July 20, 2010 > TechKnow Talk: Optical Discs: CD to Blu-ray and beyond
TechKnow Talk: Optical Discs: CD to Blu-ray and beyond
The introduction of the cassette tape (and the eight track) in the late 1960s changed the way people listened to music. Favorite tunes were now easily portable and could even be enjoyed in a car! A decade later, movies became more portable too, with the advent of VHS and Beta videocassette tapes.
In spite of the inevitable tangling of audio and video tapes, the TechKnow Guy marveled at these innovations. But today's young people are accustomed to Portable Media Player technology, including devices based on hard drives as well as optical discs that are more durable than their tape-based predecessors, and capable of storing tremendous quantities of data.
The Compact Disc (CD) is the breakthrough technology that started it all. Introduced in the mid-1980s, there were soon sufficient music CDs and CD players available to create a huge demand. By 2000, cassette tapes were becoming obsolete. Some 25 years after its debut, the CD remains the standard for audio recording.
Current CD technologies offer a capacity of about 700 Megabytes (MB) or 80 minutes of music. CDs are available in a number of formats: the most important for the typical consumer are Read-Only Memory (CD-ROM), Recordable (CD-R), and Re-Writable (CD-RW). A CD-ROM is recorded by the manufacturer with data that cannot be overwritten. A CD-R is a blank CD that can be recorded onto by the user only once, then becoming in effect a CD-ROM. A CD-RW can be used to record, delete, and re-record multiple times.
A CD is composed of a layer of clear plastic, or polycarbonate, roughly one millimeter (0.04 inch) thick. Pressed into the plastic is a spiral pattern consisting of tiny "bumps" of varying length and spacing. Over this is deposited a very thin coating of reflective metal, typically aluminum. A thin layer of acrylic is bonded onto this to protect the fragile aluminum coating. Finally, a label is affixed to the top.
The spiral track on the CD is similar to a vinyl record, but much more compact. The radial distance between spirals is about 1.5 microns (a human hair is roughly 100 microns in diameter). The data is encoded by the length and separation of the tiny bumps that lie along the spiral track. These bumps are only about 0.5 micron wide and 0.1 micron deep. It is because of these tiny dimensions that so much data can be written to a CD. If the spiral was "unrolled" into a straight line, it would be over three miles long.
A CD player spins the CD and sends a laser beam up through the plastic layer to track the spiral, not outside to inside as a vinyl record player, but from the center outward. The wavelength of the laser beam in a CD player is 0.78 micron, which is "near infrared" and not visible to the human eye.
Because the length of the track increases slightly with each rotation, the player gradually slows the rate of rotation as it moves outward, maintaining the rate of movement over the laser beam at a constant rate. The CD is spun at about 500 rpm on the innermost portion, gradually slowing to about 200 rpm at its outer edge.
The laser reflects off the aluminum coating, sending this information back to an optical sensor. Electronics then convert the sensor data to a digital signal. Essentially, a flat area is a one and a bump is a zero. Finally, the binary signal is converted into an analog signal and sent to speakers.
If the CD holds data other than music, the CD reader retains the data in digital format and may write it to a hard drive or other media. In addition to music, CDs are often used for distribution of software and offline data storage.
CD technology laid the foundation for the development of the DVD, first marketed in the late 1990s. The primary difference is that a DVD has an even greater density, allowing it to hold nearly seven times more data than a CD: about 4700 MB or 4.7 Gigabytes (GB). This allows an entire movie (in compressed format) to be encoded onto a DVD. Double-sided and double-layered DVDs are available that increase capacity to more than 8.5 GB.
The spiral tracks on a DVD are only 0.74 microns apart, half the distance of the CD tracks. If rolled out straight, the track would be over seven miles long. In addition, the bumps are even narrower and shorter than on a CD. This requires greater focusing precision of the laser beam. Thus, DVD players use a visible red laser at the shorter wavelength of 0.65 microns.
Players that read both CDs and DVDs have two sets of laser optics. Because video data is compressed on a DVD, players also include a decoder, which uncompresses the video as it is played.
As CDs sounded the death knell for cassette tapes, the VHS format quickly became obsolete in the face of the much more convenient and higher quality DVD. It took less than a decade after the release of the DVD to completely eliminate the VHS industry. Like CDs, DVDs are also available in recordable and re-writable formats. In addition to video, they are widely used for data storage. A single DVD is capable of storing an entire encyclopedia.
The term 'DVD' has engendered much confusion. Originally conceived as "Digital VideoDisc," it has been more recently identified as "Digital Versatile Disc." However, there is no consensus agreement that it currently stands for anything at all.
The advent of high-definition (HD) video in recent years has spurred the development of the Blu-ray Disc (BD). A BD is able to pack data in far greater density than even a DVD. With a distance between tracks of a miniscule 0.32 microns, and more closely situated bumps, the BD can hold more than five times the data of a DVD, or 27 GB. This is a case of necessity driving invention, as HD video contains five times the data of a standard format movie.
A BD uses different manufacturing methods than a CD or DVD because of the extreme precision required. The layer with the bumps is placed near the bottom of the disc, closer to the laser lens, to reduce error associated with tiny variations in the flatness of the disc. In addition, an even shorter wavelength laser of 0.4 microns is used. This is violet-blue in color, hence the name "Blu-Ray."
As CDs, DVDs, and BDs all have the same physical characteristics and similar data encoding methodologies, it is possible to manufacture readers that are compatible with all three discs, as long as separate optical components are included for each. Because of this, it is unlikely BD will supplant DVD as quickly as DVD vanquished VHS, since DVDs will continue to be readable in most new BD players.
The value of optical disks as long-term data storage has been much debated. How long they remain readable is dependent on several factors, including the quality of the disc, the quality of the recording device, and the conditions of storage. An unreadable condition, broadly referred to as "CD rot," may occur as quickly as two years, though in some cases discs may be readable for 20 years or more.
To be on the safe side, critical data should not be stored solely on optical disk, even under the best conditions. BDs are especially risky as backup media, as their long-term viability is as yet unknown.
3D stereoscopic BD movies are now available in limited numbers. Some employ technologies allowing them to achieve a 3D-like experience on existing equipment, but both a 3D-capable BD player and television are required to display true 3D. This will surely delay their widespread acceptance.
What's next? Employing dual-sided, multi-layered discs and/or ultra-violet lasers, BDs capable of storing 400-500 GB may be available in a few years. Looking further ahead, holographic and multi-laser, multi-layer technologies may someday be capable of producing discs in the 3-10 terabyte range (3000-10,000 GB). Imagine a disc containing 400 HD movies or 10 million books! Such discs may also deliver software to drive some extremely realistic gaming and alternate reality experiences.