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Optical disc
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The bottom surface of a 12 cm compact disc (CD-R), showing characteristic iridescence.
The optical lens of a compact disc drive.
Computer memory and data storage types
General
Volatile
RAM
Historical
Non-volatile
ROM
NVRAM
Early-stage NVRAM
Analog recording
Optical
In development
Historical
Optical discs
General
Optical media types
Standards
See also

An optical disc is a flat, usually[note 1] disc-shaped object that stores information in the form of physical variations on its surface that can be read with the aid of a beam of light. Optical discs can be reflective, where the light source and detector are on the same side of the disc, or transmissive, where light shines through the disc to be detected on the other side. They may contain analog or digital information, or a mixture of the two. Their main uses are the distribution of media and data, and long-term archival storage.

Design and technology

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The encoding material sits atop a thicker substrate (usually polycarbonate) that makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track.

The data are stored on the disc with a laser or stamping machine, and can be accessed when the data path is illuminated with a laser diode in an optical disc drive that spins the disc at speeds of about 200 to 4,000 RPM or more, depending on the drive type, disc format, and the distance of the read head from the center of the disc (outer tracks are read at a higher data speed due to higher linear velocities at the same angular velocities).

Most optical discs exhibit a characteristic iridescence as a result of the diffraction grating formed by their grooves.[1][2] This side of the disc contains the actual data and is typically coated with a transparent material, usually lacquer.

The reverse side of an optical disc usually has a printed label, sometimes made of paper but often printed or stamped onto the disc itself. Unlike the 31⁄2-inch floppy disk, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems. Blu-rays have a coating called durabis that mitigates these problems.

Optical discs have been offered between 7.6 and 30 cm (3.0 and 11.8 in) in diameter, with 12 cm (4.7 in) becoming the dominant size beginning 1997.[3]. The so-called program area that contains the data commonly starts 25 millimeters away from the center point.[4] A typical disc is about 1.2 mm (0.047 in) thick, while the track pitch (distance from the center of one track to the center of the next) ranges from 1.6 μm (for CDs) to 320 nm (for Blu-ray discs).

Recording types

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An optical disc is designed to support one of three recording types: read-only (such as CD and CD-ROM), recordable (write-once, like CD-R), or re-recordable (rewritable, like CD-RW). Write-once optical discs commonly have an organic dye (may also be a (phthalocyanine) azo dye, mainly used by Verbatim, or an oxonol dye, used by Fujifilm[5]) recording layer between the substrate and the reflective layer. Rewritable discs typically contain an alloy recording layer composed of a phase change material, most often AgInSbTe, an alloy of silver, indium, antimony, and tellurium.[6] Azo dyes were introduced in 1996 and phthalocyanine only began to see wide use in 2002. The type of dye and the material used on the reflective layer on an optical disc may be determined by shining a light through the disc, as different dye and material combinations have different colors.

Blu-ray Disc recordable discs do not usually use an organic dye recording layer, instead using an inorganic recording layer. Those that do are known as low-to-high (LTH) discs and can be made in existing CD and DVD production lines, but are of lower quality than traditional Blu-ray recordable discs.

File systems

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File systems specifically created for optical discs are ISO9660 and the Universal Disk Format (UDF).

ISO9660 can be extended using the "Joliet" extension to store longer file names than standalone ISO9660. The "Rock Ridge" extension can store even longer file names and Unix/Linux-style file permissions, but is not recognized by Windows and by DVD players and similar devices that can read data discs.

For cross-platform compatibility, multiple file systems can co-exist on one disc and reference the same files.[7][8]

Usage

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Optical discs are most commonly used for digital preservation, storing music (particularly for use in a CD player), video (such as for use in a Blu-ray player), or data and programs for personal computers (PC), as well as offline hard copy data distribution due to lower per-unit prices than other types of media. The Optical Storage Technology Association (OSTA) promoted standardized optical storage formats.

Libraries and archives enact optical media preservation procedures to ensure continued usability in the computer's optical disc drive or corresponding disc player.

File operations of traditional mass storage devices such as flash drives, memory cards and hard drives can be simulated using a UDF live file system.

For computer data backup and physical data transfer, optical discs such as CDs and DVDs are gradually being replaced with faster, smaller solid-state devices, especially the USB flash drive.[9] This trend is expected to continue as USB flash drives continue to increase in capacity and drop in price.[citation needed]

Additionally, music, movies, games, software and TV shows purchased, shared or streamed over the Internet has significantly reduced the number of audio CDs, video DVDs and Blu-ray discs sold annually. However, audio CDs and Blu-rays are still preferred and bought by some, as a way of supporting their favorite works while getting something tangible in return and also since audio CDs (alongside vinyl records and cassette tapes) contain uncompressed audio without the artifacts introduced by lossy compression algorithms like MP3, and Blu-rays offer better image and sound quality than streaming media, without visible compression artifacts, due to higher bitrates and more available storage space.[10] However, Blu-rays may sometimes be torrented over the internet, but torrenting may not be an option for some, due to restrictions put in place by ISPs on legal or copyright grounds, low download speeds or not having enough available storage space, since the content may weigh up to several dozen gigabytes. Blu-rays may be the only option for those looking to play large games without having to download them over an unreliable or slow internet connection, which is the reason why they are still (as of 2020) widely used by gaming consoles, like the PlayStation 4 and Xbox One X. As of 2020, it is unusual for PC games to be available in a physical format like Blu-ray.

Optical discs are typically stored in special cases, sometimes called jewel cases. Discs should not have any stickers and should not be stored together with paper; papers must be removed from the jewel case before storage. Discs should be handled by the edges to prevent scratching, with the thumb on the inner edge of the disc. The ISO Standard 18938:2014 is about best optical disc handling techniques. Optical disc cleaning should never be done in a circular pattern, to avoid concentric cirles from forming on the disc. Improper cleaning can scratch the disc. Recordable discs should not be exposed to light for extended periods of time. Optical discs should be stored in dry and cool conditions to increase longevity, with temperatures between -10 and 23 °C, never exceeding 32 °C, and with humidity never falling below 10%, with recommended storage at 20 to 50% of humidity without fluctuations of more than ±10%.[11]

Durability

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Optical discs are not vulnerable to water.

Although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage, if handled improperly.

Optical discs are not prone to uncontrollable catastrophic failures such as head crashes, power surges, or exposure to water like hard disk drives and flash storage, since optical drives' storage controllers are not tied to optical discs themselves like with hard disk drives and flash memory controllers, and a disc is usually recoverable from a defective optical drive by pushing an unsharp needle into the emergency ejection pinhole, and has no point of immediate water ingress and no integrated circuitry.

Security

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As the media itself only is accessed through a laser beam and has no internal control circuitry, it cannot contain malicious hardware in the same way as so-called rubber-duckies or USB killers. Like any data storage media, optical discs can contain malicious data, they are able to contain and spread malware - as happened in the case of the Sony BMG copy protection rootkit scandal in 2005 where Sony misused discs by pre-loading them with malware.[12]

Many types of optical discs are factory-pressed or finalized write once read many storage devices and would therefore not be effective at spreading computer worms that are designed to spread by copying themselves onto optical media, because data on those discs can not be modified once pressed or written. However, re-writable disc technologies (such as CD-RW) are able to spread this type of malware.[13]

History

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Main article: History of optical storage media
An earlier analog optical disc recorded in 1935 for Lichttonorgel [de] (sampling organ)
Lichttonorgel & optical disc system

The first recorded historical use of an optical disc was in 1884 when Alexander Graham Bell, Chichester Bell and Charles Sumner Tainter recorded sound on a glass disc using a beam of light.[14]

Optophonie is a very early (1931) example of a recording device using light for both recording and playing back sound signals on a transparent photograph.[15]

An early analogue optical disc system existed in 1935, used on Welte's Lichttonorgel [de] sampling organ.[16]

An early analog optical disc used for video recording was invented by David Paul Gregg in 1958[17] and patented in the US in 1961 and 1969. This form of optical disc was a very early form of the DVD (U.S. patent 3,430,966). It is of special interest that U.S. patent 4,893,297, filed 1989, issued 1990, generated royalty income for Pioneer Corporation's DVA until 2007 —then encompassing the CD, DVD, and Blu-ray systems. In the early 1960s, the Music Corporation of America bought Gregg's patents and his company, Gauss Electrophysics.

American inventor James T. Russell has been credited with inventing the first system to record a digital signal on an optical transparent foil that is lit from behind by a high-power halogen lamp. Russell's patent application was first filed in 1966 and he was granted a patent in 1970. Following litigation, Sony and Philips licensed Russell's patents (then held by a Canadian company, Optical Recording Corp.) in the 1980s.[18][19][20]

Both Gregg's and Russell's disc are floppy media read in transparent mode, which imposes serious drawbacks, after this were developed four generations of optical drive that includes Laserdisc (1969), WORM (1979), Compact Discs (1984), DVD (1995), Blu-ray (2005), HD-DVD (2006), more formats are currently under development.

