Compact Disc

The compact disc ( CD shortly, English for compact disc ) is an optical storage, the beginning of the 1980s, was introduced for digitally storing music by Philips / PolyGram, Sony, and alternative to vinyl records.

Later, the format of the Compact Disc has been extended so that they could save not only music (CD -DA). As CD -ROM since it is also used for storing data for computers. As a recordable CD -R they sparked in the late 1990s from the Compact Cassette as the preferred audio recording media in the private sector, but has been largely replaced in the 2000s by the MP3 technology.

4.1 Physical formats
4.2 Typical sizes

History

In the 1970s, engineers investigated very many electronics companies in the field of digital audio recording. The first prototypes were based on magnetic storage media, such as the classic audio cassette. The first device on the market was an extension of the Betamax video recorder from Sony is an analog -to-digital or digital -to-analog converter ( PCM modulator and demodulator ) in 1977. This is recorded by the video recorder in place of a video signal, the PCM signal, which - by means of appropriate coding in rows or images ( frames) - from the point of view of a video recorder look like a video signal. The chunky device and the noise in the recording failed to convince the consumers. Sony developed special techniques to eliminate the noise. To test this method, were made secretly at a rehearsal of a concert by Herbert von Karajan recordings in September 1978. Karajan was subsequently invited by Sony to assess the pictures.

At the same time, the team worked to Lou Ottens at the company Philips on the optical recording of image signals, which should revolutionize the video technology. Among other things, the image plate was presented at the radio exhibition Berlin, which had the format of an LP about and was reproduced by a correspondingly large player. Soon the idea of using this technology for digital sound developed. Both companies were suddenly faced with a problem. They had the new optical disc ( laser disc ) similar to the record scheduled with a diameter of 30 cm. When recording moving images could it accommodate about 30 minutes of video. For audio data but enough capacity for 13 hours and 20 minutes. Sony was clear that the business model of the music industry would collapse if such amounts should be marketed to music to consumers. After the Compact Cassette (Audio Cassette ) had been developed by the Philips company alone in 1963, now both companies have attempted to bring about a common standard. The key for the playing time of the CD diameter was established by the Philips leadership as follows: The Compact Cassette was a great success, the CD should not be much larger. The Compact Cassette had a diagonal of 11.5 cm, the engineers made the CD 0.5cm taller. All kinds of modern legends about the determination of these parameters; one of the most popular is the following:

After a few differences Sony suggested that the new CD at least should cover Ludwig van Beethoven's Ninth Symphony in its entirety. This proposal had to do with Sony's damaligem vice president Norio Oga, who was a trained opera singer and has always wanted to be able to hear Beethoven's Ninth Symphony without disturbing change the audio medium. Ogas favorite version, conducted by Herbert von Karajan, lasts 66 minutes, the technician talked to the then longest available version of Wilhelm Furtwängler. The recording from 1951 has a playing time of exactly 74 minutes. 74 minutes meant twelve centimeters in diameter of the optical disk. The developers of Philips responded with skepticism, since such a large disc would not fit in the suit pockets. Then reasonably Sony developers suits from around the world from, with the result that for twelve inches square was everywhere. Thus Beethoven had set a new standard.

A similar version of the story is officially distributed by Philips; the influence of Beethoven on the CD playing time but in some cases disputed.

In 1980, established by Philips and Sony for audio recordings of the " Red Book " standard. The diameter of the inner hole of the CD (15 mm ) was determined rather by accident by the Dutch Philips developers. The measure used was the time of its world's smallest coin, the Netherlands ten - cent piece ( the so-called Dubbeltje ), which had a developer in determining the diameter of it. On the Radio Exhibition in Berlin in 1981, the CD was first introduced to the public. The following year, on 17 August 1982 began in Langenhagen near Hannover, in the production of the then Polygram, even before October 1, 1982, the first produced the world's first industrial production of the last ABBA album The Visitors, and in series CD player could be offered on the market. 1983 the cost of a compact disc between 30 and 45 DM, 700 titles were available. In the same year some 70,000 CD players were sold in the Federal Republic of Germany with a purchase price in 1984 was 650-1800 DM. In 1988, already 100 million audio CDs were produced worldwide. Starting this year, there were systems that could be burned CDs, and no longer, as had to be sprayed to the point.

