Computer graphics: The fundamental characteristic which determines the clarity of a computer screen is number of picture elements which are called pixels. On a computer screen these pixels are arranged in a rectangular grid and the size of the grid varies considerably among computers. For example, if each character is represented by a rectangle of 9 lines and 9 columns then a 80 character by 24 line text display would have a total of 216 lines and 720 columns. For graphic pictures from satellites, 256, 512 and 1024 lines and columns are grid sizes frequently used. On computer screens and television sets there are 4 columns for every 3 lines. In contrast, the aspect ration of movie screens is 16 to 9 (broader field of vision). Currently 640 columns and 480 lines is a fairly common size grid for 14 inch computer screens. For computer assisted design more detail is desirable, hence workstation screens are 16 inch or larger with 1280 columns by 846 lines or greater. For this purpose the new IBM PC2 offer cards with grids up to 1024 by 1024. The number of bits associated with each pixel determines the number of colors that can be represented. For example, black and white, 4, 16, and 256 colors would be represented by pixel of 1,2,4, and 8 bits respectively. Computer graphics are digital, however as humans prefer an analog signal it is sometimes converted to analog just before display.
To avoid flicker on a computer screen the image must be redrawn about 60 times a second. The number of bits of information which must be processed each second to output to a computer screen is the refresh rate times the number of columns times the numbers of rows times the number of bits used to represent each pixel. For current PC's this number can as large as 60 x 640 x 480 x 4 = 73,728,000 bits per second.
The trend in computer screens is higher and higher resolution and greater number of colors. Since for most purposes no more than ten thousand colors are required for quality pictures, this limit may become common. Currently television type monitors are cheaper and better than flat screen displays, such as those in notebook computers. In the future quality flat screen displays may displace television type monitors. Currently, there are many types of competing flat screen technologies.
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To communicate over analoy phone lines, computer digital information must be converted to an analog signal. This requires a modem. To understand digital to analog conversion and modems, click here for Rockwell Corporation's Baudman. To return to the notes remember to click `back' at the top of your screen. You may have to click several times depending how deeply you delve into these files.
(1) Commercial TV: TV has a single light waveform (analog). The single waveform corresponds to the position of the electron beam as it moves across the TV screen in a 525 (US standard) line zigzag pattern. On one pass the electron beam draws the odd lines and on the next the even lines(interlacing). The entire picture is refreshed, that is redrawn, 30 times a second. Color is represented by adding red, blue, and green light. Current TV is a low resolution device with about 300 columns and 200 lines. Note that when text is presented on a TV screen you almost never see more than 20 characters in a line. If they tried to present 80 characters, all you would see is a blur. TV looks realistic because it is an analog device which displays millions of colors.
To digitize a TV picture the signal is sampled at twice the frequency humans can discriminate, which is about 5 million cycles per second, and the colors represented by a 24 bit word. The resulting raw digital signal varyies from 90 million bits to 220 million depending on the standard. The digital signal is converted back to analog for viewing, a process which functions to smooth out the discrete digital points.
Currently there is a commercial battle between Japan, Europe and the US for the next generation of high definition television, HDTV. Japan and Europe both adopted analog standards with approximately double the resolution of current TV. The US to leapfrog the competition has organized a competition between major firms. The final standard is likely to be digital. HDTV will come on line in the next twenty years.