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The Bull Gamma 60, initially designed in 1957 and first released in 1960, was the first computer designed with multiprogramming in mind. Its architecture featured a central memory and a Program Distributor feeding up to twenty-five autonomous processing units with code and data, and allowing concurrent operation of multiple clusters.
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Another such computer was the LEO III, first released in 1961. During batch processing, several different programs were loaded in the computer memory, and the first one began to run. When the first program reached an instruction waiting for a peripheral, the context of this program was stored away, and the second program in memory was given a chance to run. The process continued until all programs finished running.
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The use of multiprogramming was enhanced by the arrival of virtual memory and virtual machine technology, which enabled individual programs to make use of memory and operating system resources as if other concurrently running programs were, for all practical purposes, nonexistent.
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Multiprogramming gives no guarantee that a program will run in a timely manner. Indeed, the first program may very well run for hours without needing access to a peripheral. As there were no users waiting at an interactive terminal, this was no problem: users handed in a deck of punched cards to an operator, and came back a few hours later for printed results. Multiprogramming greatly reduced wait times when multiple batches were being processed.
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Early multitasking systems used applications that voluntarily ceded time to one another. This approach, which was eventually supported by many computer operating systems, is known today as cooperative multitasking. Although it is now rarely used in larger systems except for specific applications such as CICS or the JES2 subsystem, cooperative multitasking was once the only scheduling scheme employed by Microsoft Windows and classic Mac OS to enable multiple applications to run simultaneously. Cooperative multitasking is still used today on RISC OS systems.
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As a cooperatively multitasked system relies on each process regularly giving up time to other processes on the system, one poorly designed program can consume all of the CPU time for itself, either by performing extensive calculations or by busy waiting; both would cause the whole system to hang. In a server environment, this is a hazard that makes the entire environment unacceptably fragile.
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Preemptive multitasking allows the computer system to more reliably guarantee to each process a regular "slice" of operating time. It also allows the system to deal rapidly with important external events like incoming data, which might require the immediate attention of one or another process. Operating systems were developed to take advantage of these hardware capabilities and run multiple processes preemptively. Preemptive multitasking was implemented in the PDP-6 Monitor and Multics in 1964, in OS/360 MFT in 1967, and in Unix in 1969, and was available in some operating systems for computers as small as DEC's PDP-8; it is a core feature of all Unix-like operating systems, such as Linux, Solaris and BSD with its derivatives, as well as modern versions of Windows.
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At any specific time, processes can be grouped into two categories: those that are waiting for input or output , and those that are fully utilizing the CPU . In primitive systems, the software would often "poll", or "busywait" while waiting for requested input . During this time, the system was not performing useful work. With the advent of interrupts and preemptive multitasking, I/O bound processes could be "blocked", or put on hold, pending the arrival of the necessary data, allowing other processes to utilize the CPU. As the arrival of the requested data would generate an interrupt, blocked processes could be guaranteed a timely return to execution.
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The earliest preemptive multitasking OS available to home users was Sinclair QDOS on the Sinclair QL, released in 1984, but it was not a big success. Commodore's Amiga, released the following year, was the first commercially successful home computer to use the technology, and its multimedia abilities make it a clear ancestor of contemporary multitasking personal computers. Microsoft made preemptive multitasking a core feature of their flagship operating system in the early 1990s when developing Windows NT 3.1 and then Windows 95. In 1988 Apple offered A/UX as a UNIX System V-based alternative to the Classic Mac OS. In 2001 Apple switched to the NeXTSTEP-influenced Mac OS X.
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A similar model is used in Windows 9x and the Windows NT family, where native 32-bit applications are multitasked preemptively. 64-bit editions of Windows, both for the x86-64 and Itanium architectures, no longer support legacy 16-bit applications, and thus provide preemptive multitasking for all supported applications.
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Another reason for multitasking was in the design of real-time computing systems, where there are a number of possibly unrelated external activities needed to be controlled by a single processor system. In such systems a hierarchical interrupt system is coupled with process prioritization to ensure that key activities were given a greater share of available process time.
