Compact disk

Principle of operation

The compact disk rests on a method of optical character reading: a beam of coherent light (Laser) comes to strike the disc in rotation. The irregularities (cavities) in the reflective surface of this one produce variations binary S. the considered ray is recorded by a Capteur. When it is used as support for musical listening (first uses) binary information is then transformed into a signal Analogique by a converter.

As of its appearance, this support was promoted by its inventive S and the musical editors like offering better a sound quality than the other existing supports (in particular the Disques vinyl). These qualities are disputed today and of new supports appeared (SACD - Super Audio Compact Disc or DVD - has - DIGITAL Versatile Audio Disc). One in addition notes a renewal of popularity of the support vinyl.

History

The compact disk was invented jointly by the firms Philips and Sony Corporation (but especially Philips which invested much in research on the optical recording since the Années 1950) with, also, the participation of Hitachi for the audionumerical one (audio CD) in 1979. When the two companies decided to work together in 1979, the project provided that platinums laser would be equipped with the most powerful electronic chips ever marketed for a product general public. In 1980, a " deliver orange" specified the features of the new disc and the division of the patents between the two competitors: in Philips design of CD (on the basis of their experiment of the technology of the Laserdisc ) and of the lenses which allow the reading; in Sony the definition of the format used to digitize the music and the method of correction of errors. Among the principal members of the team, most known are Pieter Kramer (directing optical research laboratory of Philips in the Années 1970) and Kees A. Schouhamer Immink for Philips and Toshitada DOI for Sony.

The first prototypes produced by Philips measured 115 mms in diameter, with a Codage on 14 bits and a capacity of 60 minute S. Sony insisted that one adopts a coding on 16 bits and a 74 minutes duration, which increased the size of the disc with 120 Misters According to the rumors, the capacity of MINOR ROAD 12 centimetres was increased at 74 minutes, at the request of Herbert von Karajan, so that the slowest version of the 9 {{E}} symphony of Beethoven holds on only one CD. Sony indicates that it is at the request of the wife of its president, for these same reasons.

Philips and Sony announced at the end of August 1982 that they were ready to leave their new product and began the sales with the autumn. The industrial production began on August 17th, 1982 with Langenhagen, close to Hanover (the F.R.G.). The first turntable was sold with the Japan on October 1st, 1982 accompanied by the album " 52nd Street" of Billy Joel.

The success of CD progressive, is limited initially to the album The Visitors of ABBA (Polygram, label of Philips), and to a recording of the Alpine Symphony of Richard Strauss directed by Karajan. Indeed, CD especially passes in the first times for a support reserved to the music lovers traditional, thanks to the sound quality which it offers. Some 200 titles, traditional primarily, thus are produced by Philips. It is the marketing of the Brothers album in Arms, of the group Dire Straits (first entirely numerical album), which democratizes CD: the album is sold to more than one million specimens. There is not any more a doubt that CD is the sound support of the future.

As of 1986, platinums laser were sold better than the others and in 1988 the sales CD exceeded those of vinyls. CD was a broad success and quickly replaced the Disques vinyl like musical support, in particular thanks to following qualities:

Absence of wear due to the reading (the optical character reading removes the mechanical contact and thus the deterioration of the support by friction). In reality, the real average lifespan of the supports is disputed, certain granting to the compact disks an life expectancy only ten years (degradations can be: in stripes, oxidation…), but the careful users will be able to longer preserve their CD in good state during good.
Sizes of the support: its 12 centimetres diameter confer a portability to him that the Microsillon did not have. A second format of 8 centimetres, is also standardized to him.
the thickness is of 1.2 mm nominal.
theoretical Quality of sound reproduction higher than the cassettes audio and Discs vinyls. (More important Dynamics, exact reproduction with each reading thanks to the system of correction of error, demanding etc However some audiophiles or music lovers prefer the sound of vinyl which they consider more musical and more naturalness.)
Retour to complete listening without having to turn over the audio support in the reader with an access without mechanical handling, which had not seen since the disappearance of the cartridges 8 tracks. The cassettes audios and the discs vinyls were recorded on two sides, one was to thus turn over them to semi-listening.
the CD-R (virgin CD to engrave) have same dimensions, and can be used to store data Red Book (which defines the Audio standard for CD, such as the 44.1 Khz of Sampling rate and 16 bits of resolution). There exist the CD-ROMs “PC” which are designed for a use with an engraver in a PC, and the CD-ROMs “Audio” which are designed for the recorders of house (which cannot read CD-ROMs “PC”). They are more expensive besides because they contain a percentage for the Royalty who is transferred with SACEM in France or SABAM in Belgium.