First-generation

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At the start optical discs were read-only media used to store broadcast-quality analog video, and later digital media such as music or computer software. The LaserDisc format stored analog video signals for the distribution of home video, but commercially lost to the VHS videocassette format, due mainly to its high cost and non-re-recordability; other first-generation disc formats were designed only to store digital data and were not initially capable of use as a digital video medium.

Most first-generation disc devices had an infrared laser reading head. The minimum size of the laser spot is proportional to the wavelength of the laser, so wavelength is a limiting factor upon the amount of information that can be stored in a given physical area on the disc. The infrared range is beyond the long-wavelength end of the visible light spectrum, so it supports less density than shorter-wavelength visible light. One example of high-density data storage capacity, achieved with an infrared laser, is 700 MB of net user data for a 12 cm compact disc.

Other factors that affect data storage density include: the existence of multiple layers of data on the disc, the method of rotation (Constant linear velocity (CLV), Constant angular velocity (CAV), or zoned-CAV), the composition of lands and pits, and how much margin is unused is at the center and the edge of the disc.

Sony and Philips developed the first generation of the CDs in the mid-1980s with the complete specifications for these devices. With the help of this kind of technology the possibility of representing the analog signal into digital signal was exploited to a great level. For this purpose, the 16-bit samples of the analog signal were taken at the rate of 44,100 samples per second. This sample rate was based on the Nyquist rate of 40,000 samples per second required to capture the audible frequency range to 20 kHz without aliasing, with an additional tolerance to allow the use of less-than-perfect analog audio pre-filters to remove any higher frequencies.[21] The first version of the standard allowed up to 74 minutes of music or 650 MB of data storage.

Types of Read-only Optical Discs:

Laserdisc

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In the Netherlands in 1969, Philips Research physicist, Pieter Kramer invented an optical videodisc in reflective mode with a protective layer read by a focused laser beam U.S. patent 5,068,846, filed 1972, issued 1991. Kramer's physical format is used in all optical discs.

In 1975, Philips and MCA began to work together, and in 1978, commercially much too late, they presented their long-awaited Laserdisc in Atlanta. MCA delivered the discs and Philips the players. However, the presentation was a commercial failure, and the cooperation ended.

In Japan and the U.S., Pioneer succeeded with the Laserdisc until the advent of the DVD. In 1979, Philips and Sony, in consortium, successfully developed the audio compact disc.

CD-ROM

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The CD-ROM format was developed by Sony and Philips, introduced in 1984, as an extension of Compact Disc Digital Audio and adapted to hold any form of digital data. The same year, Sony demonstrated a LaserDisc data storage format, with a larger data capacity of 3.28 GB.[22]


Types of recordable Optical Discs

Magneto-optical drive

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Magneto-optical discs are erasable media, they can be written and read many times; the media and drives were first introduced in late 1987 and early 1988 by Sharp, MCI, Sony and others, all using SCSI interface. Capacity ranged from 512 MB on 130 mm media to 160 Mb on 90 mm media.[23] By 1998 there were more than 50 models offered by 12 vendors with media of 86 and 130 mm in diameter and offering capacities up to 2,600 MB, almost all using SCSI interface.[24]

WORM drive

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In 1979, Exxon STAR Systems in Pasadena, CA built a computer controlled WORM drive that utilized thin film coatings of Tellurium and Selenium on a 12" diameter glass disk. The recording system utilized blue light at 457 nm to record and red light at 632.8 nm to read. STAR Systems was bought by Storage Technology Corporation (STC) in 1981 and moved to Boulder, CO. Development of the WORM technology was continued using 14" diameter aluminum substrates. Beta testing of the disk drives, originally labeled the Laser Storage Drive 2000 (LSD-2000), was only moderately successful. Many of the disks were shipped to RCA Laboratories (now David Sarnoff Research Center) to be used in the Library of Congress archiving efforts. The STC disks utilized a sealed cartridge with an optical window for protection U.S. patent 4,542,495.

Second-generation

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Second-generation optical discs were for storing great amounts of data, including broadcast-quality digital video. Such discs usually are read with a visible-light laser (usually red); the shorter wavelength and greater numerical aperture[25] allow a narrower light beam, permitting smaller pits and lands in the disc. In the DVD format, this allows 4.7 GB storage on a standard 12 cm, single-sided, single-layer disc; alternatively, smaller media, such as the DataPlay format, can have capacity comparable to that of the larger, standard compact 12 cm disc.[26]

DVD-ROM

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In 1995, a consortium of manufacturers (Sony, Philips, Toshiba, Panasonic) developed the second generation of the optical disc, the DVD.[27] The DVD disc appeared after the CD-ROM had become widespread in society.

Third-generation

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Third-generation optical discs are used for distributing high-definition video and videogames and support greater data storage capacities, accomplished with short-wavelength visible-light lasers and greater numerical apertures. Blu-ray Disc and HD DVD uses blue-violet lasers and focusing optics of greater aperture, for use with discs with smaller pits and lands, thereby greater data storage capacity per layer.[25] In practice, the effective multimedia presentation capacity is improved with enhanced video data compression codecs such as H.264/MPEG-4 AVC and VC-1.

Announced but not released:

Blu-ray and HD-DVD

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The third generation optical disc was developed in 2000–2006 and was introduced as Blu-ray Disc. First movies on Blu-ray Discs were released in June 2006.[31] Blu-ray eventually prevailed in a high definition optical disc format war over a competing format, the HD DVD. A standard Blu-ray disc can hold about 25 GB of data, a DVD about 4.7 GB, and a CD about 700 MB.

Comparison of various optical storage media

Fourth-generation

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The following formats go beyond the current third-generation discs and have the potential to hold more than one terabyte (1 TB) of data and at least some are meant for cold data storage in data centers:[32][dubious – discuss]

Announced but abandoned:

Announced but not released:

In 2004, development of the Holographic Versatile Disc (HVD) commenced, which promised the storage of several terabytes of data per disc. However, development stagnated towards the late 2000s due to lack of funding.

In 2006, it was reported that Japanese researchers developed ultraviolet ray lasers with a wavelength of 210 nanometers, which would enable a higher bit density than Blu-ray discs.[33] As of 2022, no updates on that project have been reported.

Overview of optical types

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Name Capacity Experimental[Note 1] Years[Note 2]
LaserDisc (LD) N/A 1971–2007
Write Once Read Many Disk (WORM) 0.2–6.0 GB 1979–1984
Compact disc (CD) 0.7–0.9 GB 1982–present
Electron Trapping Optical Memory (ETOM) 6.0–12.0 GB 1987–1996
MiniDisc (MD) 0.14–1.0 GB 1989–2025
Magneto Optical Disc (MOD) 0.1–16.7 GB 1990–present
Digital Versatile Disc (DVD) 4.7–17 GB 1995–present
LIMDOW (Laser Intensity Modulation Direct OverWrite) 2.6 GB 10 GB 1996–present
GD-ROM 1.2 GB 1997–2006
Fluorescent Multilayer Disc 50–140 GB 1998-2003
Versatile Multilayer Disc (VMD) 5–20 GB 100 GB 1999-2010
Hyper CD-ROM 1 PB 100 EB 1999–present
DataPlay 500 MB 1999-2006
Ultra Density Optical (UDO) 30–60 GB 2000–present
Forward Versatile Disc (FVD) 5.4–15 GB 2005–2006[34]
Enhanced Versatile Disc (EVD) DVD 2002-2004
HD DVD 15–51 GB 1 TB[citation needed] 2002-2008
Blu-ray Disc (BD) 25 GB
50 GB		 2002–present
BDXL 100 GB, 128 GB 1 TB 2010–present
Professional Disc for Data (PDD) 23 GB 2003-2006
Professional Disc 23–128 GB 2003–present
Digital Multilayer Disk 22-32 GB 2004–2007
Multiplexed Optical Data Storage (MODS-Disc) 250 GB–1 TB 2004–present
Universal Media Disc (UMD) 0.9–1.8 GB 2004–2014
Holographic Versatile Disc (HVD) 6.0 TB 2004–2012
Protein-coated disc (PCD) 50 TB 2005–2006
M-DISC 4.7 GB (DVD format)
25 GB (Blu-ray format)
50 GB (Blu-ray format)
100 GB (BDXL format) [35]		 2009–present
Archival Disc 0.3-1 TB 2014–2024
Ultra HD Blu-ray 50 GB
66 GB
100 GB
128 GB		 2015–present
Notes
  1. ^ Prototypes and theoretical values.
  2. ^ Years from (known) start of development till end of sales or development.

