Multisync monitor

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. In contrast, fixed frequency monitors can only synchronise with a specific set of scan rates.

Multiscan monitors appeared during the late 1980s, 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 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.

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, by identifying the scan rate from the received video signal itself.

IBM's 1987 VGA standard, in turn, expanded to three scan rates, requiring another new monitor which decoded the scan rate from the polarity of both the horizontal and vertical sync pulses, but soon VGA began to be extended by third-party vendors and VESA into Super VGA, and the number of possible resolutions a graphics card could output began to increase beyond what this approach could communicate.

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

A typical screen resolution of the late 1990s was 1024x768 at 85 Hz, requiring a horizontal scan rate over 68 kHz, yet during system boot the POST display and operating system splash screen would be displayed at the standard VGA 31 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

Depending on the design of the monitor, there may be several discrete frequencies supported. For example, a monitor designed for use with CGA, EGA and VGA standards could elect to support 15.7 kHz, 21.8 kHz and 31.5 kHz horizontal scan rates. Alternatively, the monitor may be designed to support a continuous range from 15 kHz to 31 kHz or more (with the original NEC Multisync, giving its name to the type, supporting 15 to 35 kHz and, at least unofficially, being just-about capable of SVGA and XGA).

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 kHz up to 100 kHz+, omitting support for the older 15 kHz through 29 kHz standards.

Fixed-frequency 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.

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.

The terms "multisync" or "multiscan" do not apply to LCD monitors in the same way they do to CRT monitors. LCD monitors are 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 scaler.

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 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 referred to by the term "120 Hz" (or the applicable refresh rate) rather than "multiscan."

External links

• Multiscanning monitor webopedia.