Compact disc
For the Public Image Ltd album called "Compact Disc" on certain editions, see Album (album).
A compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. It is the standard playback format for commercial audio recordings today.
A standard compact disc, often known as an "audio CD" to differentiate it from later variants, stores audio data in a format compliant with the red book standard. An audio CD consists of several stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard compact discs have a diameter of 120 mm, though 80 mm versions exist in circular and "business-card" forms. The 120 mm discs can hold 74 minutes of audio, and versions holding 80, 90 or even 99 minutes have been introduced. The 80 mm discs are used as "CD-singles" or novelty "business-card CDs". They hold about 20 minutes of audio. Compact disc technology was later adapted for use as a data storage device, known as a CD-ROM.
The design of the CD was originally conceived as an evolution of the gramophone record, rather than primarily as a data storage medium. Only later did the concept of an 'audio file' arise, and the generalising of this to any data file. As a result, the original CD format has a number of limitations; no built-in track names or disc naming for example. Online services such as CDDB were developed to work around these shortcomings in the computer age.
History
In the early 1970s, using video Laserdisc technology, Philips' researchers started experiments with "audio-only" optical discs, initially with wideband frequency modulation FM and later digitized PCM audio signals. The compact disk was thus developed by Philips from its own 12 inch Philips Laservision disks. At the end of the 1970s, Philips, Sony, and other companies presented prototypes of digital audio discs.
In 1979 Philips and Sony decided to join forces, setting up a joint task force of engineers whose mission was to design the new digital audio disc. Prominent members of the task force were Kees Immink and Toshitada Doi. After a year of experimentation and discussion, the taskforce produced the "Red Book", the Compact Disc standard. Philips contributed the general manufacturing process, based on the video Laserdisc technology. Philips also contributed the Eight-to-Fourteen Modulation, EFM, which offers both a long playing time and a high resilience against disc handling damage such as scratches and fingerprints; while Sony contributed the error-correction method, CIRC. The Compact Disc Story, 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. According to Philips, the Compact Disc was thus "invented collectively by a large group of people working as a team."[1]
The Compact Disc reached the market in late 1982 in Asia and early the following year in other markets. 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. The far larger popular and rock music industries were slower to adopt the new format, especially in the huge consumer markets in Europe and the United States. This "highbrow niche" status of the CD format changed dramatically in May, 1985, when UK rock band Dire Straits, then under contract by Philips' Polygram, released the album Brothers in Arms. One of the first all-digital rock recordings and the first by a major act, Brothers in Arms played to the strengths of the CD by offering more and longer tracks, running ten minutes longer than the album's concurrent LP and cassette releases. It spurred the sale of compact disc players like no other recording before it, helped to drive down the price of players, induced other acts and record labels to release more music on CD and firmly established the format in the mind of the average consumer. [2]
From its origins as a music format, Compact Disc has grown to encompass other applications. Two years later, in June 1985, the CD-ROM (read-only memory) was introduced. Developed by Sony and Philips, this CD-ROM standard was initially known as the High Sierra standard, and now ISO 9660. This defined the 12cm diameter of the disks, identical to that of CD audio - the data structure of the information stored on the disks and a technique ensuring reliable reproduction of data as standards for the CD-Rom industry. With this it was now possible to disseminate massive amounts (for the time) of computer data instead of digital sound.
Since then, a newer format named CD-ROM XA had been agreed between Philips, Sony and Microsoft, as a bridge between CD-ROM as used with a PC-compatible computer using MS-DOS, and in a CD-I machine. Software conforming to the CD-ROM XA standard could be played in both. The essential difference between the two is that where CD-ROM is largely text-based with a limited amount of pictures and animation, CD-I offers in it's later versions full motion video and audio and enables the user to interact more with the software. Sony and Matsushita collaborated with Philips to market and promote CD-I on Philips' CD-I machines in 1991.
