Whatís a screen?
The computerísscreen is an ordinary TV (the same kind you watch Bill Cosby on) or resembles a TV. The screen shows what you typed on the keyboard and also shows the computerís responses.
If your computer is old and primitive (such as an Apple 2 or Radio Shack Color Computer or Commodore 64 or Commodore VIC or Atari 800), you can attach it to an ordinary television set. Hereís how.
Look at your TVís antenna. Wires run from the antenna to two screws, which are on the back of the TV:
Loosen the two screws, to release the antenna. When you buy a TV computer, the salesperson also gives you aswitch box. Attach that box to the two screws you loosened:
The salesperson also gives you an electrical cord looking like this:
Itís called anRCA cord, because RCA invented it. Plug one end of that cord into your computer, and plug the opposite end into the switch box.
Attach the antennaís wires to the screws on the switch box:
The switch box has a switch on it. If you move the switch toward the antenna, you have a normal TV, so you can watch Bill Cosby. If you move the switch toward the computerís RCA cord, your TVís controlled by the computer so that the computer can write messages on your TV screen.
So by moving the switch, you can make your TV act either normal or computerized. Your family will argue about which way to move the switch.
That switch box is the same kind used by video-game machines. When you buy a TV computer, the salesperson will give you the switch box and RCA cord, free!
To use the computer, flip the computerís switch to channel 3 or 4, then turn your TVís dial to the same channel.
To get a sharp picture on your TV screen, avoid the channel used by your local TV station. For example, if you live in Boston, CBS hogs channel 4, so avoid channel 4; put your computer and TV on channel 3 instead.
Although most computers (such as Commodore and Radio Shack) use channels 3 and 4, some computers (such as Atari) use channels 2 and 3 instead, and some other computers use channels 10 and 33 and 34 instead.
If the image on your TV screen looks fuzzy ó so that you can barely read the computerís writing ó adjust the TVís "fine tuning" knob.
What the TV can do
Besides writing messages on your TVís screen, the computer can also draw its own pictures on the TV! And if your TV has color, youíll see the pictures in color!
When you watch Bill Cosby on TV, his faceís size depends on the size of your TVís screen. If your TVís screen is tiny (less than 12 inches), his face looks small; if your TVís screen is 25 inches, his face looks bigger; and if you have a projection TV with a gigantic 60-inch screen, his face looks gigantic. The sameís true for the messages & pictures sent to the TV by the computer: the bigger the TVís screen, the more magnified the computerís messages & pictures.
How much is displayed
The computer makes the TVís screen show lots of words, numbers, and formulas. Those words, numbers, and formulas are made ofcharacters: each character is a letter of the alphabet, a digit, or any other symbol you can type.
The ideal TV computer would make the TV display 25 lines of info, with each line of info containing 40 characters, so the total number of characters you see on the screen simultaneously is "25 times 40", which is 1000.
But most TV computers are less than perfect: they display slightly fewer than 25 lines of info and slightly fewer than 40 characters per line, so the total number of characters you see on the TV screen simultaneously is slightly less than 1000.
Acomputer monitor resembles a TV but produces a sharper picture and costs more.
Like a TV, a computer monitor uses a picture tube. The tube in a TV or monitor is called acathode-ray tube (CRT).
The monitor can be eitherstand-alone or built-in. A stand-alone monitor looks like a TV but has no antenna and no dial for selecting channels: the only channel you get is "computer". It doesnít need a switch box: instead, the computerís RCA cord (or similar cord) plugs directly into a hole in the monitor. Before buying a computer that uses a stand-alone monitor, ask whether the computerís price includes the monitor: the monitor might cost extra.
Abuilt-in monitor is a screen permanently attached to the rest of the computer: the unit containing the computerís main circuits also contains the monitor.
When buying a TV, you ask for either "color" or "black-and-white". Similarly, when buying a computer monitor, ask for eithercolor or monochrome. A color monitor displays all colors of the rainbow; a monochrome monitor displays just black-and-light.
Four kinds of monochrome monitors are popular:
Apaper-white monitor displays black and white.
Anamber monitor displays black and yellow.
Agreen-screen monitor displays black and light green.
