Display Resolution

We have recently examined television display technologies old and new; now we'll look at the spatial resolution capabilities of the display technologies that are commonly used in television and video today.
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We have recently examined television display technologies old and new; now we'll look at the spatial resolution capabilities of the display technologies that are commonly used in television and video today.

Although static spatial resolution receives most of the attention in advertising and marketing, dynamic spatial resolution can be affected by the type of display technology used. For example, an inherent characteristic of liquid crystal devices is slow response time, which can manifest itself as "lag," or the smearing of moving images, while color wheels or other strobing techniques can cause edge contouring and other disturbing artifacts.

It must quickly be pointed out that these shortcomings have been overcome to a significant degree in high-quality contemporary displays. Resolution, in the context we will use it, is the ability to render distinguishable the individual parts of an image.

CRT RESOLUTION

Cathode ray tubes are completely analog devices, and unlike other display technologies we have discussed, they do not have discretely ad-dressed pixels that de-fine a native resolution. A color CRT has a matrix of red, green and blue phosphor dots, while a monochrome CRT has a continuous coating of a single [nom-inally] white phosphor over the entire screen. The resolution capability of a given color CRT depends on a number of factors-some of the important ones being the dot pitch (the distance between dot centers, which effectively defines the size of the dots themselves), how tightly the electron beam is focused and the electron beam's scanning speed. A phosphor dot is not a discrete pixel; the sweeping electron beam does not uniquely turn each individual dot on and off, as is the case with discrete pixel types of displays. Rather, as the beam scans across the face of the tube, the areas of phosphor that the beam strikes fluoresce with an intensity proportional to the beam's instantaneous amplitude. The size of the phosphor area illuminated by the beam depends on the focus of the electron beam and the distance between adjacent dots of the same color. If the electron beam is sufficiently well-focused, the tube will be able to resolve an area smaller than a single dot: We may think of a dot as a tiny area of continuous phosphor. If the electron beam's focus is sufficiently diffuse, on the other hand, it may simultaneously illuminate more than a single dot. The dot pitch plays a significant role in determining a color CRT's resolution capability, because it determines the distance between adjacent dots of the same color. If we consider just red dots, for example, one red dot is not directly adjacent to another red dot. There are, rather, green and blue dots that are to some degree positioned between the two red dots. The result is a gap between adjacent red dots, and the size of this gap influences the attainable resolution. The farther apart the red dots, for example, the less red resolution the screen is capable of displaying, and likewise for the green and blue dots. The smallest dot pitch found in a typical top-quality monitor tube is about 0.22 mm, or about 4.45 dots per millimeter, which is about 115 dots per inch. Such a display with a width of 36 inches has about 4,156 dots in a horizontal line. It is safe to say that it is possible for a CRT monitor to display the resolution of HDTV, either 1280 x 720 or 1980 x 1020, if the proper conditions of dot pitch, electron beam focus and scanning speed are met. It must also be pointed out that many CRTs, sometimes even those found in nominally HDTV displays, are not capable of displaying a resolution of 1920 x 1080.

RESOLUTION FLEXIBLE

The other display technologies previously discussed are more straightforward in their resolution characteristics, because they all employ discrete, individually addressed pixels. This imparts to such a display a native resolution, specified by the number of pixels it contains. Unlike CRTs, which are, within limits, flexible in their resolution capabilities, when you want to display a resolution other than the native pixel resolution on a display containing discrete pixels, the image must be re-scaled.

What resolutions are currently available in advanced displays? Some of the important standard display resolutions include:

VGA 640 x 480

SVGA 800 x 600

XGA 1024 x 768

WXGA 1366 x 768

UXGA 1600 x 1200

WUXGA 1920 x 1080

QXSGA 2560 x 2048

Most commercial liquid crystal display (LCD) HDTV panels have resolutions of 1280 x 720 or 1366 x 768, and there is at least one 54-inch flat panel LCD on the market that claims a native resolution of 1920 x 1080. Although they are not used for television, there are some monochrome LCD monitors available, developed for medical imaging, that have resolutions of 2560 x 2048 (QXSGA), which is slightly more than 5 megapixels.

Many available 42-inch plasma display panels (PDPs) have resolutions of 852 x 480, which puts them in the 480p resolution category, but there is a 54-inch PDP available that claims a resolution of 1024 x 1024 (vertical resolution would really be less than 1024, as either a 16:9 or a 4:3 image, if scaled to fill the screen horizontally, would not fill the screen vertically. 1024 x 1024 would be a 1:1 aspect ratio), and a 70-inch PDP that claims 1920 x 1080. Plasma displays, like LCD displays, become increasingly costly to manufacture as their pixel count increases, because the increasing pixel count and correspondingly decreasing pixel size significantly increases the complexity of the manufacturing process.

CRT projectors are subject to the conditions and caveats enumerated above for direct-view CRTs, with the recognition that the tubes used for projection are smaller than those used for direct view. Projector CRTs have round imaging screens, and are monochrome tubes with a continuous phosphor coating across the screen, not phosphor dots separated by a shadow mask. CRT projectors have resolutions up to 1920 x 1080, but it should be noted that they are scanned at 1080i.

LCD projectors use liquid crystal microdisplays as their image generators, and, as is the case with all LCD displays, are progressively scanned. Most rear projection LCD sets have resolutions of 1280 x 720, although there is one model available at 1920 x 1080. Most current front projection sets are also microdisplay-based; they range in resolution from 858 x 480 to 1920 x 1080. The DLP projectors currently used for Digital Cinema projection in movie theaters, have resolutions of 1280 x 1020, although consumer Digital Micro Device (DMD) projectors with 1920 x 1080 resolution capability are reported to now exist.

This is an overview of the resolution capabilities of the various display technologies commonly used for SDTV and HDTV display today. Many readers are aware that display resolution is not the only factor, and maybe not even the primary factor, in determining picture quality. It has been observed in tests that even discriminating viewers preferred pictures with higher contrast ratios over the same pictures displayed at higher resolutions but lower contrast ratios. Having said that, the resolution capabilities of television and video displays are increasing constantly. Soon the argument that "you can't display HDTV it in its full resolution, anyway," will be a thing of the past.