A CD can store around 640 megabytes of data, several hundred times as much as the most popular software distribution medium of the time, the floppy disk. A user-recordable CD for data storage, CD-R, was introduced in the early 1990s, and it became the de facto standard for exchange and archiving of computer data and music. The CD and its later extensions have been extremely successful: in 2004 the annual worldwide sales of CD-Audio, CD-ROM, and CD-R reached about 30 billion discs.
|== Physical details ==
Compact discs are made from a 1.2 mm thick disc of very pure polycarbonate plastic - a similar material used to make bullet-proof windscreens and crash helmets. A thin layer of Super Purity Aluminium is applied (or rarely gold, used for its data longevity, such as in some limited-edition audiophile CDs) to the surface to make it reflective, which is protected by a film of lacquer (to avoid the aluminium from oxidising). The lacquer can be printed with a label. Common printing methods for compact discs are silkscreening and offset printing. CDs are available in two sizes. By far the most common is 120 mm in diameter, with a 74-minute audio capacity and a 650 MB data or an 80-minute audio capacity and a 700 MB data (See storage capacity; this form factor has also erroneously been called "CD5" since it is 4 3/4 inches in diameter, about five inches across). Such a standard disc weighs 15 grams. 80 mm discs are also available, a format which is mainly used for audio CD singles in some regions (e.g. Japan), much like the old vinyl single. Each such "miniCD" or "Maxi CD" can hold 21 minutes of music, or 180 MB of data (this form factor has also been called "CD3", since it is about three inches across).
There is a 15 mm hole in the centre of the disc, usually used by some form of clamp or clip device within the player to hold it in place and allow it to be rotated by a motor. The Diameter of this hole was agreed upon eventually as the same diameter as a 10 Dutch cent coin as this being a denominating currency of Dutch based company Philips.
The information on a standard CD is encoded as a spiral track of pits 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 long. The spacing between the tracks is 1.6 μm. To grasp the scale of the pits and land of a CD, if the disc is enlarged to the size of a stadium, a pit would be approximately the size of a grain of sand. The spiral begins at the center of the disc and proceeds outwards to the edge, which allows the different size formats available.
A CD is read by focusing a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer. The difference in height between pits and lands is one quarter to one sixth of the wavelength of the laser light, leading to a half-wavelength or less phase difference between the light reflected from a pit and from its surrounding land. The destructive interference thus reduces the intensity of the reflected light compared to when the laser is focused on just a land. By measuring this intensity with a photodiode, one is able to read the data from the disc. The pits and lands themselves do not represent the zeroes and ones of binary data. Instead a change from pit to land or land to pit indicates a one, while no change indicates a zero. 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 audio data stored on the disc. Data CDs, unlike Audio CDs, require the removal of a third layer of Error Correcting Code, before the data is accessible in a useful format. This is because computer software could be rendered non-functional by a single incorrect bit, which is not the case for audio data.
Figure 1, page 8a, of the Red Book specifies many mechanical parameters including the pit depth. It specifies that the pit depth should be less than (and, thus, not equal) 130 nm. However, the Red Book implicitly specifies the pit depth by specifying the strength of both the push-pull radial tracking signal and full aperture detection signal. For a maximum full aperture signal, the optimum pit depth is λ/4n = 130 nm (refractive index n=1.5, λ=780 nm). For a maximum push-pull radial tracking signal the best choice is λ/8n = 65 nm. Most CD manufacturers, dependent on the exact pit geometry such as the slope of the pit edges etc, choose a pit depth of around 90-100 nm, (which is around λ/6n) yielding a sound trade-off between the quality of the push-pull radial tracking and full aperture detection signal.
Pits are much closer to the label side of a disc so that defects and dirt on the clear side can be out of focus during playback. Discs are consequently much easier to ruin by scratching the label side, whereas clear-side scratches can be repaired by refilling them with plastic of similar index of refraction.
Other unique shapes and smaller form factors have also been sold or given away as promotional items. Examples include Business Card CDs in the shape of a rectangular card and CDs shaped like the map of a country etc, although such discs are not always compatible with all CD players — they will work with any machine where the disc is inserted by manually clipping it onto the spindle (the mechanism used in virtually all portable CD players), but may not necessarily be inserted into drives which load the disc from a tray, or pull it into a slot. Irregularly shaped, non rotationally symmetric discs with an offset centre of mass may also cause damaging vibration if played in computer CD drives, which can operate at a much higher rotational velocity than stand-alone audio CD players. Some irregularly shaped discs will work with tray loading CD drives if they include a circular ridge on their underside which centers them on the part of the tray designed to hold 80 mm CDs, assuming the tray has such a feature.
It should be noted that the term "pit" is somewhat misleading as the laser beam reads the disc thorough the polycarbonate base and thus sees the pits as "ridges". "Pits" are pits on the reverse (label) side of the disc.
Manufacturing Process
 |
Mastering Process: First, in a clean room, a glass master is prepared by coating a perfectly flat piece of half inch thick circular glass with a layer of nickel. The nickel is transferred by exciting the nickel to a plasma state whereupon a thin layer of nickel will adhere to the glass. The glass is coated with an emulsion. Source material is encoded into the appropriate format whereupon a computer controlled machine "burns" the pits into the emulsion layer of the glass master. The glass master produced is quality checked before it moves to the next stage.