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As multitasking greatly improved the throughput of computers, programmers started to implement applications as sets of cooperating processes . This, however, required some tools to allow processes to efficiently exchange data.
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Threads were born from the idea that the most efficient way for cooperating processes to exchange data would be to share their entire memory space. Thus, threads are effectively processes that run in the same memory context and share other resources with their parent processes, such as open files. Threads are described as lightweight processes because switching between threads does not involve changing the memory context.
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While threads are scheduled preemptively, some operating systems provide a variant to threads, named fibers, that are scheduled cooperatively. On operating systems that do not provide fibers, an application may implement its own fibers using repeated calls to worker functions. Fibers are even more lightweight than threads, and somewhat easier to program with, although they tend to lose some or all of the benefits of threads on machines with multiple processors.
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Some systems directly support multithreading in hardware.
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Essential to any multitasking system is to safely and effectively share access to system resources. Access to memory must be strictly managed to ensure that no process can inadvertently or deliberately read or write to memory locations outside the process's address space. This is done for the purpose of general system stability and data integrity, as well as data security.
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In general, memory access management is a responsibility of the operating system kernel, in combination with hardware mechanisms that provide supporting functionalities, such as a memory management unit . If a process attempts to access a memory location outside its memory space, the MMU denies the request and signals the kernel to take appropriate actions; this usually results in forcibly terminating the offending process. Depending on the software and kernel design and the specific error in question, the user may receive an access violation error message such as "segmentation fault".
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In a well designed and correctly implemented multitasking system, a given process can never directly access memory that belongs to another process. An exception to this rule is in the case of shared memory; for example, in the System V inter-process communication mechanism the kernel allocates memory to be mutually shared by multiple processes. Such features are often used by database management software such as PostgreSQL.
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Inadequate memory protection mechanisms, either due to flaws in their design or poor implementations, allow for security vulnerabilities that may be potentially exploited by malicious software.
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Use of a swap file or swap partition is a way for the operating system to provide more memory than is physically available by keeping portions of the primary memory in secondary storage. While multitasking and memory swapping are two completely unrelated techniques, they are very often used together, as swapping memory allows more tasks to be loaded at the same time. Typically, a multitasking system allows another process to run when the running process hits a point where it has to wait for some portion of memory to be reloaded from secondary storage.
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Processes that are entirely independent are not much trouble to program in a multitasking environment. Most of the complexity in multitasking systems comes from the need to share computer resources between tasks and to synchronize the operation of co-operating tasks.
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Various concurrent computing techniques are used to avoid potential problems caused by multiple tasks attempting to access the same resource.
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Bigger systems were sometimes built with a central processor and some number of I/O processors, a kind of asymmetric multiprocessing.
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Over the years, multitasking systems have been refined. Modern operating systems generally include detailed mechanisms for prioritizing processes, while symmetric multiprocessing has introduced new complexities and capabilities.
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3-D computer graphics, contrary to what the name suggests, are most often displayed on two-dimensional displays. Unlike 3-D film and similar techniques, the result is two-dimensional, without visual depth. More often, 3-D graphics are being displayed on 3-D displays, like in virtual reality systems.
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3-D graphics stand in contrast to 2-D computer graphics which typically use completely different methods and formats for creation and rendering.
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3-D computer graphics rely on many of the same algorithms as 2-D computer vector graphics in the wire-frame model and 2-D computer raster graphics in the final rendered display. In computer graphics software, 2-D applications may use 3-D techniques to achieve effects such as lighting, and similarly, 3-D may use some 2-D rendering techniques.
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The objects in 3-D computer graphics are often referred to as 3-D models. Unlike the rendered image, a model's data is contained within a graphical data file. A 3-D model is a mathematical representation of any three-dimensional object; a model is not technically a graphic until it is displayed. A model can be displayed visually as a two-dimensional image through a process called 3-D rendering, or it can be used in non-graphical computer simulations and calculations. With 3-D printing, models are rendered into an actual 3-D physical representation of themselves, with some limitations as to how accurately the physical model can match the virtual model.
