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 to) 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 consequently suffer more damage because of defects such as scratches on the label side, whereas clear-side scratches can be repaired by refilling them with plastic of similar index of refraction.
Disc shapes and diameters
In reverse fashion to that of a vinyl record, the digital data on a CD begins at the center of the disc and proceeds outwards to 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 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. 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 184 MB of data (this form factor has also been called "CD3", since it is about three inches across).
Other non-standard shapes and smaller form factors have also been sold or given away as promotional items. All of these unique shapes must fit within the 120 mm ring or the 80 mm ring that is standard on tray drives. Any shape falling between the 80 mm ring and the 120 mm ring of a tray drive, such as a credit card-sized CD business cards, must include a method of locating the disc in the tray during load and unload. This is usually a circular ridge on their underside. Irregularly shaped, non rotationally symmetric discs with an offset centre of mass may cause damaging vibration if played in computer CD drives, which may operate at a much higher rotational velocity than stand-alone audio CD players. Even symmetrical rectangular discs often cause far more vibration than standard circular ones.
Audio format
The format of the audio disc, known as the "Red Book" standard, was laid out by Sony and Philips in 1981. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate. Four-channel sound is an allowed option within the Red Book format, but has never been implemented.
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.
Storage capacity and playing time
The original target storage capacity for a CD was 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.[2] Kees Immink of Philips refutes this.[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.
Main physical parameters
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 approximately 500 rpm at the inside of the disc, and approximately 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 tolerated by most players (though some old ones fail). 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. Even higher capacities on non-standard discs (up to 99 minutes) are available at least as recordables, but generally the tighter the tracks are squeezed the worse the compatibility will be.
Data structure
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. A frame comprises 33 bytes, of which 24 are audio bytes (six full stereo samples), eight CIRC-generated error correction bytes, and one subcode byte. The eight bits of a subcode byte are available for control and display. Under Eight-to-Fourteen Modulation (EFM) rules, each data/audio byte is translated into 14-bit EFM words, which alternates with 3-bit merging words. In total we have 33*(14+3) = 561 bits. A 27-bit unique synchronization word is added, so that the number of bits 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. Data in a CD-ROM are organized in both frames and sectors, where a CD-ROM sector contains 98 frames, and holds 98×24 = 2352 (user) bytes.
CD-ROM
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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 (CDR) drive.
Manufacture
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Replicated CDs are mass-produced using a hydraulic injection mold, where a glass master disc is created and used to make "stampers." Small granules of raw plastic are fed into the barrel while under heat and increasing amount of pressure melt the plastic and force the liquified material into the mold cavity. Equipped with a metal stamper, with the pattern of pits (for pre-recorded discs) the mold closes, allowing the plastic to cools 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 removed.
Recordable CD
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Recordable compact discs, CD-Rs, 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 color 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 stand-alone DVD players can.
Copy protection
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The Red Book audio specification, except a simple 'anti-copy' bit in the subcode, does not include any serious copy protection mechanism. Starting in early 2002, attempts were made by record companies to market "copy-protected" non-standard compact discs. 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. Moreover, there has been great public outcry over copy-protected discs because many see it as a threat to fair use.
References
See also