The TV Picture

Ok, we all spend hours watching TV each week, and let’s admit it, for some of us each day. We enjoy breathtaking visuals and stories, whether real or fictitious, that move us to tears, laughter, and sometimes even fear. We sit in the comfort of our living rooms as we are taken on adventure after adventure without the slightest hint of the special magic that occurs every 60th of a second inside our television sets enabling us to be entertained this way each and every day.

So, how does it all work? Television would be nothing but an oversized radio without its picture. Tiny picture elements called pixels are used together on the screen to compose each image we see, like the dots of paint that make up a Monet painting. Analog or traditional TVs have around 211, 000 pixels. In great contrast, HDTVs have at least 900,000 pixels, and some have up to 2,000,000 pixels. Resolution is the number of rows of horizontal pixels compared to the columns of vertical pixels, in other words, the number of pixels going across the screen verses the number of pixels stacked up to make up the picture. More pixels equal more visual information, and that equals a sharper picture, or higher resolution. HDTVs have an advantage over analog TVs in that they have two high resolutions (1280 by 720 or 1920 by 1080). Analog TVs have only one smaller resolution of 640 by 480. Because of their higher resolution, HDTVs can produce an image that is 6 times as dense as one produced by analog TVs.

Now that we know what makes up the picture, we need to know how it’s made up. TVs create pictures by drawing, or scanning, horizontal lines in sequence across the screen with an electron beam that stimulates the phosphorescent screen coating creating patterns of light on the surface. A scan line is each horizontal line drawn by the electron beam. Two types of scanning are used to display our pictures on the screen, interlaced and progressive.

Interlaced scanning is a traditional method used by analog TVs that uses two alternating passes of the electron beam to draw 525 scan lines (480 visible on screen) that combine to make an image. This type of scanning takes two passes to fill the screen with the necessary lines that make up the picture. Odd lines (lines 1-479) are drawn in sequence on the 1st pass while even lines (lines 2-480) are drawn on the 2nd pass. One of these two groups of lines is called a field. The two fields are interlaced to make the picture. The combination of these two fields is called a frame. A TV using interlaced scanning draws one field every 1/60th of a second equaling 30 frames per second.

Progressive scanning on the other hand is a major advantage of digital television. The scan lines are drawn at much higher speeds to complete a progressive scan of all 480 visible lines in sequence every 1/60th of a second (the same amount of time it takes interlaced scanning to fill only half the screen’s scan lines). This is similar to what happens on your computer monitor.

The results are a sharper picture with none of the striping seen on conventional TVs and a brighter picture due to the fact that twice as much screen area is being illuminated in the same period of time. Both of these characteristics increase in importance with the increase of screen size on your TV; the better these results are, the better the preservation of the quality of the picture will be as the screen size is increased. Also, the “flicker” seen in traditional analog TVs that use interlaced scanning is reduced with progressive scanning, which may in turn reduce eye fatigue.

An added benefit to progressive scanning is that the motion artifacts, or distortions that are present in interlaced scanning when tracking fast-moving images, are not as apparent in progressive scans. Why? True video sources generate 60 images per second. Although interlaced scanning displays each of the 60 stills, each uses only half the available scan lines on the screen. The image definition becomes blurry with movement, only becoming clear again when the image is stationary. Because progressive scanning can reproduce the entire frame for each still, the images for moving objects appear much more focused.

One last item you might find useful when it comes to scanning is a de-interlacer, a.k.a. progressive-scan upconverter, interlaced-to-progressive [I/P] converter, or line doubler. Most digital TVs are manufactured with one. A de-interlacer converts incoming interlaced signals (analog TV) to a progressive scan format using intelligent technology to estimate the content of the missing lines from each interlaced field.

So, now we know the secret undertakings behind the screen of each television we watch. Regardless of format, screen type, or source, all TVs use pixels and scanning to make up every picture we see on the screen.

	Analog	SDTV	EDTV	HDTV	HDTV
Pixels	211,000	307,200	337,920	921,600	2,073,600
Resolution	640 x 480	640 x 480	704 x 480	1280 x 720	1920 x 1080
Scanning	Interlace 480i	Interlace 480i	Progressive 480p	Progressive 720p	Interlace 1080i
Aspect Ratio	4:3	4:3	4:3 or 16:9	16:9	16:9
Quality	Current	Good	Better	Best	Best

 

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