Paragraph numbers 1984-1991 and 2001-2012 in the Federal Republic of Germany:

Note: The Year of the biggest paragraph is highlighted in gray.

Production

CDs composed of polycarbonate, and a thin metal layer (for example, aluminum evaporation ) with a protective paint and inks. They are - in contrast to records - not pressed, but injected into injection molding machines in the form of the father - die. The equipment for the production of optical disks are still called press shop.

Operation

The information on the CD, called " program " are arranged on a spiraling outward trail, they occupy a maximum of 85% of the total CD. The program area reflects light with clear color phenomena because of its microstructure, the pits. Length and spacing of these small depressions form a serial digital code which represents the stored information. On an audio CD, a maximum of 99 tracks can be stored to each disc has a table of contents (TOC, table of contents ) and one of the information embedded time code, text overlays and further information can be optionally applied. The scanning of the disc without contact forth across a track tracked laser interference detector of the reflecting bottom. The clock frequency of the read data is dependent on the rotational speed of the CD, which is restricted in favor of a constant data clock rate for the traveling outward scanning system, so that a constant linear velocity (CLV ) results. In similar to the analog record method one speaks of constant angular velocity ( CAV).

Building a CD

With a CD data using a stored inside out ongoing spiral track are (ie in reverse from the record ). The spiral track consists of pits ( pits ) and lands (areas) that are applied to the polycarbonate. The pits have a length from 0.833 to 3.054 microns and a width of 0.5. The spiral track has a length of about six kilometers. Depending on how the disc is created, the created pit. In the industrial production are first photochemically a glass master and then it is manufactured by galvanic means, one or more stampers (negative). A polycarbonate disc is then so in pressing plants by spraying method and coined the reflection and added protection layer.

A CD is therefore for the most part made of polycarbonate. In the reflection layer consists of an aluminum film.

Between the imprint and the layer of aluminum ( thickness of the reflection layer 50 to 100 nm) is not a protective lacquer layer to protect the aluminum against external influences. The degree is the imprint which is applied to the screen printing process (up to six colors). Alternatively can be used here, too, the offset printing process.

Data rate of a CD

For single speed (1 × ) the data is read with 150 kbytes / s; this speed is also used when playing audio CDs. Data CDs can be read with higher data rates depending on the drive used.

CD -RW ( Compact disc rewritable )

A CD-RW medium has, in principle, the same layers as a CD-R medium. However, the reflective layer is a silver -indium- antimony -tellurium alloy, which has a polycrystalline structure and reflecting properties in the original state. Writing the write beam using its maximum power, and heating the material at points 500 and 700 ° C. This leads to a liquefying of the material. In this state, the alloy loses its polycrystalline structure assumes an amorphous state and loses its reflectivity. The polycrystalline state of the volume forms the trenches, the amorphous surveys. The scanning signal during reading so is not caused by cancellation or reinforcement of the laser - light by superposing the reflected light with the emitted as pressed discs (interference (Physics) ), but as writable CDs of given or not given (or weaker ) reflection of the laser beam. For deletion of the data carrier the write beam heats the amorphous regions with low power to about 200 ° C. The alloy is not liquefied, but returns to the polycrystalline state and is thus again able to reflection.