Recordable and writable optical discs

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Main article: Optical disc recording technologies

There are numerous formats of optical direct to disk recording devices on the market, all of which are based on using a laser to change the reflectivity of the digital recording medium in order to duplicate the effects of the pits and lands created when a commercial optical disc is pressed. Formats such as CD-R and DVD-R are "Write once read many" or write-once, while CD-RW and DVD-RW are rewritable, more like a magnetic recording hard disk drive (HDD).

Media technologies vary, for example, M-DISC media uses a rock-like layer to retain data for longer than conventional recordable media. While being read-only compatible with existing DVD and Blu-ray drives, M-DISC media can only be written to using a stronger laser specifically made for this purpose, which is built into fewer optical drive models.

Surface error scanning

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Error rate measurement on a DVD+R. The error rate is still within a healthy range.

Optical media can predictively be scanned for errors and media deterioration well before any data becomes unreadable.[36] Optical formats include some redundancy for error correction, which works until the amount of error exceeds a threshold. A higher rate of errors may indicate deteriorating and/or low quality media, physical damage, an unclean surface and/or media written using a defective optical drive.

Precise error scanning requires access to the raw, uncorrected readout of a disc, which is not always provided by a drive. As a result, support of this functionality varies per optical drive manufacturer and model. On ordinary drives without this functionality, it is possible to still look for unexpected reduction in read speed as an indirect, much less reliable measure.[37]

Optical media, such as CDs and DVDs, can be scanned to detect errors and signs of deterioration well before data becomes unreadable. These formats include built-in error correction mechanisms, which function by adding redundant data. However, once the rate of errors surpasses the correction threshold, the media becomes vulnerable to failure. A high error rate can signal physical deterioration, low-quality manufacturing, surface contamination, or data recorded by a faulty optical drive.

Accurate error scanning requires access to a disc's raw, uncorrected readout. However, not all optical drives provide this capability, and support for this feature can vary significantly between manufacturers and drive models. On drives lacking raw data access, users may rely on a less precise method: monitoring unexpected reductions in read speed, though this is a far less reliable indicator of disc health.

Several specialized tools are available for performing error scans on optical media. Popular programs include Nero DiscSpeed, K-Probe, Opti Drive Control (previously known as "CD Speed 2000"), and DVD Info Pro for Windows. For cross-platform users, QPxTool is available to help monitor and maintain optical media integrity. Each of these tools allows for detailed analysis of the error rates and conditions affecting optical discs.

Error types

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There are different types of error measurements, including so-called "C1", "C2" and "CU" errors on CDs, and "PI/PO (parity inner/outer) errors" and the more critical "PI/PO failures" on DVDs. Finer-grain error measurements on CDs supported by very few optical drives are called E11, E21, E31, E21, E22, E32.

"CU" and "POF" represent uncorrectable errors on data CDs and DVDs respectively, thus data loss, and can be a result of too many consecutive smaller errors.[38]

Due to the weaker error correction used on Audio CDs (Red Book standard) and Video CDs (White Book standard), C2 errors already lead to data loss. However, even with C2 errors, the damage is inaudible to some extent.

Blu-ray discs use so-called LDC (Long Distance Codes) and BIS (Burst Indication Subcodes) error parameters. According to the developer of the Opti Drive Control software, a disc can be considered healthy at an LDC error rate below 13 and BIS error rate below 15.[39]

Optical disc manufacturing

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Main article: CD manufacturing

Optical discs are made using replication. This process can be used with all disc types. Recordable discs have pre-recorded vital information, like manufacturer, disc type, maximum read and write speeds, etc. In replication, a cleanroom with yellow light is necessary to protect the light-sensitive photoresist and to prevent dust from corrupting the data on the disc.

A glass master is used in replication. The master is placed in a machine that cleans it as much as possible using a rotating brush and deionized water, preparing it for the next step. In the next step, a surface analyzer inspects the cleanliness of the master before photoresist is applied on the master.

The photoresist is then baked in an oven to solidify it. Then, in the exposure process, the master is placed in a turntable where a laser selectively exposes the resist to light. At the same time, a developer and deionized water are applied to the disc to remove the exposed resist. This process forms the pits and lands that represent the data on the disc.

A thin coating of metal is then applied to the master, making a negative of the master with the pits and lands in it. The negative is then peeled off the master and coated in a thin layer of plastic. The plastic protects the coating while a punching press punches a hole into the center of the disc, and punches excess material.

The negative is now a stamper - a part of the mold that will be used for replication. It is placed on one side of the mold with the data side containing the pits and lands facing out. This is done inside an injection molding machine. The machine then closes the mold and injects polycarbonate in the cavity formed by the walls of the mold, which forms or molds the disc with the data on it.

The molten polycarbonate fills the pits or spaces between the lands on the negative, acquiring their shape when it solidifies. This step is somewhat similar to record pressing.

The polycarbonate disc cools quickly and is promptly removed from the machine, before forming another disc. The disc is then metallized, covered with a thin reflective layer of aluminum. The aluminum fills the space once occupied by the negative.

A layer of varnish is then applied to protect the aluminum coating and provide a surface suitable for printing. The varnish is applied near the center of the disc, and the disc is spun, evenly distributing the varnish on the surface of the disc. The varnish is hardened using UV light. The discs are then silkscreened or a label is otherwise applied.[40][41][42]

Recordable discs add a dye layer, and rewritable discs add a phase change alloy layer instead, which is protected by upper and lower dielectric (electrically insulating) layers. The layers may be sputtered. The additional layer is between the grooves and the reflective layer of the disc. Grooves are made in recordable discs in place of the traditional pits and lands found in replicated discs, and the two can be made in the same exposure process.[43][44][45][46][47] In DVDs, the same processes as in CDs are carried out, but in a thinner disc. The thinner disc is then bonded to a second, equally thin but blank, disc using UV-curable Liquid optically clear adhesive, forming a DVD disc.[48][5][49][50] This leaves the data in the middle of the disc, which is necessary for DVDs to achieve their storage capacity. In multi layer discs, semi reflective instead of reflective coatings are used for all layers except the last layer, which is the deepest one and uses a traditional reflective coating.[51][52][53]

Dual layer DVDs are made slightly differently. After metallization (with a thinner metal layer to allow some light to pass through), base and pit transfer resins are applied and pre-cured in the center of the disc. Then the disc is pressed again using a different stamper, and the resins are completely cured using UV light before being separated from the stamper. Then the disc receives another, thicker metallization layer, and is then bonded to the blank disc using LOCA glue. DVD-R DL and DVD+R DL discs receive a dye layer after curing, but before metallization. CD-R, DVD-R, and DVD+R discs receive the dye layer after pressing but before metallization. CD-RW, DVD-RW and DVD+RW receive a metal alloy layer sandwiched between 2 dielectric layers. HD-DVD is made in the same way as DVD. In recordable and rewritable media, most of the stamper is composed of grooves, not pits and lands. The grooves contain a wobble frequency that is used to locate the position of the reading or writing laser on the disc. DVDs use pre-pits instead, with a constant frequency wobble.[44]

Blu-ray

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HTL (high-to-low type) Blu-ray discs are made differently. First, a silicon wafer is used instead of a glass master.[54] The wafer is processed in the same way a glass master would.

The wafer is then electroplated to form a 300-micron thick nickel stamper, which is peeled off from the wafer. The stamper is mounted onto a mold inside a press or embosser.

The polycarbonate discs are molded in a similar fashion to DVD and CD discs. If the discs being produced are BD-Rs or BD-REs, the mold is fitted with a stamper that stamps a groove pattern onto the discs, in lieu of the pits and lands found on BD-ROM discs.

After cooling, a 35 nanometre-thick layer of silver alloy is applied to the disc using sputtering.[55][56][57] Then the second layer is made by applying base and pit transfer resins to the disc, and are pre-cured in its center.

After application and pre-curing, the disc is pressed or embossed using a stamper and the resins are immediately cured using intense UV light, before the disc is separated from the stamper. The stamper contains the data that will be transferred to the disc. This process is known as embossing and is the step that engraves the data onto the disc, replacing the pressing process used in the first layer, and it is also used for multi layer DVD discs.

Then, a 30 nanometre-thick layer of silver alloy is then sputtered onto the disc and the process is repeated as many times as required. Each repetition creates a new data layer. (The resins are applied again, pre-cured, stamped (with data or grooves) and cured, silver alloy is sputtered and so on)

BD-R and BD-RE discs receive (through sputtering) a metal (recording layer) alloy (that is sandwiched between two dielectric layers, also sputtered, in BD-RE), before receiving the 30 nanometre metallization (silver alloy, aluminum or gold) layer, which is sputtered. Alternatively, the silver alloy may be applied before the recording layer is applied. Silver alloys are usually used in Blu-rays, and aluminum is usually used on CDs and DVDs. Gold is used in some "Archival" CDs and DVDs, since it is more chemically inert and resistant to corrosion than aluminum, which corrodes into aluminum oxide, which can be seen in disc rot as transparent patches or dots in the disc, that prevent the disc from being read, since the laser light passes through the disc instead of being reflected back into the laser pickup assembly to be read. Normally, aluminum does not corrode since it has a thin oxide layer that forms on contact with oxygen. In this case, it can corrode due to its thinness.

