Karbosguide.com - Module 7a.3

The cathode ray tube

The contents:

The CRT Greater resolutions

Next page Previous page

The CRT

The inside of the glass surface we look at is coated with tiny phosphorous dots. They are arranged in groups of three – a red, a green and a blue phosphorescent dots. Together they make a pixel. These dots light up, when hit by electrons from the electron gun. Each of the mini dots is hit by one electron beam.

The more powerful the beam is, the brighter the dots light up. They start being black, but end in full power red, green and blue.

The electron beams are guided by electromagnets, which bend the beams, so they hit the exact desired phosphorus dot.

The electron beams sweep across the screen very fast. Each of the three electron guns must scan its intended color mini dots continually, from left to right, line by line from top to bottom, typically about 70 to 85 times per second. The beam intensity can be adjusted for every mini dot, to compose the resulting color.

A typical screen image could consist of 480,000 pixels. That is called a 800 x 600 image. There are 800 dots in each horizontal line, and there are 600 lines from top to bottom of the screen. That adds up to 480,000 pixels.

Greater resolutions

Top

The greater the number of pixels in the screen image, the better the resolution. And with a higher resolution, the image may appear sharper.

The lowest resolution seen in modern PCs is found in text based DOS screen images, which are 640 x 480 pixels. That is called a VGA image. VGA was the standard, until Windows came on the market. Back in the eighties, there were even lower standards, like CGA, which I will not even describe.

As the PCs got more powerful, around 1990 a demand developed for better screen resolutions. Windows is a graphic environment, and it works fine in all screen resolutions. The same programs work as well in 640 x 480 as in higher resolutions. Many DOS games also demanded better screen quality. Anyway, VGA was the last "real standard" working on any PC. Screen resolution was since improved relative to VGA, and the term SVGA (Super VGA) came into use. Later came XGA and other names, which each described different resolutions.

Actually, the terms SVGA and XGA are not used much anymore. Instead we are looking at resolution, image frequency and color depth. But, let us stay with the resolution. It ties in with screen size, the bigger the screen the bigger the possible resolution. Below, you see a table with different resolutions:

Standard	Resolution	Number of pixels	Recommended CRT screen size	Recommended TFT screen size
VGA	640 x 480	307,200	14"	n/a
SVGA	800 x 600	480,000	15", 17"	10.4", 12"
SVGA	1024 x 768	786,432	17", 19"	13.3" - 15"
XGA	1152 x 864	995,328	17", 19", 21"	n/a
Vesa 1280	1280 x 1024	1,310,720	19", 21"	17.3", 18.3"
Vesa 1600	1600 x 1200	1,920,000	21" and bigger	n/a (yet)

Screen and resolution need to be matched

The individual monitor can be set to different resolutions. However, not all resolutions are suitable. On a small screen the icons get too small, if you choose too high a resolution. Therefore, resolution and screen size must be matched!

You cannot judge a monitor just by its resolution. Equally important is refresh rate and color depth. They will be described later. But first something on screen size.