Stamper Process: Next the glass master is used to create nickel stampers using an electroplating technique. Multiple stampers can be made from one glass master. Each stamper is quality checked. This process is also done in a clean room environment.
Pressing: Each stamper is mounted in an injection moulding machine. Melted polycarbonate resin is injected into the chamber and the CD is pressed using up to 40 tons of pressure. The chamber opens and a robotic arm grabs the disc and transfers it to the next stage. At this point the disc is clear, so a coating of aluminum is applied to the disc for reflectivity. A laquer is spin coated onto the disc and the disc is tranfered to a spindle. The discs are sampled by QC to ensure quality product.
Printing and Packaging: The label is printed onto the disc using a one to six color process (in the case of silk screening), then the printed discs are loaded into a packaging machine that combines a jewel box, tray card, the disc, and booklet. The finished assembly has security stickers applied, and is shrinkwrapped with marketing stickers applied. Sometimes the spindle of 150 discs are shrinkwrapped together in bulk. Bulk packaging can be done before or after printing.
Audio format
The format of the audio disc, known as the "Red Book" / Sony standard, was laid out by Sony and Philips in 1981. Philips is responsible for the licensing program of the intellectual property pertinent to the Compact Disc including the "Compact Disc Digital Audio" logo that appears on the disc. In broad terms the format is a two-channel (four-channel sound is an allowed option within the Red Book format, but has never been implemented) 16-bit PCM encoding at a 44.1 kHz sampling rate. Reed-Solomon error correction allows the CD to be scratched to a certain degree and still be played back.
The sampling rate of 44.1 kHz is inherited from a method of converting digital audio into an analog video signal for storage on video tape, which was the most affordable way to get the data from the recording studio to the CD manufacturer at the time the CD specification was being developed. A device that turns an analog audio signal into PCM audio, which in turn is changed into an analog video signal is called a PCM adaptor. This technology could store six samples (three samples per each stereo channel) in a single horizontal line. A standard NTSC video signal has 245 usable lines per field, and 59.94 fields/s, which works out at 44,056 samples/s. Similarly PAL has 294 lines and 50 fields, which gives 44,100 samples/s. This system could either store 14-bit samples with some error correction, or 16-bit samples with almost no error correction. There was a long debate over whether to use 14 or 16 bit samples and/or 44,056 or 44,100 samples/s when the Sony/Philips task force designed the compact disc; 16 bits and 44.1 kilosamples per second prevailed. The Sony PCM-1610 and PCM-1630 are well known examples of PCM adaptors used in conjunction with the Sony U-matic VCR.
Storage capacity
During development the target storage capacity for the CD was sufficient for one hour of audio content, and a disc diameter of 11.5 cm was sufficient. However, according to Philips, Sony vice-president Norio Ohga suggested extending the capacity to 74 minutes to accommodate a complete performance of Beethoven's 9th Symphony on a single disk [3]. The extra playing time required changing to a 12 cm disc.
According to a Sunday Tribune interview [4] the story is slightly more involved. At that time (1979) Philips owned Polygram, one of the world's largest distributors of music. Polygram had set up a large experimental CD disc plant in Hanover, Germany, which could produce huge amounts of CDs having, of course, a diameter of 11.5cm. Sony did not yet have such a facility. If Sony had agreed on the 11.5cm disc, Philips would have had a significant competitive edge in the market. Sony was aware of that, did not like it, and something had to be done. The long-playing time of Beethoven's Ninth imposed by Ohga was used to push Philips to accept 12cm, so that Philips' Polygram lost its edge on disc fabrication.
The 74-minute playing time of a CD, being more than that of most long-playing vinyl albums, was often used to the format's advantage during the early years when CDs and LPs vied for commercial sales. CDs would often be released with one or more bonus tracks, enticing consumers to buy the CD for the extra material. However, attempts to combine double LPs onto one CD occasionally resulted in an opposing situation in which the CD would actually offer fewer tracks than the LP equivalent.
The main parameters of the CD (taken from the September 1983 issue of the compact disc specification) are as follows:
- Scanning velocity: 1.2–1.4 m/s (constant linear velocity) - Equivalent to about 500 rpm at the inside of the disc, or about 200 rpm at the outside edge.
- Track pitch: 1.6 μm.
- Disc diameter 120 mm.
- Disc thickness: 1.2 mm.
