出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2014/01/31 12:49:15」(JST)
「CD」はこの項目へ転送されています。その他の用法については「CD (曖昧さ回避)」をご覧ください。 |
コンパクトディスク Compact Disc, CD |
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コンパクトディスク
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メディアの種類 | 光ディスク |
記録容量 | 640MiB、700MiB |
読み込み速度 | 1.2Mbps (1411.2kbps、1倍速) |
読み取り方法 | 780nm 近赤外線レーザー |
策定 | フィリップス、ソニー |
主な用途 | 音声、映像、データ(ゲームソフトを含む) |
ディスクの直径 | 12cm、8cm |
大きさ | 120×120×1.2mm (12cmCD) 80×80×1.2mm (8cmCD) |
上位規格 | スーパーオーディオCD (SACD) DVDオーディオ |
関連規格 | CD-DA CD-V |
テンプレートを表示 |
コンパクトディスク(Compact Disc、CD(シーディー))とはデジタル情報を記録するためのメディアである。光ディスク規格の一つでレコードに代わり音楽を記録するため、ソニーとフィリップスが共同開発した。2011年現在ではコンピュータ用のデータなど、派生規格の普及により音楽以外のデジタル情報収録(画像や動画など)にも用いられる。音楽CDについてはCD-DAも参照。
コンパクトディスクの外見は直径12cmまたは8cm、厚さ1.2mmの円盤状でプラスチックで作られている。プラスチックの材質は一般的なものはポリカーボネートで、ほかにAPO(非晶質ポリオレフィン)やガラスを使用したものもある。読み取りには780nmの赤外線レーザーが用いられ、照射したレーザー光の反射を読み取る。レーザー光を反射させるためコンパクトディスクは、鏡のような役割を持ちレーザー光を反射する厚さ約80nmのアルミニウム蒸着膜と厚さ約10μmの保護層、レーベルなどの印字膜の複数の層を重ねた構造になっている。
ディスクには細かいくぼみが彫られており、このパターンによってデジタル情報を表現している。このくぼみ(読み取り面から見れば出っ張りになる)をピットといい、ピットのない部分をランドという。ランドの部分に当ったレーザー光は反射してそのまま戻ってくるが、ピットがある部分に当ったレーザー光はランドからの反射波と1/2波長の位相差をもつため干渉して打ち消しあい暗くなる。この明暗によりデジタル信号を読み取り、これをアナログ信号に戻して音声として出力する。ピットの幅は0.5μmで長さは0.83μmから0.3μm単位で3.56μmまで9種類、ピットから次のピットまでの距離も同じ9種類である。またピットの列をトラックというがこのトラックは1.6μm間隔で、内側から外側に向かって渦巻状に並んでいる。CDの虹色のような光沢は、この規則正しく並んだトラックで回折した光が干渉することによる構造色である。
データをピット列として記録するにはEFM (eight-to-fourteen modulation) という変調方式が用いられる。また誤り訂正はCIRC(英語版) (cross-interleaved reed-solomon code) による。短いバーストエラーからの誤り訂正を行う符号としてリードソロモン符号を提案したのはフィリップス社のCD開発チーム責任者である。
コンピュータのデータ保管等、1bitの誤りも許されない用途には追加の誤り検出、訂正が行われている。
音楽用途の場合、デジタルのPCM形式で最大79分58秒、99トラックの音楽が記録できる。また規格上1トラック中には99インデックス(位置決め標識)を設けることができ、2000年頃までのプレーヤーは、インデックスを扱うことのできるものが多数存在した。 現在でもインデックスが記録されているディスクは多数存在する(特にクラシック)が インデックスサーチが出来るプレーヤーが殆ど無くなってしまったので、インデックスが記録されたディスクを持っているユーザーは、不便を感じているようである。 記録層にアルミニウムのかわりに金を使用したものもあり、「ゴールドディスク」と呼ばれる。
近年はダウンロード配信が増えてきている事もあり、次世代メディアへの置き換えは進んでいない。SACDやDVDオーディオが登場した2013年現在でも音楽供給媒体としてはいまだにCDが主流である。
上から印刷層、保護層、反射・記録層、樹脂層で記録層の部分は印刷面から10μm (0.01mm)、樹脂層から約1.2mmの所にある。そのため、印刷面からの衝撃に弱く鉛筆やボールペン等、フェルト以外の油性マーカーで記入を行うと記録層にダメージが加わり音飛びなどの症状が出ることもある。最悪の場合読み込めなくなる可能性も考えられる。印刷層側に深い傷が入ったり湿度の高い場所に放置すると、記録層をのぞき反射層までがはがれることがある。ちなみにDVDの記録層は印刷面からも樹脂層からも0.6mm、Blu-ray Discでは印刷面から1.1mm、樹脂層から0.1mmである。レーベルのデザインによるが、2012年現在では反射層と印刷層が穴の部分まで拡大されたものが主流となっている。
「A面/B面」も参照
レコードでは表をA面・裏をB面と呼んでいたが、CDには1面しかないのでポピュラー音楽のCDシングルなどの場合はレコードでのA面曲に相当するものを「タイトル曲」、B面曲に相当するものを「カップリング曲」などと呼び分けている。後者は「… をカップリングしている」を意味する英語の「coupling with …」を短縮した「c/w」と表記されることもある。
ただし、「タイトル曲」を2曲入れたCDシングルはCDであっても「両A面」という呼び方をすることが多い。