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William Fetter was credited with coining the term computer graphics in 1961 to describe his work at Boeing. An early example of interactive 3-D computer graphics was explored in 1963 by the Sketchpad program at Massachusetts Institute of Technology's Lincoln Laboratory. One of the first displays of computer animation was Futureworld , which included an animation of a human face and a hand that had originally appeared in the 1971 experimental short A Computer Animated Hand, created by University of Utah students Edwin Catmull and Fred Parke.
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3-D computer graphics software began appearing for home computers in the late 1970s. The earliest known example is 3D Art Graphics, a set of 3-D computer graphics effects, written by Kazumasa Mitazawa and released in June 1978 for the Apple II.
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3-D computer graphics production workflow falls into three basic phases:
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3-D modeling – the process of forming a computer model of an object's shape
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Layout and CGI animation – the placement and movement of objects within a scene
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3-D rendering – the computer calculations that, based on light placement, surface types, and other qualities, generate an image
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The model describes the process of forming the shape of an object. The two most common sources of 3-D models are those that an artist or engineer originates on the computer with some kind of 3D modeling tool, and models scanned into a computer from real-world objects . Models can also be produced procedurally or via physical simulation. Basically, a 3-D model is formed from points called vertices that define the shape and form polygons. A polygon is an area formed from at least three vertices . A polygon of n points is an n-gon. The overall integrity of the model and its suitability to use in animation depend on the structure of the polygons.
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Before rendering into an image, objects must be laid out in a 3D scene. This defines spatial relationships between objects, including location and size. Animation refers to the temporal description of an object . These techniques are often used in combination. As with animation, physical simulation also specifies motion.
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Materials and textures are properties that the render engine uses to render the model. One can give the model materials to tell the render engine how to treat light when it hits the surface. Textures are used to give the material color using a color or albedo map, or give the surface features using a bump map or normal map. It can be also used to deform the model itself using a displacement map.
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Rendering converts a model into an image either by simulating light transport to get photo-realistic images, or by applying an art style as in non-photorealistic rendering. The two basic operations in realistic rendering are transport and scattering . This step is usually performed using 3-D computer graphics software or a 3-D graphics API. Altering the scene into a suitable form for rendering also involves 3-D projection, which displays a three-dimensional image in two dimensions. Although 3-D modeling and CAD software may perform 3-D rendering as well , exclusive 3-D rendering software also exists
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3-D computer graphics software produces computer-generated imagery through 3-D modeling and 3-D rendering or produces 3-D models for analytic, scientific and industrial purposes.
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There are many varieties of files supporting 3-D graphics, for example, Wavefront .obj files and .x DirectX files. Each file type generally tends to have its own unique data structure.
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Each file format can be accessed through their respective applications, such as DirectX files, and Quake. Alternatively, files can be accessed through third-party standalone programs, or via manual decompilation.
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3-D modeling software is a class of 3-D computer graphics software used to produce 3-D models. Individual programs of this class are called modeling applications or modelers.
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3-D modeling starts by describing 3 display models : Drawing Points, Drawing Lines and Drawing triangles and other Polygonal patches.
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3-D modelers allow users to create and alter models via their 3-D mesh. Users can add, subtract, stretch and otherwise change the mesh to their desire. Models can be viewed from a variety of angles, usually simultaneously. Models can be rotated and the view can be zoomed in and out.
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3-D modelers can export their models to files, which can then be imported into other applications as long as the metadata are compatible. Many modelers allow importers and exporters to be plugged-in, so they can read and write data in the native formats of other applications.
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Most 3-D modelers contain a number of related features, such as ray tracers and other rendering alternatives and texture mapping facilities. Some also contain features that support or allow animation of models. Some may be able to generate full-motion video of a series of rendered scenes .
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Computer aided design software may employ the same fundamental 3-D modeling techniques that 3-D modeling software use but their goal differs. They are used in computer-aided engineering, computer-aided manufacturing, Finite element analysis, product lifecycle management, 3D printing and computer-aided architectural design.
