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A multiple-sync (multisync) monitor, also known as a multiscan or multimode monitor, is a raster-scan analog video
monitor Monitor or monitor may refer to: Places * Monitor, Alberta * Monitor, Indiana, town in the United States * Monitor, Kentucky * Monitor, Oregon, unincorporated community in the United States * Monitor, Washington * Monitor, Logan County, Wes ...
that can properly synchronise with multiple horizontal and vertical scan rates. In contrast, fixed frequency monitors can only synchronise with a specific set of scan rates. They are generally used for computer displays, but sometimes for television, and the terminology is mostly applied to CRT displays although the concept applies to other technologies. Multiscan computer monitors appeared during the mid 1980s, offering flexibility as computer video hardware shifted from producing a single fixed scan rate to multiple possible scan rates. "MultiSync" specifically was a trademark of one of NEC's first multiple-sync monitors.


Computers


History

Early
home computer Home computers were a class of microcomputers that entered the market in 1977 and became common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a s ...
s output video to ordinary televisions or
composite monitor A composite monitor or composite video monitor is any analog video display that receives input in the form of an analog composite video signal to a defined specification. A composite video signal encodes all information on a single conductor; a ...
s, utilizing television display standards such as
NTSC NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...
,
PAL Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
or
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''sequential colour memory''), is an analog color television system that was used in France, Russia and some other countries or territories of Europe and Africa. ...
. 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 A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a electronic visual display, visual display, support electronics, power supply, Housing (engineering), housing, electri ...
s still often relied on fixed scan rates. IBM's original 1981 PC, for instance, was sold with a choice of two video cards ( MDA and 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 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. This monitor as well as others that could be manually switched between these two sync rates were known as dual-scan displays. The NEC Multisync was released in 1985 for use with the IBM PC, supporting a wide range of sync frequencies including those for CGA, EGA, various extended forms of those standards marketed by third party vendors, and standards yet to be released. IBM's 1987
VGA Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the IBM PC compatible industry within three years. T ...
standard, in turn, expanded to three fixed scan rates. At this point, PC and Mac owners with multiple graphics cards required unique monitors for each of them, and by the late 80s all of the below computer video standards required monitors which supported a small number of specific frequencies: #
PAL Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
,
NTSC NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...
, CGA: ~15.7 kHz horizontal scan, 50 or 60 Hz vertical scan # EGA: 15.7 kHz (CGA compatible mode) or 21.8 kHz horizontal scan, 60 Hz vertical scan #
VGA Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the IBM PC compatible industry within three years. T ...
: 31.5 kHz horizontal scan, 60 or 70 Hz vertical scan. No support for CGA/EGA timings. CGA/EGA resolutions are transmitted to the monitor at VGA compatible timings. #
XGA The eXtended Graphics Array (usually called XGA) is a graphics card manufactured by IBM and introduced for the IBM PS/2 line of personal computers in 1990 as a successor to the IBM 8514, 8514/A. It supports, among other modes, a display resol ...
: 35.5 kHz horizontal scan, 87 Hz (43.5 Hz interlaced) vertical scan (plus VGA modes) # Many different display formats for Macintosh, Sun, NeXT, and other microcomputers After 1987's
VGA Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the IBM PC compatible industry within three years. T ...
. the IBM market began to develop
Super VGA Super VGA (SVGA) or Extended VGA is a broad term that covers a wide range of computer display standards that extended IBM's VGA specification. When used as shorthand for a resolution, as VGA and XGA often are, SVGA refers to a resolution of 800& ...
cards which used many different scan rates, culminating in the 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 Hz, to at least 1600x1200 at 85 Hz. 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 720x400 at 70 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 kHz. In response, VESA established a standardized list of display resolutions, refresh rates, and accompanying timing for hardware manufacturers. 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, and this in turn was superseded by VESA's Coordinated Video Timings standard.


Implementation

Early multisync monitors designed for use with systems having a small number of specific frequencies, like CGA, EGA and VGA, or built-in Macintosh graphics, supported limited fixed frequencies. On the IBM PC, these were signaled from the graphics card to the monitor through the polarities of one or both H- and V-sync signals sent by the video adapter. Later designs supported a continuous range of scan frequencies, such as the NEC Multisync which supported horizontal scan rates from 15 to 31 kHz derived from the sync signal timing rather than the polarity of the sync signals. Displays like these could be used on multiple platforms and video cards as long as the frequencies were within range. Modern monitors produced using the VESA frequency standards generally support arbitrary scan rates between specific minimum and maximum horizontal and vertical rates. Most modern multiscan computer monitors have a minimum horizontal scan frequency of 31 kHz. In both multisync and fixed-sync monitors, timing is important to prevent image distortion and even damage to components. Most modern multiscan monitors are
microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...
controlled and will refuse to attempt to synchronise to an unsupported scan rate, which usually protects them from damage.


Non-CRT monitors

The multisync concept applies to non-CRT monitors, such as
LCD A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers to display information. Liquid crystals do not em ...
s, but is implemented differently. LCD monitors are 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. This is accomplished either by
scaling Scaling may refer to: Science and technology Mathematics and physics * Scaling (geometry), a linear transformation that enlarges or diminishes objects * Scale invariance, a feature of objects or laws that do not change if scales of length, energ ...
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 majority of LCDs accept only 60 Hz to 75 Hz vertical scan rates. In recent years, LCD monitors designed for gaming have appeared on the market offering vertical scan rates of 120 Hz and up. These monitors are usually referred to by their specific max refresh rate.


Television

CRT televisions are typically designed to operate only with the video standard of the country they are sold in (
PAL Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
,
NTSC NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...
,
SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''sequential colour memory''), is an analog color television system that was used in France, Russia and some other countries or territories of Europe and Africa. ...
), but some sets, particularly broadcast monitors, can operate on multiple standards.


References

{{reflist


External links


Multiscanning monitor
webopedia. Electronic display devices Graphics hardware