Screens


What's a screen?
The computer's screen 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 computer's responses.

Televisions
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.
Here's how.
Look at your TV's 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 a switch box. Attach that box to the two screws you loosened:










The salesperson also gives you an electrical cord looking like this:


It's called an RCA 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 antenna's 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 computer's RCA cord, your
TV's 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 computer's switch to channel 3 or 4, then turn your TV's 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 - so that you can barely read the computer's writing -
adjust the TV's "fine tuning" knob.
What the TV can do
Besides writing messages on your TV's screen, the computer can also draw its own pictures on
the TV! And if your TV has color, you'll see the pictures in color!
When you watch Bill Cosby on TV, his face's size depends on the size of your TV's screen. If
your TV's screen is tiny (less than 12 inches), his face looks small; if your TV's 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 same's true for the messages & pictures sent to the TV by the computer:
the bigger the TV's screen, the more magnified the computer's messages & pictures.
How much is displayed
The computer makes the TV's screen show lots of words, numbers, and formulas. Those words,
numbers, and formulas are made of characters: 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.

Monitors
A computer 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 a
cathode-ray tube (CRT).
The monitor can be either stand-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 doesn't
need a switch box: instead, the computer's 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
computer's price includes the monitor: the monitor might cost extra.
A built-in monitor is a screen permanently attached to the rest of the computer: the unit
containing the computer's main circuits also contains the monitor.
Colors
When buying a TV, you ask for either "color" or "black-and-white". Similarly, when buying a
computer monitor, ask for either color 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:
A         paper-white monitor      displays black and white.
An   amber monitor                      displays black and yellow.
A         green-screen monitor     displays black and light green.
A         gray-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.
Screen size
The typical color monitor's screen is 14 inches (measured diagonally). The typical monochrome
monitor's 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.
Fat cables
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 a transistor-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 a red-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 called CGA
monitors; the better ones are called EGA monitors; the best ones are called VGA color monitors.
(I'll reveal more details about those monitors on pages 68 and 69.)
If your monitor's cable is not fat, the picture isn't sharp. For example, if your monitor's cable is
just a thin RCA cord, your monitor's called a composite 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.

Video terminals
A video-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
maxicomputer's main console. The other 199 people typically sit at 199 VDT's, which are in
different rooms or even different cities.

Liquid crystals
If your computer is tiny, it comes with a tiny screen, called a liquid-crystal display (LCD). That's
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; that's 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 be backlit. 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 you'll have trouble
reading the message they're 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 use supertwist crystals, which you can read from any angle; but they're 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.
An LCD plate (or LCD overhead-projection panel) is a special LCD screen that you put on an
overhead projector, which projects the LCD's 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 the Sharp 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.

42
Buyer's guide: screens

41
Buyer's guide: screens



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