Read

The scanning of a CD by means of a laser diode ( wavelength of 780 nm), wherein the CD is read from the bottom. The laser beam is reflected on the CD, and bundled with a half mirror in an array of several photodiodes. The mirror must therefore be semi-transparent, because the laser beam on its way to CD there must pass. The photodiodes register fluctuations in brightness. The brightness fluctuations are caused partially due to destructive interference of the laser beam with itself: The focus of the laser beam is about two to three times as large as the width of a pit. Being a Pit read, then the laser beam in part from the pit and partly from the surrounding country is reflected. Then two partial waves are coming back, which have a slightly different path. The height difference between pit and land is chosen so that the propagation time difference is about half a wavelength ( see also section " Operation " ), so that because of destructive interference decreases the intensity of the reflected light. In addition, in the pit part of the light is scattered away at its edges. Thus, the photodiode register on the pits a reduced brightness. Since the discs are pressed from the upper side, the pits ( recesses ) are visible from the bottom side as a hill.

A special light guide onto the photodiode, for example by a Astigmata on a square array of four photodiodes, can by subtraction of the signals of different photodiodes addition to the useful (sum of all signals) and control variables for tracking and focus ( right distance between the CD and optical reading system ) are determined.

The optics with the laser moves when playing from the first to the last track - as opposed to the record - from the inside out. In addition, the CD has no fixed angular velocity ( rotational speed ); this is the current position of the read head adapted so that the web velocity (CLV ), and not, as in the record, the angular velocity (CAV ) is constant. When the read head reads on the outside on the CD, the CD is so rotated slowly. In this way can be carried out anywhere in the full CD recording density, and there is a constant stream secured as it is needed for audio CDs. In the Red Book two different speeds are fixed, 1.2 m / s and 1.4 m / s Thus, according to seasons of 74:41 minutes and 64:01 minutes, making maximum use of all tolerances 80:29 min possible. This corresponds to a rotational speed of 500 min - 1 at the beginning of the disc (inner trace ) to 200 min-1 on the outer edge of the disc. The rotational speed is controlled by a control circuit based on the fill level of a FIFO buffer. Therefore, no change has to take place ( either manually or automatically ) depending on the used linear speed. Through the above-mentioned buffer to fluctuations in speed do not affect the playback speed.

Many modern CD -ROM drives, from about a 32- fold reading speed, reading data CDs, however, with a constant rotational speed in order to avoid the time-consuming acceleration and deceleration of the CD when jumping back and forth the reading position. Characterized a data CD is dependent on the data rate of the position of the read head. Indicated on the packaging speed is almost always the maximum, not the average.

By the mechanical strength of the disc of increasing the read speed by increasing the rotational speed is limited. So-called " 52 -fold " drives spin the disc up to 10,000 min -1. At these speeds, even the smallest imbalance of the CD lead to strong vibrations, on the one hand clearly audible and could permanently damage both the drive and media to the other.

Data coding

To record the user data on the CD they have with an appropriate channel coding (more precisely, line coding ) are coded, which must comply with the peculiarities of the medium ( in this case the optical sensing system and the shape and size of the pits ) take into account. Wherein CD is the so-called Eight -to- Fourteen Modulation ( EFM). This ensures that every two to ten bars changes the polarity of the read-out signal. This happens when the laser in the track a transition from a depression ( pit) to a section without depression ( country ) happened or vice versa.

The background is as follows: The sections with grooves or without grooves must be long enough to allow the laser to detect the change. If you were to write a bit pattern directly to disk, incorrect values would in an alternating signal (1010101010101010 ... ) read since the laser could not read reliably the transition from 1 to 0 or from 0 to 1 and these transitions not only in plastic could ' be pressed. EFM inflates the signal from eight to 14 bits. In addition there are a further three padding bits, which are chosen so that the above- mentioned requirement that every two to ten bars changes the polarity, is also fulfilled between the 14 -bit symbols. Through this modulation, although the data rate on the one hand increases arithmetically, but on the other hand, can be chosen so high the data rate that unmodulated data could not be resolved in pits and country; a pit can not be less than its width, although may also vary within a fraction of the width of the length of its own - this fact is exploited by encoding. It is therefore a design decision that is responsible ( among others) of the playing time. Further provided by the coding for the signal of the photo diodes does not contain a DC component; wherein the signal processing is significantly simplified.

