About the Midterm

Midterm 1 will be Monday, October 7th. It will be held in the SB Union Auditorium to ensure enough space for everyone to be comfortable.

The test will cover all the material we have had in lecture prior to the midterm, covered by Chapters 1 to 6 of Shelly-Cashman-Waggoner. Reading the text to augment the on-line lecture notes is a good idea.

The level of the questions is likely to be at that of the short answer questions in the text. To study, I urge you to work through the ``What you should know'' and ``Test your knowledge'' sections of the book, as well as reviewing the lecture notes.

I will have an optional question/answer review session Wednesday after class.

Printers

The most common output devices are the monitor and printer.

Printers generate `hard copy'. With modern WYSIWYG software, you should need to print fewer drafts of intermediate work than in years past.

The primary types of printers today are:

• Dot-matrix printers, which form a character out of a combination of discrete dots.
• Ink-jet printers, which spray ink drops at the paper to form overlapping dots.
• Laser printers, which like xerox machines work by the electrostatic attraction of toner particles to the page.

Becoming less important are printers which, like typewriters, are incapable of changing fonts in software. All three of the above are capable of printing arbitrary graphics and alphabets, given the right software.

The primary differences between printers is resolution, measured in dots-per-inch; the speed, measured in pages-per-minutes; and the cost, measured dollars.

The difference in device speeds is both a function of printing technology and the rate at which you can get images into the device. Laser printers work best with a fair amount of on-board memory, to store up pages to be printed.

Although improvements in printer resolution are continuing, eventually the human eye will not detect the difference. Current printers are 300-600 dots per inch, while photos are approximately 1000 dots per inch.

Display Devices

A monitor is the most common computer display device. Electronic display screens are either CRTs or LCDs:

• Cathode-ray tubes (CRTs) are the screen device behind televisions. Electron guns aim at light emitting phosphors on the inner surface of the screen.
• Liquid crystal displays (LCDs) are the screen device behind digital watches. The work by passing electricity through a chemical which changes light transmission on being charged.

Both LCDs and CRTs compose images of dots, or picture elements or pixels. Resolution on both are substantially lower than on good printers, only 100 dots-per-inch or so.

LCDs are used primarily for portable computers, since they are cool and light. They are still too expensive to replace CRTs, but maybe will some day.

When I was a student, there were printing terminals (so called teletypes) but happily these seem to have disappeared.

How does a TV work?

Movies and TVs update images typically 30 times per second, because that is sufficient to fool the eye into perceiving steady motion. Alternate lines (interlaced) of the images are updated to make the motion smoother.

Updating at a slower rate produces flicker, which is very annoying.

The electron gun sweeps across the display in scan lines. The phosphors stay lit for the fraction of a second necessary to appear to be always illuminated. In the 1940's, computer memories were build out of CRTs, and used the decaying light to "remember" bits.

Color TV uses three electron guns, each aiming at a different color phosphor.

Broadcast signals direct the TV how to move and power the electron guns. The graphics "card" converts the digital images to be displayed into such analog signals.

Analog signals are `real number' functions, which can take on any values. Digital data must be represented as binary numbers. Although today TV is analog, the "high-definition television" of the future will be digital, requiring your TV to be a computer.

Graphics adapters

The stuff to be displayed on a monitor must be read every 1/30th of a second. Thus the image to be displayed must be stored in a fast memory.

To avoid slowing down the CPU, this memory is usually under the control of a special graphics processor, which is responsible for keeping the screen refreshed.

When the CPU wants to change the contents of the screen, it has to write its desired changes into the screen memory on the graphics card.

How much memory does the screen take? Typically, you have about pixels.

How much space a pixel takes up depends upon how many possible colors each pixel can be. Allowing 3 bits resolution for each of the three electron guns given colors at about one byte/pixel. More colors require more memory.

Different graphics cards support different graphics standards, hence CGA, VGA, Super VGA, etc. You must make sure you software and graphics card match each other and your monitor.

Modern bit-mapped displays could not have been possible without the decrease in memory prices.

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