1枚のコンパクトディスクは(ディスク全面を使った場合)、CD-ROM形式の場合約650–700MiBの容量を持つ (1MiB = 1024×1024 Byte)。CD-DA形式での収録時間は約74分–80分である。
コンパクトディスクは650MiBでは約333,000セクタ、700MiBでは約360,000セクタからなる。1セクタは2,352バイトで、1セクタあたりのデータ容量はCD-ROMで2,048バイト、CD-DAで2,352バイトである。CD-ROMはCD-DAより厳密なエラー訂正が必要となるため、2,352バイトのうち304バイトをヘッダやエラー訂正などに割り当てていることからCD-DAより容量が少なくなる。一部では800MiBを超える容量のものもあるが、一部の機器では読み取れない場合がある。
なお、この650MiBという容量は以下の計算式によって求められる。CD-DA形式では音楽データをサンプリング周波数44.1kHz、ビット深度16bit、チャンネル数2.0chステレオで記録している(1秒分の音楽データを44,100回に分割し、1回あたり16bitを費やして記録している)。このため、1秒分のデータ量は16×44,100×2÷8=176,400バイトである。これが74分だと176,400×60×74=783,216,000バイトとなり、これは約747MiBとなる。全領域に音楽データだけを記録するならこれだけの記録が可能だが、CD-ROMの場合はエラー訂正用データ等が入るため、使用できる容量は783,216,000÷2352×2048=681,984,000バイトとなり、これが約650MiBとなる。
記録トラックの幅を狭めれば容量は増やせるが、古い音楽CDプレーヤーにはまれに74分記録されたCD-Rは再生できても80分以上のものは再生できないという互換性の問題が存在する。
CD初期の最大収録時間(74分42秒)が決まったいきさつについて、開発元のソニーによれば以下の通りである。開発の過程でカセットテープの対角線と同じでDINに適合する11.5cm(約60分)を主張するフィリップスに対し、当時ソニー副社長で声楽家出身の大賀典雄が「オペラ一幕分、あるいはベートーベンの第九が収まる収録時間」(12cm、74分)を主張して調査した結果クラシック音楽の95%が75分あれば1枚に収められることからそれを押し通した[1]。その大きな要因となったのが、指揮者のカラヤンであった。
開発当時、大賀典雄は、親交のあったカラヤンに、11.5cm(60分)と12cm(74分)との二つの規格で二者択一の段階に来ていることを話すと、カラヤンは「ベートーベンの交響曲第九番が1枚に収まったほうがいい」と提言した。指揮者によって変わるが、カラヤンの「第九」は約63分–69分であり、ほとんどの指揮者による演奏時間は60分を超えているからだ。結果的に74分(最大80分も可能)という収録時間は、1951年にライヴ録音されたフルトヴェングラー指揮のいわゆる「バイロイトの第九」(演奏時間およそ74分32秒)や、それに匹敵する長さであるカール・ベームやレナード・バーンスタインの演奏も、コンパクトディスク1枚に収めることが可能になった。
この話は、大賀がフィリップスを説得するためにカラヤンの名を引き合いに出したという見方があるが、カラヤンが音楽媒体のディジタル化を望んでいたことは事実である。
また、8cmCD (CD SINGLE) の最大収録時間は約22分程度である。これは、CDビデオのオーディオパートとビデオパートを分けてそれぞれ開発した際に由来している。8cmというサイズはケースに収納したときレコードのシングル盤ケースのちょうど半分のサイズとなるため、小売店でレコード用の棚を使いまわせるだろうと考えたためである。
2013年5月現在の収録時間最長の音楽CDは、DGレーベルにピアニストのマリア・ジョアン・ピレシュが録音したシューベルト:ピアノ・ソナタ第21番、第16番 (DG 4778107)の83分24秒である。規格上は97分26秒まで可能である。
音楽CD(CD-DA形式)の再生時のデータの転送速度は等倍速で1倍速 (150kB/s) を基準として最大記録時間は640MiBのディスクで約72分、650MiBのディスクで約74分、700MiBのディスクで約80分、最新の800MiBのディスクで約90分となる。この音楽CDの1倍速を基準として、ディスクのデータ転送速度を表すのに「○倍速」という言い方をする。
当初から音声・映像記録媒体として開発された。物理フォーマットは先に決まっており、音声記録ディスクの論理仕様が先行して策定された。少し遅れてビデオ記録用としてCDビデオが策定されたが、普及しなかった。後にデータ記録用としてCD-ROM、ビデオ記録用としてビデオCDなどの論理仕様が策定された。これらと対比して音声記録ディスクをCD-DAという。
さらに記録にピットを用いずに、レーザーによる媒体の物理的変化を利用して同等なデジタルデータの書き込みを行う方式が開発された。CD-Rはエンドユーザがデータの追加記録ができる。また、記録してしまった領域を取り戻し、空き領域として記憶領域を再利用することができないCD-Rに対して、CD-RWはデータの消去を可能にし、書き換えができる。
CDの技術を踏まえて音質の向上、あるいは著作権管理機能の強化を目指したディスク媒体の開発が引き続き行われている。オーディオ分野で実用化されたものとしてはスーパーオーディオCD (SACD)、DVDオーディオなどが開発されたがどれもCD-DAを代替するまでの普及には至っていない。
CDの寿命としては、蒸着した反射膜の寿命、基板となるポリカーボネートの寿命、そしてCD-Rの場合には色素の寿命の観点がある。全般として直射日光や高温・多湿を嫌う。
2012年現在、スパッタリング法によってアルミニウムの反射膜を形成する方法が主流となっているが、アルミニウムを用いるCDは環境にもよるが、20–30年が限度と見積られており、現在長期的な保存を可能とした製品の開発が急務となっている。その一方で、メーカー側などでは80年前後保存が可能とする主張もある[2]。なお反射膜に金を用いた場合、100年前後保存が可能と見積られているが、コストの問題など解決しなければならない課題がある。安価なものは印刷・反射層が端からはがれてきたり、水分が反射膜に浸透してアルミニウムが錆びてしまい反射の機能を失うなど、短寿命のものが多い。
