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Optical Disc

A flat, typically circular disc designed to encode binary data—a concept that, while seemingly straightforward, has underpinned decades of digital information storage and distribution. This isn't about some anatomical feature of your eye, for that you'd need the optic disc. And if you're thinking "optical media" broadly, for light transmission, see Optical medium; for the wider definition of storage, you'll want optical storage. But here, we're discussing the tangible, iridescent circles that once dominated our digital lives.

Observe the bottom surface of a 12 cm compact disc (CD-R), and you’ll see its characteristic iridescence—a fleeting aesthetic bonus born from the very physics of data storage. The precision of the optical lens within a compact disc drive is what allows these discs to function, translating microscopic physical variations into usable data.

For those who enjoy a comprehensive, if somewhat exhaustive, overview of how we've attempted to cling to data over the eons, here's a rather extensive list of computer memory and data storage types. Try to keep up.

Computer memory and data storage types

General

Volatile

Historical

Non-volatile

NVRAM

Early-stage NVRAM

Analog recording

Optical

In development

Historical

The world of optical discs is, predictably, segmented.

Optical discs

General

Optical media types

Standards

See also

An optical disc is, in its most fundamental form, a flat, typically disc-shaped object that stores information through physical variations on its surface. These variations are then interpreted with the assistance of a focused beam of light. They can be reflective, meaning the light source and its corresponding detector are situated on the same side of the disc, or transmissive, where light passes through the disc to be detected on the opposite side. These discs are capable of holding either analog or digital information, or even a peculiar blend of both. Their primary utility has historically been for the widespread distribution of various media and data, along with providing a relatively robust option for long-term archival storage.

Design and technology

The core of the data encoding material rests upon a considerably thicker substrate, most commonly composed of polycarbonate. This substrate forms the substantial bulk of the disc and also serves a rather practical purpose as a dust-defocusing layer—a small mercy in a world prone to microscopic annoyances. The intricate encoding pattern isn't haphazard; it meticulously follows a continuous, spiral path that gracefully traverses the entire surface of the disc, extending from the innermost track outwards.

The actual data is inscribed onto the disc's surface either by a precise laser or through a high-volume stamping machine. Accessing this data involves illuminating the data path with a laser diode housed within an optical disc drive. This drive, with a mechanical diligence that might impress some, spins the disc at speeds ranging from approximately 200 to a staggering 4,000 revolutions per minute (RPM), or even faster, depending on the drive's specific type, the disc's format, and the proximity of the read head to the disc's center. It's worth noting that outer tracks are read at a higher data speed; a simple consequence of their higher linear velocities even when maintaining the same angular velocities.

Most optical discs, if you bother to look closely, display a distinct iridescence. This isn't for show, but rather a direct byproduct of the diffraction grating meticulously formed by their grooves. This particular side of the disc, which holds the actual data, is typically safeguarded by a transparent material, often a layer of lacquer.

The opposing side of an optical disc usually bears a printed label, sometimes a simple paper adhesive, though more frequently it's directly printed or stamped onto the disc itself. Unlike the more robust, if ultimately obsolete, 3½-inch floppy disk, the majority of optical discs lack an integrated protective casing. This inherent vulnerability leaves them prone to data transfer issues stemming from scratches, the inevitable grime of fingerprints, and various other environmental hazards. A notable exception exists in the form of Blu-ray discs, which feature a specialized coating known as durabis, specifically designed to mitigate these common problems. Perhaps some lessons are eventually learned.

Optical discs have been manufactured in a range of diameters, from a compact 7.6 cm (3.0 in) to a sprawling 30 cm (11.8 in). However, the 12 cm (4.7 in) diameter firmly established itself as the dominant standard around 1997. The so-called "program area"—the section containing the actual data—typically commences about 25 millimeters from the disc's central point. A standard disc measures approximately 1.2 mm (0.047 in) in thickness. The track pitch, which is the precise distance from the center of one data track to the center of the next, varies significantly depending on the format, ranging from 1.6 micrometers (for standard CDs) down to a remarkably fine 320 nanometers (for Blu-ray discs).

Recording types

An optical disc is not a one-trick pony; it's engineered to support one of three distinct recording capabilities. There are the venerable read-only discs, such as the original CD and CD-ROM, which offer content for consumption but not alteration. Then come the recordable discs—the "write-once" variety, like the CD-R—allowing a single, irreversible inscription of data. Finally, there are the re-recordable, or rewritable, discs, exemplified by the CD-RW, which permit multiple cycles of writing and erasing.