It should be noted that Compact Disc is a Registered trademark by the firm Dutchwoman Koninklijke Philips Electronics N.V and that the latter refuses the use of the term deposited for any audio disc protected from the copy.

Types of discs

One distinguishes several types of compact disks:

CD audio (CDDA or HALF-VALUE LAYER): Compact Audio DIGITAL Disc or in French Compact Audio Disc .
CD-ROM ( Compact Disc Read-Only Memory ), officially cédérom in French: data-processing memory support.
CD-WORM : ( Cd-Write Ounce Read Many ) CD Alternative which can be engraved only once by l utilisator (on an engraver of CD-ROM) and then read on n´importe which reader of CD-ROM.
Extra CD (or Enhanced CD or Disc improved ): compact disk joining together in first a session containing only the audio tracks and a session containing only data. This type of disc is readable in 99,99% of the Cd readers (car radios, hi-fi systems,…). CD OpenDisc are technically Extra CD.
CD in mixed mode: compact disk joining together in first a session containing only data and a session containing only audio tracks. This type of disc is not readable on all the Cd readers (car radios, hi-fi systems,…). It is this type of disc which some majors create by putting the symbol " Copy-controlled".
CD-R : Compact Disc Recordable , writable Disc.
CD-RW : Compact Disc Rewritable , rewriteable Disc.
CD+G : Compact Disc + Graphics , Compact disk and Images
VCD: Video Compact Disc , video Compact disk.
SVCD : Super Video Compact Disc , Super compact video disc.

The reading devices for Cd-audio are not designed to read the CD-ROMs; a contrario , the readers of CD-ROMs can also read the Cd-audio. There exist also “hybrid” CD containing audio information (readable by an audio reader) and information of other types (text, video, images, etc), readable by a reader of CD-ROM (CD in mixed mode and Extra CD referred to above).

Lately, with the arrival of the method of audio compression MPEG-1 To bush-hammer 3 shortened MP3, of the audio readers being able to read tracks MP3 on a CD-R (W) and to play them as traditional audio CD was developed. The interest of format MP3 is that it makes it possible to store up to ten times more music than on audio CD with a more or less perceptible degradation of sound quality according to the Bitrate to which the disc/the piece was compressed.

Physical details

The compact disks consist of a wafer of Polycarbonate of 1,2 millimetre thickness covered with a fine layer of Aluminum (at the beginning, it was of a layer of Or and it is still the case currently on the discs at long life of life) protected by a film from lacquer. This film can also be printed to illustrate the disc. The techniques of impression are the offset and the Sérigraphie.

Information on standard CD is coded on a track of cells in spiral moulded in polycarbonate. Each cell measures approximately between 125 [[Nanometer Nm]] (0.000000125 m) and 500 Nm broad and varies between 833 Nm and 3.5 [[µm]] in length. Space between the tracks is of 1.6 µm. To give an idea of dimensions, if the disc were put on a stage scale of foot, an cell would have the size of a grain of sand. The spiral starts in the center of the disc to finish in periphery, which authorizes several sizes of discs.

CD is read by a diode Laser of 780 Nm wavelength through the layer of polycarbonate (diameter of the spot: 1.04 µm). The difference in depth between an cell (hollow) and the plane surface (bump) are of a quarter the wavelength of the laser, which makes it possible to have a dephasing of a half-length of wave between a reflection of the laser in an cell and on the plane surface. The destructive Interférence caused by this reflection reduces the intensity of reflected light in an cell compared with a reflection on the plane surface. By measuring this intensity with a photodiode, one is able to read the data on the disc.

