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Tektronix 4010
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==Principles of operation== [[File:Tektronics 4010 US map.jpg|thumb|A map of the contiguous United States on the Tektronix 4010. The typewritten note at the bottom of the display bezel warns against [[screen burn-in]]]] Conventional modern [[video]] displays consist of a series of images, or ''frames'', representing single snapshots in time. When the frames are updated rapidly enough, changes in those images provide the illusion of continuous motion.<ref>{{cite journal | url=http://vision.arc.nasa.gov/publications/TemporalSensitivity.pdf | title=Temporal Sensitivity | first=Andrew | last=Watson | year=1986 | journal=Sensory Processes and Perception | url-status=dead | archiveurl=https://web.archive.org/web/20160308001647/http://vision.arc.nasa.gov/publications/TemporalSensitivity.pdf | archivedate=2016-03-08 }}</ref> Computer displays, where the image is generally static for extended periods of time (for example, a page of text), required a stationary, more-precise, flicker-free image, as compared to television displays available at the time. A modern solution is to use additional hardware and [[computer memory]] to store the image between each update, a section of memory known as a [[framebuffer]].<ref name="Shoup_SuperPaint">{{cite web |url=http://accad.osu.edu/~waynec/history/PDFs/Annals_final.pdf |title=SuperPaint: An Early Frame Buffer Graphics System |author=Richard Shoup |publisher=IEEE Annals of the History of Computing |year=2001 |url-status=dead |archiveurl=https://web.archive.org/web/20040612215245/http://accad.osu.edu/~waynec/history/PDFs/Annals_final.pdf |archivedate=2004-06-12 }}</ref> In the 1960s, memory based on [[core memory|core]] was extremely expensive, generally priced in dollars or cents per bit. Solid-state memory was even more expensive, and could only be used for a handful of high-speed working storage registers in data processing hardware. If one wanted to store a screen of text at 80 columns by 25 lines and using 7-bit [[ASCII]], one would require 80{{times}}25{{times}}{{val|7|u=bits}} = {{val|14000 |u=bits}}, making the price of the terminal prohibitive. The cost would be even more if the terminal needed to display graphics. For instance, a graphics terminal supporting 1-bit points (on/off) at {{val|1024 |by| 768}} resolution would require 1024{{times}}768{{times}}{{val|1 |u=bit}} = {{val|786432 |u=bits}} of memory, likely more than the cost of the computer it connected to. One solution to reduce the amount of memory required was to represent the image not as dots, but straight-line "vectors". In this case, only the endpoints have to be stored in memory, and additional hardware draws between them to produce the display. A coordinate within that same 1,024 resolution space requires {{val|10|u=bits}} (2<sup>10</sup>), so if a display can hold 1000 vectors in total, it requires {{nowrap|1000 vectors Γ 2 ends Γ 2 coordinates}} per end {{nowrap|(X and Y) Γ {{val|10|u=bits}}}} = {{val|40000|u=bits}}. The [[IBM 2250]] graphics terminal used this solution, and sold for {{US$|280000|1970|long=no}}.<ref>[http://bitsavers.informatik.uni-stuttgart.de/topic/graphics/ComputerDisplayReview_Mar70.pdf "Computer Display Review"], Keydata Corp., March 1970, pp. V.1980, V.1964</ref> Tektronix had originally developed their storage tubes in the late 1950s as a way to store images on oscilloscope displays for study, although the same system had already been used in [[radar]] displays. The basic concept used a conventional CRT layout, but with two sets of [[electron gun]]s. One, the ''flood gun'', provided a constant flow of low-energy electrons covering the entire screen, causing it to glow faintly. The second source, the ''write gun'', resembled the normal gun of a black and white TV, and its beam was moved about on the display surface in the conventional fashion by using electromagnetic coils.<ref name="tek3"/> However, this write gun was set to a higher than normal energy. When its beam struck the screen, it caused an effect known as [[photoemission]] that expelled electrons from the light-emitting [[phosphor]]s and towards the front of the display, where they were drained away by a thin transparent electrode. The "written" patch of the phosphor now contained fewer electrons than normal, giving it a positive charge relative to its surroundings. This caused more of the electrons from the flood gun to be continuously attracted to that spot, which kept it emitting an intermediate intensity of light. Thus, a complex image could be "stored" in the same light-emitting phosphors which made the image visible to the user.<ref name="tek3"/>
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