Error correction and error concealment

In order not to adversely affect scratches on the readability of the data, the data is secured using Reed -Solomon error correction, so that bit errors can be detected and corrected. Furthermore, consecutive data bytes are distributed by interleaving at greater length. The Cross Interleaved Reed -Solomon code (CIRC ) is thus able to correct an error of up to 3500 bits ( corresponding to a track length of about 2.4 mm) and errors of up to 12000 bit ( about 8.5 mm gauge length) to cover in the audio CD. Wherein the masking of the error is not corrected, but an attempt is made to make it inaudible, for example by interpolation. In case of severe scratching of the disc from the bottom but the readability is limited or impossible.

A distinction is made between C1 and C2 errors. C1 error give singular individual errors (for example, small scratches ), C2 larger block error which could not be corrected by the first correction step. The error of type C1 can be reported by only a few drives, for example, on Plextor or Lite-On based with special software ( Cdrtools, Plextools, k- sample). C2 error can be determined by most drives, and there is software for so-called C2 - scans, for example readcd, Nero CD Speed and CD -Doctor.

Information that can be derived from C1 or C2 errors, provide information on the state of the optical data carriers ( affected by aging, scratches, etc. ), about the basic reading or burning quality of an optical drive, depending on the speed (eg, a C2 - statistics of many media) and on the quality of fresh burnt media for long- term data storage (large C1-/C2-Werte after burning have a limited long-term safety data back ). One problem is that is difficult or impossible to separate the causes of errors. For example, a blank type that produces a specific burner bad values still reach good results with another burner type. In addition, the C2 - information can be used to access a computer to close while transferring audio material on the quality, was read out on the audio data from the CD. This allows critical points possibly to be read again or the other way around re-reading are restricted to the critical points.

When interpreting for the media is that new CDs should have a maximum of 250 C1 errors per second and no C2 errors. A frequent occurrence of C2 errors can be an indicator of a progressive aging of the medium represent. For data backup, a copying to a new medium is recommended. Such data backup should be done immediately after the purchase of media, Kopierschutz' mechanism are equipped with one ( "Un - CDs " ), as these are mostly produced intentionally with well over 250 C1 errors per second and therefore have a small amount of otherwise harmless scratches can make such media unreadable. Furthermore, the use of a repair or sprays of grinding and polishing machines is possible to save a damaged CD or DVD. However, satisfactory results can not be guaranteed in every case.

CD formats

Physical formats

CDs are available in two different sizes, the most common is the version with a diameter of 120 mm and 15 grams weight, less often the mini - CD with a diameter of 80 mm and 30 % of the storage capacity at a weight of 6.7 grams.

In addition, there are discs that have a shape other than a circular disk. These so-called shape CDs found but because of playback problems ( imbalance, no indentation in slot drives) only a small spread.

Typical sizes

The format specifications of the audio CD ( CD -DA short ), known as the "Red Book " standard, designed by the Dutch electronics company Philips. Philips also has the right to license the "Compact Disc Digital Audio" logo. The music information are in 16 -bit stereo ( quantization with 216 = 65,536 steps) and a sampling rate of 44.1 kilohertz saved.

The specifications of the CD -ROM are defined in the " Yellow Book " standard. A cross-platform file system of the CD -ROM has been written by the ISO standard ISO 9660. His successor is UDF.

There are several ways, audio CD contents and CD -ROM content on a disc to combine. The easiest way is to get a data track with the CD- ROM contents as the first track on the CD ( Mixed Mode CD, some manufacturers also "Enhanced CD" called ). The now virtually worthless advantage that the CD - ROM portion can also be read in only single-session compatible CD -ROM drives, is the comparatively large disadvantage of the visibility of this data tracks for regular audio CD player contrary - in particular, because some older CD players also allow playback of CD -ROM data as an audio data. This is expressed, depending on the volume setting in deafening and for the speakers potentially hazardous noise.

