Optical disc
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.
The encoding material sits atop a thicker substrate (usually polycarbonate) that makes up the bulk of the disc and forms a dust defocusing layer.
[6][7] 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.
[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.
[10] 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.
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.
[14] An early analogue optical disc system existed in 1935, used on Welte's Lichttonorgel [de] sampling organ.
[15] An early analog optical disc used for video recording was invented by David Paul Gregg in 1958[16] and patented in the US in 1961 and 1969.
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.
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.
Following litigation, Sony and Philips licensed Russell's patents (then held by a Canadian company, Optical Recording Corp.) in the 1980s.
From the start optical discs were used to store broadcast-quality analog video, and later digital media such as music or computer software.
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.
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.
Many of the disks were shipped to RCA Laboratories (now David Sarnoff Research Center) to be used in the Library of Congress archiving efforts.
Although this technology was written up in Video Pro Magazine's December 1994 issue promising "the death of the tape", it was never marketed.
Second-generation optical discs were for storing great amounts of data, including broadcast-quality digital video.
[23] In 1995, a consortium of manufacturers (Sony, Philips, Toshiba, Panasonic) developed the second generation of the optical disc, the DVD.
[22] 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.
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.
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).
Precise error scanning requires access to the raw, uncorrected readout of a disc, which is not always provided by a drive.
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.
[34] Optical media, such as CDs and DVDs, can be scanned to detect errors and signs of deterioration well before data becomes unreadable.
A high error rate can signal physical deterioration, low-quality manufacturing, surface contamination, or data recorded by a faulty optical drive.
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.
Popular programs include Nero DiscSpeed, K-Probe, Opti Drive Control (previously known as "CD Speed 2000"), and DVD Info Pro for Windows.
Blu-ray discs use so-called LDC (Long Distance Codes) and BIS (Burst Indication Subcodes) error parameters.
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.
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.
