Physical details

Diagram of CD layers illustrates:

A. A polycarbonate disc layer has the data encoded by using bumps.

B. A reflective layer reflects the laser back.

C. A lacquer layer is used to prevent oxidation.

D. Artwork is screen printed on the top of the disc.

E. A laser beam reads the polycarbonate disc, is reflected back, and read by the player.

A CD is made from 1.2 mm thick, almost-pure polycarbonate plastic and weighs approximately 16 grams. From the center outward components are at the center (spindle) hole, the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the information (data) area, and the rim. A thin layer of aluminum or, more rarely, gold is applied to the surface to make it reflective, and is protected by a film of lacquer that is normally spin coated directly on top of the reflective layer, upon which the label print is applied. Common printing methods for CDs are screen-printing and offset printing. CD data are stored as a series of tiny indentations known as “pits”, encoded in a spiral track molded into the top of the polycarbonate layer. The areas between pits are known as “lands”. Each pit is approximately 100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in length.










The optical lens of a CD drive

The distance between the tracks, the pitch, is 1.6 µm. A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in intensity in the light reflected. By measuring the intensity change with a photodiode, the data can be read from the disc. The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, Non-return-to-zero, inverted (NRZI) encoding is used: 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 data stored on the disc. CDs are susceptible to damage from both daily use and environmental exposure. 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. Consequently, CDs suffer more scratch damage on the label side whereas scratches on the clear side can be repaired by refilling them with similar refractive plastic, or by careful polishing. Initial music CDs were known to suffer from "CD rot", or "laser rot", in which the internal reflective layer degrades. When this occurs the CD may become unplayable.










Disc shapes and diameters

A Mini-CD is 8 centimetres in diameter. The digital data on a CD begin at the center of the disc and proceeds toward the edge, which allows adaptation to the different size formats available. Standard CDs are available in two sizes. By far the most common is 120 mm in diameter, with a 74- or 80-minute audio capacity and a 650 or 700 MB data capacity. This diameter has also been adopted by later formats, including Super Audio CD, DVD, HD DVD, and Blu-ray Disc. 80 mm discs ("Mini CDs") were originally designed for CD singles and can hold up to 21 minutes of music or 184 MB of data but never really became popular. Today, nearly every single is released on a 120 mm CD, called a Maxi single. "Shaped CD" Novelty CDs are also available in numerous shapes and sizes, and are used mostly for marketing. A common variant is a "business card" CD, a single with portions removed at the top and bottom to more closely resemble a business card. Physical size Audio Capacity CD-ROM Data Capacity Note 12 cm 74–80 min 650–703 MB Standard size 8 cm 21–24 min 185–210 MB Mini-CD size 85x54 mm - 86x64 mm ~6 min 10-65 MB "Business card" size Logical formats Audio CD Main article: Red Book (audio CD standard) The logical format of an audio CD (officially Compact Disc Digital Audio or CD-DA) is described in a document produced by the format's joint creators, Sony and Philips in 1980. The document is known colloquially as the "Red Book" after the color of its cover. The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel. Four-channel sound is an allowable option within the Red Book format, but has never been implemented. Monaural audio has no existing standard on a Red Book CD; mono-source material is usually presented as two identical channels on a 'stereo' track. The selection of the sample rate was primarily based on the need to reproduce the audible frequency range of 20 Hz - 20 kHz. The Nyquist–Shannon sampling theorem states that a sampling rate of more than double the maximum frequency of the signal to be recorded is needed, resulting in a required rate of at least 40 kHz. The exact sampling rate of 44.1 kHz was inherited from a method of converting digital audio into an analog video signal for storage on U-matic video tape, which was the most affordable way to transfer data from the recording studio to the CD manufacturer at the time the CD specification was being developed. The 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 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/stereo channel. Similarly, PAL has 294 lines and 50 fields, which gives 44,100 samples/s/stereo channel. 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-bit (Philips) or 16-bit (Sony) quantization, and 44,056 or 44,100 samples/s (Sony) or around 44,000 samples/s (Philips). When the Sony/Philips task force designed the Compact Disc, Philips had already developed a 14-bit D/A converter, but Sony insisted on 16-bit. In the end, 16 bits and 44.1 kilosamples per second prevailed. Philips found a way to produce 16-bit quality using their 14-bit DAC by using four times oversampling.