As a further development of the data track was provided with an index position of 0, which this is not approached by the CD player easily (i- Trax ). The audio begins, as with simple audio CDs at index position 1 of track 1 ( The problem for the playback compatibility might be the fact that changes to the audio in the track of mode of CD - ROM Mode 1. )

Today, virtually exclusively multisession CDs for this purpose used - the audio data is stored in the first session, while the CD -ROM data are contained in a second session, which is not read by audio CD players ( CD-Extra, CD -Plus). Of course, a multisession CD -ROM drive is required for the CD - ROM part.

A mixed form is the CD G ( CD Graphics). This CD represents at the same time to the music graphical data, such as the lyrics on screen, generates a common application of this format is karaoke. In a normal CD player, the CD G and regular audio CD is playable. On special devices ( more recently on some DVD players ) is to the music and the graphics on the screen visible. The additional data are stored in the subcode of the CD, i.e., in contrast to the content of data tracks is not readily visible to an operating system.

Other hand, found significantly more often CDs with CD text. Here, in the subcode of the CD (usually in Lead - in ) stored additional information, such as title and artist. This information is then displayed by appropriate devices during playback of the CD.

Other CD formats are:

CD -i
CDTV
Kodak Photo CD
Kodak Picture CD
Fujicolor CD
Image CD
Video CD

There are also so-called HDCD CDs. These are real 20-bit music information encoded (rather than 16) and should sound better in conjunction with corresponding CD players. HDCD CDs are fully compatible with "normal" CD players.

Further developments of the CD, the DVD-Audio and Super Audio Compact Disc (SACD ). DVD media provides much greater storage capacities of 4.7 ( one layer ) to 8.5 gigabytes ( two layers). The main advantage is no longer playing time, but that the audio in 5.1 sound format. While the Super Audio CD and DVD Audio only be used for audio data with the DVD, various data types are possible ( DVD Data, DVD -Video, DVD - Audio, DVD-ROM, DVD /-R (W) ). However, the DVD has not been enforced in the audio range.

A further developed by Sony version of the CD was the Double Density Compact Disc ( DDCD ). The storage capacity is twice the storage capacity of 640 MB CD. She was available in two versions ( a recordable DDCD -R and rewritable DDCD -RW ), but could not prevail against the DVD itself. Another variant is developed by Sony Blu- spec CD, which was introduced in 2008 in the market.

The CD- Video format (as opposed to the Video CD) no compact disc, but a LV / LD ( image plate ) with analog video data and digital audio data.

Copy protection

The CD standards do not see a copy before since the time of its establishment was not foreseeable in the early 1980s, that Recordable digital storage media with the necessary data capacity for the final consumer would be affordable in the near future - the copying was so easily prevented by there was nothing where the data could realistically be copied. There remained only the poorer quality and not any repeatable analog copy on audio cassette, which was also taken as records in purchasing. The advent of CD burners, high capacity disks, compression methods such as MP3 and Internet exchanges in the 1990s has changed this situation significantly.

Since the year 2001, media are sold in Germany that contain a copy protected to the digital readout of the audio data (and thus copying the data ) prevent. Although they were partly also referred to as audio CD, but do not meet the provisions of the Red Book and are therefore in this sense, no real audio CDs. These CDs are therefore referred to as the " Un - CDs " ( not CDs).

The copy protection is realized by error, or a second incorrect session to be introduced. Deviations from the Red Book standard are possible, but rare. The "click to protect " arises from the fact that the errors should cause no longer can play the discs in the CD drive of a PC. Thus, the copying is to be prevented. Some CD drives and most DVD drives let that not influence and can read anyway, so this idea of " copy protection " is ultimately useless data.