CD-Rでは記録面に真夏の昼間の日差しを当て続けると色素が変化し読み込めなくなったり、質の悪い媒体の場合には蛍光灯に含まれる紫外線で変化するものもある。また高温・多湿の環境に置くと、ごく短時間でも印刷・反射層が端からはがれてくる事がある。
ディスクに用いられるポリカーボネートは湿気に遇うと加水分解する欠点があり、徐々に白濁していく。これにより情報を読み取るレーザーが通らなくなり、情報を読めなくなる。ディスクの寿命としては前述の反射膜や色素の寿命がよく取りざたされるが、環境によってはポリカーボネートの透明度で寿命が定まる場合もある。
なお、この欠点を積極的に活かし開封後数週間程度で白濁するように製造された媒体もある。これにより、音楽や映像のソフトウェアを再生できる日数を制限する。
温度や湿度変化の影響が比較的少ないガラス製のCDが開発・発売され、保存性の改善が期待されている。2008年には液晶パネル用のポリカーボネートを使用したスーパー・ハイ・マテリアルCD (SHM-CD) とハイ・クオリティCD (HQCD) が開発・発売。さらにブルーレイディスクの技術を応用したブルースペックCDも開発・発売されている。
コンパクトディスクの仕様・規格は対象とする範囲や目的によって複数の規格に分かれており、各規格基準書の表紙の色によってそれぞれが呼び分けられている[3]。
(以下、「規格名 / 対象範囲」)
この記録メディアに「コンパクト」という言葉が使用された理由は、フィリップス社の意向によるところが大きい。
開発段階でフィリップス社が提示した試作品は、コンパクトカセットの対角線と同じ直径11.5cmで、名称の一貫性が図られていた。ただしその後ソニー側の提案で収録時間を延長したため、実際には直径12cmとなった。
また、レーザーディスクの総本山がフィリップス社であり、そのディスクサイズが30cmだったことにも由来する[要説明]。
その後の技術革新で各種記録メディアの小型化・高密度化なども進んだ。しかし、スーパーオーディオCD、DVD、DVDオーディオ、次世代DVD (Blu-ray Disc、HD DVD) などの光ディスクはいずれも直径12cmである。
後継(あるいは後継を狙った)規格の中には、その名に「CD」が入ったものもあるが、レッドブック等に不適格であり、「コンパクトディスクの種類」ではない。
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ウィキメディア・コモンズには、コンパクトディスクに関連するカテゴリがあります。 |
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The readable surface of a compact disc includes a spiral track wound tightly enough to cause light to diffract into a full visible spectrum
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Media type | Optical disc |
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Encoding | Various |
Capacity | Typically up to 700 MiB (up to 80 minutes audio) |
Read mechanism | 780 nm wavelength (infrared and red edge) semiconductor laser, 1200 Kib/s (1×) |
Write mechanism | 1200 Kib/s (1×) |
Developed by | Philips, Sony |
Usage | Audio and data storage |
Optical discs |
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General
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Optical media types
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Standards
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See also
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Compact disc, or CD for short, is a digital optical disc data storage format. The format was originally developed to store and play back sound recordings only (CD-DA), but was later adapted for storage of data (CD-ROM). Several other formats were further derived from these, including write-once audio and data storage (CD-R), rewritable media (CD-RW), Video Compact Disc (VCD), Super Video Compact Disc (SVCD), Photo CD, PictureCD, CD-i, and Enhanced Music CD. Audio CDs and audio CD players have been commercially available since October 1982.
