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Multisync monitor

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A multiple-sync (multisync) monitor, also known as a multiscan or multimode monitor, is a raster-scan video monitor, generally used for computer displays, that can properly synchronise with multiple horizontal and vertical scan rates.[1][2] In contrast, fixed frequency monitors can only synchronise with a specific set of scan rates.

Multiscan monitors appeared during the late 1980s,[3] offering flexibility as computers moved from earlier standards such as composite video and CGA, which utilized a single set refresh rate, to standards implementing multiple scan rates such as EGA, VGA and SVGA.

The term "MultiSync" derives from an NEC trademark for one of the first multiple-sync monitors. Sond uses the terms "Multiscan" and "MultiFormat" instead, and Samsung uses the "SyncMaster" trademark.

History

Early home computers output video to ordinary televisions or composite monitors, 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.

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

Released in 1984, IBM's EGA required the use of a monitor which supported the scan rate of CGA as well as a second scan rate for EGA's new video modes.[4] This monitor as well as others that could be manually switched between these two sync rates were known as dual-scan displays.[5]

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.[6]

IBM's 1987 VGA standard, in turn, expanded to three fixed scan rates, and in all, by the late 80s all of the below standards required unique fixed-frequency monitors:

  1. PAL, NTSC, CGA: ~15.7 kHz horizontal scan, 50 or 60 Hz vertical scan
  2. EGA: 15.7 kHz or 21.8 kHz horizontal scan, 60 Hz vertical scan (plus CGA modes)
  3. VGA: 31.5 kHz horizontal scan, 60 or 70 Hz vertical scan (plus EGA/CGA modes)
  4. XGA: 35.5 kHz horizontal scan, 87 Hz (43.5 Hz interlaced) vertical scan (plus VGA modes)

Soon after VGA's release, it began to be extended by third-party vendors and VESA, developing into Super VGA, which implemented both higher resolutions and different refresh rates for a single given resolution.

By the late 1990s, monitors were available with specs ranging from 1024x768 at 60 Hz, to at least 1600x1200 at 85 Hz.[7] In addition to these higher resolutions and frequencies, during system boot the 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.

In response, VESA established a standardized list of display resolutions, refresh rates, and accompanying timing for hardware manufacturers.[8] 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,[9] and this in turn was superseded by VESA's Coordinated Video Timings standard.

Implementation

Early multisync monitors designed for use with CGA / EGA / VGA standards supported limited fixed frequencies, e.g. 15.7 kHz, 21.8 kHz and 31.5 kHz horizontal rates, detected via the polarity of one or both H- and V-sync signals sent by the video adapter.[4]

Later designs supported a continuous range of scan frequencies, such as the original NEC Multisync which supported horizontal scan rates from 15 to 31 kHz[6] derived from the sync signal timing rather than the polarity of the sync signals.[10]

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.[11]

In both multisync and fixed-sync monitors, timing is important to prevent image distortion and even damage to components.[12] Most modern multiscan monitors are microprocessor controlled[13] 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 LCDs, 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 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.[14] These monitors are usually referred to by their specific max refresh rate.

References

  1. ^ "13 What's the difference between fixed frequency and multisynchronous monitors?". 070808 stason.org
  2. ^ "Standards FAQ". VESA - Interface Standards for The Display Industry. Retrieved 2020-08-16. Multimode monitors can measure the incoming sync signal frequencies and thus sync to any frequency within their range of operation.{{cite web}}: CS1 maint: url-status (link)
  3. ^ "MultiSync 25th Anniversary – The Evolution of the MultiSync".
  4. ^ a b IBM Enhanced Color Display Manual (PDF). p. 1.
  5. ^ Inc, InfoWorld Media Group (1988-08-22). InfoWorld. InfoWorld Media Group, Inc. {{cite book}}: |last= has generic name (help)
  6. ^ a b Inc, InfoWorld Media Group (1986-10-27). InfoWorld. InfoWorld Media Group, Inc. {{cite book}}: |last= has generic name (help)
  7. ^ Inc, InfoWorld Media Group (1997-12-15). InfoWorld. InfoWorld Media Group, Inc. {{cite book}}: |last= has generic name (help)
  8. ^ Inc, Ziff Davis (1993-07). PC Mag. Ziff Davis, Inc. {{cite book}}: |last= has generic name (help); Check date values in: |date= (help)
  9. ^ "Standards FAQ". VESA - Interface Standards for The Display Industry. Retrieved 2020-08-16. 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.{{cite web}}: CS1 maint: url-status (link)
  10. ^ "PC Mag 1987-03-31 : Free Download, Borrow, and Streaming". Internet Archive. Retrieved 2020-08-16.
  11. ^ "Converters | RetroRGB". Retrieved 2020-08-16.
  12. ^ "Standards FAQ". VESA - Interface Standards for The Display Industry. Retrieved 2020-08-16. 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.{{cite web}}: CS1 maint: url-status (link)
  13. ^ "Standards FAQ". VESA - Interface Standards for The Display Industry. Retrieved 2020-08-16. In order to identify the mode, most present day multiple frequency monitors use a simple microcontroller to measure syncs.{{cite web}}: CS1 maint: url-status (link)
  14. ^ "List of 120Hz monitors – Includes 144Hz, 240Hz Blur Busters".
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