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After producing video, studios then edit or composite the video using programs such as Adobe Premiere Pro or Final Cut Pro at the mid-level, or Autodesk Combustion, Digital Fusion, Shake at the high-end. Match moving software is commonly used to match live video with computer-generated video, keeping the two in sync as the camera moves.
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Use of real-time computer graphics engines to create a cinematic production is called machinima.
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Not all computer graphics that appear 3D are based on a wireframe model. 2D computer graphics with 3D photorealistic effects are often achieved without wireframe modeling and are sometimes indistinguishable in the final form. Some graphic art software includes filters that can be applied to 2D vector graphics or 2D raster graphics on transparent layers. Visual artists may also copy or visualize 3D effects and manually render photorealistic effects without the use of filters.
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Some video games use 2.5D graphics, involving restricted projections of three-dimensional environments, such as isometric graphics or virtual cameras with fixed angles, either as a way to improve performance of the game engine or for stylistic and gameplay concerns. By contrast, games using 3D computer graphics without such restrictions are said to use true 3D.
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Although most laptop manufacturers no longer have optical drives bundled with their products, external drives are still available for purchase separately.
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As of 2021, most of the optical disc drives on the market are DVD-ROM drives and BD-ROM drives which read and record from those formats, along with having backward compatibility with CD, CD-R and CD-ROM discs; compact disc drives are no longer manufactured outside of audio devices. Read-only DVD and Blu-ray drives are also manufactured, but are less commonly found in the consumer market and mainly limited to media devices such as game consoles and disc media players. Over the last ten years, laptop computers no longer come with optical disc drives in order to reduce costs and make devices lighter, requiring consumers to purchase external optical drives.
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Optical disc drives are an integral part of standalone appliances such as CD players, DVD players, Blu-ray Disc players, DVD recorders, and video game consoles. As of 2017, the PlayStation and Xbox consoles are the only home video game consoles that are currently using optical discs as its primary storage format, as the Wii U's successor, the Nintendo Switch, began using game cartridges, while the PlayStation Portable is the only handheld console to use optical discs, using Sony's proprietary UMD format. They are also very commonly used in computers to read software and media distributed on disc and to record discs for archival and data exchange purposes. Floppy disk drives, with capacity of 1.44 MB, have been made obsolete: optical media are cheap and have vastly higher capacity to handle the large files used since the days of floppy discs, and the vast majority of computers and much consumer entertainment hardware have optical writers. USB flash drives, high-capacity, small, and inexpensive, are suitable where read/write capability is required.
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Disc recording is restricted to storing files playable on consumer appliances , relatively small volumes of data for local use, and data for distribution, but only on a small scale; mass-producing large numbers of identical discs by pressing is cheaper and faster than individual recording .
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To support 8 centimetre diameter discs, drives with mechanical tray loading have an indentation in the tray. It can however only be used in horizontal operation.
Slot loading drives, frequently used in game consoles and car radios, might be able to accept 8 centimetre discs and center the disc automatically.
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Optical discs are used to back up relatively small volumes of data, but backing up of entire hard drives, which as of 2015 typically contain many hundreds of gigabytes or even multiple terabytes, is less practical. Large backups are often instead made on external hard drives, as their price has dropped to a level making this viable; in professional environments magnetic tape drives are also used.
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Some optical drives also allow predictively scanning the surface of discs for errors and detecting poor recording quality.
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With an option in the optical disc authoring software, optical disc writers are able to simulate the writing process on CD-R, CD-RW, DVD-R and DVD-RW, which allows for testing such as observing the writing speeds and patterns with different writing speed settings and testing the highest capacity of an individual disc that would be achievable using overburning, without writing any data to the disc.
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Few optical drives allow simulating a FAT32 flash drive from optical discs containing ISO9660/Joliet and UDF file systems or audio tracks , for compatibility with most USB multimedia appliances.
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Optical drives for computers come in two main form factors: half-height and slim type . They exist as both internal and external variants.
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Half-height optical drives are around 4 centimetres tall, while slim type optical drives are around 1 cm tall.