Standard CDs have a diameter of 120 millimetres (4.7 in) and can hold up to about 80 minutes of uncompressed audio or 700 MiB (actually about 703 MiB or 737 MB) of data. The Mini CD has various diameters ranging from 60 to 80 millimetres (2.4 to 3.1 in); they are sometimes used for CD singles, storing up to 24 minutes of audio or delivering device drivers.
At the time of the technology's introduction it had much greater capacity than computer hard drives common at the time. The reverse is now true, with hard drives far exceeding the capacity of CDs.
In 2004, worldwide sales of CD audio, CD-ROM, and CD-R reached about 30 billion discs. By 2007, 200 billion CDs had been sold worldwide.[1] Compact discs are increasingly being replaced or supplemented by other forms of digital distribution and storage, such as downloading and flash drives, with audio CD sales dropping nearly 50% from their peak in 2000.[2]
The Compact Disc is an evolution of LaserDisc technology. Prototypes were developed by Philips and Sony independently from the mid-to-late 1970s. The two companies then collaborated to produce a standard format and related player technology which was made commercially available in 1982.
In 1974, an initiative was taken by L. Ottens, a director of the audio industry group within the Philips Technology Corporation in Eindhoven, the Netherlands. A seven-person project group was formed to develop an optical audio disc with a diameter of 20 cm with a sound quality superior to that of the large and fragile vinyl record.[3] In March 1974, during a meeting of the audio group, two engineers from the Philips research laboratory recommended the use of a digital format on the 20 cm optical disc, because an error-correcting code could be added.[3] It wasn't until 1977 that the directors of the group decided to establish a laboratory with the mission of creating a small optical digital audio disc and a small player. They chose the term "compact disc" in line with another Philips product, the compact cassette.[3] Rather than the original 20 cm size, the diameter of this compact disc was set at 11.5 cm, the diagonal measurement of a compact cassette.[3]
Meanwhile, Sony first publicly demonstrated an optical digital audio disc in September 1976. In September 1978, the company demonstrated an optical digital audio disc with a 150 minute playing time, 44,056 Hz sampling rate, 16-bit linear resolution, and cross-interleaved error correction code—specifications similar to those later settled upon for the standard Compact Disc format in 1980. Technical details of Sony's digital audio disc were presented during the 62nd AES Convention, held on March 13–16, 1979, in Brussels.[4] Just before that, on March 8, 1979 Philips publicly demonstrated a prototype of an optical digital audio disc at a press conference called "Philips Introduce Compact Disc"[5] in Eindhoven, Netherlands.[6] Thirty years later, on March 6, 2009, Philips received an IEEE Milestone award with the following citation: "On 8 March 1979, N.V. Philips' Gloeilampenfabrieken demonstrated for the international press a Compact Disc Audio Player. The demonstration showed that it is possible by using digital optical recording and playback to reproduce audio signals with superb stereo quality. This research at Philips established the technical standard for digital optical recording systems."[7]
Sony executive Norio Ohga, who later became the CEO and chairman of Sony, was convinced of the format's commercial potential and pushed further development despite widespread skepticism.[8]
Later in 1979, Sony and Philips set up a joint task force of engineers to design a new digital audio disc. Led by Kees Schouhamer Immink and Toshitada Doi, the research pushed forward laser and optical disc technology that began independently by the two companies.[5] After a year of experimentation and discussion, the task force produced the Red Book CD-DA standard. First published in 1980, the standard was formally adopted by the IEC as an international standard in 1987, with various amendments becoming part of the standard in 1996.
Philips contributed the general manufacturing process, based on video LaserDisc technology. Philips also contributed eight-to-fourteen modulation (EFM), which offers a certain resilience to defects such as scratches and fingerprints, while Sony contributed the error-correction method, CIRC.