Agray-scale monitor displays many shades of gray.
A color TV costs more than black-and-white. Similarly, a color monitor (that displays all the colors of the rainbow) costs more than a monochrome monitor.
Most monochrome monitors cost about $100. Most color monitors cost between $200 and $500.
The typical color monitorís screen is 14 inches (measured diagonally). The typical monochrome monitorís screen is 12 inches, but some monochrome monitors use a smaller screen (9-inch) to make the monitor be smaller, weigh less, and be easier to carry.
If the cable running from the monitor to the computer is fat (so it contains many wires), the monitor produces a sharp image.
A monochrome monitor with a fat cable is called atransistor-transistor-logic monitor (TTL monitor). Its cable contains one wire to transmit the fundamental picture, plus additional wires for further enhancements.
A color monitor with a fat cable is called ared-green-blue monitor (RGB monitor). Its cable contains one wire to transmit red, a second wire to transmit green, a third wire to transmit blue, plus additional wires for further enhancements.
For the IBM PC, you can buy three kinds of RGB monitors: the cheapest are calledCGA monitors; the better ones are called EGA monitors; the best ones are called VGA color monitors. (Iíll reveal more details about those monitors on pages 68 and 69.)
If your monitorís cable is not fat, the picture isnít sharp. For example, if your monitorís cable is just a thin RCA cord, your monitorís called acomposite monitor; its picture is fuzzy, though not as fuzzy as a plain TV.
Most computers (such as the IBM PC) can make a monitor display 80 characters per line. To fit so many characters on a line, the characters are made tiny. To display the tiny characters clearly, the monitor must either have a fat cable (to produce a sharp picture) or be monochrome; it must not be a composite color monitor. (If you try to display tiny characters on a composite color monitor, the characters are hard to read because the fuzzy colors bleed into each other.)
The typical monitor displays 25 80-character lines, so you see 2000 characters simultaneously.
Avideo-display terminal (VDT) is a monitor that communicates with a large computer and has an attached keyboard.
If 200 people are using a maxicomputer simultaneously, only one of them is sitting at the maxicomputerís main console. The other 199 people typically sit at 199 VDTís, which are in different rooms or even different cities.
If your computer is tiny, it comes with a tiny screen, called aliquid-crystal display (LCD). Thatís the kind of screen you see on digital watches, pocket calculators, pocket computers, notebook computers, and laptop computers.
Since an LCD screen uses little electricity, it can run on batteries. A traditional picture tube cannot run on batteries. If your computer system runs on batteries, its screen is an LCD.
An LCD screen displays black characters on a white background. The screen consists of thousands of tiny crystals. Each crystal is normally white, but temporarily changes to black when an electrical charge passes through it.
A traditional picture tube emits light; thatís why you can watch TV even in a dark room. But an LCD screen does not emit light; you cannot read an LCD screen in a dark room. You must turn on a light, to see which of the crystals are white and which are black.
Some LCD screens come with a light to help you see the screen. If the light is behind the LCD surface, the screen is said to bebacklit. Since the light consumes electricity, it quickly runs down the battery, which you must recharge often.
Although the crystals can change from white to black, they appear black only if you look at them from the correct angle, and if the light in the room is positioned correctly. If you move your head or the light, the black crystals will appear very light gray instead of black, and youíll have trouble reading the message theyíre trying to display. So if you have trouble seeing the message on an LCD screen, move your head or the light or the screen, until the message darkens. The fanciest LCD screens usesupertwist crystals, which you can read from any angle; but theyíre expensive and consume more electricity.
Laptops use LCD screens instead of traditional picture tubes, because LCD screens consume less electricity, weigh less, and are less bulky. Desktops stay with traditional picture tubes, because the image on the typical LCD screen has poor contrast and resolution and responds too slowly to computer commands.
AnLCD plate (or LCD overhead-projection panel) is a special LCD screen that you put on an overhead projector, which projects the LCDís image onto the wall of your office or classroom or auditorium, so that the image becomes several feet across.
The nicest low-cost LCD plate is theSharp QA-75. It can display many shades of gray. It sells for about $1500. It attaches to the IBM PC, and you can buy a cable to connect it to a Mac.