- Inner radius program area: 25 mm.
- Outer radius program area: 58 mm.
The program area is 86.05 cm², so that the length of the recordable spiral is 86.05/1.6 = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or around 650 MB of data on a CD-ROM. If the disc diameter were 115 mm, the maximum playing time would have been 68 minutes, i.e., six minutes less. A disc with data appearing slightly more densely is allowable. Using a linear velocity of 1.2 m/s and a track pitch of 1.5 micrometre leads to a playing time of 80 minutes, or a capacity of 700 MB. This is the limit for most conventional audio CDs today.
Another technique to increase the capacity of a disc is store data in the lead out groove that is normally used to indicate the end of a disk, and an extra minute or two of recording is often possible. However, these discs can cause problems in playback when the end of the disc is reached.
CD capacities are always given in binary units. A "700 MB" (or "80 minute") CD has a nominal capacity of about 700 MiB, but DVD capacities are given in decimal units. A "4.7 GB" DVD has a nominal capacity of about 4,489 MiB.
Data structure
Under a microscope, all that is visible is a series of various-sized pits arranged in a long spiral, starting near the inner hole. As bit-times are counted off, a transition (pit-to-land, or land-to-pit) is interpreted as a "1" bit, while a constant region (all-land or all-pit) is interpreted as a "0" bit. Each 14 consecutive bits are grouped and decoded using Eight-to-Fourteen Modulation to get a byte. Each 14-bit EFM word alternates with a 3-bit merging word.
The smallest entity in the CD audio format is called a frame. A frame can accommodate six complete 16-bit stereo samples, i.e. 2×2×6 = 24 bytes. Data in a CD-ROM are organized in both frames and sectors. A frame comprises 33 bytes, of which 24 are audio bytes (six full stereo samples), eight error correction, CIRC-generated, bytes plus one subcode byte. The eight bits of a subcode byte are available for control and display. In total we have 33*(14+3) = 561 channel bits. A 27-bit unique synchronization word is added, so that the number of channel bit in a frame totals 588. The synchronization word cannot occur in the normal bit stream, and can thus be used to identify the beginning of a frame.
A CD-ROM sector contains 98 frames, and holds 98×24 = 2352 bytes. The CD-ROM is in essence a data disc, which cannot rely on error concealment, and it requires therefore a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM has a third layer of Reed-Solomon error correction. Note that the CIRC error correction system used in the CD audio format has two interleaved layers. A Mode-1 CD-ROM, which has the full third layer error correction capability, contains a net 2048 bytes of the available 2352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM is 44.1k×2048/(6×98) = 153.6 kB/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB.
A 1x speed CD drive reads 75 consecutive sectors per second.
A CD-ROM (data) sector contains 2352 bytes:
- 12 bytes: sync
- 4 bytes: sector ID
- 2 048 bytes: user data
- 4 bytes: error detection
- 8 bytes: null
- 276 bytes: error correction
Subcode
Besides digital audio, a CD contains digital data called "subcode", which is multiplexed with the digital audio. The data in a CD are arranged in frames. A frame comprises 33 bytes, of which 24 are audio bytes (six full stereo samples), eight error correction, CIRC-generated, bytes plus one subcode byte. The eight bits of a subcode byte are available for control and display. The eight bits are used as eight different subcoding channels, and given letters designating their usage: P, Q, …, W. Thus each channel has a bit rate of 7.35 (=44.1/6) kbit/s.
In each sector there are 2352 bytes (24×98) of audio content data and 96 bytes of subchannel data.
The 96 bytes of subchannel information in each sector contain four packets of 24 bytes apiece:
1 byte for command, 1 byte for instruction, 2 bytes for parityQ, 16 bytes for data, and 4 bytes parityP.
Each of the 96 subchannel data bytes can be thought of as being divided into eight bits. Each of these bits corresponds to a separate stream of information. These streams are called "channels", and are labeled starting with the letter P, like so:
| Channel | P | Q | R | S | T | U | V | W |
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Channel P is a simple pause/music flag, which can be used for low-cost search systems. Quite a few players ignore it in favor of the Q Channel.
Channel Q is used for control purposes of more sophisticated players. It contains positioning information, the Media Catalog Number (MCN), and International Standard Recording Code (ISRC). The ISRC is used by the media industry, and contains information about the country of origin, the year of publication, owner of the rights, as well as a serial number, and some additional tags:
- Data
- This track contains Data (rather than audio). Can be used for muting in audio CD players.
- Copy Flag
- Used by the Serial Copy Management System to indicate permission to digitally copy the track.