The Compact Disc Story,[9] told by a former member of the taskforce, gives background information on the many technical decisions made, including the choice of the sampling frequency, playing time, and disc diameter. The task force consisted of around four to eight persons,[10][11] though according to Philips, the Compact Disc was "invented collectively by a large group of people working as a team."[12]
Philips established the Polydor Pressing Operations plant in Langenhagen near Hannover, Germany, and quickly passed a series of milestones.
The Japanese launch was followed in March 1983 by the introduction of CD players and discs to Europe,[17] and North America (where CBS Records released sixteen titles).[18] This event is often seen as the "Big Bang" of the digital audio revolution. The new audio disc was enthusiastically received, especially in the early-adopting classical music and audiophile communities, and its handling quality received particular praise. As the price of players gradually came down, the CD began to gain popularity in the larger popular and rock music markets. The first artist to sell a million copies on CD was Dire Straits, with its 1985 album Brothers in Arms.[19] The first major artist to have his entire catalogue converted to CD was David Bowie, whose 15 studio albums were made available by RCA Records in February 1985, along with four Greatest Hits albums.[20] In 1988, 400 million CDs were manufactured by 50 pressing plants around the world.[21]
The CD was planned to be the successor of the gramophone record for playing music, rather than primarily as a data storage medium. From its origins as a musical format, CDs have grown to encompass other applications. In June 1985, the computer-readable CD-ROM (read-only memory) and, in 1990, CD-Recordable were introduced, also developed by both Sony and Philips.[22] Recordable CDs are an alternative to tape for recording music and copying music albums without defects introduced in compression used in other digital recording methods. Other newer video formats such as DVD and Blu-ray use the same physical geometry as CD, and most DVD and Blu-ray players are backward compatible with Audio CD.
By the early 2000s, the CD had largely replaced the audio cassette player as standard equipment in new automobiles, with 2010 being the final model year for any car in the US to have a factory-equipped cassette player.[23] With the increasing popularity of portable digital audio players and solid state music storage, CD players are being phased out of automobiles in favor of minijack auxiliary inputs and connections to USB devices.
Meanwhile, with the advent and popularity of digital audio formats, such as the 256 kbit m4a, sales of CDs began dropping in the 2000s. For example, during the eight-year period ending in 2008, despite overall growth in music sales and one anomalous year of increase, major-label CD sales declined overall by 20% [24] although independent and DIY music sales may be tracking better according to figures released March 30, 2009 and CDs still sell greatly nonetheless.[25]
A CD is made from 1.2 millimetres (0.047 in) thick, polycarbonate plastic and weighs 15–20 grams.[26] From the center outward, components are: the center spindle hole (15 mm), the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the program (data) area, and the rim. The inner program area occupies a radius from 25 to 58 mm.
A thin layer of aluminium or, more rarely, gold is applied to the surface making it reflective. The metal is protected by a film of lacquer normally spin coated directly on the reflective layer. The label is printed on the lacquer layer, usually by screen printing or offset printing.
CD data is represented as tiny indentations known as "pits", encoded in a spiral track moulded into the top of the polycarbonate layer. The areas between pits are known as "lands". Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in length. The distance between the tracks, the pitch, is 1.5 µm.
Scanning velocity is 1.2–1.4 m/s (constant linear velocity) – equivalent to approximately 500 rpm at the inside of the disc, and approximately 200 rpm at the outside edge. (A disc played from beginning to end slows down during playback.)
The program area is 86.05 cm2 and the length of the recordable spiral is (86.05 cm2 / 1.6 µm) = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or 650 MB of data on a CD-ROM. A disc with data packed slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m/s and a narrower track pitch of 1.5 µm increases the playing time to 80 minutes, and data capacity to 700 MB.
A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in the way the light is reflected. By measuring the intensity change with a photodiode, the data can be read from the disc.
The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, non-return-to-zero, inverted encoding is used: a change from pit to land or land to pit indicates a one, while no change indicates a series of zeros. There must be at least two and no more than ten zeros between each one, which is defined by the length of the pit. This in turn is decoded by reversing the eight-to-fourteen modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon Coding, finally revealing the raw data stored on the disc. These encoding techniques (defined in the Red Book) were originally designed for the CD Digital Audio, but they later became a standard for almost all CD formats (such as CD-ROM).
