数 や 浮 動 小 数 点 型 、 固 定 小 数 点 型 な ど と し て 扱 え る 。 
  • 文 字 は 、 文 字 コ ー ド で 文 字 と コ ー ド を 対 応 さ せ る こ と が で き る 。 
  • 音 声 は 、 PCMな ど で デ ジ タ ル 化 で き る 。 楽 譜 情 報 を 電 子 化 し た も の は MIDI、 MMLな ど 。 
  • 絵 、 映 像 は 、 光 を RGBな ど 色 の 成 分 に 分 解 し 、 各 色 の 明 る さ な ど を 数 値 化 す る 。 
  • 図 形 は 、 ベ ク タ 形 式 に よ る 。 こ の 形 式 は 、 狭 義 に は 線 分 の 始 点 と 終 点 の 座 標 を 数 値 で 記 録 す る 。 広 義 に は 、 各 種 の 図 形 に 対 し て 、 例 え ば 円 な ら 、 「 図 形 コ ー ド = 円 、 中 心 座 標 、 半 径 」 を 記 録 す る 。 こ れ ら の デ ー タ か ら の 例 え ば 円 を 描 く こ と は 図 形 表 示 ソ フ ト ウ ェ ア に 任 せ る 。 
  • ※ 参照元:https://ja.wikipedia.org


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