Instead, the error on the " copy-protected " CD cause problems on numerous normal audio CD players and many car radio with integrated CD unit. This can these media either did not play or only partially, partly caused even serious hardware defects, such as when the CD player, the firmware crashes and the medium is impossible to eject. Also often suffer from the sound quality and the life of the player under the copyright protection.

Since 1 November 2003, the manufacturers in Germany are legally required by § 95d of the Copyright Act, to identify copy-protected media as such. Such labeling, however, is to be found hardly what problems can occur in an individual case with car radios, MP3 CD players, DVD players and other devices.

Since the copy protection in practice is unlikely to be effective over and over again leads to problems when playing and also had a partial reluctance result, more and more labels have the concept of " copy-protected CD" abandoned from around 2009. Ordinary, unprotected CDs are increasingly being re-released after the Red Book, especially since so also can be saved royalties for copy protection.

Manufacturer's specifications and production

Most CDs are on the inner ring of the scanning with information on the manufacturer, the country of production (for example, Made in Germany by EDC, Made in France by PDO or Mastered by DADC Austria ) and other identifiers ( for example, catalog no. , IFPI identifier, source Identification Code ( SID) ) provided. These identifiers are typically on an approximately 5 mm wide circle ( the mirror ) and with the naked eye difficult to detect.

Just for collectors of CDs such references are sometimes very important because you firstly distinguish it legally produced CD of a Black Copy and recognize other " special pressures " can. Often CDs of an artist with the same content are produced in different countries. The pads can be of different heights and therefore valuable for collectors and fans.

Recordable CDs

Recordable CDs are available in a write-once variant (CD -R: CD recordable ) and in a multi- rewritable variant (CD -RW: CD rewritable ). While the reflective properties of a CD -R which a normal CD almost the same and this can therefore also be read in older CD drives should the read head output signal of a CD -RW is much weaker, so that these media only by appropriately appointed (newer) drives or players can be read.

To write to a CD no ordinary CD player can be used. It is a so-called CD - burner '(or a CD recorder ) is necessary. CD burners can describe not only CDs but also read. Therefore, pure CD - ROM readers for computers are now virtually disappeared from the market.

The ISO 9660 file format of a CD -ROM does not allow for subsequent changes. In addition, recordable CDs - unlike hard drives - not be written in blocks. Therefore, only a memory map must be created that contains an exact copy of the CD data to be burned. This image can then be ( as a track ) " fired " on the CD in a single pass. For this special CD burning programs are needed. Current burning programs dominate the creation of the image of " on-the- fly", that is, the ISO image is generated during writing.

However, one can, as long as the CD is not closed ( " finalized ") was, with another write operation later in another track ( ie normally in another session) to create a new file system on the CD. The directories of the new file system can refer to files in the older tracks. As always, the file system of the last track is used during normal operation, it is possible to add files, rename to " delete " and "overwrite" to. Of course, the occupied space not be used again. With special software (for example IsoBuster Windows or ISO Master on Linux) can also be accessed on the older file systems, ie, that are " deleted " files or the older versions of overwritten ' files so still accessible ( multi-session CD ).

Alternatively, the file systems can be considered (analogous to partitions of a hard disk) as different virtual drives ( multivolume CD) to the tracks on a CD. This method has been used, for example the Mac OS in versions 8 and 9 up, but is otherwise not very common.

CD - RW discs can be described theoretically in blocks. This must also be supported by the CD writer. Since the used on CD- ROMs ISO - 9660 file format does not support subsequent changes to files, it has its own file system called UDF was introduced, which is also used on DVDs. This format allows, for example, a floppy disk, directly to store files on the CD.