CDs are susceptible to damage during handling and from environmental exposure. Pits are much closer to the label side of a disc, enabling defects and contaminants on the clear side to be out of focus during playback. Consequently, CDs are more likely to suffer damage on the label side of the disc. Scratches on the clear side can be repaired by refilling them with similar refractive plastic, or by careful polishing. The edges of CDs are sometimes incompletely sealed, allowing gases and liquids to corrode the metal reflective layer and to interfere with the focus of the laser on the pits.[27] The fungi Geotrichum candidum, found in Belize, has been found to consume the polycarbonate plastic and aluminium found in CD's. [28] [29]
The digital data on a CD begins at the center of the disc and proceeds toward the edge, which allows adaptation to the different size formats available. Standard CDs are available in two sizes. By far, the most common is 120 millimetres (4.7 in) in diameter, with a 74- or 80-minute audio capacity and a 650 or 700 MB (737,280,000 bytes) data capacity. This capacity was reportedly specified by Sony executive Norio Ohga so as to be able to contain the entirety of London Philharmonic Orchestra's recording of Beethoven's Ninth Symphony on one disc.[8] This diameter has been adopted by subsequent formats, including Super Audio CD, DVD, HD DVD, and Blu-ray Disc. 80 mm discs ("Mini CDs") were originally designed for CD singles and can hold up to 24 minutes of music or 210 MB of data but never became popular.[citation needed] Today, nearly every single is released on a 120 mm CD, called a Maxi single.[citation needed]
Novelty CDs are also available in numerous shapes and sizes, and are used chiefly for marketing. A common variant is the "business card" CD, a single with portions removed at the top and bottom making the disk resemble a business card.
Physical size | Audio Capacity | CD-ROM Data Capacity | Definition |
---|---|---|---|
120 mm | 74–80 min | 650–700 MB | Standard size |
80 mm | 21–24 min | 185–210 MB | Mini-CD size |
80x54 mm – 80x64 mm | ~6 min | 10-65 MB | "Business card" size |
The logical format of an audio CD (officially Compact Disc Digital Audio or CD-DA) is described in a document produced by the format's joint creators, Sony and Philips in 1980. The document is known colloquially as the Red Book CD-DA after the color of its cover. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel. Four-channel sound was to be an allowable option within the Red Book format, but has never been implemented. Monaural audio has no existing standard on a Red Book CD; mono-source material thus is usually presented as two identical channels in a standard Red Book stereo track (i.e. mirrored mono), MP3 CDs however can also have audio file formats with mono sound.
CD-Text is an extension of the Red Book specification for audio CD that allows for storage of additional text information (e.g., album name, song name, artist) on a standards-compliant audio CD. The information is stored either in the lead-in area of the CD, where there is roughly five kilobytes of space available, or in the subcode channels R to W on the disc, which can store about 31 megabytes.
Compact Disc + Graphics is a special audio compact disc that contains graphics data in addition to the audio data on the disc. The disc can be played on a regular audio CD player, but when played on a special CD+G player, can output a graphics signal (typically, the CD+G player is hooked up to a television set or a computer monitor); these graphics are almost exclusively used to display lyrics on a television set for karaoke performers to sing along with. The CD+G format takes advantage of the channels R through W. These six bits store the graphics information.
CD + Extended Graphics (CD+EG, also known as CD+XG) is an improved variant of the Compact Disc + Graphics (CD+G) format. Like CD+G, CD+EG utilizes basic CD-ROM features to display text and video information in addition to the music being played. This extra data is stored in subcode channels R-W. Very few, if any, CD+EG discs have been published.
Super Audio CD (SACD) is a high-resolution read-only optical audio disc format that was designed to provide higher fidelity digital audio reproduction than the Red Book. Introduced in 1999, it was developed by Sony and Philips, the same companies that created the Red Book. SACD was in a format war with DVD-Audio, but neither has replaced audio CDs. The SACD standard is referred to the Scarlet Book standard.
Titles in the SACD format can be issued as hybrid discs; these discs contain the SACD audio stream as well as a standard audio CD layer which is playable in standard CD players, thus making them backward compatible.
CD-MIDI is a format used to store music-performance data which upon playback is performed by electronic instruments that synthesize the audio. Hence, unlike the original Red Book CD-DA, these recordings are not digitally sampled audio recordings. The CD-MIDI format is defined as an extension to the original Red Book.
For the first few years of its existence, the CD was a medium used purely for audio. However, in 1985 the Yellow Book CD-ROM standard was established by Sony and Philips, which defined a non-volatile optical data computer data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive.
Video CD (VCD, View CD, and Compact Disc digital video) is a standard digital format for storing video media on a CD. VCDs are playable in dedicated VCD players, most modern DVD-Video players, personal computers, and some video game consoles.
The VCD standard was created in 1993 by Sony, Philips, Matsushita, and JVC and is referred to as the White Book standard.