Write-once optical discs typically incorporate an organic dye as their recording layer, strategically placed between the disc's substrate and its reflective layer. This dye can be a phthalocyanine or an azo dye (the latter predominantly favored by Verbatim) or even an oxonol dye, a choice often seen with Fujifilm. Rewritable discs, by contrast, generally feature an alloy recording layer. This alloy is composed of a specialized phase change material, most frequently AgInSbTe, which is an alloy meticulously crafted from silver, indium, antimony, and tellurium. Azo dyes, for their part, made their debut in 1996, while phthalocyanine only gained widespread adoption around 2002. Identifying the specific type of dye and the reflective layer material on an optical disc can often be achieved simply by shining a light through it, as different combinations yield distinct color profiles.

Interestingly, Blu-ray Disc recordable media generally eschew the organic dye recording layer, opting instead for an inorganic alternative. Those rare exceptions that do utilize an organic dye are colloquially known as low-to-high (LTH) discs. While these LTH discs benefit from being manufacturable on existing CD and DVD production lines, their quality is generally considered inferior to that of traditional Blu-ray recordable discs. A small price for convenience, perhaps, but a price nonetheless.

File systems

When it comes to organizing the digital chaos on these discs, specific file systems were developed. The most prominent are ISO9660 and the Universal Disk Format (UDF).

ISO9660, in its base form, is rather restrictive. However, it can be expanded using the "Joliet" extension, which generously allows for longer file names than its standalone counterpart. For those operating in the Unix/Linux ecosystem, the "Rock Ridge" extension takes things a step further, supporting even longer file names and providing familiar Unix/Linux-style file permissions. Regrettably, this particular extension often goes unrecognized by Windows systems, and certainly by the array of DVD players and similar devices that might otherwise read data discs.

For the sake of practical cross-platform compatibility—a concept often more aspirational than actual—multiple file systems can be made to coexist on a single disc, all referencing the same underlying files. A surprisingly elegant solution to a perennial problem, if you ask me.

Usage

Optical discs are most commonly employed for digital preservation—a noble, if often overlooked, endeavor—as well as for the storage of music (primarily for use in a CD player), video (such as for a Blu-ray player), or even programs and various forms of data for personal computers (PC). They also found significant use in offline hard copy data distribution, largely due to their comparatively lower per-unit prices compared to other available media types at the time. The Optical Storage Technology Association (OSTA) once actively championed standardized optical storage formats, a valiant effort in a fragmented industry.

To ensure the continued usability of these discs, libraries and archives diligently implement various optical media preservation procedures. This is to guarantee that the data remains accessible, whether in a computer's optical disc drive or the relevant disc player, for generations who might wonder what a "disc" even was.

The familiar file operations typically associated with traditional mass storage devices like flash drives, memory cards, and hard drives can actually be simulated quite effectively on optical discs by utilizing a UDF live file system. It’s an interesting workaround, if not exactly mainstream.

However, for day-to-day computer data backup and physical data transfer, the reign of optical discs like CDs and DVDs is, shall we say, waning. They are being steadily supplanted by faster, more compact solid-state devices, with the USB flash drive being the most prominent usurper. This trend is not merely expected but practically guaranteed to persist, as USB flash drives continue their relentless march towards ever-increasing capacities and ever-decreasing prices. It's almost as if progress is inevitable, even when it's inconvenient.

Furthermore, the proliferation of music, movies, games, software, and television shows purchased, shared, or streamed over the Internet has significantly, and perhaps irrevocably, diminished the annual sales figures for audio CDs, video DVDs, and Blu-ray discs. And yet, a curious resistance persists: audio CDs and Blu-rays are still actively sought out and purchased by a dedicated minority. For them, it's a tangible way to support their favorite artists and creators, and to receive something physical in return. More pragmatically, audio CDs (alongside vinyl records and cassette tapes) offer the distinct advantage of containing uncompressed audio, free from the often-audible artifacts introduced by lossy compression algorithms like MP3. Similarly, Blu-rays deliver a superior image and sound quality compared to streaming media, exhibiting fewer visible compression artifacts due to their higher bitrates and the considerably greater storage space available on the physical disc.

Of course, Blu-rays can also be acquired through less conventional means, such as BitTorrent, though this path is often fraught with complications due to restrictions imposed by ISPs on legal or copyright grounds, or simply the agony of slow download speeds. Not to mention the sheer volume of storage space required, as some content can easily span several dozen gigabytes. For those seeking to play large games without the ordeal of downloading them over an unreliable or sluggish internet connection, Blu-rays remain, as of 2020, a viable, even necessary, option. This is precisely why they are still widely utilized by gaming consoles such as the PlayStation 4 and Xbox One X. It is, however, increasingly uncommon for PC games to be offered in a physical format like Blu-ray in the current market.