Then, the 98 micron-thick cover layer is applied using UV-curable liquid optically clear adhesive, and a 2 micron-thick hard coat (such as Durabis) is also applied and cured using UV light. In the last step, a 10 nanometre-thick silicon nitride barrier layer is applied to the label side of the disc to protect against humidity.[45][55][58][59] Blu-rays have their data very close to the read surface of the disc, which is necessary for Blu-rays to achieve their capacity.

Discs in large quantities can either be replicated or duplicated. In replication, the process explained above is used to make the discs, while in duplication, CD-R, DVD-R or BD-R discs are recorded and finalized to prevent further recording and allow for wider compatibility.[60] (See Optical disc authoring). The equipment is also different: replication is carried out by fully automated purpose-built machinery whose cost is in the hundreds of thousands of US dollars in the used market,[61] while duplication can be automated (using what's known as an autoloader[62]) or be done by hand, and only requires a small tabletop duplicator.[63]

Specifications

[edit]
Base (1×) and (current) maximum speeds by generation
Generation Base Max
(Mbit/s) (Mbit/s) ×
1st (CD) 1.17 65.6 56×
2nd (DVD) 10.57 253.6 24×
3rd (BD) 36 504 14×[64]
4th (AD) ? ? 14×
Capacity and nomenclature[65][66]
Designation Sides Layers
(total)		 Diameter Capacity
(cm) (GB)
BD SS SL 1 1 8 7.8
BD SS DL 1 2 8 15.6
BD SS SL 1 1 12 25
BD SS DL 1 2 12 50
BD SS TL 1 3 12 100
BD SS QL 1 4 12 128
CD–ROM 74 min SS SL 1 1 12 0.682
CD–ROM 80 min SS SL 1 1 12 0.737
CD–ROM SS SL 1 1 8 0.194
DDCD–ROM SS SL 1 1 12 1.364
DDCD–ROM SS SL 1 1 8 0.387
DVD–1 SS SL 1 1 8 1.46
DVD–2 SS DL 1 2 8 2.66
DVD–3 DS SL 2 2 8 2.92
DVD–4 DS DL 2 4 8 5.32
DVD–5 SS SL 1 1 12 4.70
DVD–9 SS DL 1 2 12 8.54
DVD–10 DS SL 2 2 12 9.40
DVD–14 DS DL/SL 2 3 12 13.24
DVD–18 DS DL 2 4 12 17.08
DVD–R 1.0 SS SL 1 1 12 3.95
DVD–R (2.0), +R, –RW, +RW SS SL 1 1 12 4.7
DVD-R, +R, –RW, +RW DS SL 2 2 12 9.40
DVD–RAM SS SL 1 1 8 1.46
DVD–RAM DS SL 2 2 8 2.65
DVD–RAM 1.0 SS SL 1 1 12 2.58
DVD–RAM 2.0 SS SL 1 1 12 4.70
DVD–RAM 1.0 DS SL 2 2 12 5.16
DVD–RAM 2.0 DS SL 2 2 12 9.40

See also

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References

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Optical disc

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An optical disc is a flat, circular storage medium, typically 12 cm in diameter and made of polycarbonate plastic coated with a thin reflective layer, that encodes digital data as microscopic pits and lands in a continuous spiral track, which are read and written using a low-powered laser beam to detect variations in light reflectivity.[1][2] This technology enables high-capacity, removable data storage with capacities ranging from hundreds of megabytes to over 100 gigabytes per disc, depending on the format, and relies on principles of diffraction and interference for precise data retrieval.[2][3] The origins of optical disc technology trace back to the late 1950s, when American inventor David Paul Gregg conceived the videodisc concept in 1958 and secured key patents in 1961 and 1969, laying foundational principles for laser-based video and audio storage.[4] Independently, James T. Russell developed similar ideas around the same time, contributing to early prototypes in the 1960s.[1] Commercialization accelerated in the 1970s with the introduction of the LaserDisc in 1978 by MCA Discovision, an analog video format licensed from Gregg's patents, followed by the digital Compact Disc (CD) in 1982, developed collaboratively by Philips and Sony for audio distribution with a capacity of about 700 MB.[4][1] Subsequent innovations included IBM's magneto-optical discs in the 1970s, which used magnetic fields alongside lasers for rewritability, and the Digital Versatile Disc (DVD) in 1995, offering 4.7 GB (single-layer) to 17 GB (dual-layer) for video and data.[5] The Blu-ray Disc, introduced in 2006 by a consortium including Sony and Philips, utilized a blue-violet laser for higher densities, achieving 25 GB (single-layer) to 128 GB (multi-layer).[2] Optical discs function through a laser diode emitting a focused beam that scans the disc's surface at speeds of 200–500 RPM, with photodetectors converting reflected light patterns into binary code (0s for pits, 1s for lands), while writing involves higher-intensity lasers to etch or alter the recording layer.[3][2] Variants include read-only formats like CD-ROM and DVD-ROM for software and media distribution, write-once options such as CD-R and BD-R for permanent archiving, and rewritable types like CD-RW and BD-RE for repeated use.[1] These discs have been pivotal in consumer electronics for audio CDs, video DVDs and Blu-rays, and data backups, prized for their durability (up to 100 years of readability), immunity to electromagnetic interference, and low cost per gigabyte, though access speeds are slower than modern solid-state drives and they remain susceptible to physical damage like scratches.[5][3] Despite the shift toward flash memory and cloud storage by the 2020s, optical discs persist in applications requiring long-term archival stability, such as government records and scientific data preservation.[5]

Fundamentals

Definition and principles

An optical disc is a flat, circular storage medium that encodes digital data in the form of microscopic pits or marks on its surface, which are read using low-power laser light to detect variations in reflection.[2] These discs, typically made of polycarbonate, allow for high-density data storage by leveraging the precise focusing of a laser beam onto a spiral track of pits and lands, where pits represent depressions and lands are the flat areas between them.[6] The fundamental principle of operation involves directing a focused laser beam at the disc's reflective layer while the disc rotates, causing the beam to interact with the pit-land transitions. When the laser strikes a land, it reflects strongly back to a photodetector, whereas reflection from a pit is diminished due to phase interference, resulting in intensity variations that are converted into electrical signals representing binary data (0s and 1s).[2] This process enables non-contact reading, with data retrieval speeds determined by the disc's rotation and the laser's tracking mechanism.[6] Key physical limitations arise from the diffraction of light, which sets the minimum resolvable pit size and thus storage density; the approximate minimum pit width is given by
d≈[λ](/page/Lambda)2⋅NA, d \approx \frac{[\lambda](/page/Lambda)}{2 \cdot \mathrm{NA}},
where [λ](/page/Lambda)[\lambda](/page/Lambda) is the laser wavelength and NA\mathrm{NA} is the numerical aperture of the objective lens.[7] Shorter wavelengths and higher NA values allow for smaller pits and greater data capacity. Compared to magnetic media, optical discs offer advantages such as higher storage density, easy removability for portability, and immunity to magnetic fields, ensuring data integrity in environments where magnetic interference is a concern.[5]

Physical components

An optical disc consists of multiple layered components designed to facilitate the storage and retrieval of data through laser interaction. The primary structure includes a transparent polycarbonate substrate that forms the base, onto which data patterns are imprinted or prepared for recording. This substrate is typically produced via injection molding, where molten polycarbonate is pressed into a mold to create microscopic pits for read-only discs or shallow grooves for writable variants, ensuring precise data topography essential for optical readability.[8][9] Overlying the substrate is a thin reflective layer, commonly aluminum, which mirrors the incident laser beam to enable data detection by reflecting light back to the optical pickup. This metallic coating, applied via sputtering, achieves a minimum reflectance of 70% to ensure reliable signal strength. A protective lacquer layer, approximately 7 micrometers thick, coats the reflective surface to shield it from environmental factors like oxidation and mechanical damage, while maintaining optical clarity.[8][10] For recordable discs such as CD-R, an additional organic dye layer—typically cyanine or phthalocyanine-based—is inserted between the polycarbonate substrate and the reflective aluminum, allowing laser-induced chemical changes to form data marks mimicking pits. In these discs, the substrate features a pre-grooved spiral track rather than discrete pits, with the groove serving as a guide for laser tracking via wobble modulation at around 22 kHz.[11][9] Standard optical discs measure 120 mm in diameter with a tolerance of ±0.3 mm and a thickness of 1.2 mm ±0.1 mm, the latter critical for maintaining laser focus depth and preventing read errors due to substrate variations. The central hub includes a 15 mm diameter hole (+0.1 mm/-0.0 mm) surrounded by a clamping zone spanning 22 to 33 mm in diameter, designed for secure spindle attachment in drives while minimizing radial and axial runout to under 140 micrometers.[8] Data is organized in a single continuous spiral track starting from the inner radius, with a track pitch of 1.6 μm ±0.1 μm; in read-only discs, this track alternates between pits (typically 0.83 to 3.0 micrometers long) and lands, modulating reflected light to encode binary data. These components collectively support the optical principles of interference and reflection outlined in the disc's foundational design.[8]