- Four Channel Audio
- The track uses four channel audio. This is very rarely used on Compact Discs.
- Pre-Emphasis
- The audio track was recorded with pre-emphasis. Used very rarely on Compact Discs.
Channels R…W are unused by Red-Book compliant CDs, and have been used for extensions to the standard.
CD-Text
CD-Text is part of the CD+G extension to the Red Book standard for audio CDs. It allows for storage of additional information (e.g. album name, song name, and artist) on a standards-compliant audio CD. The information is stored in the lead-in area of the CD, where there is roughly five kilobytes of space available, or in the R through W Subchannels on the disc, which are not used by strict Red Book CDs. About 31 megabytes of information can be stored there. The text is stored in a format usable by the Interactive Text Transmission System (ITTS). ITTS is also used by Digital Audio Broadcasting or the MiniDisc.
Note that the CD+G or “karaoke” extension also uses the R-W subchannels or subcodes to store low resolution graphics.
The SPARS Code for audio CDs
Many CDs, especially classical music and many popular recordings, come with a three-letter code printed on the back known as the SPARS (acronym for Society of Professional Audio Recording Studios) Code, where "A" stands for analog and "D" stands for digital. The first letter represents how the album was recorded, the second how it was mixed, and the third how it was transferred (inevitably a D, as the CD is a digital medium). Almost all early CDs are "AAD" (analog recording and mixing, digital transfer to CD) as a result. Often this code was accompanied by a short description such as "Full Digital Recording" for DDD and "Digitally Mixed Analog Recording" for ADD.
Commercial digital recording of classical and jazz music began in the early 1970s, pioneered by Japanese companies such as Denon, although experimental recordings exist from the 1960s. The first 16-bit PCM recording in the United States was made by Thomas Stockham at the Santa Fe Opera in 1976 on a Soundstream recorder. In most cases there was no mixing stage involved; a stereo digital recording was made and used unaltered as the master tape for subsequent commercial release. These unmixed digital recordings are still described as DDD since the technology involved is purely digital. (Unmixed analog recordings are likewise usually described as ADD to denote a single generation of analog recording).
The first digitally recorded (DDD) popular music album was Ry Cooder's Bop Till You Drop, recorded in late 1978. It was unmixed, being recorded straight to a two-track 3M digital recorder in the studio. Many other top recording artists were early adherents of digital recording. Stevie Wonder adopted the technology in early 1979 for Journey Through the Secret Life of Plants and used it on all later recordings. Others, such as former Beatles producer George Martin, felt that the multitrack digital recording technology of the early 1980s had not reached the sophistication of analog systems. Martin used digital mixing, however, to eliminate the distortion and noise that an analog master tape would introduce (thus ADD). An early example of an analog recording that was digitally mixed is Fleetwood Mac's 1979 release Tusk.
By the time the compact disc was introduced worldwide digital recording and mixing was becoming commonplace among recording artists and producers known for their interest in fidelity. Two examples from 1982 are Signals by Rush and The Nightfly by Donald Fagen.
A few examples of DAD recordings exist, mostly of works that were originally recorded digitally but later remixed by artists who preferred to work with analog technology. A notable example is Herb Alpert's Rise album from 1979.
The originally CD-only label Ryko extended this system to the other media when it began making LPs and cassettes so that a digital recording on an LP would be DDA, and so forth.
Three-Letter Codes
- DDD: digital tape recorder used during session recording, mixing and/or editing, and mastering (transcription).
- ADD: analog tape recorder used during session recording, digital tape recorder used during subsequent mixing and/or editing and during mastering (transcription).
- DAD: digital tape recorder used during session recording, analog tape recorder used during subsequent mixing and/or editing, digital tape recorder used during mastering (transcription).
- AAD: analog tape recorder used during session recording and subsequent mixing and/or editing, digital tape recorder used during mastering (transcription).
CD-ROM
For its first few years of existence, the compact disc was purely an audio format. However, in 1985 Yellow Book CD-ROM standard was established by Sony and Philips, which defined a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive.
Recordability
Injection moulding is used to mass produce compact discs. A "stamper" is made from the original media (audio tape, data disc, etc.) by writing to a glass disc (referred to as a glass master) coated with a photosensitive dye with a laser. This dye is then etched, leaving the data track. It is then plated to make a positive version of the CD. Polycarbonate is liquified and injected into the mold cavity where the stamper transfers the pattern of pits and lands to the polycarbonate disc. The disc is then metallized with aluminum and lacquer coated.