Optical discs are typically housed in specialized containers, often referred to as "jewel cases"—a rather optimistic descriptor for plastic. For optimal longevity, discs should ideally be free of any stickers and, critically, should not be stored alongside paper inserts; any papers must be removed from the jewel case prior to long-term storage. When handling, one should always grasp the disc by its edges, with a thumb resting on the inner edge, to prevent the inevitable scourge of scratches. The ISO Standard 18938:2014 provides comprehensive guidelines on the best optical disc handling techniques, for those who truly care. When cleaning, never—and I mean never—do so in a circular pattern, as this can induce concentric scratches that will absolutely interfere with readability. Improper cleaning is a direct path to a damaged disc. Recordable discs, particularly, should not be subjected to extended periods of light exposure. For maximum longevity, optical discs demand dry and cool storage conditions, with temperatures maintained between -10 and 23 °C, never to exceed 32 °C. Humidity should never dip below 10%, with a recommended range of 20 to 50%, and fluctuations should be kept within a tight ±10% margin. Because apparently, even inanimate objects have very specific environmental demands.

Durability

Optical discs, surprisingly, are not inherently vulnerable to water. A small victory, I suppose.

While it's true that optical discs possess a greater degree of durability compared to earlier audio-visual and data storage formats—which, let's be frank, isn't a particularly high bar—they are still quite susceptible to both environmental degradation and the wear and tear of daily use, especially if handled with the typical human disregard for delicate objects.

Crucially, optical discs are not prone to the kind of uncontrollable, catastrophic failures that plague other storage technologies. You won't find them suffering from head crashes, succumbing to violent power surges, or instantly failing upon exposure to water in the same dramatic fashion as hard disk drives and flash storage. This is primarily because an optical drive's storage controllers are not inextricably linked to the optical discs themselves, unlike with hard disk drives and flash memory controllers. Should a disc become stuck in a defective optical drive, it's usually recoverable with the simple, if slightly barbaric, act of pushing an unsharp needle into the emergency ejection pinhole. Furthermore, they possess no immediate points of water ingress and lack integrated circuitry, making them somewhat more resilient to certain forms of accidental destruction. A minor miracle, perhaps.

Security

Given that the media itself is accessed solely via a laser beam and lacks any internal control circuitry, an optical disc cannot, by its very nature, harbor malicious hardware in the same insidious manner as, say, a so-called "rubber-ducky" or a USB killer. However, like any medium capable of storing data, optical discs are perfectly capable of containing and subsequently spreading malware. This unfortunate reality was vividly demonstrated in the infamous Sony BMG copy protection rootkit scandal of 2005, where Sony, in a spectacular display of corporate hubris, pre-loaded discs with malware, effectively turning customer purchases into vectors for digital infection. Because nothing says "customer appreciation" like a hidden rootkit.

Many types of optical discs are either factory-pressed or finalized as write once read many storage devices. This design makes them rather ineffective at propagating computer worms specifically engineered to spread by copying themselves onto optical media, precisely because the data on such discs cannot be modified once it's been etched or written. However, the advent of re-writable disc technologies, such as CD-RW, regrettably provides a fertile ground for this very type of malware to spread. Progress, as always, comes with its own set of complications.

History

For a more exhaustive dive into the timeline of these shiny circles, refer to the Main article: History of optical storage media.

An earlier analog optical disc, dating back to 1935, was notably recorded for the Lichttonorgel de (sampling organ)—a fascinating precursor to modern digital sampling.

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The earliest documented historical application of an optical disc emerged in 1884. In this pioneering effort, Alexander Graham Bell, his cousin Chichester Bell, and Charles Sumner Tainter successfully recorded sound onto a glass disc, employing a beam of light in their innovative method. This was a rather elegant, if primitive, start.

Optophonie, a remarkably early example from 1931, showcased a recording device that harnessed light for both the inscription and playback of sound signals, utilizing a transparent photographic medium. A testament to early ingenuity, even if it didn't quite catch on.

An early analog optical disc system, as mentioned, existed in 1935, finding its niche within Welte's Lichttonorgel de sampling organ—a quaint, if niche, application of the technology.

A significant leap in analog optical disc technology for video recording was made by David Paul Gregg in 1958, with his invention subsequently patented in the US in 1961 and 1969. This particular iteration of the optical disc stands as a very early precursor to the modern DVD (U.S. patent 3,430,966). Of particular note is U.S. patent 4,893,297, filed in 1989 and issued in 1990, which generated substantial royalty income for Pioneer Corporation's DVA until 2007, encompassing the technologies behind the CD, DVD, and Blu-ray systems. In the early 1960s, the formidable Music Corporation of America (MCA) recognized the potential, acquiring Gregg's patents and his company, Gauss Electrophysics.

American inventor James T. Russell has been widely credited with devising the very first system capable of recording a digital signal onto an optical transparent foil. This ingenious setup was illuminated from behind by a high-power halogen lamp. Russell's initial patent application was lodged in 1966, and he was formally granted the patent in 1970. Following a period of litigation, industry giants Sony and Philips eventually licensed Russell's patents (which were, by the 1980s, held by a Canadian entity, Optical Recording Corp.). A rather common trajectory for groundbreaking inventions, it seems.

