Multisync monitor: Difference between revisions

Revision as of 20:48, 22 October 2020

Revision as of 07:44, 15 December 2020

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Early [[home computer]]s output video to ordinary televisions or [[composite monitor]]s, utilizing television display standards such as [[NTSC]], [[PAL]] or [[SECAM]]. These display standards had fixed scan rates, and only used the vertical and horizontal sync pulses embedded in the video signals to ensure synchronization, not to set the actual scan rates.

Early dedicated [[Computer monitor|computer monitors]] still often relied on fixed scan rates. IBM's original 1981 [[IBM Personal Computer|PC]], for instance, was sold with a choice of two video cards ([[IBM Monochrome Display Adapter|MDA]] and [[Color Graphics Adapter|CGA]]) which were intended for use with custom IBM monitors which still used fixed scan rates,. The CGA timings were identical to NTSC television, whereas the MDA card used a custom timing for higher resolution to provide better text quality. Early Macintosh monitors also used fixed scan rates.

In 1984, IBM's [[Enhanced Graphics Adapter|EGA]] added a second resolution which necessitated the use of a monitor supporting two scan rates, the original CGA rate as well as a second scan rate for the new video modes.<ref name=":0">{{Cite book|last=|first=|url=http://minuszerodegrees.net/oa/OA%20-%20IBM%20Enhanced%20Color%20Display%20(5154).pdf|title=IBM Enhanced Color Display Manual|publisher=|year=|isbn=|___location=|pages=1}}</ref> This monitor as well as others that could be manually switched between these two sync rates were known as dual-scan displays.<ref>{{Cite book|last=Inc|first=InfoWorld Media Group|url=https://books.google.com/books?id=YToEAAAAMBAJ&lpg=PA39&dq=dual-sync%20EGA&pg=PA39#v=onepage&q&f=false|title=InfoWorld|date=1988-08-22|publisher=InfoWorld Media Group, Inc.|language=en}}</ref>

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# [[PAL]], [[NTSC]], [[Color Graphics Adapter|CGA]]: ~15.7&nbsp;kHz horizontal scan, 50 or 60&nbsp;Hz vertical scan

# [[Enhanced Graphics Adapter|EGA]]: 15.7&nbsp;kHz (CGA compatible mode) or 21.8&nbsp;kHz horizontal scan, 60&nbsp;Hz vertical scan (plus CGA modes)

# [[VGA]]: 31.5&nbsp;kHz horizontal scan, 60 or 70&nbsp;Hz vertical scan. (plusNo EGAsupport for CGA/EGA timings. CGA/EGA resolutions are transmitted to the monitor at VGA compatible modes)timings.

# [[XGA]]: 35.5&nbsp;kHz horizontal scan, 87&nbsp;Hz (43.5&nbsp;Hz interlaced) vertical scan (plus VGA modes)

# Many different display formats for Macintosh, Sun, NeXT, and other microcomputers

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After 1987's [[Video Graphics Array|VGA]]. the IBM market began to develop [[Super VGA]] cards which used many different scan rates, culminating in the [[VESA BIOS Extensions|VBE]] which established standardized methods for outputting many different resolutions from one card, eventually becoming the [[Generalized Timing Formula]] which permitted graphics cards to output arbitrary resolutions.

By the late 1990s, graphics cards for microcomputers were available with specs ranging from 1024x768 at 60&nbsp;Hz, to at least 1600x1200 at 85&nbsp;Hz.<ref>{{Cite book|last=Inc|first=InfoWorld Media Group|url=https://books.google.com/books?id=DDwEAAAAMBAJ&lpg=PA14-IA1&dq=infoworld%2085hz&pg=PA14-IA1#v=onepage&q&f=false|title=InfoWorld|date=1997-12-15|publisher=InfoWorld Media Group, Inc.|language=en}}</ref> In addition to these higher resolutions and frequencies, during system boot on systems like the IBM PC, the display would operate at standard low resolution, such as the PC standard of 320x200720x400 at 70&nbsp;Hz. A monitor capable of displaying at both resolutions would need to be able to horizontally scan in a range from at least 31 to 68&nbsp;kHz.

In response, VESA established a standardized list of display resolutions, refresh rates, and accompanying timing for hardware manufacturers.<ref>{{Cite book|last=Inc|first=Ziff Davis|url=https://books.google.com/books?id=eX8w8B-OhIIC&lpg=PA151&dq=vesa%20display%20monitor%20timings&pg=PA177#v=onepage&q&f=false|title=PC Mag|date=July 1993|publisher=Ziff Davis, Inc.|language=en}}</ref> This was superseded by VESA's [[Generalized Timing Formula]], which provided a standard method to derive the timing of an arbitrary display mode from its sync pulses,<ref>{{Cite web|last=|first=|date=|title=Standards FAQ|url=https://vesa.org/vesa-standards/standards-faq/|url-status=live|archive-url=|archive-date=|access-date=2020-08-16|website=VESA - Interface Standards for The Display Industry|language=en-US|quote=Q: How will GTF help the monitor automatically set itself to any timing format? / A: GTF defines the relationship between syncs and video signals at any frequency of operation. The display can measure the incoming sync frequency, and thus can predict where the image will start and finish, even though it may not have been preset at that operating point.}}</ref> and this in turn was superseded by VESA's [[Coordinated Video Timings]] standard.