Technology and Operation

Reading and recording mechanisms

Optical disc drives employ a laser-based reading mechanism to retrieve data stored as microscopic pits and lands on the disc surface. A semiconductor laser diode emits a coherent beam of light, typically in the infrared, red, or blue spectrum depending on the disc format. This beam is collimated and directed toward the disc by the optical pickup unit.[12] An objective lens, with a high numerical aperture, focuses the laser beam to a diffraction-limited spot size of approximately 0.5 to 1 micrometer in diameter onto the reflective layer of the spinning disc.[12] The pits and lands cause variations in the reflected light intensity due to differences in reflectivity; the returning beam passes back through the objective lens and is split by a beam splitter to reach a photodetector array, such as a photodiode integrated circuit, which converts the optical signal into electrical pulses representing the binary data.[12] Servo systems ensure precise operation during reading by maintaining focus, tracking, and speed control. Focus servo actuators adjust the objective lens vertically using electromagnetic coils or piezoelectric elements to keep the beam at the optimal focal depth, compensating for disc tilt or surface irregularities.[12] Tracking servo systems use radial actuators to position the pickup head along the data spiral track, guided by signals from the photodetector that detect groove wobble or pre-embossed tracks.[12] These closed-loop feedback mechanisms, often implemented via digital signal processing, enable reliable data access even at high rotation speeds.[12] Key drive components facilitate the mechanical aspects of reading. The spindle motor, a brushless DC motor, rotates the disc clamp to hold and spin the media.[12] In constant linear velocity (CLV) mode, common for CDs and DVDs, the motor varies the angular speed to maintain a constant tangential data rate, typically from 500 rpm at the inner radius to 200 rpm at the outer radius for a 1x CD speed of 1.2 m/s linear velocity.[13][14] Constant angular velocity (CAV) mode, used in some hybrid schemes like Z-CLV for faster access, maintains a fixed rotation rate, such as up to 10,000 rpm, resulting in higher data rates at outer tracks but lower disc utilization efficiency.[13] The pickup head assembly integrates the laser diode, objective lens, actuators, and photodetector, sliding radially via a coarse stepper or voice coil motor to access different tracks.[12] Recording mechanisms in writable optical discs rely on laser-induced thermal effects to modify the recording layer, creating readable pits or marks. In phase-change media, used in rewritable formats like CD-RW and DVD-RW, a high-power laser pulse rapidly heats a chalcogenide alloy layer, such as Ge₂Sb₂Te₅, above its melting point (around 600°C) to form amorphous marks upon rapid cooling; lower-power pulses then recrystallize these marks for erasure by annealing at 150–250°C.[15] This reversible amorphous-crystalline transition alters reflectivity, mimicking pressed pits, with cycle times under 100 ns enabling data rates up to 53 Mbit/s in DVD formats.[15][16] For write-once formats like CD-R and DVD-R, organic dye recording (also called ablative or pit-forming) uses a laser to heat a photosensitive dye layer, causing localized decomposition or bubbling that deforms the reflective metal underneath, forming non-reflective pits without altering reflectivity directly.[17] The dye, often cyanine- or azo-based, changes optical properties irreversibly under laser exposure at powers of 5–15 mW, with the process optimized by multi-pulse write strategies to control mark edges.[17] Multi-layer discs, such as dual-layer DVDs, increase capacity by stacking semi-transparent recording layers, accessed sequentially by focusing the laser at different depths using the same wavelength but adjusting focus servo for varying reflectivities (around 18–45%).[18] Advanced multi-layer magneto-optical discs employ multi-wavelength readout, where layers tuned to specific wavelengths (e.g., TbFeCo materials responsive to dual lasers) allow selective addressing to minimize crosstalk and enhance density.[19]

Data encoding and file systems

Data on optical discs is encoded using channel modulation techniques to convert digital information into a physical pattern of pits and lands that can be reliably read by a laser beam. For compact discs (CDs), Eight-to-Fourteen Modulation (EFM) is employed, where each 8-bit byte of data is mapped to a 14-bit codeword selected from a lookup table to ensure a minimum of two and a maximum of ten consecutive zeros between ones, forming a run-length limited (RLL(2,10)) code that minimizes errors from timing jitter and optimizes the signal's DC balance.[8] This modulation is preceded by merging bits to further enforce the run-length constraints, resulting in channel frames of 588 bits plus synchronization patterns.[8] As part of the encoding process, error correction is integrated to enhance data reliability, particularly for CDs where Cross-Interleaved Reed-Solomon Coding (CIRC) is applied. CIRC uses two stages of Reed-Solomon codes: a C1 code (32 symbols, 28 data, 4 parity) on interleaved frames and a C2 code (28 symbols, 24 data, 4 parity) with cross-interleaving across 108 frames to distribute burst errors, allowing correction of up to 3,800 consecutive symbol errors.[8] Data is organized into sectors, with CD-ROM Mode 1 sectors comprising 2,352 bytes total, including 2,048 bytes of user data, auxiliary fields for error detection, and parity for CIRC.[8] Logical block addressing (LBA) facilitates access, numbering sectors sequentially from zero.[8] File systems provide the logical organization atop this encoded structure, enabling hierarchical storage and retrieval. The ISO 9660 standard, adopted for CD-ROMs, defines a volume and file structure supporting directory hierarchies up to eight levels deep in its primary form, with files addressed via extents of logical blocks (typically 2,048 bytes each) and filenames restricted to an 8.3 format using uppercase ASCII characters for broad compatibility.[20][21] Joliet extensions, implemented as a supplementary volume descriptor, enhance ISO 9660 by supporting Unicode (UCS-2) characters and longer filenames up to 64 characters, while maintaining backward compatibility with the base standard.[20] For DVDs and Blu-ray discs, the Universal Disk Format (UDF) serves as the primary file system, building on ISO 9660 principles but offering greater flexibility for rewritable and write-once media through features like virtual allocation tables and pseudo-overwrite capabilities.[22] UDF supports hierarchical directories without the depth limits of early ISO 9660, logical block addressing for efficient access, and bridge formats that combine it with ISO 9660 for hybrid compatibility, making it suitable for both data interchange and multimedia applications across operating systems.[22]

Error detection and correction

Optical discs employ error detection and correction techniques to mitigate data errors arising from manufacturing defects, scratches, dust, or read imperfections, ensuring reliable data retrieval. These methods primarily rely on cyclic redundancy checks (CRC) for detection and Reed-Solomon (RS) codes for correction, operating over the Galois field GF(2^8) to handle byte-level errors efficiently.[23] In this field, each symbol represents one byte (8 bits), allowing the codes to correct multiple symbol errors per block while detecting additional ones.[23] Detection begins at the sector level with parity checks and CRC polynomials appended to data blocks. For instance, in CD-ROM sectors, a 32-bit error detection code (EDC) uses a CRC computed over the user data and auxiliary fields with the polynomial P(x) = (x^{16} + x^{15} + x^2 + 1)(x^{16} + x^2 + x + 1), enabling identification of bit flips or bursts within the 2,048-byte data payload.[8] This CRC flags erroneous sectors for correction attempts, while simpler parity checks in frame-level encoding provide initial validation. If uncorrectable, the system may skip affected areas in read-only modes or interpolate missing samples in audio applications to conceal errors from scratches exceeding correctable limits.[8] Correction algorithms center on RS codes configured as product codes with interleaving to combat burst errors common in optical media. In compact discs (CDs), the Cross-Interleaved Reed-Solomon Code (CIRC) processes 24-byte audio frames into 32-byte F1 frames using a C1 code RS(32,28) that corrects up to 2 byte errors per frame, followed by interleaving across 108 frames and a C2 code RS(28,24) that corrects up to 2 more bytes after de-interleaving.[8] For CD-ROM Mode 1 data sectors, in addition to CIRC, a supplementary scheme provides a 4-byte EDC and a 276-byte ECC field consisting of 172 P-parity bytes (computed using RS(26,24) codes, t=1, applied row-wise) and 104 Q-parity bytes (using RS(45,43) codes, t=1, applied column-wise) over a 43×48 byte block that includes sync, header, subheader, and user data bytes, enabling correction of additional single-symbol errors per row or column.[8] Similar structures appear in DVDs, where the RSPC uses inner RS(182,172) (t=5) and outer RS(208,192) (t=8) codes with deeper 16-row interleaving to handle denser data pits and higher error susceptibility.[24] Performance is gauged by metrics like the Block Error Rate (BLER), which measures erroneous frames before correction. CD specifications mandate an average BLER below 220 per second at the C1 decoder input, with bursts limited to fewer than 7,500 consecutive uncorrectable frames to prevent audible artifacts or data loss.[8] In production, surface scanning tools map defects like pits or blemishes to optimize mastering and verify compliance, aiding in preemptive error mitigation. During playback, if errors exceed thresholds (e.g., via flagged erasures from C1 failures), the system prioritizes correction over speed, ensuring data integrity across file system sectors.[8]

