Multisync monitor: Difference between revisions

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Computers that did not use ordinary televisions 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, fixed scan-rate IBM monitors.

Released in 1984, IBM's [[Enhanced Graphics Adapter|EGA]] required the use of a dual-sync monitor which supported the scan rate of CGA as well as a second scan rate for EGA's new video modes. The monitor detected the current mode from the polarity of the vertical sync signal sent by the video adapter.<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>

The first multisync monitor - the NEC Multisync - was released in 1985 to provide support for CGA, EGA, the various extended forms of those standards marketed by third party vendors, and standards yet to be released,.<ref name=":1">{{Cite book|last=Inc|first=InfoWorld Media Group|url=https://books.google.com/books?id=mTwEAAAAMBAJ&lpg=PA61&dq=multisync&pg=PA61#v=onepage&q&f=false|title=InfoWorld|date=1986-10-27|publisher=InfoWorld Media Group, Inc.|language=en}}</ref> by identifying the scan rate from the received video signal itself.<ref>{{Cite web|title=PC Mag 1987-03-31 : Free Download, Borrow, and Streaming|url=https://archive.org/details/PC-Mag-1987-03-31|access-date=2020-08-16|website=Internet Archive|language=en}}</ref>

IBM's 1987 [[Video Graphics Array|VGA]] standard, in turn, expanded to three fixed scan rates, requiring another new monitor which decoded the scan rate from the polarity of both the horizontal and vertical sync pulses,<ref>{{Cite book|last=|first=|url=http://classiccomputers.info/down/IBM_PS2/documents/PS2_Hardware_Interface_Technical_Reference_May88.pdf|title=IBM PS/2 Hardware Interface Technical Reference|publisher=|year=|isbn=|___location=|pages=13-100}}</ref> but soon VGA began to be extended by third-party vendors and [[Video Electronics Standards Association|VESA]] into [[Super VGA]], andwhich theimplemented numberboth of possiblehigher resolutions aand graphicsdifferent card[[Refresh couldrate|refresh outputrates]] beganfor to increase beyond what thisa approachsingle couldgiven communicateresolution.

By the late 1990s, monitors were available with specs ranging from 1024x768 at 60 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 the [[Power-on self-test|POST]] display on their host systems would operate at the standard VGA mode of 320x200 at 70Hz, so a monitor of this type would need to be able to horizontally scan in a range from at least 31 to 68kHz.

VESA established a standardized list of display resolutions and accompanying timing for manufacturers to use.

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=1993-07|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.

A typical screen resolution of the late 1990s was 1024x768 at 85&nbsp;Hz, requiring a horizontal scan rate over 68&nbsp;kHz, yet during system boot the [[Power-on self-test|POST]] display and [[operating system]] splash screen would be displayed at the standard VGA 31&nbsp;kHz. Many [[MS-DOS]] and [[Windows]] computer games of the time would also switch to a lower resolution for greater compatibility, more colours, improved performance or to reduce the [[video memory]] required by the [[frame buffer]].

== Implementation ==

DependingEarly onmultisync the design of the monitor, there may be several discrete frequencies supported. For example, a monitormonitors designed for use with CGA, / EGA and/ VGA standards couldsupported electlimited tofixed supportfrequencies, e.g. 15.7&nbsp;kHz, 21.8&nbsp;kHz and 31.5&nbsp;kHz horizontal scan rates. Alternatively, thedetected monitorvia maythe bepolarity designedof to support a continuous range from 15&nbsp;kHz to 31&nbsp;kHzone or moreboth (withH- theand originalV-sync NECsignals Multisync,sent giving its name toby the type,video supporting 15 to 35&nbsp;kHz and, at least unofficially, being just-about capable of SVGA and XGA)adapter. <!--ref linksname=":0" to/> product manuals for old multisyncs would be useful here, or usenet -->

Later designs supported a continuous range of scan frequencies, such as the original NEC Multisync which supported horizontal scan rates from 15&nbsp;to 31&nbsp;kHz<ref name=":1" /> derived from the sync signal timing rather than the polarity of the sync signals.<ref>{{Cite web|title=PC Mag 1987-03-31 : Free Download, Borrow, and Streaming|url=https://archive.org/details/PC-Mag-1987-03-31|access-date=2020-08-16|website=Internet Archive|language=en}}</ref>

A multiscan monitor does not need to support all possible video formats. Most modern multiscan computer monitors support a nearly-continuous range of horizontal scan frequencies from 31&nbsp;kHz up to 100&nbsp;kHz+, omitting support for the older 15&nbsp;kHz through 29&nbsp;kHz standards.<ref>{{cite web|title=comp.sys.amiga.misc usenet post circa 1992|url=https://groups.google.com/forum/#!search/Multisync/comp.sys.amiga.misc/ktCKO1yqiaY/WB2ky14SDEYJ}}</ref><ref>{{cite web|title=comp.sys.acorn.hardware usenet post circa 2010|url=https://groups.google.com/forum/#!search/Multisync/comp.sys.acorn.hardware/ZFi8regDZeo/gEGWCHGWLm8J}}</ref>

