The technology that uses lasers to read and write information stored as pits in a reflective coating is known as optical storage. The most common form of optical storage is the compact disc, also known as CD. Other optical storage technologies include DVDs, Blu-ray Discs, and LaserDiscs.
How Optical Storage Works
Optical storage technologies like CDs, DVDs, and Blu-ray Discs store data in the form of microscopic pits etched into a layer of reflective metal. A laser is used to precisely burn these tiny pits into the surface of the disc during the writing process. During reading, a lower-power laser beam reflects off the surface of the disc. The pits scatter the reflected laser light differently than the flat spaces between pits. An optical sensor detects these differences in reflectivity, allowing the original data to be reconstructed.
On a CD, the pits and lands (flat spaces) along a track together represent binary data, with lands representing 0s and pits representing 1s. Pits are formed by a laser burning microscopic holes into the reflective metal layer of a CD blank. The minimum length of a pit is approximately 0.83 microns.
The width of a CD track is 0.5 microns. With a track pitch (distance between track centerlines) of 1.6 microns, CDs can pack approximately 600 tracks per millimeter. This extremely high track density is what gives CDs the ability to store so much information in a relatively small space.
CD Capacity
A standard 120 mm CD can hold up to 74-80 minutes of audio. This storage capacity comes from:
- A track pitch of 1.6 microns
- A minimum pit length of about 0.83 microns
- A rotation speed of 200 to 500 rpm
- A standard data transfer rate of 150 kilobytes per second
For general data storage, the maximum capacity of a CD is around 700 MB. Some CD formats can hold slightly more data by using more efficient encoding methods.
DVD and Blu-ray Capacity
Later optical disc formats like DVD and Blu-ray built upon the basic CD format to offer vastly expanded storage capacity:
- A standard single-layer DVD holds 4.7 GB.
- A single-layer Blu-ray Disc holds up to 25 GB.
- Dual-layer and multilayer DVDs and Blu-ray Discs boost capacity even further, up to 128GB for the latest multilayer BDXL discs.
These greater capacities are achieved by using shorter wavelength lasers and focusing optics, allowing data tracks to be packed even more densely on the disc surface.
History of Optical Storage
The earliest forms of optical data storage emerged in the 1960s and 70s. These systems used analog methods to store audio and video content. The first optical storage technology to become popular with consumers was the LaserDisc format, introduced in 1978. Though LaserDisc players could not record content, the format was capable of exceptional video quality and gained a loyal following among home theater enthusiasts.
The Advent of CDs
The compact disc format was co-developed by Sony and Philips throughout the late 1970s and early 1980s. CDs represented a major breakthrough as the first highly-successful optical storage medium to rely on digital rather than analog methods of encoding data. The primary motivation behind CD development was to create a digital audio format capable of high fidelity, convenience, and resistance to playback issues like hiss and pops.
Though CD audio was the initial focus, Sony and Philips also saw great potential for CD-ROMs as a new standard data storage and distribution format for computers. CD-ROM drives began appearing on some desktop computers by the late 1980s, though adoption was gradual. By the early 90s, CD-ROMs became ubiquitous on new PCs.
The DVD Era
Through the 1990s, new optical disc formats emerged to meet the demands of ever-expanding multimedia computing power. DVDs arrived in 1996, offering far greater capacity than CDs. This expanded space made DVDs well-suited for storing feature length video. DVD players and discs quickly became the standard home video format by the early 2000s.
Recordable and rewritable DVD formats also became widely adopted for data storage and backup. Though CD-R and CD-RW discs were available, DVD-Rs offered far more capacity and speed. High-capacity dual layer DVD recordable formats further boosted optical disc storage potential for data archiving and distribution.
Blu-ray
Even as DVD settled in as the dominant optical format for movies and data storage in the early 2000s, work was underway on a higher-capacity high definition optical disc format. Sony took the lead in developing what would become known as Blu-ray Disc, increasing storage capacity to 25GB on a single-layer disc. With more than five times the capacity of a DVD, Blu-ray was well-suited for storing and distributing ultra high definition video content.
Blu-ray also employs a blue-violet laser with a much shorter 405nm wavelength compared to the 650nm red laser used by DVDs. This narrower beam focus allows pits to be etched more precisely and compactly on the disc surface. Blu-ray discs began rolling out commercially around 2006. Blu-ray players and media have since largely replaced DVDs as the optical disc format of choice for movies, games, and data storage.