Labelprint

Stand for the label imprint with the CD, as well as for the DVD, various printing techniques:

Screen printing: In screen printing is possible up to six label colors, it can be selected spot color (HKS or Pantone). Screen printing is currently the most common variant to print CDs or DVDs, but is more and more displaced by offset printing. Screen printing is suitable for pressed CDs and DVDs, even Rohlingsbedruckung is possible. In screen printing, the colors are very brilliant.
Dry offset printing: In dry offset are four possible label colors (CMYK ), combined with screen printing up to six ( CMYK offset, in addition white full face and a spot color screen printing ). Due to the higher resolution compared to screen printing Offset printing is ideal for photorealistic renderings. Since early 2004, offset printing is possible not only for pressed CDs and DVDs, but also for Blank CD and DVD media.
Thermal Transfer: In this printing method is transferred to a special printing ink of a specific color band by spot heating using a printing head on the disc. For technical reasons, this printing method is more suitable for headlines and logos. In practice, it is used for small print runs ( home-burned CDs and DVDs).
Thermoretransferdruck: The Thermoretransferdruck is the evolution of thermal transfer printing. The label design is first printed using thermal transfer printing process on a transfer belt and then applied a film of it on the CD. By this technique, a better resolution is possible. For example, a photo-realistic printing can already be achieved with short runs.
Inkjet printing: Some manufacturers offer blanks with papery coated surface. Such blanks, which are usually referred to as " printable", etc., can be printed with quite respectable results in full color in suitable inkjet printers. Photo-realistic image reproduction is the rule.
LightScribe method: In this method, the laser of a LightScribe -enabled CD burner burns on the front of the appropriate blanks any grayscale graphic that is designed by means of appropriate software and transferred to the burner. The burning process takes about 15 minutes. Recently, also colored blanks are available.
Labelflash: Alternative DVD labeling method
DiscT @ 2: CD-R labeling method

Conservation

The compact disc consists mainly of the valuable plastic polycarbonate. A classified recycling not worth it for the production of new compact discs, however, the very high quality raw material in medicine, used the PC and the auto industry. Several companies offer collection systems. It tanks are provided free of charge. Collection (for example, companies or municipalities) thus have no risk, but only need to maintain an appropriate space for the collection. The German Telekom will own CDs back in their shops, AOL CDs can be shipped freight at AOL.

Destruction of data

Since a CD can also contain sensitive information, it must be safe procedures to make the data unreadable before disposal, it is because the data is no longer needed or because they need to be deleted is.

Ineffective methods

Paint is virtually ineffective.
Scratching on the bottom is, if the scratches are not very deep, almost completely ineffective. Even with many deep scratches and significant parts of the data can be recovered with special programs and tools often. Scratching on the top helps, but even there can not scratched sections with special procedures are still read.

Effective method

CD Burner - For some time, offer a variety of manufacturers of CD burners also additional functions ( Smart - Erase) in the drives, which previously written CD-R media can be " overburning " again to the data stored on it finally destroy. This does not work with pressed CDs.
Breaking (possibly with a hammer ) is a relatively safe method, the safety is higher, the smaller the parts. However, there is risk of injury to hands and eyes from flying sharp-edged splinters polycarbonate, which is why you should wear safety glasses and put the CD in an envelope or bag.
Cutting or shredding - for extremely sensitive data exist special CD shredder, shred CDs and other optical media into such small pieces that the media can be considered as reliable destroyed, the recyclability of the disk is preserved. Also, some conventional paper shredder can shred CDs. The suitability of a particular model for this purpose is the instructions refer to.
Rub - It rubs the label side of the CD while on a rough surface (such as a rasp or a brick or similar. ) Until the CD and thus is completely transparent unusable.
Microwave oven or burning - works for both pressed and at home-burned CDs. The metal layer will be destroyed due to the heat generation by the electromagnetic waves, or of the fire. This method is basically very effective and quite suitable in an emergency, but it caused harmful vapors. In addition, there is a danger of fire.

The mechanical and thermal processes described above are not fully recommended, since the coating chemicals are released in part as very small particles in the ambient air by such treatment of the media. From a health point of Smart Erase is the only acceptable method.

171997