Both Gregg's and Russell's initial disc designs utilized floppy media, read in a transparent mode—a configuration that, while innovative, presented considerable practical drawbacks. From these foundational, somewhat imperfect beginnings, four distinct generations of optical drives subsequently emerged: the Laserdisc (1969), WORM (1979), Compact Discs (1984), DVD (1995), Blu-ray (2005), and HD-DVD (2006). More formats, ever striving for incremental improvement, are perpetually under development.

First-generation

In their nascent stages, optical discs were predominantly read-only media. Their initial purpose was to store broadcast-quality analog video, a task they performed with a certain analog charm. Later, they evolved to accommodate digital media, such as music or computer software, marking a significant shift in their utility. The LaserDisc format, for instance, was designed to store analog video signals, primarily for the distribution of home video. However, it ultimately lost the commercial battle to the more practical VHS videocassette format. This defeat was largely attributable to its prohibitive cost and, critically, its inability to be re-recorded—a fatal flaw in an era that craved home recording. Other first-generation disc formats were conceived solely for the storage of digital data and were not initially equipped to function as a digital video medium.

The majority of first-generation disc devices were equipped with an infrared laser reading head. The physical limitation here is that the minimum size of the laser spot is directly proportional to the wavelength of the laser being used. Consequently, wavelength acts as a fundamental limiting factor on the sheer amount of information that can be meticulously packed into a given physical area on the disc. The infrared spectrum, residing beyond the longer-wavelength end of visible light, inherently supports less data density than its shorter-wavelength, visible light counterparts. A prime example of the high-density data storage capacity achievable with an infrared laser is the 700 megabytes of net user data that a standard 12 cm compact disc could accommodate.

Beyond wavelength, several other factors critically influence data storage density. These include the ingenious concept of multiple layers of data on the disc, the chosen method of rotation (be it Constant linear velocity (CLV), Constant angular velocity (CAV), or the more sophisticated zoned-CAV), the precise composition and arrangement of "lands" and "pits" that encode the data, and, of course, the amount of unused margin at both the center and the edge of the disc. Every millimeter matters, it seems.

It was Sony and Philips, in a collaborative effort that would define a generation, who meticulously developed the first generation of CDs in the mid-1980s, complete with the exhaustive specifications that would guide their widespread adoption. This technological leap dramatically exploited the potential of representing analog signals in a digital format. To achieve this, 16-bit samples of the analog signal were captured at a rate of 44,100 samples per second. This specific sample rate was not arbitrary; it was carefully derived from the Nyquist rate of 40,000 samples per second, which is the theoretical minimum required to accurately capture the full audible frequency range up to 20 kHz without introducing undesirable aliasing artifacts. The additional tolerance was prudently included to allow for the use of less-than-perfect analog audio pre-filters, ensuring any higher, inaudible frequencies were effectively removed. The inaugural version of this standard generously allowed for up to 74 minutes of music or a respectable 650 megabytes of data storage.

Types of Read-only Optical Discs:

Laserdisc

In the Netherlands in 1969, a Philips Research physicist, Pieter Kramer, made a significant contribution with his invention of an optical videodisc. This disc operated in reflective mode, featuring a protective layer and designed to be read by a precisely focused laser beam (U.S. patent 5,068,846, filed 1972, issued 1991). Kramer's fundamental physical format, it turns out, underpins virtually all subsequent optical discs. A foundational piece of technology, often overlooked.

By 1975, Philips and MCA had forged a collaborative partnership. In 1978, with a timing that proved commercially disastrous, they unveiled their much-anticipated Laserdisc in Atlanta. MCA was responsible for the discs, while Philips provided the players. However, the launch was a commercial flop, and the cooperation, predictably, dissolved.

Despite this initial setback, in Japan and the U.S., Pioneer managed to achieve a measure of success with the Laserdisc, maintaining its presence until the eventual advent of the DVD. Meanwhile, in 1979, Philips and Sony, forming a powerful consortium, successfully developed the audio compact disc—a product that would, unlike the Laserdisc, truly revolutionize media consumption.

CD-ROM

The CD-ROM format, a collaborative effort between Sony and Philips, was introduced in 1984. It emerged as a logical extension of the Compact Disc Digital Audio standard, cleverly adapted to accommodate and store virtually any form of digital data. In the very same year, Sony also demonstrated a LaserDisc data storage format. While impressive, this larger format boasted a data capacity of 3.28 gigabytes, a figure that, while substantial for its time, would eventually be dwarfed by subsequent innovations.