Optical Drives and Players
Playback and recording of optical discs requires an optical drive designed for reading that specific media format. Optical drives use a laser pickup assembly and spinning motor mechanism to read data from or write data to discs. Optical drives usually also incorporate decoding circuitry and connections to convert stored data into usable information for the computer or playback device.
CD Drives
The first CD players designed exclusively for audio playback appeared in 1982 before CDs were officially released to the public. These first units were very expensive, costing well over $1000. Prices came down as production ramped up, and dedicated CD players quickly replaced vinyl record players and tape decks in homes worldwide during the 1980s.
The earliest CD-ROM drives designed for computers also carried high price tags, limiting adoption at first. By the late 1980s, improving technology helped make CD-ROM drives standard equipment in many PCs. CD-RW drives for writing CDs did not become commonplace until the late 90s.
DVD Drives
After the first commercial DVD players hit the market in Japan in 1996, adoption of DVD players and drives was rapid. DVD player prices dropped precipitously between 1997 and 1999. As prices fell below $100, DVD players began replacing VHS players in homes in large numbers. By 2000, nearly every new computer shipped with a DVD-ROM drive as standard.
Recordable DVD drives lagged a bit, not appearing in PCs until around 2003. Dual-layer DVD writers became available by 2004, and within just a few years dual layer DVD-R/RW drives had largely supplanted CD writers as the optical recording format of choice for computers.
Blu-ray Drives
Sony’s first Blu-ray player intended for consumers became available in 2006 for approximately $1000. But Blu-ray was slow to unseat DVD, owing to factors like initial high player prices and confusion arising from the Blu-ray/HD-DVD format war that delayed wide adoption. Only in 2008 did Blu-ray players fall below the $200 price threshold needed to spur mass popularity.
From 2010 onward, Blu-ray players and Blu-ray-equipped gaming consoles like the PS3 became common in homes. On the PC side, Blu-ray drives started appearing commonly in high-end desktops and laptops around 2009.
Challenges for Optical Storage
While optical storage boomed for decades as the leading way to distribute content to consumers, the technology faces stiff challenges in the modern era from emerging solutions:
Internet Bandwidth
Widespread broadband internet access enables distribution of HD video over the web, lessening the advantage of large optical disc capacities. Streaming and direct download services for movies, TV, and music have become huge business, with Netflix alone accounting for 15% of all internet bandwidth use globally as of 2020.
Solid State Memory
For data storage and transfer, flash-based USB drives and SSDs offer certain advantages over optical media like CDs and DVDs. Solid state memory provides much faster data access and overall performance. And with no moving parts, it is not subject to the mechanical failures that can sometimes afflict optical disc drives.
Cloud Storage
The availability of inexpensive, high-capacity cloud storage diminishes the importance of optical discs for data archiving and transfer. Services like Dropbox, Google Drive, Microsoft OneDrive, and Apple iCloud allow terabytes of data to be easily stored and shared entirely over the internet.
The Future of Optical Storage
While optical disc usage has declined, optical storage technology still offers unique benefits that virtually guarantee it will remain in use for specialty applications for the foreseeable future.
Archival Storage
For reliable long-term data storage measured in decades, properly handled and stored optical discs still have advantages over tape, hard drives, and flash memory. Optical media is inherently immune to data degradation issues like bit rot that affect other storage mediums over time.
High Capacities
Though cloud and streaming services have reduced the need for physical distribution of large video and music libraries, discs still offer a way to conveniently access this content offline. Blu-ray discs can hold multiple times more data than the largest mainstream consumer thumb drives available.
Durability
Compared to fragile magnetic tape reels and cassettes, optical discs are reasonably hardy if protected from scratches and abrasion. Many discs from the CD era still read perfectly fine decades later. This makes optical storage a good choice for storing important data and backups that need to remain readable for years.
New Disc Technologies
Work continues on developing optical disc formats with enhanced capacities. M-DISC uses an inorganic recording layer and special optical drive to create write-once discs capable of surviving for hundreds of years with proper handling. Other technologies like Holographic Versatile Disc could boost capacities, though mainstream adoption has proved elusive.
Conclusion
Since the audio CD emerged in the early 1980s, optical storage has remained an important part of the consumer technology landscape. For much of that period, formats like CDs, DVDs, and Blu-ray discs were the dominant way people accessed digital content and stored important data. Though the role of optical storage has diminished lately with the rise of streaming media and cloud storage, its unique advantages mean CDs, DVDs, Blu-ray discs, and future optical technologies will continue serving key archival and distribution functions for the foreseeable future.