Overall picture quality is intended to be comparable to VHS video. Poorly compressed VCD video can sometimes be lower quality than VHS video, but VCD exhibits block artifacts rather than analog noise, and does not deteriorate further with each use, which may be preferable.
352x240 (or SIF) resolution was chosen because it is half the vertical, and half the horizontal resolution of NTSC video. 352x288 is similarly one quarter PAL/SECAM resolution. This approximates the (overall) resolution of an analog VHS tape, which, although it has double the number of (vertical) scan lines, has a much lower horizontal resolution.
Super Video CD (Super Video Compact Disc or SVCD) is a format used for storing video media on standard compact discs. SVCD was intended as a successor to VCD and an alternative to DVD-Video, and falls somewhere between both in terms of technical capability and picture quality.
SVCD has two-thirds the resolution of DVD, and over 2.7 times the resolution of VCD. One CD-R disc can hold up to 60 minutes of standard quality SVCD-format video. While no specific limit on SVCD video length is mandated by the specification, one must lower the video bit rate, and therefore quality, to accommodate very long videos. It is usually difficult to fit much more than 100 minutes of video onto one SVCD without incurring significant quality loss, and many hardware players are unable to play video with an instantaneous bit rate lower than 300 to 600 kilobits per second.
Photo CD is a system designed by Kodak for digitizing and storing photos on a CD. Launched in 1992, the discs were designed to hold nearly 100 high quality images, scanned prints and slides using special proprietary encoding. Photo CDs are defined in the Beige Book and conform to the CD-ROM XA and CD-i Bridge specifications as well. They are intended to play on CD-i players, Photo CD players and any computer with the suitable software irrespective of the operating system. The images can also be printed out on photographic paper with a special Kodak machine. This format is not to be confused with Kodak Picture CD, which is a consumer product in CD-ROM format.
The Philips Green Book specifies a standard for interactive multimedia compact discs designed for CD-i players (1993). CD-i discs can contain audio tracks which can be played on regular CD players, but CD-i discs are not compatible with most CD-ROM drives and software. The CD-i Ready specification was later created to improve comaptibility with audio CD players, and the CD-i Bridge specification was added to create CD-i compatible discs than can be accessed by regular CD-ROM drives.
Phillips defined a format similar to CD-i called CD-i Ready, which puts CD-i software and data into the pregap of track 1. This format was supposed to be more compatible with older audio CD players.
Enhanced Music CD, also known as CD Extra and CD Plus, is a format which combines audio tracks and data tracks on the same disc by putting audio tracks in a first session and data in a second session. It was developed by Phillips and Sony and it is defined in the Blue Book.
Vinyl Disc is the hybrid of a standard Audio CD and the vinyl record. The vinyl layer on the disc's label side can hold approximately three minutes of music.
Replicated CDs are mass-produced initially using a hydraulic press. Small granules of heated raw polycarbonate plastic are fed into the press. A screw forces the liquefied plastic into the mold cavity. The mold closes with a metal stamper in contact with the disc surface. The plastic is allowed to cool and harden. Once opened, the disc substrate is removed from the mold by a robotic arm, and a 15 mm diameter center hole (called a stacking ring) is created. The time it takes to "stamp" one CD, is usually 2 to 3 seconds.
This method produces the clear plastic blank part of the disc. After a metallic reflecting layer (usually aluminium, but sometimes gold or other metal) is applied to the clear blank substrate, the disc goes under a UV light for curing and it is ready to go to press. To prepare to press a CD, a glass master is made, using a high-powered laser on a device similar to a CD writer. The glass master is a positive image of the desired CD surface (with the desired microscopic pits and lands). After testing, it is used to make a die by pressing it against a metal disc.
The die is a negative image of the glass master: typically, several are made, depending on the number of pressing mills that are to make the CD. The die then goes into a press and the physical image is transferred to the blank CD, leaving a final positive image on the disc. A small amount of lacquer is applied as a ring around the center of the disc, and rapid spinning spreads it evenly over the surface. Edge protection lacquer is applied before the disc is finished. The disc can then be printed and packed.
Manufactured CDs that are sold in stores are sealed via a process called "polywrapping" or shrink wrapping.
The most expensive part of a CD is the jewel case. In 1995, material costs were 30 cents for the jewel case and 10 to 15 cents for the CD. Wholesale cost of CDs was $0.75 to $1.15, which retailed for $16.98.[30] On average the store received 35 percent of the retail price, the record company 27 percent, the artist 16 percent, the manufacturer 13 percent, and the distributor 9 percent.[30] When 8-track tapes, cassette tapes, and CDs were introduced, each was marketed at a higher price than the format they succeeded, even though the cost to produce the media was reduced. This was done because the apparent value increased. This continued from vinyl to CDs but was broken when Apple marketed MP3s for $0.99, and albums for $9.99. The incremental cost, though, to produce an MP3 is infinitely small.[31]
Recordable Compact Discs, CD-Rs, are injection molded with a "blank" data spiral. A photosensitive dye is then applied, after which the discs are metalized and lacquer-coated. The write laser of the CD recorder changes the color of the dye to allow the read laser of a standard CD player to see the data, just as it would with a standard stamped disc. The resulting discs can be read by most CD-ROM drives and played in most audio CD players. CD-Rs follow the Orange Book standard.