Types of recordable Optical Discs

Magneto-optical drive

Magneto-optical discs represented a significant leap forward as erasable media, capable of being written to and read from numerous times. These media and their corresponding drives were first introduced to the market in late 1987 and early 1988 by a cohort of technology companies including Sharp, MCI, and Sony, all of whom utilized the ubiquitous SCSI interface. Initial capacities ranged from a respectable 512 megabytes on 130 mm media down to 160 megabytes on the smaller 90 mm media. By 1998, the market had expanded considerably, with over 50 models offered by 12 different vendors. These devices supported media with diameters of both 86 mm and 130 mm, providing capacities that had grown to an impressive 2,600 [megabytes](/Megabyte], with almost all still relying on the dependable SCSI interface.

WORM drive

In 1979, Exxon STAR Systems, based in Pasadena, California, pioneered the construction of a computer-controlled WORM drive. This innovative system leveraged thin film coatings of Tellurium and Selenium applied to a 12-inch diameter glass disk. The recording process employed blue light at a precise wavelength of 457 nanometers, while reading was performed using red light at 632.8 nanometers. STAR Systems was subsequently acquired by Storage Technology Corporation (STC) in 1981 and relocated to Boulder, Colorado. Development of the WORM technology continued, transitioning to 14-inch diameter aluminum substrates. Beta testing of these disk drives, initially branded as the Laser Storage Drive 2000 (LSD-2000), met with only moderate success. Many of the discs produced were eventually shipped to RCA Laboratories (now known as the David Sarnoff Research Center) for use in the Library of Congress's archiving initiatives. The STC disks uniquely utilized a sealed cartridge, incorporating an optical window for enhanced protection (U.S. patent 4,542,495). A rather specialized, if ultimately niche, solution.

Second-generation

The second generation of optical discs arrived with the ambitious goal of storing significantly larger quantities of data, including the then-novel concept of broadcast-quality digital video. These discs typically rely on a visible-light laser for reading, most commonly red. The shorter wavelength of this red laser, coupled with a greater numerical aperture, allows for the creation of a much narrower light beam. This precision, in turn, permits the inscription of smaller pits and lands on the disc's surface. In the context of the DVD format, this technological advancement translates to a substantial 4.7 gigabytes of storage capacity on a standard 12 cm, single-sided, single-layer disc. Alternatively, smaller media, such as the DataPlay format, could achieve capacities comparable to their larger, standard 12 cm compact disc counterparts, demonstrating that size isn't always everything.

DVD-ROM

In 1995, a powerful consortium of manufacturers—comprising Sony, Philips, Toshiba, and Panasonic—collectively developed the second generation of the optical disc, which they aptly named the DVD. The DVD disc emerged onto the technological landscape after the CD-ROM had already achieved widespread adoption in society. It was, in essence, the next logical step in the evolution of consumer media and data storage, arriving with an air of inevitability.

Third-generation

Third-generation optical discs were specifically engineered for the distribution of high-definition video and sophisticated videogames, necessitating significantly greater data storage capacities. This leap in capacity was achieved through the deployment of shorter-wavelength visible-light lasers and optics with larger numerical apertures. Both the Blu-ray Disc and HD DVD formats, for instance, utilize blue-violet lasers and focusing optics of greater aperture. This allows them to interact with discs featuring even smaller pits and lands, thereby dramatically increasing the data storage capacity per layer. A rather elegant solution, if you ask me, to the relentless demand for more.

In practical terms, the effective capacity for multimedia presentation on these discs is further enhanced by sophisticated video data compression codecs, such as H.264/MPEG-4 AVC and VC-1. It's not just about fitting more data; it's about making that data smaller to begin with.

Announced but not released:

Blu-ray and HD-DVD

The third generation of optical discs saw its primary development between 2000 and 2006, culminating in the formal introduction of the Blu-ray Disc. The first movies released on Blu-ray Discs hit the market in June 2006, marking the official start of a rather contentious era. Blu-ray eventually emerged victorious in the protracted and often acrimonious high definition optical disc format war, decisively triumphing over its primary competitor, the HD DVD. A standard Blu-ray disc can accommodate approximately 25 gigabytes of data, while a DVD holds around 4.7 gigabytes, and a CD a mere 700 megabytes. The difference in capacity alone was, as they say, illuminating.

Comparison of various optical storage media

Fourth-generation

The following formats represent an ambitious leap beyond the current third-generation discs, exhibiting the potential to store in excess of one terabyte (1 TB) of data. At least some of these are explicitly designed for the rather unglamorous, yet critical, task of cold data storage within sprawling data centers. A dubious claim, perhaps, but one worth considering.

Announced but abandoned:

Announced but not released:

In 2004, development commenced on the ambitious Holographic Versatile Disc (HVD), which held the tantalizing promise of storing several terabytes of data on a single disc. Unfortunately, this promising development largely stagnated towards the latter half of the 2000s, primarily due to a lamentable lack of adequate funding. A familiar story, really.