Recordable compact discs are injection molded with a "blank" data spiral. A photosensitive dye is then applied, and then the discs are metallized and lacquer coated. The write laser of the CD recorder changes the characteristics of the dye to allow the read laser of a standard CD player to see the data as it would an injection molded compact disc. CD-R recordings are permanent. The resulting discs can be read by most CD-ROM drives and played in most audio CD players.
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 chemical properties of the alloy and hence change its reflectivity. A CD-RW does not have as great a difference in the reflectivity of lands and bumps as a pressed CD or a CD-R, and so many CD audio players cannot read CD-RW discs, although the majority of standalone DVD players can.
Copy protection
The Red Book audio specification does not include any copy protection mechanism. Ripping is the process by which the contents of an audio disc is copied out verbatim to a duplicate disc or re-encoded into some other format, such as MP3 or Ogg Vorbis.
An error-correcting code is included with Red Book audio to deal with small scratches or defects on the disc media. Where error correction fails on larger defects, audio CD players are expected to apply interpolation algorithms to conceal the loss of audio data.
Starting in early 2002, attempts were made by record companies to market "copy-protected" compact discs. Some of these deliberately introduced error patterns into audio tracks severe enough to defeat the error-correcting code (and hence defeat most CD-ROM drives attempting to copy the tracks as data), but not so disruptive as to prevent interpolation from working (hence allowing the same tracks to be played in audio mode without overly affecting fidelity). These discs are said to be more sensitive to disc pollution or surface damage (typically in the form of scratches) because they partially exhaust the error-correction thresholds incorporated into the Red Book standard right from the time of production.
Another copy protection method places a data track (usually containing bonus software for computer users) at the end of the disc and gives it an invalid size in the disc's table of contents. This is intended to prevent the data track from being ripped, but can be defeated by ignoring the table of contents and reading the disc sector by sector.
Philips has stated that such discs are not permitted to bear the trademarked Compact Disc Digital Audio logo because they violate the Red Book specification. It also seems likely that Philips' new models of CD recorders will be designed to be able to record from these "protected" discs. However, there has been great public outcry over copy-protected discs because many see it as a threat to fair use. For example, audio tracks on such media cannot be easily added to a personal music collection on a computer's hard disk or a portable (non-CD) music player. Also, many ordinary CD audio players, e.g. in car radios, have problems playing copy-protected media, mostly because they use hardware and firmware components also used in CD-ROM drives. The reason for this reuse is cost efficiency.
Other systems developed are Macrovision CDS-200 and Mediamax CD-3.
In any case, even if a disc cannot be directly ripped, it can still be played in audio mode, and the audio thence captured. Any loss of sound quality caused by this method is generally considered negligible. This is commonly referred to as the analog hole.
Color effects in CDs
As stated above, a CD is made up of a layer of about 1.2 mm of polycarbonate plastic which has been moulded from a master with pits burnt into it. The refractive index between pits and lands is similar to that of oil on water, and something approaching the wavelength of visible light. When light hits a CD a small amount will be reflected off the outmermost plastic layer, with the rest being allowed through. When this light hits the metallic reflective layer it will bounce back off the different layers. The light reflecting off the different heights (as well as the sides of the “pits”) will be out of phase. These phases will either interfere with each other (constructively or destructively), or have an in-between result. This has a similar effect to that of oil on water and like oil on water, the CDs are not exactly uniform in their shape (always at least very slightly warped) and hence different colors and reflectivness are produced.
Non-standard CD behaviors
Some commercially released audio discs have a "secret" bonus track. These may be an extension of the last audio track or a separate track hidden from the disc's table of contents. Either way, the hidden portion is heard when the disc is played to the end.
Other discs hide the extra material at the beginning of the disc. On most discs, the location of the first track listed in the table of contents immediately follows the table of contents itself. In this case, the hidden track is an unlisted track sandwiched between the two. To hear the hidden track, the listener must usually "rewind" the player past the beginning of the first listed track. Not all players allow this.
Name
Notwithstanding the variability of general usage between "disk" and "disc" [5], the customary spelling is "compact disc", rather than "compact disk". This may be in large degree due to its status as a Philips trademark under that spelling. [citation needed]
References
- Kees Immink, The Compact Disc Story, Journal of the Audio Engineering Society, 46(5), pp. 458-465, May 1998 [6].
- Kenneth C. Pohlmann (1992). The Compact Disc Handbook. Middleton, Wisconsin: A-R Editions. ISBN 895793008.
See also
