In computing, an optical disc drive (ODD) is a disk drive that uses laser light or electromagnetic waves near the light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only read from discs, but recent drives are commonly both readers and recorders. Recorders are sometimes called burners or writers. Compact discs, DVDs, and Blu-ray discs are common types of optical media which can be read and recorded by such drives.
A typical generic CDROM Drive.
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.
Disc recording is generally restricted to small-scale backup and distribution, being slower and more materially expensive per unit than the moulding process used to mass-manufacture pressed discs.
The most important part of an optical disc drive is an optical path, placed in a pickup head (PUH), usually consisting of semiconductor laser, a lens for guiding the laser beam, and photodiodes detecting the light reflection from disc's surface.
Initially, CD 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).
Two main servomechanisms are used, the first one to maintain a correct distance between lens and disc, and ensure the laser beam is focused on a small laser spot on the disc. The second servo moves a head along the disc's radius, keeping the beam on a groove, a continuous spiral data path.
On read only media (ROM), during the manufacturing process the groove, made of pits, is pressed on a flat surface, called land. Because the depth of the pits is approximately one-quarter to one-sixth of the laser's wavelength, the reflected beam's phase is shifted in relation to the incoming reading beam, causing mutual destructive interference and reducing the reflected beam's intensity. This is detected by photodiodes that output electrical signals.
A recorder encodes (or burns) data onto a recordable CD-R, DVD-R, DVD+R, or BD-R disc (called a blank) by selectively heating parts of an organic dye layer with a laser. This changes the reflectivity of the dye, thereby creating marks that can be read like the pits and lands on pressed discs. For recordable discs, the process is permanent and the media can be written to only once. While the reading laser is usually not stronger than 5 mW, the writing laser is considerably more powerful. The higher writing speed, the less time a laser has to heat a point on the media, thus its power has to increase proportionally. DVD burners' lasers often peak at about 200 mW, either in continuous wave and pulses, although some have been driven up to 400 mW before the diode fails.
For rewritable CD-RW, DVD-RW, DVD+RW, DVD-RAM, or BD-RE media, the laser is used to melt a crystalline metal alloy in the recording layer of the disc. Depending on the amount of power applied, the substance may be allowed to melt back (change the phase back) into crystalline form or left in an amorphous form, enabling marks of varying reflectivity to be created.
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. Traditional single layer (SL) writable media are produced with a spiral groove molded in the protective polycarbonate layer (not in the data recording layer), to lead and synchronize the speed of recording head. Double-layered writable media have: a first polycarbonate layer with a (shallow) groove, a first data layer, a semi-reflective layer, a second (spacer) polycarbonate layer with another (deep) groove, and a second data layer. The first groove spiral usually starts on the inner edge and extends outwards, while the second groove starts on the outer edge and extends inwards.
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', in that the latter keep a constant angular velocity (CAV), in other words a constant number of revolutions per minute (RPM). With CAV, a higher throughput is generally achievable at an outer disc area, as compared to inner area.
On the other hand, optical drives were developed with an assumption of achieving a constant throughput, in CD drives initially equal to 150 KiB/s. It was a feature important for streaming audio data that always tend to require a constant bit rate. But to ensure no disc capacity is wasted, a head had to transfer data at a maximum linear rate at all times too, without slowing on the outer rim of disc. This had led to optical drives—until recently—operating with a constant linear velocity (CLV). The spiral groove of the disc passed under its head at a constant speed. Of course the implication of CLV, as opposed to CAV, is that disc angular velocity is no longer constant, and spindle motor need to be designed to vary speed between 200 RPM on the outer rim and 500 RPM on the inner rim.
Later CD drives kept the CLV paradigm, but 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.
There are mechanical limits to how quickly a disc can be spun. Beyond a certain rate of rotation, around 10000 RPM, centrifugal stress can cause the disc plastic to creep and possibly shatter. On the outer edge of the CD disc, 10000 RPM limitation roughly equals to 52x speed, but on the inner edge only to 20x. Some drives further lower their maximum read speed to around 40x on the reasoning that blank discs will be clear of structural damage, but that discs inserted for reading may not be. Without higher rotational speeds, increased read performance may be attainable by simultaneously reading more than one point of a data groove, but drives with such mechanisms are more expensive, less compatible, and very uncommon.
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; however, the Wii and PlayStation 3 video game consoles seem to have defeated this problem, for they are able to load standard size DVDs and 80 mm discs in the same slot-loading drive.
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 all feature top-loading drives).
These sometimes have the advantage of using spring-loaded ball bearings to hold the disc in place, minimizing damage to the disc if the drive is moved while it is spun up.
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—such drives would also inconveniently require "bare" discs to be manually inserted into an openable caddy before use.
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 Red Book audio. The outputs may be connected via a header cable to the sound card or the motherboard. At one time, computer software resembling cd players controlled playback of the CD. Today the information is extracted from the disc as data, to be played back or converted to other file formats.
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.
When the optical disc drive was first developed, it was not easy to add to computer systems. Some computers such as the IBM PS/2 were standardizing on the 3.5" floppy and 3.5" hard disk, and did not include a place for a large internal device. Also IBM PCs and clones at first only included a single ATA drive interface, which by the time the CDROM was introduced, was already being used to support two hard drives. Early laptops simply had no built-in high-speed interface for supporting an external storage device.
This was solved through several techniques:
- Early sound cards could include a second ATA interface, though it was often limited to supporting a single optical drive and no hard drives. This evolved into the modern second ATA interface included as standard equipment
- A parallel port external drive was developed that connected between a printer and the computer. This was slow but an option for laptops
- A PCMCIA optical drive interface was also developed for laptops
- A SCSI card could be installed in desktop PCs for an external SCSI drive enclosure, though SCSI was typically much more expensive than other options
The Z-CLV recording strategy is easily visible after burning a DVD-R.
Because keeping a constant transfer rate for the whole disc is not so important in most contemporary CD uses, to keep the rotational speed of the disc safely low while maximizing data rate, a pure CLV approach needed to be abandoned. Some drives work in partial CLV (PCLV) scheme, by switching from CLV to CAV only when a rotational limit is reached. But switching to CAV requires considerable changes in hardware design, so instead most drives use the zoned constant linear velocity (Z-CLV) scheme. This divides the disc into several zones, each having its own different constant linear velocity. A Z-CLV recorder rated at "52X", for example, would write at 52X on the innermost zone and then progressively decrease the speed in several discrete steps down to 20X at the outer rim.