In 2006, reports emerged from Japanese researchers detailing their development of ultraviolet ray lasers with a remarkably short wavelength of 210 nanometers. This breakthrough, it was claimed, would enable a significantly higher bit density than even Blu-ray discs could achieve. As of 2022, however, no further updates on that particular project have been reported. One can only assume it's still somewhere in the quantum ether.

Overview of optical types

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
Enhanced Versatile Disc (EVD) DVD 2002-2004
HD DVD 15–51 GB 1 TB 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) 2009–present
25 GB (Blu-ray format)
50 GB (Blu-ray format)
100 GB (BDXL format)
Archival Disc 0.3-1 TB 2014–2024
Ultra HD Blu-ray 50 GB 2015–present
66 GB
100 GB
128 GB

Notes

  1. ^ Prototypes and theoretical values.
  2. ^ Years from (known) start of development till end of sales or development.

Recordable and writable optical discs

There exists a myriad of formats for optical direct to disk recording devices on the market, all of which, despite their variations, fundamentally operate on the principle of employing a laser to alter the reflectivity of the digital recording medium. This alteration is precisely engineered to mimic the effects of the pits and lands—the microscopic data points—created when a commercial optical disc is factory-pressed. Formats such as CD-R and DVD-R fall under the category of "Write once read many" (WORM) or simply "write-once" media, meaning data can be inscribed a single time, irrevocably. In contrast, CD-RW and DVD-RW are designated as rewritable, offering the flexibility to write, erase, and rewrite data multiple times, much like a magnetic recording hard disk drive (HDD), albeit with different underlying mechanisms.

The media technologies themselves vary considerably. For instance, M-DISC media distinguishes itself by incorporating a unique "rock-like" layer. This innovative material is designed to retain data for significantly longer periods than conventional recordable media, offering a more robust archival solution. While M-DISC media maintains read-only compatibility with existing DVD and Blu-ray drives, the act of writing to it requires a more powerful laser, specifically engineered for this purpose. Such specialized lasers are, predictably, integrated into a more limited selection of optical drive models.

Surface error scanning

Observing the error rate measurement on a DVD+R reveals a critical truth: even with inherent redundancies, these discs have their limits. Here, the error rate is still within what's considered a healthy range.

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Optical media possesses a rather useful, if often overlooked, capability: it can be predictively scanned for errors and signs of media deterioration long before any data actually becomes unreadable. This foresight is crucial. All optical formats incorporate some degree of redundancy for error correction, a mechanism that functions effectively until the sheer volume of errors surpasses a predefined threshold. Once that threshold is breached, the integrity of the data becomes compromised. A persistently high rate of errors can serve as an early warning, indicating deteriorating or low-quality media, the presence of physical damage, an unclean surface, or even media that was originally written using a defective optical drive.

However, precise error scanning requires direct access to the raw, uncorrected readout from a disc—a capability that, unfortunately, is not universally provided by all optical drives. Consequently, support for this critical functionality varies significantly across different optical drive manufacturers and models. On ordinary drives that lack this direct raw data access, one might still attempt to infer disc health by monitoring for unexpected reductions in read speed. This method, however, is an indirect and considerably less reliable measure. It's like trying to diagnose a complex illness by observing a slight limp.

Fortunately, several specialized software tools are available for those who wish to perform detailed error scans on their optical media. Popular programs for Windows users include Nero DiscSpeed, K-Probe, Opti Drive Control (which was previously known as "CD Speed 2000"), and DVD Info Pro. For users seeking cross-platform solutions, QPxTool offers a viable option to help monitor and maintain the integrity of optical media. Each of these tools provides a window into the detailed error rates and underlying conditions that can affect the longevity and readability of optical discs.

Error types

There are various classifications for error measurements, each serving as an indicator of disc health. On CDs, these include the so-called "C1," "C2," and "CU" errors. For DVDs, we look at "PI/PO (parity inner/outer) errors" and the more critical "PI/PO failures." For those with a taste for granular detail, a finer-grained set of error measurements on CDs—though supported by very few optical drives—includes E11, E21, E31, E21, E22, and E32.

The "CU" errors on data CDs and "POF" (Parity Outer Failure) on DVDs represent what are known as uncorrectable errors. These are the critical indicators of actual data loss, and they typically arise from an excessive number of consecutive, smaller errors that overwhelm the disc's error correction capabilities.

Due to the comparatively weaker error correction mechanisms employed on Audio CDs (governed by the Red Book standard) and Video CDs (following the White Book standard), even the presence of C2 errors can lead directly to data loss. However, it's a curious fact that even with C2 errors, the resulting damage is often inaudible to a certain extent, a small mercy for casual listeners.

Blu-ray discs utilize a different set of error parameters, specifically LDC (Long Distance Codes) and BIS (Burst Indication Subcodes). According to the developer of the Opti Drive Control software, a Blu-ray disc can generally be considered healthy if its LDC error rate remains below 13 and its BIS error rate stays below 15. These numbers, presumably, offer a fleeting moment of clarity in an otherwise chaotic data landscape.