Durability and Security

Material degradation and lifespan

Optical discs are susceptible to several forms of material degradation that can compromise data integrity over time. One primary mechanism is the oxidation of the reflective layer, typically made of aluminum in read-only discs like CDs and DVDs, which leads to a phenomenon known as "disc rot" where the metal corrodes and forms pits that interfere with laser readability.[25] Delamination, the separation of the disc's layers such as the polycarbonate substrate from the reflective or protective coatings, often results from prolonged exposure to high humidity or temperature fluctuations, causing physical instability and data loss.[26] In recordable discs, dye fading occurs as the organic recording layer degrades, particularly under exposure to light, reducing the contrast needed for data pits and potentially rendering the disc unreadable.[27] Additionally, bit rot—subtle data corruption from environmental factors such as humidity and temperature fluctuations—can accumulate errors that eventually exceed built-in error correction limits, though this is rarer in optical media compared to magnetic storage.[28] The lifespan of optical discs varies widely based on material quality and storage conditions, with estimates ranging from 5 to 100 years or more for CDs and DVDs under optimal archival settings. Similar estimates apply to Blu-ray discs, with lifespans of 20-50 years or more under optimal conditions, though specialized archival variants like M-Disc claim up to 1000 years.[27] The ISO 18921 standard provides a method for estimating the life expectancy of CD-ROM media by assessing degradation thresholds, while ISO 18925 outlines storage practices to extend usability, recommending temperatures below 20°C and relative humidity under 50% to minimize chemical reactions.[29][30] Factors such as elevated temperatures accelerate oxidation and delamination, potentially halving lifespan for every 10°C increase above ideal levels, whereas high humidity promotes moisture ingress that exacerbates layer separation.[27] To evaluate longevity, accelerated aging tests simulate decades of environmental stress in controlled conditions, such as exposure to 80°C and 85% relative humidity for periods up to several weeks, allowing prediction of real-world failure rates.[31] Studies using these tests on various disc types have shown that recordable DVDs may retain data for 30 years at 25°C and 50% RH, but performance degrades faster in suboptimal environments.[32] Certain material choices enhance durability; for instance, gold-layered reflective coatings in recordable CDs resist oxidation far better than silver or aluminum, potentially extending lifespan by decades in archival use.[27] Avoiding direct sunlight is crucial, as UV radiation accelerates dye fading and polycarbonate yellowing, which can cause data errors within months of exposure.[25]

Protection and encryption methods

Optical discs employ various protection and encryption methods to prevent unauthorized copying, playback, and distribution of content. These measures range from analog techniques that exploit physical differences in disc manufacturing to digital encryption systems that require authentication between the disc and playback device. Analog methods, such as those used in early audio CDs, intentionally introduce irregularities in the pit and land structures to cause read errors during duplication attempts. For instance, Cactus Data Shield (CDS), developed by Midbar Technologies, embeds invalid C2 error correction pointers and hidden sectors that disrupt copying on CD-ROM drives while allowing normal playback on audio CD players.[33] This approach leverages the fact that replicated discs from consumer burners cannot accurately reproduce the precise pit variations required for error-free reading.[34] Digital copy protection became prominent with the introduction of the Content Scramble System (CSS) for DVDs in 1996, a digital rights management scheme licensed by the DVD Copy Control Association. CSS encrypts video data on the disc using a unique 40-bit key per title, combined with player authentication to prevent unauthorized extraction.[35] However, CSS was compromised in 1999 when Norwegian programmer Jon Lech Johansen reverse-engineered it to create DeCSS, a tool that decrypts DVD content for playback on Linux systems, sparking legal battles over circumvention and free speech.[36] The DeCSS controversy highlighted vulnerabilities in early encryption, leading to widespread lawsuits by the DVD CCA against distributors of the software.[36] For advanced formats like Blu-ray, the Advanced Access Content System (AACS) provides robust encryption, using symmetric keys to protect content up to the display device. AACS employs media keys derived from disc-specific identifiers and device private keys, with periodic updates to revocation lists that disable compromised players or keys.[37] Drive-level authentication in AACS verifies both the disc and player before decryption, ensuring only licensed hardware can access the content.[38] Despite these safeguards, vulnerabilities have emerged, such as key extraction from player memory dumps, allowing backups of specific titles; the AACS Licensing Administrator has responded by revoking exposed keys through updated lists embedded in new discs.[39] Blu-ray further enhances protection with BD+, an optional layer that runs virtual machine code on the player to implement dynamic countermeasures, including revocation of pirated devices. BD+ allows content providers to update protections post-manufacture, addressing threats like key leaks by altering decryption processes.[40] The Blu-ray Disc Association adopted BD+ in 2005 as part of a comprehensive system to counter evolving piracy methods.[40] Anti-piracy features extend beyond encryption to include region coding, which restricts playback to specific geographic zones on DVDs and Blu-ray discs, enforcing distribution controls. DVDs use eight regions, with players locked to one or more, preventing cross-region imports that could undermine pricing strategies.[41] Digital watermarks, imperceptible markers embedded in video streams, aid in tracing unauthorized copies; for DVDs, systems like the Millennium watermark proposed by Philips, Macrovision, and Digimarc enable forensic identification without affecting playback.[42] Some experimental anti-piracy efforts included self-destruct mechanisms, such as Disney's 2003 Flexplay DVDs, which darken after 48 hours of air exposure to limit rental-like use and reduce unauthorized retention.[43] These methods collectively aim to balance content security with user accessibility, though ongoing vulnerabilities underscore the cat-and-mouse nature of optical disc protection.

Historical Development

Early inventions and first-generation formats

The earliest precursors to optical disc technology emerged in the 19th century through efforts to visually capture and reproduce sound waves using light-sensitive methods. In 1857, French inventor Édouard-Léon Scott de Martinville patented the phonautograph, a device that used a diaphragm and stylus to trace sound vibrations onto soot-covered paper, creating a visual waveform, though it lacked playback capability.[44] This laid conceptual groundwork for recording media. Similarly, in 1877, Charles Cros proposed the paléophone, an innovative idea to photographically engrave sound waveforms onto a rotating glass disc using photoengraving techniques, allowing optical playback—foreshadowing modern optical storage despite never being built.[45] Key advancements in the mid-20th century laid the foundation for practical optical discs. In 1958, American inventor David Paul Gregg conceived the videodisc concept and secured patents in 1961 and 1969 for laser-based video and audio storage. Independently, James T. Russell developed similar ideas in the 1960s, creating early prototypes that influenced subsequent technologies. The first practical optical disc systems appeared in the 1970s, with Laserdisc marking the debut of consumer video storage. Developed collaboratively by Philips and MCA (Music Corporation of America), the format was demonstrated in 1972 and commercially released in 1978 as DiscoVision, featuring 30 cm (12-inch) diameter aluminum discs coated in plastic for analog video and audio recording.[46] These discs offered approximately 425 lines of resolution in NTSC format, enabling up to 60 minutes of playback per side with superior fidelity to magnetic tapes like VHS, though playback required a laser-based reader to detect pits and lands without physical contact.[47] Laserdisc's introduction revolutionized home entertainment but remained niche due to high costs and lack of recording capability for consumers. Parallel developments in data storage introduced Write Once Read Many (WORM) optical drives in the late 1970s, targeting archival applications. These systems used laser etching on thin-film coatings to permanently store digital data on 12-inch discs, with early capacities in the range of hundreds of megabytes to 1 GB per disc.[48] By the 1980s, WORM technology scaled to terabyte levels through jukebox systems combining multiple discs, making it ideal for secure, long-term archives in industries like government and finance where data immutability was critical.[48] The Compact Disc Read-Only Memory (CD-ROM) emerged as the first standardized digital optical format in the early 1980s, building on audio CD principles. Sony and Philips finalized the Yellow Book specification in 1983, adapting the 12 cm disc to store up to 650 MB of computer data with enhanced error correction for reliable digital retrieval.[49] The first CD audio player, Sony's CDP-101, launched in October 1982 in Japan, paving the way for data applications.[50] CD-ROM adoption accelerated in computing by 1985, with the release of Grolier's Academic American Encyclopedia as the inaugural title, compressing vast reference materials onto a single disc and spurring integration into PCs for multimedia and software distribution.[51]

