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Optical disc

Flat, usually circular disc that encodes binary data

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Verbatim CD-R 700MB 52X scanned @ 1200 DPI with an Epson ET-2720
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Design and technology

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. 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. The so-called program area that contains the data commonly starts 25 millimeters away from the center point. 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

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) 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. 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

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.

Usage

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. This trend is expected to continue as USB flash drives continue to increase in capacity and drop in price.

Additionally, music, movies, games, software and TV shows purchased, shared or streamed over the Internet have 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. 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 Internet service providers 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 circles 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%.

Durability

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

As the media itself is accessed only 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, and 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.

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.

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