Optical disc manufacturing

For a more comprehensive exposition on the intricacies of mass production, one might consult the Main article: CD manufacturing.

Optical discs are predominantly created through a process known as replication. This method is versatile enough to be employed across all disc types. Recordable discs, even before any user data is written, come pre-loaded with essential information such as the manufacturer, disc type, and maximum read and write speeds—a sort of digital birth certificate. In the replication process, a meticulously maintained cleanroom, bathed in an eerie yellow light, is not merely recommended but absolutely necessary. This specific lighting protects the light-sensitive photoresist material, and the sterile environment is crucial for preventing even microscopic dust particles from corrupting the delicate data patterns on the disc.

The journey begins with a glass master. This master is first subjected to an intensive cleaning regimen, utilizing a rotating brush and deionized water to achieve the highest possible level of purity, preparing it for the subsequent, critical steps. Following this, a surface analyzer meticulously inspects the master's cleanliness before a layer of photoresist is uniformly applied.

The photoresist-coated master is then baked in an oven, a process designed to solidify the resist layer. Next, in the exposure process, the master is placed on a turntable, where a laser selectively exposes the resist to light. Simultaneously, a developer solution and deionized water are applied to the disc, effectively washing away the areas of exposed resist. This precise process is what forms the intricate pits and lands that physically represent the data on the disc.

A thin metallic coating is then meticulously applied to the master, which creates a negative impression of the master, complete with its pits and lands. This negative is then carefully peeled away from the master and subsequently coated with a thin layer of plastic. This plastic layer serves to protect the metallic coating, while a punching press precisely cuts a hole into the center of the disc and removes any excess material.

This negative, now transformed, becomes a stamper—a vital component of the mold that will be used for mass replication. It is carefully positioned on one side of the mold, with its data-bearing side (the pits and lands) facing outwards. This assembly occurs within an injection molding machine. The machine then seals the mold and injects molten polycarbonate into the cavity formed by the mold's walls. This high-pressure injection process forms, or molds, the disc, imprinting it with the data pattern.

The molten polycarbonate rapidly fills the microscopic pits, or the spaces between the lands, on the stamper. As it solidifies, it acquires their precise shape. This step bears a strong resemblance to the traditional process of record pressing in the analog world.

The polycarbonate disc cools with remarkable speed and is promptly extracted from the machine, making way for the next disc to be formed. The disc then undergoes metallization, where it's covered with an exceedingly thin, reflective layer of aluminum. This aluminum meticulously fills the spaces that were once occupied by the stamper's negative impression.

Finally, a layer of varnish is applied. This varnish serves a dual purpose: it protects the delicate aluminum coating and provides a smooth surface suitable for printing. The varnish is initially applied near the center of the disc, and as the disc spins, the varnish is evenly distributed across its surface. Ultraviolet (UV) light is then used to rapidly harden this varnish. In the concluding step, the discs are either silkscreened or otherwise labeled, preparing them for their eventual destiny.

Recordable discs introduce an additional dye layer, while rewritable discs, instead of dye, incorporate a phase change alloy layer. This alloy is meticulously protected by both upper and lower dielectric (electrically insulating) layers. These layers are often applied through a process called sputtering. This crucial additional layer is situated between the grooves and the reflective layer of the disc. In recordable discs, grooves are formed in place of the traditional pits and lands found in replicated discs, and both can be created simultaneously during the same exposure process. In the case of DVDs, the manufacturing processes largely mirror those of CDs, but are applied to a significantly thinner disc. This thinner disc is then meticulously bonded to a second, equally thin but blank, disc using a UV-curable Liquid optically clear adhesive (LOCA), thereby forming a complete DVD disc. This ingenious design places the data layer precisely in the middle of the disc, a necessity for DVDs to achieve their enhanced storage capacity. For multi-layer discs, semi-reflective coatings are used for all layers except the deepest, final layer, which employs a traditional, fully reflective coating.

Dual-layer DVDs are constructed with a slightly modified process. Following metallization (which uses a thinner metal layer to permit some light transmission), base and pit transfer resins are applied and then pre-cured at the disc's center. The disc is then pressed a second time, but with a different stamper, and the resins are fully cured using UV light before being separated from the stamper. Subsequently, the disc receives another, thicker metallization layer, and is then bonded to a blank disc using LOCA glue. DVD-R DL and DVD+R DL discs incorporate a dye layer after the curing step but before metallization. CD-R, DVD-R, and DVD+R discs receive their dye layer after pressing but before metallization. CD-RW, DVD-RW, and DVD+RW discs are equipped with a metal alloy layer sandwiched between two dielectric layers. HD-DVD manufacturing largely followed the same methodology as DVDs. In recordable and rewritable media, the majority of the stamper is composed of grooves, rather than the discrete pits and lands found on replicated discs. These grooves contain a subtle wobble frequency, which serves a critical purpose: it allows the reading or writing laser to precisely locate its position on the disc. DVDs, however, employ pre-pits instead, characterized by a constant frequency wobble.

