Optical storage
From ECDL.web
Optical disc drives are an integral part of stand-alone consumer appliances such as CD players, DVD players and DVD recorders. They are also very commonly used in computers to read software and consumer media distributed in disc form, and to record discs for archival and data exchange. Optical drives—along with flash memory—have mostly displaced floppy disk drives and magnetic tape drives for this purpose because of the low cost of optical media and the near-ubiquity of optical drives in computers and consumer entertainment hardware.
The most important part of an optical disc drive is an optical path, placed in a pickup head, usually consisting of semiconductor laser, a lens for guiding the laser beam, and photodiodes detecting the light reflection from disc's surface. Initially, CAT lasers with a wavelength of 780 nm were used, being within infrared range. For DVDs, the wavelength was reduced to 650 nm (red color), and the wavelength for Blu-Ray Disc was reduced to 405 nm (violet color).
Double-sided media may be used, but they are not easily accessed with a standard drive, as they must be physically turned over to access the data on the other side.
Double layer (DL) media have two independent data layers separated by a semi-reflective layer. Both layers are accessible from the same side, but require the optics to change the laser's focus.
Some drives support Hewlett-Packard's LightScribe photothermal printing technology for labeling specially coated discs.
Optical drives' rotational mechanism differs considerably from hard disk drives'. Optical drives were developed with an assumption of achieving a constant throughput, in CD drives initially equal to 150 KiB/s. Later CD drives evolved to achieve higher rotational speeds, popularly described in multiples of a base speed. As a result, a 4X drive, for instance, would rotate at 800-2000 RPM, while transferring data steadily at 600 KiB/s, which is equal to 4 x 150 KiB/s. For DVD base speed, or "1x speed", is 1.385 MB/s, equal to 1.32 MiB/s, approximately 9 times faster than CD's base speed. For Blu-ray drive base speed is 6.74 MB/s, equal to 6.43 MiB/s.
[edit] Computer interfaces
Most internal drives for personal computers, servers and workstations are designed to fit in a standard 5.25" drive bay and connect to their host via an ATA or SATA interface. Additionally, there may be digital and analog outputs for audio to be connected via a header cable to the sound card or the motherboard.
External drives usually have USB or FireWire interfaces. Some portable versions for laptop use power themselves off batteries or off their interface bus. Drives with SCSI interface exist, but are less common and tend to be more expensive, because of the cost of their interface chipsets and more complex SCSI connectors.
[edit] Loading mechanisms
Current optical drives use either a tray-loading mechanism, where the disc is loaded onto a motorised or manually operated tray, or a slot-loading mechanism, where the disc is slid into a slot and drawn in by motorized rollers. Slot-loading drives have the disadvantage that they cannot usually accept the smaller 80 mm discs or any non-standard sizes.
A small number of drive models, mostly compact portable units, have a top-loading mechanism where the drive lid is opened upwards and the disc is placed directly onto the spindle (for example, all PlayStation 1 consoles, portable CD players, and some standalone CD recorders).
Some early CD-ROM drives used a mechanism where CDs had to be inserted into special cartridges or caddies, somewhat similar in appearance to a 3.5" floppy diskette. This was intended to protect the disc from accidental damage by enclosing it in a tougher plastic casing, but did not gain wide acceptance due to the additional cost and compatibility concerns.
[edit] Compatibility
Most optical drives are backwards compatible with their ancestors up to CD, although this is not required by standards. A DVD lens supports a different focus for CD or DVD media with same laser.