Machine Design

Putting Monitor Specs on the View Screen

Video monitor resolution is often quoted in terms of pixels and raster lines. For instance, if resolution is 1,024 x 748, 1,024 is the number of pixels per horizontal raster line, while 728 is the number of lines. A problem with describing resolution this way is that it does not take screen size into account. A 1,024 x 728 image on a 19-in. screen is less clear than the same image on a 13-in. screen.

Alternatively, some manufacturers quote screen density in dots per inch (dpi). Vertical and horizontal pixel density usually is equal in a display, so dpi is specified as a single number. Most PC screens have dpi resolutions of between 40 and 72, although some high-resolution monochrome monitors go as high as 300 dpi.

Scan frequency, video data rate, and monitor bandwidth together also affect resolution. The horizontal and vertical scan frequencies determine the number of horizontal raster lines per screen. Video data rate is the speed, in megahertz, at which the graphics controller provides pixels. It determines the number of pixels per scan line. Monitor bandwidth describes the rate, in megahertz, at which the video amplifier can accept pixels to produce the required resolution.

Screen flicker occurs when the screen phosphors are not rewritten fast enough, so that the images fades considerably before it is refreshed. Refresh rate and whether screens are interlaced or noninterlaced affect whether the operator sees flicker.

On interlaced displays, only half the raster lines refresh every time the screen is updated. Noninterlaced displays refresh all raster lines every update. Flicker may be more pronounced when static images display on interlaced screens. Most monitors today are noninterlaced, although some multiscan monitors provide interlaced displays in their high-resolution modes. Multiscan monitors have adjustable scan frequencies.

On noninterlaced displays larger than 13 in., a refresh rate greater than the typical 60-Hz rate may be needed to avoid flicker. Larger computer screens place more image in peripheral vision, which is sensitive to flicker. Ergonometric studies show that most users can detect flicker when using a 19-in. screen refreshed at 60 Hz. When refresh rate rises to 70 Hz, however, less than 10% of users notice flicker on the same size screen.

Dot pitch is the distance between the triads that make up each pixel in a color display. Color triads are groupings of red, green, and blue phosphor dots that are turned on and off to form the colors in a display. According to experts, the best dot pitch for a high-resolution screen is 0.21 mm, while dot pitch greater than 0.36 mm is considered inferior.

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