Blu-ray

HTL (high-to-low type) Blu-ray discs are manufactured using a distinctly different process. First, a silicon wafer is utilized as the master, rather than the traditional glass master. This wafer undergoes processing in a manner analogous to how a glass master would be prepared.

The wafer is then subjected to electroplating to form a 300-micron thick nickel stamper, which is subsequently peeled away from the wafer. This stamper is then carefully mounted onto a mold within a press or embosser.

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

After cooling, a remarkably thin, 35 nanometre-thick layer of silver alloy is applied to the disc using the sputtering technique. Subsequently, the second layer is constructed by applying base and pit transfer resins to the disc, which are then pre-cured at its center.

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

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

BD-R and BD-RE discs receive a metal (recording layer) alloy (which, in BD-RE, is carefully sandwiched between two dielectric layers, also sputtered) before receiving the 30 nanometre metallization layer, typically made of silver alloy, aluminum, or gold, which is applied via sputtering. Alternatively, the silver alloy might be applied before the recording layer itself. Silver alloys are generally the preferred choice for Blu-rays, while aluminum is commonly used for CDs and DVDs. Gold finds its niche in some "Archival" CDs and DVDs, primarily due to its superior chemical inertness and resistance to corrosion compared to aluminum. Aluminum, when it corrodes, forms aluminum oxide, which manifests as transparent patches or dots on the disc—a phenomenon commonly known as disc rot. These corroded areas prevent the disc from being read, as the laser light simply passes through instead of being reflected back to the laser pickup assembly. Normally, aluminum forms a thin, protective oxide layer on contact with oxygen, preventing further corrosion. However, in the context of optical discs, its extreme thinness makes it vulnerable.

Finally, a 98 micrometer-thick cover layer is applied using UV-curable liquid optically clear adhesive, followed by a 2 micrometer-thick hard coat (such as Durabis), which is also cured using UV light. In the very last step, a 10 nanometre-thick silicon nitride barrier layer is applied to the label side of the disc, offering crucial protection against humidity. Blu-rays are designed with their data layers positioned remarkably close to the read surface of the disc, a critical factor in achieving their impressive storage capacity.

Discs produced in large quantities can either be replicated or duplicated. In replication, the elaborate process detailed above is employed to manufacture the discs from scratch. Duplication, by contrast, involves recording and then finalizing CD-R, DVD-R, or BD-R discs. Finalization is a crucial step that prevents any further recording and ensures wider compatibility with various players and drives. (For more on this, see Optical disc authoring). The equipment used also differs significantly: replication is carried out by fully automated, purpose-built machinery, the cost of which can run into the hundreds of thousands of US dollars even on the used market. Duplication, however, can be automated using what's known as an autoloader, or even performed manually, requiring only a relatively small tabletop duplicator. A stark difference in scale and investment, reflecting the varying demands of mass production versus smaller-batch creation.

Specifications

Base (1×) and (current) maximum speeds by generation

Generation Base (Mbit/s) Max (Mbit/s) ×
1st (CD) 1.17 65.6 56×
2nd (DVD) 10.57 253.6 24×
3rd (BD) 36 504 14×
4th (AD) ? ? 14×

Capacity and nomenclature

Designation Sides Layers (total) Diameter (cm) Capacity (GB)
BD SS SL 1 8
BD SS DL 1 2
BD SS SL 1 1
BD SS DL 1 2
BD SS TL 1 3
BD SS QL 1 4
CD–ROM 74 min SS SL 1 1
CD–ROM 80 min SS SL 1 1
CD–ROM SS SL 1 1
DDCD–ROM SS SL 1 1
DDCD–ROM SS SL 1 1
DVD–1 SS SL 1 1
DVD–2 SS DL 1 2
DVD–3 DS SL 2 2
DVD–4 DS DL 2 4
DVD–5 SS SL 1 1
DVD–9 SS DL 1 2
DVD–10 DS SL 2 2
DVD–14 DS DL/SL 2 3
DVD–18 DS DL 2 4
DVD–R 1.0 SS SL 1 1
DVD–R (2.0), +R, –RW, +RW SS SL 1 1
DVD-R, +R, –RW, +RW DS SL 2 2
DVD–RAM SS SL 1 1
DVD–RAM DS SL 2 2
DVD–RAM 1.0 SS SL 1 1
DVD–RAM 2.0 SS SL 1 1
DVD–RAM 1.0 DS SL 2 2
DVD–RAM 2.0 DS SL 2 2

See also