Field Rate - Cathode Ray Tubes

Cathode Ray Tubes

In a CRT, the scan rate is controlled by the vertical blanking signal generated by the video controller, ordering the monitor to position the beam at the upper left corner of the raster, ready to paint another frame. It is limited by the monitor's maximum horizontal scan rate and the resolution, since higher resolution means more scan lines.

The refresh rate can be calculated from the horizontal scan rate by dividing the scanning frequency by the number of horizontal lines multiplied by 1.05 (since about 5% of the time it takes to scan the screen is spent moving the electron beam back to the top). For instance, a monitor with a horizontal scanning frequency of 96 kHz at a resolution of 1280 × 1024 results in a refresh rate of 96,000 / (1024 × 1.05) ≈ 89 Hz (rounded down).

CRT refresh rates have historically been an important factor in electronic game programming. Traditionally, one of the principles of video/computer game programming is to avoid altering the computer's video buffer except during the vertical retrace. This is necessary to prevent flickery graphics (caused by altering the picture in mid-frame) or screen tearing (caused by altering the graphics faster than the electron beam can render the picture). Some video game consoles such as the Famicom/NES did not allow any graphics changes except during the retrace (the period when the electron guns shut off and return to the upper left corner of the screen).

While flicker does not occur on LCD displays, it is still necessary to avoid modifying graphics data except during the retrace phase to prevent tearing from an image that is rendered faster than the display operates (LCDs normally always refresh at 60 frame/s).

CRTs have the unique ability to use light guns and pens. These are devices with a photosensor that detects the electron beam and sends a signal to the attached computer. This can be used to determine if a specific graphics object is on the screen. The light gun is a larger variant used in arcade games and some consoles. Unlike light pens, they are held at a distance from the screen.

Light pens and guns cannot be used on fixed-pixel displays because they have no electron beam to detect. Pen tablets and touchscreen LCDs are used as a substitute for them, but the latter require a specially-designed LCD panel and are mostly only found in point-of-service monitors. The Nintendo DS is an example of a video game system that has a touchscreen LCD.