CD-R recordings are designed to be permanent. Over time the dye's physical characteristics may change, however, causing read errors and data loss until the reading device cannot recover with error correction methods. The design life is from 20 to 100 years, depending on the quality of the discs, the quality of the writing drive, and storage conditions. However, testing has demonstrated such degradation of some discs in as little as 18 months under normal storage conditions.[32][33] This failure is known as disc rot, for which there are several, mostly environmental, reasons.[27]
The recordable audio CD is designed to be used in a consumer audio CD recorder. These consumer audio CD recorders use SCMS (Serial Copy Management System), an early form of digital rights management (DRM), to conform to the AHRA (Audio Home Recording Act). The Recordable Audio CD is typically somewhat more expensive than CD-R due to lower production volume and a 3% AHRA royalty used to compensate the music industry for the making of a copy.[34]
High-capacity recordable CD is a higher density recording format that can hold 90 or 99 minutes of audio on a 12 cm (5 in) disc (compared to about 80 minutes for Red Book audio), or 30 minutes of audio on an 8 cm (3 in) disc (compared to about 24 minutes for Red Book audio).[35] The higher capacity is incompatible with some recorders and recording software.[36]
CD-RW is a re-recordable medium that uses a metallic alloy instead of a dye. The write laser in this case is used to heat and alter the properties (amorphous vs. crystalline) of the alloy, and hence change its reflectivity. A CD-RW does not have as great a difference in reflectivity as a pressed CD or a CD-R, and so many earlier CD audio players cannot read CD-RW discs, although most later CD audio players and stand-alone DVD players can. CD-RWs follow the Orange Book standard.
The ReWritable Audio CD is designed to be used in a consumer audio CD recorder, which won't (without modification) accept standard CD-RW discs. These consumer audio CD recorders use the Serial Copy Management System (SCMS), an early form of digital rights management (DRM), to conform to the United States' Audio Home Recording Act (AHRA). The ReWritable Audio CD is typically somewhat more expensive than CD-RW due to (a) lower volume and (b) a 3% AHRA royalty used to compensate the music industry for the making of a copy.[34]
Due to technical limitations, the original ReWritable CD could be written no faster than 4x speed. High Speed ReWritable CD has a different design that permits writing at speeds ranging from 4x to 12x. Original CD-RW drives can only write to original ReWritable CDs. High Speed CD-RW drives can typically write to both original ReWritable CDs and High Speed ReWritable CDs. Both types of CD-RW discs can be read in most CD drives. Higher speed CD-RW discs, Ultra Speed (16x to 24x write speed) and Ultra Speed+ (32x write speed), are now available.
The Red Book audio specification, except for a simple 'anti-copy' statement in the subcode, does not include any copy protection mechanism. Known at least as early as 2001,[37] attempts were made by record companies to market "copy-protected" non-standard compact discs, which cannot be ripped, or copied, to hard drives or easily converted to MP3s. One major drawback to these copy-protected discs is that most will not play on either computer CD-ROM drives, or some standalone CD players that use CD-ROM mechanisms. Philips has stated that such discs are not permitted to bear the trademarked Compact Disc Digital Audio logo because they violate the Red Book specifications. Numerous copy-protection systems have been countered by readily available, often free, software.
This "see also" section may contain an excessive number of suggestions. Please ensure that only the most relevant suggestions are given and that they are not red links, and consider integrating suggestions into the article itself. (February 2013) |
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CD1 | CD21 | CD41 | CD61 | CD81 |
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CD6 | CD26 | CD46 | CD66 | CD86 |
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CD9 | CD29 | CD49 | CD69 | CD89 |
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CD13 | CD33 | CD53 | CD73 | CD93 |
CD14 | CD34 | CD54 | CD74 | CD94 |
CD15 | CD35 | CD55 | CD75 | CD95 |
CD16 | CD36 | CD56 | CD76 | CD96 |
CD17 | CD37 | CD57 | CD77 | CD97 |
CD18 | CD38 | CD58 | CD78 | CD98 |
CD19 | CD39 | CD59 | CD79 | CD99 |
CD20 | CD40 | CD60 | CD80 | CD100 |
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