The hollows and the bumps do not represent the 0 and the 1 of the binary data. It is the passage of a hollow to a bump or a bump to a hollow which indicates a 1 . If there is no passage bump-hollow, then it is about a 0 . One calls that a face .

Then, these data pass to the moulinette EFM (Eight-to-Fourteen Modulation) used during coding the audionumerical data in numerical data for audio CD, for finally obtaining the rough audionumerical data.

Manufacturing method

The industrial production of CD is done according to various stages:

the Prématriçage
the creation of the matrix of glass
the manufacture of the matrices of
production series production

1. The prématriçage corresponds to the transcription of information of the customer on a tape with 9 tracks, while passing by a phase of correction of errors, and formatting of the files to the format ISO 9660 in the case of a CD-ROM. The essential goal of the prématriçage is the calculation of the Detecting Code and the Correct Code. These codes are contained on 288 bytes joined with 2 [[Kilooctet KB]] of information more of information of synchronization and heading. This process makes it possible to prevent the errors of transmission. Once this stage passed, it has there no more no modification of the data to register.

2. The creation of the Disque stamps, also called matrix of glass, consists with the marking of the data on a disc of glass. The starting point of the disc matrix is a strongly polished pane, whose characteristics of surface resemble of close with an astronomical mirror. This glass plate is covered with a substrate sensitive to the light, called photosensitive resin. The cover of the plate by a process of rotation (deposit by centrifugation) ensures an absolutely plane and uniform layer of 120 Nm thickness. It is the thickness of this layer which determines the depth of the hollows.

The inscription of the data is carried out thanks to an apparatus emitting a laser beam which is activated and decontaminated at the rate/rhythm of information. The ray thus modulated mark the photosensitive layer of the glass plate. The disc of glass is then placed in a developer. The sites deteriorated by the ray are washed thus revealing the first hollows.

After drying of the disc matrix follows the vacuum vaporization of a layer silver plated of 100 Nm, called Galvanoplastie. At this stage, the disc matrix is readable by a special reader who allows to control the quality of the recording.

3. Galvanization is an operation which creates the matrix of production starting from the matrix of glass. The matrix of glass is plunged in a bath of galvanization comprising a Anode Nickel. The silver plated layer of the matrix of glass is transformed into Cathode. The current thus created involves a displacement of the nickel ions on the anode, covering little by little the glass plate of a layer of nickel. The separation of the layer of nickel of its drinking glass holder brings the destruction of this last. So at this stage of the operation the quality standards are not respected, all the preceding process is to be remade. The layer of nickel, copies drawn directly from the matrix of glass, is named original or copy father : it is a reproduction into negative original. To avoid a loss of this original, one makes of it a copy called copy mother , which is then used to draw the submatrices. The submatrices are, like the original, as negative and serve to print the data on the plastic discs during their manufacture. They are perforated in the center and polished with endorsement. The quality of the back of the matrix has a great influence on the noise which will be perceived by the photoreceivers of the readers of CD-ROM. Maximum average roughness is of 600 Nm. Like the air, the cleanliness of water is important for the final quality of the product.

4. The mass production of the compact disks can be done by moulding injection or pressure. This first principle consists of the injection of the Polycarbonate liquid in the matrix; the second system, has as a principle the impression of the basins in the still hot disc by pressing. The polycarbonate was retained in the design of CD for its properties such as the optical purity, the transparency and a constant index of refraction. The discs thus obtained see their face marked by the data, then metallized by a layer of aluminum from 40 to 50 Nm. With this intention, aluminum is atomized in a vacuum space, and settles slowly on the disc. Atomization is obtained by warming, or cold, by a process of cathode sputtering. The layer of aluminum thus deposited is finally protected by the application from a protective varnish, using the process of deposit by centrifugation. The varnish becomes thus a uniform layer of 10 µm thickness. Before conditioning, a label is printed on varnish by the principle of the Sérigraphie.

CD thus product ensures a longevity of about a century if however it is well treated by its owner and his successors. In comparison, a CD-R has one lifespan of about a decade, because of its sensitivity to the luminous rays.