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Half-height optical drives operate upwards of twice the speeds as slim type optical drives, because speeds on slim type optical drives are constrained to the physical limitations of the drive motor's rotation speed rather than the performance of the optical pickup system.
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Because half-height demand much more electrical power and a voltage of 12 V DC, while slim optical drives run on 5 volts, external half height optical drives require separate external power input, while external slim type are usually able to operate entirely on power delivered through a computer's USB port. Half height drives are also faster than Slim drives due to this, since more power is required to spin the disc at higher speeds.
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Half-height optical drives hold discs in place from both sides while slim type optical drives fasten the disc from the bottom.
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Half height drives fasten the disc using 2 spindles containing a magnet each, one under and one above the disc tray. The spindles may be lined with flocking or a texturized silicone material to exert friction on the disc, to keep it from slipping. The upper spindle is left slightly loose and is attracted to the lower spindle because of the magnets they have. When the tray is opened, a mechanism driven by the movement of the tray pulls the lower spindle away from the upper spindle and vice versa when the tray is closed. When the tray is closed, the lower spindle touches the inner circumference of the disc, and slightly raises the disc from the tray to the upper spindle, which is attracted to the magnet on the lower disc, clamping the disc in place. Only the lower spindle is motorized. Trays in half height drives often fully open and close using a motorized mechanism that can be pushed to close, controlled by the computer, or controlled using a button on the drive. Trays on half height and slim drives can also be locked by whatever program is using it, however it can still be ejected by inserting the end of a paper clip into an emergency eject hole on the front of the drive. Early CD players such as the Sony CDP-101 used a separate motorized mechanism to clamp the disc to the motorized spindle.
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Slim drives use a special spindle with spring loaded specially shaped studs that radiate outwards, pressing against the inner edge of the disc. The user has to put uniform pressure onto the inner circumference of the disc to clamp it to the spindle and pull from the outer circumference while placing the thumb on the spindle to remove the disc, flexing it slightly in the process and returning to its normal shape after removal. The outer rim of the spindle may have a texturized silicone surface to exert friction keeping the disc from slipping. In slim drives most if not all components are on the disc tray, which pops out using a spring mechanism that can be controlled by the computer. These trays cannot close on their own; they have to be pushed until the tray reaches a stop.
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The most important part of an optical disc drive is an optical path, which is inside a pickup head . The PUH is also known as a laser pickup, optical pickup, pickup, pickup assembly, laser assembly, laser optical assembly, optical pickup head/unit or optical assembly. It usually consists of a semiconductor laser diode, a lens for focusing the laser beam, and photodiodes for detecting the light reflected from the disc's surface.
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Initially, CD-type lasers with a wavelength of 780 nm were used. For DVDs, the wavelength was reduced to 650 nm , and for Blu-ray Disc this was reduced even further to 405 nm .
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Two main servomechanisms are used, the first to maintain the proper distance between lens and disc, to ensure the laser beam is focused as a small laser spot on the disc. The second servo moves the pickup head along the disc's radius, keeping the beam on the track, a continuous spiral data path. Optical disc media are 'read' beginning at the inner radius to the outer edge.
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Near the laser lens, optical drives are usually equipped with one to three tiny potentiometers that can be turned using a fine screwdriver. The potentiometer is in a series circuit with the laser lens.
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The laser diode used in DVD writers can have powers of up to 100 milliwatts, such high powers are used during writing. Some CD players have automatic gain control to vary the power of the laser to ensure reliable playback of CD-RW discs.
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Readability may vary among optical drives due to differences in optical pickup systems, firmwares, and damage patterns.
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On factory-pressed read only media , during the manufacturing process the tracks are formed by pressing a thermoplastic resin into a nickel stamper that was made by plating a glass 'master' with raised 'bumps' on a flat surface, thus creating pits and lands in the plastic disk. 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 beam, causing mutual destructive interference and reducing the reflected beam's intensity. This is detected by photodiodes that create corresponding electrical signals.