Modern monitors produced using the VESA frequency standards generally support arbitrary scan rates up to a specific maximum horizontal and vertical rate. Most modern multiscan computer monitors have a minimum horizontal scan frequency of 31kHz.<ref>{{Cite web|title=Converters {{!}} RetroRGB|url=https://www.retrorgb.com/converters.html|access-date=2020-08-16|language=en-US}}</ref>

Fixed-frequency [[Cathode ray tube|CRT]] monitors, and multiscan CRT monitors that only support a set of frequencies, may upon receiving scan frequencies outside design limits cause damage to the monitor. This is especially true for horizontal scanrate, which in CRT monitors is associated with higher voltages and power levels. A resonant circuit is commonly employed, restricting the allowable horizontal scan rates to one or more very narrow ranges. Not all fixed-frequency monitors are vulnerable to damage, but it is never safe to assume this. Most modern multiscan monitors are [[microprocessor]] controlled and will refuse to attempt to synchronise to an unsupported scan rate, which usually protects them from damage.

In both multisync and fixed-sync monitors, timing is important to prevent image distortion and even damage to components.<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=Sync signals for displays drastically affect the quality, performance and even reliability of CRT displays. Even small differences in timing parameters can significantly affect image position and size, causing problems for the user. Difference in blanking times can lead to excessive power dissipation and electrical stress in the scanning circuits, or at the other extreme, incomplete or distorted images being displayed.}}</ref> Most modern multiscan monitors are [[microprocessor]] controlled<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=In order to identify the mode, most present day multiple frequency monitors use a simple microcontroller to measure syncs.}}</ref> and will refuse to attempt to synchronise to an unsupported scan rate, which usually protects them from damage.

It is important to remember that when the "refresh rate" setting in a computer operating system is increased, both the horizontal and vertical scan rates increase together. Even although monitors are relatively tolerant of a range of vertical refresh rates, it may take the horizontal scan rate out of range and potentially cause damage.

<!-- In a fixed frequency monitor, even a little bit off horizontal frequency can cause a significant current increase, particularly if the frequency is too low. The range of faults are far wider than burned out transformers as the previous version of this article suggested. The flyback, yoke or power transformer could burn out, but other faults can occur instead or in addition to transformer faults, for example a blown HOT, damaged caused by out-of-range power supply voltages, and arcing damage to the tube or flyback assembly from too high EHT or focus voltages. -->

== Non-CRT monitors ==

The terms "multisync" or "multiscan" do not apply to [[LCD]] monitors in the same way they do to [[Cathode ray tube|CRT]] monitors. LCD monitors are [[Fixed pixel display|fixed-pixel monitors]]. For compatibility with boot screens and legacy software, stand-alone LCD monitors are usually required to support VGA scan rates in addition to their native resolution. In order to support the lower resolutions, a circuit is used to convert the incoming signal to the monitor's native resolution. The resulting image either appears in a small 1:1 pixel mapped window, or more commonly is stretched to fill the screen. In the latter case, the circuitry is referred to as a [[Video scaler|scaler]]. <!-- scaler article has all the citations -->

The multisync concept applies to non-CRT monitors, such as [[LCD|LCDs]], but is implemented differently.

LCD monitors are [[Fixed pixel display|fixed-pixel displays]], where the number of rows and columns displayed on the screen are constant, set by the construction of the panel. When the input signal has a resolution that does not match the number of pixels in the display, the LCD controller must still populate the same number of image elements.

While stand-alone LCD monitors generally accept a wide range of horizontal scan rates, the same is not true of the vertical scan rate. The vast majority of LCDs accept only 60&nbsp;Hz to 75&nbsp;Hz vertical scan rates. In recent years, LCD monitors designed for gaming have appeared on the market offering vertical scan rates of 120&nbsp;Hz and up.<ref>{{cite web|title=List of 120Hz monitors{{Snd}} Includes 144Hz, 240Hz Blur Busters|url=http://www.blurbusters.com/faq/120hz-monitors/}}</ref> These monitors are referred to by the term "120&nbsp;Hz" (or the applicable refresh rate) rather than "multiscan."<!-- Any others? -->

This is accomplished either by [[Video scaler|scaling]] the image up or down as needed, creating a picture that does not have a 1:1 relationship between LCD image elements and pixels in the original image, or by displaying the image unscaled in the center of the monitor, filling the spaces on all sides with black pixels.

While stand-alone LCD monitors generally accept a wide range of horizontal scan rates, the same is not true of the vertical scan rate. The vast majority of LCDs accept only 60&nbsp;Hz to 75&nbsp;Hz vertical scan rates. In recent years, LCD monitors designed for gaming have appeared on the market offering vertical scan rates of 120&nbsp;Hz and up.<ref>{{cite web|title=List of 120Hz monitors{{Snd}} Includes 144Hz, 240Hz Blur Busters|url=http://www.blurbusters.com/faq/120hz-monitors/}}</ref> These monitors are usually referred to by thetheir termspecific "120&nbsp;Hz" (or the applicablemax refresh rate) rather than "multiscan."<!-- Any others? -->

== References ==