Second-generation advancements

The second-generation optical disc advancements of the 1990s, epitomized by the DVD format, addressed the storage limitations of first-generation CDs by dramatically increasing capacity to support multimedia content such as full-length films. Developed as a successor to the compact disc, DVD-ROM was finalized in 1995 with a single-layer capacity of 4.7 GB and dual-layer configurations reaching up to 8.5 GB per side, allowing for approximately two hours of high-quality video.[https://www.ic.unicamp.br/~celio/mc326/hw/dvdintroduction.pdf] This leap was facilitated by MPEG-2 compression, which enabled efficient encoding of video and audio data onto the disc while maintaining superior picture and sound quality compared to analog formats like VHS.[https://www.ic.unicamp.br/~celio/mc326/hw/dvdintroduction.pdf] Standardization of the DVD format was achieved through the establishment of the DVD Forum in 1995, an industry consortium that unified competing proposals to prevent a fragmented market.[https://www.pctechguide.com/dvd/history-of-dvd-development-and-birth-of-the-dvd-forum] The primary rivals included the MultiMedia Compact Disc (MMCD), championed by Sony and Philips for its focus on multimedia versatility, and the Super Density (SD) format, promoted by Toshiba, Matsushita, and Time Warner for higher data density suited to video applications.[https://www.pctechguide.com/dvd/history-of-dvd-development-and-birth-of-the-dvd-forum] The resulting DVD-ROM specification represented a compromise, predominantly adopting SD's structure while incorporating MMCD elements, thus ensuring broad compatibility across consumer electronics and computing sectors.[https://www.pctechguide.com/dvd/history-of-dvd-development-and-birth-of-the-dvd-forum] Central to these improvements were optical enhancements that boosted data density: DVDs employed a red laser with a 650 nm wavelength—shorter than the 780 nm infrared laser used in CDs—enabling finer pit and track dimensions.[https://hometheaterhifi.com/volume_3_1/v3edtpre.html] Additionally, the disc's substrate was reduced to 0.6 mm thickness per layer (with two bonded to form a 1.2 mm disc), compared to the CD's single 1.2 mm substrate, which allowed for a narrower track pitch of 0.74 µm and supported the higher capacities without increasing overall disc size.[https://hometheaterhifi.com/volume_3_1/v3edtpre.html][https://www.ic.unicamp.br/~celio/mc326/hw/dvdintroduction.pdf] DVD's commercial rollout began in Japan on November 1, 1996, with initial players and titles like the film Twister, followed by a U.S. launch on March 24, 1997, in select markets.[https://www.historyofinformation.com/detail.php?id=1093] Adoption accelerated rapidly, driven by its use in home video distribution—where titles such as Titanic sold millions of units—and in software packaging for PCs, with over 6 million DVD-ROM drives shipped in the U.S. by late 1998.[https://www.mediaplaynews.com/25-years-of-digital-entertainment-part-one-the-disc-that-changed-the-world/] By 1999, DVD players had penetrated 4 million U.S. households, and disc shipments exceeded 98 million units annually, solidifying the format's role in mainstream multimedia consumption.[https://www.mediaplaynews.com/25-years-of-digital-entertainment-part-one-the-disc-that-changed-the-world/]

Third- and fourth-generation formats

The third generation of optical disc formats emerged in the early 2000s to enable high-definition video storage, overcoming the capacity constraints of second-generation DVDs for standard-definition content. Blu-ray Disc, specified in 2002 by the Blu-ray Disc Association (BDA), utilizes a shorter-wavelength 405 nm blue-violet laser to read denser pits, achieving 25 GB on a single-layer disc and 50 GB on dual-layer variants—more than five times the storage of single-layer DVDs.[52] This design prioritized high-definition playback, supporting resolutions up to 1080p with advanced audio codecs like Dolby TrueHD. Toshiba simultaneously promoted HD DVD as a rival standard starting in 2002, offering 15 GB per single-layer disc through a comparable 405 nm laser but with a 0.65 numerical aperture for easier manufacturing and lower costs. The format war between the BDA's Blu-ray and the DVD Forum's HD DVD escalated from 2005 to 2008, influenced by content licensing, pricing, and hardware ecosystems; Blu-ray's adoption in Sony's PlayStation 3 console proved pivotal, securing exclusive studio support. Toshiba conceded on February 19, 2008, halting HD DVD development and production amid mounting losses.[53][54] Fourth-generation formats, developed from the 2010s onward, emphasized ultra-high-capacity archival storage beyond consumer video needs. Sony and Panasonic formalized the Archival Disc standard in 2013 for professional long-term data preservation, employing multi-layer land-and-groove recording with a 405 nm laser to target over 300 GB per write-once disc; prototypes reached this capacity by 2015, with early cartridge-based systems using multiple 25 GB discs to achieve up to 300 GB total, and later developments scaling to multi-terabyte capacities.[55] Holographic storage offered a volumetric alternative, as demonstrated by InPhase Technologies' 2005 prototypes storing 300 GB per disc via multiplexed interference patterns in thick photopolymer layers for parallel data access. Despite early promise, persistent issues with media stability and read/write speeds resulted in commercial failure, leading to the company's closure in 2010 without market-ready products.[56] Ultra HD Blu-ray, an extension of the third-generation standard, was completed by the BDA in 2015 and launched in 2016 to support 4K resolution content, featuring 100 GB triple-layer discs with data rates up to 128 Mbit/s, HDR10, and wide color gamuts for enhanced visual fidelity. By November 2025, optical disc technologies face significant decline driven by streaming platforms and digital alternatives, with recordable media demand eroding as cloud storage proliferates; sales of Blu-ray and similar formats have slumped, and Sony ceased production of recordable Blu-ray media in February 2025.[57] Archival innovations persist, however, including 300 GB prototypes for compliance storage and projections for 1 PB optical cartridges by the 2030s to meet enterprise needs.[58][59][60]

Optical Disc Formats

Read-only discs

Read-only optical discs are pre-manufactured media where data is stamped into the disc during production, rendering them immutable and suitable for mass distribution of software, music, and video content. These formats represent successive generations of optical storage technology, each increasing data density through advancements in laser wavelength, pit size, and layering techniques. The primary examples include Compact Disc Read-Only Memory (CD-ROM) for data and audio, Digital Versatile Disc Read-Only Memory (DVD-ROM) for enhanced video and data, and Blu-ray Read-Only Memory (BD-ROM) for high-definition applications. The Compact Disc (CD) format, introduced in the early 1980s, serves as the foundational read-only optical disc. Audio CDs adhere to the Red Book standard, encoding stereo pulse-code modulation (PCM) audio at a sampling rate of 44.1 kHz and 16-bit depth, enabling up to 74 minutes of playback on a standard 120 mm disc. For data storage, CD-ROM discs offer capacities of 650 MB on 74-minute variants or 700 MB on 80-minute ones, utilizing sectors of 2,048 bytes in Mode 1 for error-corrected computer data or 2,336 bytes in Mode 2 for interleaved audio and video under the CD-ROM XA extension. These discs are commonly used for music albums and software distribution due to their reliability and low production costs. DVD-ROM discs, launched in the mid-1990s, significantly expand capacity through shorter-wavelength lasers and multi-layer construction. A single-sided, single-layer DVD-5 holds 4.7 GB, while dual-layer variants (DVD-9) reach 8.5 GB; double-sided configurations, such as DVD-10 (single-layer both sides) or DVD-18 (dual-layer both sides), achieve up to 17 GB. DVD-Video, a specialized read-only format, supports standard-definition video at 480p resolution in NTSC regions or 576p in PAL regions, with MPEG-2 compression for feature-length films typically fitting on single-layer discs. Applications include movie distribution and large software packages, benefiting from the format's backward compatibility with CD drives. Blu-ray ROM discs, developed in the 2000s, employ blue-violet lasers for even higher densities, starting at 25 GB for single-layer and 50 GB for dual-layer discs, with triple-layer options extending to 100 GB for Ultra HD content. These support high-definition video at 1080p resolution, while Ultra HD Blu-ray variants enable 4K (2160p) playback with advanced codecs like H.265/HEVC. Blu-ray ROM excels in archiving high-resolution media and interactive content, such as games and 4K films.
FormatSingle-Layer CapacityMulti-Layer MaxTypical Video ResolutionKey Applications
CD-ROM/Audio650-700 MBN/AAudio (44.1 kHz/16-bit)Music, software distribution
DVD-ROM/Video4.7 GB17 GB (dual-side dual-layer)480p/576pMovies, data backups
Blu-ray ROM25 GB100 GB1080p/4KHD/4K video, large files
Capacity progression across generations approximates a 1:7:25 ratio for baseline single-layer discs (CD:DVD:Blu-ray), reflecting exponential improvements in optical precision and enabling broader use cases from personal music libraries to professional video archiving.