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An optical disk recorder encodes data onto a recordable CD-R, DVD-R, DVD+R, or BD-R disc by selectively heating parts of an organic dye layer with a laser.
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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. DVD lasers operate at voltages of around 2.5 volts.
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The higher the 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.
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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 into crystalline form or left in an amorphous form, enabling marks of varying reflectivity to be created.
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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.
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Double layer or dual layer 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 writable media are produced with a spiral groove molded in the protective polycarbonate layer , to lead and synchronize the speed of recording head. Double-layered writable media have: a first polycarbonate layer with a groove, a first data layer, a semi-reflective layer, a second polycarbonate layer with another groove, and a second data layer. The first groove spiral usually starts on the inner edge and extends outwards, while the second groove start on the outer edge and extends inwards.
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Some drives support Hewlett-Packard's LightScribe, or the alternative LabelFlash photothermal printing technology for labeling specially coated discs.
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Zen Technology and Sony have developed drives that use several laser beams simultaneously to read discs and write to them at higher speeds than what would be possible with a single laser beam. The limitation with a single laser beam comes from wobbling of the disc that may occur at high rotational speeds; at 25,000 RPMs CDs become unreadable while Blu-rays cannot be written to beyond 5,000 RPMs. With a single laser beam, the only way to increase read and write speeds without reducing the pit length of the disc is by increasing the rotational speed of the disc which reads more pits in less time, increasing data rate; hence why faster drives spin the disc at higher speeds. In addition, CDs at 27,500 RPMs may explode causing extensive damage to the disc's surroundings, and poor quality or damaged discs may explode at lower speeds.
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In Zen's system , a diffraction grating is used to split a laser beam into 7 beams, which are then focused into the disc; a central beam is used for focusing and tracking the groove of the disc leaving 6 remaining beams that are spaced evenly to read 6 separate portions of the groove of the disc in parallel, effectively increasing read speeds at lower RPMs, reducing drive noise and
stress on the disc. The beams then reflect back from the disc, and are collimated and projected into a special photodiode array to be read. The first drives using the technology could read at 40x, later increasing to 52x and finally 72x. It uses a single optical pickup.
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In Sony's system the drive has 4 optical pickups, two on each side of the disc, with each pickup having two lenses for a total of 8 lenses and laser beams. This allows for both sides of the disc to be read and written to at the same time, and for the contents of the disc to be verified during writing.
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The rotational mechanism in an optical drive differs considerably from that of a hard disk drive's, in that the latter keeps a constant angular velocity , in other words a constant number of revolutions per minute . With CAV, a higher throughput is generally achievable at the outer disc compared to the inner.
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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 was wasted, a head had to transfer data at a maximum linear rate at all times too, without slowing on the outer rim of the disc. This led to optical drives—until recently—operating with a constant linear velocity . The spiral groove of the disc passed under its head at a constant speed. The implication of CLV, as opposed to CAV, is that disc angular velocity is no longer constant, and the spindle motor needed to be designed to vary its speed from between 200 RPM on the outer rim and 500 RPM on the inner, keeping the data rate constant.
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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 4× CLV drive, for instance, would rotate at 800-2000 RPM, while transferring data steadily at 600 KiB/s, which is equal to 4 × 150 KiB/s.
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For DVDs, base or 1× speed is 1.385 MB/s, equal to 1.32 MiB/s, approximately nine times faster than the CD base speed. For Blu-ray drives, base speed is 6.74 MB/s, equal to 6.43 MiB/s.
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Because keeping a constant transfer rate for the whole disc is not so important in most contemporary CD uses, a pure CLV approach had to be abandoned to keep the rotational speed of the disc safely low while maximizing data rate. Some drives work in a partial CLV 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 scheme. This divides the disc into several zones, each having its own constant linear velocity. A Z-CLV recorder rated at "52×", for example, would write at 20× on the innermost zone and then progressively increase the speed in several discrete steps up to 52× at the outer rim. Without higher rotational speeds, increased read performance may be attainable by simultaneously reading more than one point of a data groove, also known as multi-beam, but drives with such mechanisms are more expensive, less compatible, and very uncommon.