Storage capacity and playing time

The partners aimed at a playing time of 60 minutes with a disc diameter of 100 mm (Sony) or 115 mm (Philips).[6] Von Karajan suggested extending the capacity to 74 minutes to accommodate Wilhelm Furtwängler's recording of Beethoven’s 9th Symphony from the 1951 Bayreuth Festival.[18] [19] The extra 14-minute playing time subsequently required changing to a 120 mm disc. Kees Immink, Philips' chief engineer, however, denies this, claiming that the increase was motivated by technical considerations, and that even after the increase in size, the Furtwängler recording would not have fit on one of the earliest CDs.[5][6] According to a Sunday Tribune interview,[20] the story is slightly more involved. In 1979, Philips owned Polygram, one of the world’s largest distributors of music. Polygram had set up a large experimental CD plant in Hanover, Germany, which could produce huge numbers of CDs having, of course, a diameter of 115 mm. Sony did not yet have such a facility. If Sony had agreed on the 115-mm disc, Philips would have had a significant competitive edge in the market. Sony decided that something had to be done. The long playing time of Beethoven's Ninth Symphony imposed by Ohga was used to push Philips to accept 120 mm, so that Philips’ Polygram lost its edge on disc fabrication.[20] The 74-minute playing time of a CD, which was longer than the 20 minutes per side[21][22] typical of long-playing (LP) vinyl albums, was often used to the CD’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. An example is the 1987 album Kiss Me, Kiss Me, Kiss Me by The Cure, which states in the CD liner notes: "The track Hey You!!! which appears on the double album and cassette has been omitted so as to facilitate a single compact disc." The 2006 re-release of this album saw the inclusion of the missing track.[23] Another example is the original late-1980s Warner Bros. Records reissue of Fleetwood Mac's Tusk album, which substituted the long album version of "Sara" with the shorter single version. Enough complaints were lodged to eventually convince Warner Bros. to remaster the album in the mid-1990s with the original contents intact.[24]










Physical parameters

The main parameters of the CD (taken from the September 1983 issue of the Red Book) 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. (A disc played from beginning to end slows down during playback.)

• 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

• Center spindle hole diameter: 15 mm

The program area is 86.05 cm˛ and the length of the recordable spiral is (86.05 cm˛ / 1.6 µm) = 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 only 115 mm, the maximum playing time would have been 68 minutes, i.e., less six minutes. A disc with data packed slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m/s and a track pitch of 1.5 µm 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.










Data structure

The smallest entity in a CD is called a frame, which consists of 33 bytes and contains six complete 16-bit stereo samples (two bytes × two channels × six samples: equals 24 bytes). The other nine bytes consist of eight CIRC error-correction bytes and one subcode byte, used for control and display. Each byte is translated into a 14-bit word using eight-to-fourteen modulation, which alternates with three-bit merging words. In total there are 33 × (14 + 3) = 561 bits. A 27-bit unique synchronization word is added, so that the number of bits in a frame totals 588 (of which only 192 bits are music). These 588-bit frames are in turn grouped into sectors. Each sector contains 98 frames, totaling 98 × 24 = 2352 bytes of music. The CD is played at a speed of 75 sectors per second, which results in 176,400 bytes per second. Divided by two channels and two bytes per sample, this results in a sample rate of 44,100 samples per second. For CD-ROM data discs, the physical frame and sector sizes are the same. Since error concealment cannot be applied to non-audio data in case the CIRC error correction fails to recover the user data, a third layer of error correction is defined, reducing the payload to 2048 bytes per sector for the Mode-1 CD-ROM format. To increase the data-rate for Video CD, Mode-2 CD-ROM, the third layer has been omitted, increasing the payload to 2336 user-available bytes per sector, only 16 bytes (for synchronization and header data) less than available in Red-Book audio.