Recordable and rewritable discs

Recordable optical discs allow users to write data once or multiple times using laser-based mechanisms that alter the disc's recording layer, enabling personal data storage, backups, and content creation. These formats evolved from read-only predecessors, incorporating specialized materials to support writing without requiring factory-level production equipment. Write-once variants permanently etch data, while rewritable ones permit erasure and overwriting through reversible physical changes in the medium. As of 2025, production of recordable Blu-ray discs by major manufacturers like Sony has ceased, potentially limiting future availability.[61] Write-once discs, such as CD-R, DVD-R, and BD-R, rely on organic dyes or inorganic compounds that change reflectivity when heated by a laser, forming pits or marks that mimic stamped patterns for readability. The CD-R format, developed by Philips and Sony, was introduced in 1988 and uses an organic dye layer sandwiched between a polycarbonate substrate and a reflective metal coating; the laser burns the dye to create non-reflective areas for data encoding. DVD-R, pioneered by Pioneer in 1997, employs a similar organic dye recording layer for single-sided capacities of 4.7 GB, with the laser altering the dye's transparency to store information. BD-R, standardized by the Blu-ray Disc Association in 2005, supports both inorganic materials like copper-silicon alloys for enhanced stability and organic dyes in low-to-high (LTH) configurations, allowing single-layer capacities of 25 GB and multi-layer options up to 128 GB via BDXL quadruple-layer discs.[62] Rewritable discs utilize phase-change alloys that switch between crystalline (reflective) and amorphous (less reflective) states under laser heating, enabling repeated data modification. CD-RW, commercialized by Philips in 1996, features an Ag-In-Sb-Te alloy layer that supports rewriting up to 1,000 times on 650-700 MB discs. DVD-RW, introduced by Pioneer in 1999, and the competing DVD+RW format from the DVD+RW Alliance in 2002, both employ phase-change media for 4.7 GB capacities, with DVD+RW offering improved overwrite performance through groove wobbling for seamless recording without linking sectors. BD-RE, also from 2005 under the Blu-ray Disc Association, uses advanced phase-change materials for 25 GB single-layer and up to 100 GB triple-layer discs with BDXL, supporting high-definition video and data rewriting.[63] These formats have facilitated practical applications like data backups for archival purposes and video authoring for creating custom DVDs or Blu-rays, where users compile menus, tracks, and footage using compatible drives. However, compatibility challenges persist, particularly between DVD-R and DVD+R, as the former adheres to DVD Forum land/groove recording while the latter uses DVD+RW Alliance's wobbled groove structure, leading some older players to recognize only one variant reliably. Write speeds vary by format and drive, with DVD recordable media supporting up to 16x-20x rates for efficient burning, though actual performance depends on media quality and hardware.

Manufacturing and Standards

Production processes

The production of optical discs begins with the creation of a stamper, which serves as the mold for replicating data patterns. A glass master substrate is first polished to a high degree of flatness and coated with a thin layer of photoresist material. A laser beam recorder then exposes the photoresist to form the precise pattern of pits and lands representing the digital data, using a focused laser (such as a 351 nm krypton laser) to cut microscopic features. The exposed photoresist is developed in an alkaline solution to reveal the data pattern as a relief structure on the glass master.[64][65] To produce the metal stamper, a thin conductive layer, typically silver or nickel, is applied to the patterned master via sputtering or chemical deposition to enable electroforming. The master is then immersed in an electrolytic bath containing nickel sulfamate, boric acid, and nickel chloride, where a current density of up to 30 A/dm² deposits a nickel layer approximately 300 μm thick over several hours, forming the durable stamper. This electroforming process, often conducted in a class 2 vacuum environment to minimize defects, yields stampers with overall process efficiencies of 60-80%, though improvements in automation aim to increase this rate. The stamper is separated from the master, polished, and may be used to create secondary "mother" or "son" stampers for higher-volume replication, with each primary stamper capable of producing up to 30,000 disc substrates.[66][67][68][64] For read-only discs, the stamper is inserted into an injection molding machine, where molten polycarbonate resin, heated to around 300-350°C, is injected under high pressure (typically 100-150 MPa) into the mold cavity. The resin cools and solidifies rapidly (within seconds) against the stamper, replicating the pit and land structure onto one side of the transparent polycarbonate substrate, which forms the disc's base layer approximately 1.2 mm thick. This step occurs in a class 100 (ISO 5) cleanroom to prevent dust particles from contaminating the surface, ensuring defect rates below 1 per million bits; injection molding yields often exceed 95% in high-volume production due to precise temperature and pressure controls. The molded substrate is then ejected and inspected for birefringence and pit geometry.[64][65][68] Following molding, the substrate undergoes metallization in a vacuum chamber using planar magnetron sputtering, where an aluminum target is eroded by argon plasma at 500-700 V to deposit a reflective metal layer (50-100 nm thick) onto the pit side, achieving uniform reflectivity over 70%. For added protection, a thin dielectric or buffer layer may be sputtered beforehand. A protective lacquer layer, typically an acrylic resin, is then applied via spin-coating, spreading evenly across the surface before being cured with ultraviolet light to form a hard, scratch-resistant coating about 5-10 μm thick. These multilayer coatings, referencing the physical components such as the reflector and overcoat, complete the disc assembly.[66][64] Recordable and rewritable discs differ in production by omitting the pit structure during molding, resulting in a smooth polycarbonate substrate with a pre-grooved spiral track for guidance. Instead of pits, a phase-change or organic dye recording layer is added via spin-coating, where a cyanine- or azo-based dye solution is dispensed onto the substrate and rotated at high speed (up to 3000 rpm) to form a uniform 100-200 nm thick layer that absorbs laser energy for data writing. This dye layer is overcoated with a metal reflector (e.g., gold or silver for better corrosion resistance) via sputtering, followed by a UV-curable protective layer, all in the same class 100 cleanroom to maintain layer integrity and achieve comparable high yields.[17][64]

Key specifications and comparisons

Optical disc specifications are governed by international standards developed by organizations such as ECMA International and the International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC). For instance, the Compact Disc Read-Only Memory (CD-ROM) is defined in ECMA-130 and ISO/IEC 10149, which outline the mechanical, physical, and optical characteristics for 120 mm discs with capacities up to approximately 700 MB.[69][70] DVD specifications, including rewritable formats like DVD+RW, are covered in ECMA-337 and related ISO/IEC standards such as ISO/IEC 16448 for DVD read-only discs.[71][70] Blu-ray Disc (BD) standards are primarily specified in ISO/IEC 30190 for recordable discs and ISO/IEC 30191 for rewritable discs, addressing capacities from 25 GB upward. Key optical parameters differentiate formats and enable higher densities in later generations. CDs use a laser wavelength of 780 nm and a numerical aperture (NA) of 0.45, allowing a larger spot size suitable for lower capacities.[72] DVDs employ a 650 nm wavelength with an NA of 0.60, reducing the spot size for increased data density.[73] Blu-ray discs utilize a shorter 405 nm blue-violet laser and an NA of 0.85, further minimizing the spot size to support multi-layer recording and higher capacities.[74] These factors—shorter wavelengths and higher NA—directly contribute to capacity gains by enabling tighter track pitches and more layers without excessive crosstalk.[73] Capacities vary by format, layers, and recording type, as summarized below:
FormatSingle-Layer CapacityDual-Layer CapacityMaximum LayersTotal Max Capacity
CD-ROM0.7 GBN/A10.7 GB
DVD-ROM4.7 GB8.5 GB28.5 GB
Blu-ray25 GB50 GB4 (BDXL)128 GB
These values reflect standard read-only and recordable variants; factors like track pitch (1.6 μm for CD, 0.74 μm for DVD, 0.32 μm for Blu-ray) and layer count multiply base capacities.[69][71] As of 2025, research has demonstrated prototype optical discs with capacities up to 1.6 PB using advanced nano-layering, though these are not yet standardized for commercial production.[60] Comparisons across formats highlight trade-offs in performance and usability. Data transfer speeds are denoted in multiples of the base rate: 1x for CD equals 153.6 kB/s, for DVD 1.385 MB/s, and for Blu-ray approximately 4.5 MB/s; maximum speeds reach 52x for CDs, 16x-20x for DVDs, and 12x-16x for Blu-rays, balancing read/write rates with media integrity.[75] Blu-ray drives maintain backward compatibility with DVD and CD media, allowing a single device to handle all formats through multi-laser pickups, though CD/DVD drives cannot read Blu-ray discs.[76] Cost per GB has declined steadily due to production scale and material efficiencies, reaching approximately $0.01/GB for bulk Blu-ray media by 2025, making optical storage viable for archival despite competition from solid-state options.[77]

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