8.1 hardware devicesOutput devices

By Ranjit Bassi

Cathode Ray Tube• A cathode ray tube is a vacuum tube with a narrow neck

and a flat rectangular base• The screen is coated with a phosphor which emits light

when struck with an electron beam• The electron beam is produced by the electron gun• As long as the whole screen is swept with the beam

within 0.05 seconds the ‘flicker’ of the screen will not happen

• Colour CRT’s have three separate electron guns with red, blue and green colours so any colour can be created

Flat Screen• Liquid crystals can change the polarisation state of light

when an electric field is applied to them• They can be used with a light source and polarising filters to

control the amount of light that reaches a screen• A Liquid crystal display flat screen is a matrix of liquid crystal

cells• Some LCD displays are made using tiny thin film transistors

(TFT’s). Each pixel consists of a TFT, a capacitor and the liquid crystal

• Flat screens have a 30 per cent lower power consumption than CRT’s

Plasma Screen

• Plasma screen are much more useful for large screens than small screens

• Each pixel is controlled by a miniature fluorescent light• When the control voltage is applied, the gas becomes

plasma and releases ultraviolet light, which strikes phosphors on the front of the screen to emit visible light

• The greatest limitation of the plasma screens is pixel size, typically 0.81mm

• Plasma screen generate a lot of heat, enough to injure people that touch them

Speech Output

• The hardware in computer can output speech • These words can be from any program but

most common a document• This type of output would also need a certain

type of specialist speech analysis software• The hardware involved would be the sound

card and the speaker output

Electronic Paper

• Electronic paper is a flexible paper-thin display that creates images

• It creates images using tiny coloured beads that move or rotate in response to an electric field

• An electrically charged pencil can be used to write on the display and it can be fed through a device that erases the image and writes a new page

• E Ink and Xerox are US companies that produces this paper

Impact Printer

• These printers use an inked ribbon to mark paper with an impression of a character

• In a dot-matrix printer the ribbon is struck hard up to 24 metal pins that form the outline of the character, these printers are also very loud

• These printers use multi-part stationary, this consists of several layers of paper and each layer is carbonised on the reverse side so that the marks made on the top copy appear on all copies below

• Used for printing payslips or even credit card pins

Non-impact printerInkjet printer

• Non-impact printers transfer ink using electrostatics or some other non-impact technique

• These printers can print high quality text, graphics and photographic images in colour

• Inkjet printer use 4 colour cartridges which are cyan, magenta, yellow and black

• To produce the correct colour the colours must first be layered on top of white light

Non-impact printerLaser printer

• A laser printer prints whole pages at a time• A processor in the laser printer generates a bitmap of the page in

raster memory from the page description • A negative charge is applied to the photosensitive drum at the

heart of the laser printer. One or more laser beams are directed onto the rotating drum’s surface

• The resulting pattern of charges on the drum’s surface is an image of the page to be printed

• By rolling pressing the rotating drum over a sheet of paper, the toner is transferred onto the paper

• The paper is passed through heated rollers that squeeze the paper and fuse the toner to the paper

Bitmap and TrueType fonts

• Computers represent characters and numeric codes in ASCII, Unicode, etc.

• A character font may be printed by bitmap or from a description of its straight lines and curves

• This method allows character fonts to be scaled accurately because the computer uses geometric description of the font to simply draw an outline shape at a specific size, then fills it with ink to create the character

• TrueType fonts are independent of the resolution of the screen or printer

Plotter• A plotter is an output device that moves a pen across paper in a

continuous movement so that a two-dimensional drawing can be made

• Pens of different colours are parked in holders on one side of the plotter and may be picked up or parked by the drawing arm under program control

• In a flatbed plotter, the paper is placed flat in a platen. The pen may move in the X direction and the Y direction at the same time

• In a drum plotter, a roll of paper at least 1m wide is fed over a drum so the perforated edges of the paper engage with sprockets on the drum.

• Paper is then rolled around over the drum• Drum plotters mostly used in engineering drawings