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Both DVDs and CDs have been known to explode when damaged or spun at excessive speeds. This imposes a constraint on the maximum safe speeds at which drives can operate.
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The reading speeds of most half-height optical disc drives released since c. 2007 are limited to ×48 for CDs, ×16 for DVDs and ×12 for Blu-ray Discs. Writing speeds on selected write-once media are higher.
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Some optical drives additionally throttle the reading speed based on the contents of optical discs, such as max. 40× CAV for the Digital Audio Extraction of Audio CD tracks, 16× CAV for Video CD contents and even lower limitations on earlier models such as 4× CLV for Video CDs.
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Current optical drives use either a tray-loading mechanism, where the disc is loaded onto a motorized tray , a manually operated tray , or a slot-loading mechanism, where the disc is slid into a slot and drawn in by motorized rollers. Slot-loading optical drives exist in both half-height and slim type form factors.
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With both types of mechanisms, if a CD or DVD is left in the drive after the computer is turned off, the disc cannot be ejected using the normal eject mechanism of the drive. However, tray-loading drives account for this situation by providing a small hole where one can insert a paperclip to manually open the drive tray to retrieve the disc.
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Slot-loading optical disc drives are prominently used in game consoles and vehicle audio units. Although allowing more convenient insertion, those have the disadvantages that they cannot usually accept the smaller 80 mm diameter discs or any non-standard sizes, usually have no emergency eject hole or eject button, and therefore have to be disassembled if the optical disc cannot be ejected normally. However, some slot-loading optical drives have been engineered to support miniature discs. The Nintendo Wii, because of backward compatibility with GameCube games, and PlayStation 3 video game consoles are able to load both standard size DVDs and 80 mm discs in the same slot-loading drive. Its successor's slot drive however, the Wii U, lacks miniature disc compatibility.
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There were also some early CD-ROM drives for desktop PCs in which its tray-loading mechanism will eject slightly and user has to pull out the tray manually to load a CD, similar to the tray ejecting method used in internal optical disc drives of modern laptops and modern external slim portable optical disc drives. Like the top-loading mechanism, they have spring-loaded ball bearings on the spindle.
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A small number of drive models, mostly compact portable units, have a top-loading mechanism where the drive lid is manually opened upwards and the disc is placed directly onto the spindle . 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.
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Unlike tray and slot loading mechanisms by default, top-load optical drives can be opened without being connected to power.
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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 inch micro 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. Ultra Density Optical , Magneto-optical drives, Universal Media Disc , DataPlay, Professional Disc, MiniDisc, Optical Disc Archive as well as early DVD-RAM and Blu-ray discs use optical disc cartridges.
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All optical disc-drives use the SCSI-protocol on a command bus level, and initial systems used either a fully featured SCSI bus or as these were somewhat cost-prohibitive to sell to consumer applications, a proprietary cost-reduced version of the bus. This is because conventional ATA-standards at the time did not support, or have any provisions for any sort of removable media or hot-plugging of disk drives. Most modern internal drives for personal computers, servers, and workstations are designed to fit in a standard 5+1⁄4-inch drive bay and connect to their host via an ATA or SATA bus interface, but communicate using the SCSI protocol commands on software level as per the ATA Package Interface standard developed for making Parallel ATA/IDE interfaces compatible with removable media. Some devices may support vendor-specific commands such as recording density , laser power setting , ability to manually hard-limit rotation speed in a way that overrides the universal speed setting , and adjusting the lens and tray movement speeds where a lower setting reduces noise, as implmenented on some Plextor drives, as well as the ability to force overspeed burning, meaning beyond speed recommended for the media type, for testing purposes, as implemented on some Lite-On drives. Additionally, there may be digital and analog outputs for audio. The outputs may be connected via a header cable to the sound card or the motherboard or to headphones or an external speaker with a 3.5mm AUX plug cable that many early optical drives are equipped with. At one time, computer software resembling CD players controlled playback of the CD. Today the information is extracted from the disc as digital data, to be played back or converted to other file formats.
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