Audio format

The format of data, known under the name of standard Red Book , was drawn up by Dutch Electronics of the Philips group which has the rights of CDDA and of the logo which appears on the discs. In technical terms, it is about a stereo track encodée in PCM with a resolution of 16 bits (linear in amplitude, without compression logarithmic curve of the high amplitudes) with a sampling rate of 44.1 Khz.

The samples are then gathered in frame , each frame comprises 6 stereo samples (6×2×16 bits = 192 bits is 24 bytes), plus 8 bytes of correction of error and one 1 byte of subcode , is a total of 33 bytes by frame . The correct code is added to allow the reading of a disc striped within the limits of the reasonable one, they are 2 Reed-Solomon code after and of an interlacing of the data carried out between 2 codings. The byte subcode is used to form 8 check mark channels (each channel having a binary debit of 7.35 kbps), in standard CD only the first 2 channels is used and is used for to indicate the beginnings of tracks, time, the preemphasis, the authorization of copy, the number of channels (stereo or quadraphony, but although the bit of indication of quadraphony exists in the standard, the way whose these additional channels must be coded is not defined and it is thus not used), the 6 other channels are used in the extensions as the CD+G (allows the insertion of the words for karaokés) or Cd-Text (name of the tracks, authors, interpreters).

The sampling rate of 44.1 Khz is inherited a method of conversion numerical of an audio signal into video signal for a recording to video cassette which was the only support offering a sufficient band-width to record the quantity of data necessary to an audionumerical recording. This technology can store 6 samples (3 per channel in stereophony) per horizontal line. A video signal NTSC has 245 lines usable by screen and 59,94 fields a second which function with: 44,056 samples a second. In the same way, a video signal STAKE or SECAM has 294 lines and 50 fields which also makes it possible to deliver: 44,100 samples a second. This system could also store samples of 14 bits with corrections of error or samples of 16 bits without correction of error. There be thus a long debate between Philips and Sony concerning the frequency and the resolution of sampling. Philips wanting to use the 44,100 Hz used in Europe and a resolution of 14 bits having already developed CNA 14 bits and Sony wanting to impose the 44,056 Hz used on Japan and the United States and a resolution of 16 bits.

Anecdote: they is thus for this that the first turntables CD were equipped with CNA 14 bits (TDA1540), Philips having found the means of using them in 16 bits by an oversampling 4×, the CNA thus functioned to 176.4 Khz instead of 44.1 Khz and was preceded by a numerical filter. This frequency 4 times higher made it possible to have a low-pass filter with a slope much more progressive than with the competitor CNA. The behavior in the frequencies close to 20,000 Hz was linear with less rotation of phase and the sound was all the more pure.

Storage capacity and speed

The specifications of the compact disk recommend a linear Speed of 1.22 m/s and a step between the tracks of 1.59 µm. That led to 74 minutes audio CD on a disc of 120 mm or approximately 650 Mo of data on a CD-ROM. Nevertheless, in order to authorize variations in the manufacture of the supports, there is a tolerance in the density of the tracks. By deliberately manufacturing discs of high density, one can increase the capacity and remain very close to the specifications of CD. While using a linear velocity of 1.1975 m/s and a step enters the tracks of 1.497 µm, one reaches a new maximum capacity 79 minutes and 40 seconds or 702 Mo. Although these discs have a light variation of manufacture, they are very often read by the readers and only a very low number of readers rejects them.

There exist recordable discs of 90 and 99 minutes, that by increasing the density of the tracks. But of other problems arise. First is that the maximum capacity that a disc can announce itself, in agreement with the specifications of the CD-R, is lower than 80 minutes. Second is that the markers of time between 90 and 99 minutes on the discs are normally reserved to indicate to the reader whom it reads the beginning of the disc and not the end. These problems are function of the manufacturers of discs, the engravers and the software of engraving. That means that the discs of more than 80 minutes are reserved for a niche market. Another technique to increase the capacity of a disc is to write in the preamble and the end of the disc which are normally designed to indicate the limits of the disc. That makes it possible to extend the capacity of one or two minutes, but that can cause problems of reading when the end of the disc is reached.

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