2. HUMAN COMPUTER INTERACTION 3: OUTPUT UNITS Printers, monitors and special purpose units. Focus on character (and image) formation and transfer.

3. 1. Introduction. §The purpose of any output unit is §(1) to translate binary into useful information, §(2) display printing characters as characters, §(3) execute non-printing control characters, such as tab, line feed, form feed, scroll etc.

4. 2.Printers. §The purpose of printers is to produce “hardcopy” of documents or graphics. So much for the “paperless office”! §There are three main printer concepts: §(A) Printer speed, §(B) Character formation method, §(C) Character transfer method.

5. 2A Printer speed. §One measure of printer speed is how much is printed at any one time. §The early printers were character printers printing one character after another e.g. daisy wheel, dot matrix. §Businesses used line printers, e.g. the chain printer. §The norm is now page printers e.g. laser printers. Speed in pages per minute.

6. 2B. Character formation method. §How are the symbols on the page created? §2 main methods: §(1) Full form. Characters are fully formed in the H/W e.g. daisy wheel / golf ball. §(2) Simulation. There are no pre-formed characters. They are created “on the fly” e.g. as an arrangement of dots (dot matrix).

7. 2B continued. §See diagrams. §How is a character created by a dot matrix print head? §A column at a time. §How does the printer know which pins to fire and not fire for each column? §The S/W driving the printer maintains a matrix of 0’s and 1’s.

8. 2B continued. §Comparison between full form e.g. daisy wheel and simulation e.g. dot matrix. §(1) Quality of printing. §(2) Speed of printing. §(3) Flexibility of fonts. §(4) Graphic capability.

9. 2C Character transfer method. §How does the image get to the paper? §Two methods: §(1) Impact method: uses a physical mechanism [e.g. a hammer to strike a ribbon]. Includes typewriters, daisy wheels, chain printers, impact dot matrixes. §Note: not all dot matrix printers use impact.

10. 2C Character transfer continued. §(2) Non-impact. No mechanism strikes the paper. §Examples: (A) ink jet printers. Electric current behind the nozzles heats ink until it squirts onto the page, §(B) laser printers. Each dot of a character is converted into a light signal sent by a laser.

11. How do laser printers work? §1. Each dot is a light signal. §2. Light signals are directed at a photo- sensitive drum which rotates. §3. Where light hits the drum it magnetizes a spot. This attracts toner to that spot. §4. The drum passes over paper. §5.Heat and pressure transfer toner to paper. See diagrams / CD.

13. Important Note: §Laser printers still use a dot matrix to represent characters, but: §(a) It is a larger matrix (better resolution), §(b) Impact is not used (no pins), §(c) an entire page is prepared very rapidly before transferring it to paper.

14. 3.Monitors. §Synonyms for monitors? CRTs, VDTs, VDU’s, screens. §3 main concepts for monitors. §(A) Size, §(B) Character formation method, §(C) Character transfer method.

15. 3A. Size. §The size of the tube affects the viewable area. A standard screen is 15” but for some applications, 17”, 18” or 20” are needed because of the amount that must be displayed on one screen. E.g. for CAD / CAM, art, weather systems.

16. 3B. Character formation method. §The character formation method is similar to printers. §CRT screens contain 100,000’s of dots called “Pixels” = picture elements. §E.g. SuperVGA 800 * 600 = 480,000 pixels.

17. 3B. Character formation continued. §A pixel is really a positively charged phosphor dot K+. §It can be lit up when struck by electrons from an electron gun in the back of the CRT. §How are shapes created? §By selectively lighting up pixels.

18. 3B. Character formation continued. §How does the gun know which pixels to light up? §S/W supplies a matrix of 0’s and 1’s. §1 -- light up; §0 -- don’t light up.

19. 3C.Character transfer method. §A non-impact method is used (nothing mechanical strikes the screen, only electrons). §See diagram of monochrome screen. §Problems: How do we maintain a whole screen with only one gun? Since phosphor only glows for a fraction of a second, how do we keep an unchanged image constant?

20. 3C.Character transfer continued. §Answer: §A very fast refresh rate. The entire screen is “re-drawn” 60-80 times each second. Note the cycle speed in Hz. §Industry standard is 72Hz. The human eye detects flicker even at 60Hz!

21. Color versus monochrome. §Monochrome. §(1) Actually get two colors: background and foreground. §(2) Each pixel glows one color but can change intensity. §(3) Simulate different colors by grayscaling—also used for monochrome printing.

22. Color versus monochrome. §Color. §(1) Pixel has three parts (Red, Green and Blue), like a palette. §(2) 3 electron guns needed to mix the colors. §(3) Why color?

23. 4.Special purpose units. §See your text book for more specific information, photographs etc. §(1) Plotters, §(2) Voice synthesizers, §(3) Music. §Special sound cards and speakers needed for good quality.