Output Devices Monitors The most commonly used

Output Devices Monitors The most commonly used
Output Devices
Monitors
The most commonly used output device.
2 important hardware devices determine the quality of the image that
you see,
• The monitor itself
• Video Controller
2 basic types of monitors used with pc’s
• CRT Cathode Ray Tube – works in the same way as a tv screen using
a large vacuum tube.
• Flat Panel Display – primarily used with portable computers and are
becoming more popular with desktops.
Monitors can be categorised by the way they display colours,
• Monochrome monitors – one colour displayed against a contrasting
background.
• Greyscale monitors – display various intensities of grey against a
white background.
• Colour monitors – display anywhere from 16 to 16million colours.
CRT
How a typical CRT works
1. Electron gun shoots streams of electrons towards the screen.
2. Magnetic coil guides the stream of electrons across and down the
screen.
3. Phosphor dots on the back of the screen glow when the electron
beam hits them.
Screen’s phosphor coating is organised into a grid of dots. The smallest
number of dots that the gun can focus on is called a pixel. Modern
monitors can focus on a pixel as small as a single phosphor dot.
Electron gun systematically aims at every pixel on the screen starting at
the top left corner and scanning to the right.
As the electron gun scans, the circuitry driving the monitor adjusts the
intensity of each beam to determine whether a pixel is on or off.
Colour monitor has 3 electron guns representing red, green and blue,
although the beams are colourless. The phosphors in each pixel have a
triangle representing each colour.
When the beams of the 3 guns
combine to focus on a pixel the phosphors light up. Colour displayed
depends on the combination of the intensities of the 3 beams.
A CRT monitor contains a shadow-mask which is a fine metal mesh fitted
to the shape and size of the screen. The holes in the shadow-mask’s
mesh are used to align the electron beams to ensure that they strike
precisely the correct phosphor dot. In most shadow masks these holes
are arranged in triangles.
Flat Panel Monitors
2 major disadvantages associated with CRT monitors,
• Big, difficult to move. Flat panel monitors are comparatively
lightweight.
• CRT’s require a lot of power. Not practical for notebook computers
which have a built in battery.
Most common type of flat panel monitor is the liquid crystal display (LCD).
Creates an image with a special kind of liquid crystal that is normally
transparent but becomes opaque when charged with electricity.
Disadvantage
Liquid crystal does not emi t light so there is not enough contrast
between the images and the background to make them legible
under all conditions.
Solution
Backlight the screen, however additional power required.
Another disadvantage
Limited viewing angle.
Two main categories of LCD displays,
1. Active matrix
2. Passive matrix
Passive matrix LCD
Relies on transistors for each row and each column of pixels creating a
grid that defines the location of each pixel. Colour displayed by a pixel is
determined by electricity coming from transistors at end of each row and
the top of each column.
Advantage
Less expensive
Disadvantage
• Narrow viewing angle
• Does not refresh the pixels very often
• Move the pointer very quickly and it disappears, this effect
is known as submarining.
• Animated graphics appear blurry.
Most notebooks using passive matrix technology refer to their screens as
dual-scan LCD. Scanning the pixels twice as often lessens problem of
refresh rate. Thus submarining and blurry graphics are less troublesome
than before.
Active Matrix LCD
Technology assigns a transistor to each pixel and each pixel is turned on
or off individually. Enhancement allows pixels to be refreshed much more
rapidly so submarining is not a problem. Also there is a wider viewing
angle.
Often called thin film transistor (TFT) displays because many active
matrix monitors are based on TFT technology, which employs as many as 4
transistors per pixel.
More complex à more expensive!
Other types of monitors
•
Paper white displays
Produces a very high contrast between the monitors white
background and the displayed text or graphics. Utilise a
special technology called super twist to create higher
contrasts.
•
Electro luminescent (ELD) displays
Similar to LCD but use a phosphorescent film between 2
sheets of glass. A grid of wires sends current through the
film to create an image.
•
Plasma/Gas Plasma displays
Thin displays created by sandwiching a special gas (neon or
xenon) between 2 sheets of glass.
When the gas is
electrified via a small grid of electrodes, it glows.
Controlling the amount of voltage applied at various points on
the grid means each point acts as a pixel to display an image.
Expensive but higher quality and larger than LCDs.
Comparing Monitors
Need to check several specifications
1. Size
2. Resolution
3. Refresh rate
4. Dot Pitch
Monitor Size
Monitors are measured diagonally in inches across the front of the
screen. The picture on a 17” monitor is a little over 15”. The standard
size is 17”.
Resolution
The number of pixels on the screen expressed as a matrix, eg. 640x480
à 640 pixels horizontally and 480 vertically.
Actual resolution is determined by the video controller not the monitor so
monitors specify a range of resolutions.
In the mid 80’s IBM established the Video Graphics Array (VGA)
standard of 640x480 pixels. Super VGA (SVGA) standard extended the
resolution to 800x600 and 1024x768.
Refresh Rate
This is neither obvious nor standard. The number of times per second
that the electron guns scan every pixel on the screen and is measured in
Hertz (Hz), or in cycles per second. The monitor refreshes itself at least
a dozen times every second.
If the screen is not refreshed often it appears to flicker, which is one of
the main causes of eyestrain. A refresh rate of 72Hz or higher should
not cause eyestrain.
Dot Pitch
The distance between the phosphor dots that make up a pixel. If the
dots are not close enough the image will not be crisp. Difficult to detect
differences in the dot pitch but blurry pixels will cause eyestrain.
Distance between dots = dot pitch average
= 0.28mm.
Video Controller
An intermediary device between the CPU and the monitor. Contains the
video dedicated memory and other circuitry necessary to send info to the
monitor. Consists of a circuit board whi ch is attached to the computer’s
motherboard. Controls the refresh rate, resolution and the number of
colours that can be displayed.
E.g. Max. amount of colour at 1024x768, computer must send 2,359,296
bytes to the monitor for each screen.
Result à Video controller has high power and importance.
The microprocessor on the video controller frees the CPU from the
burden of calculations for displaying graphics. Most video controllers
include at least 4MB of Video RAM (VRAM). VRAM is dual ported à Can
send data to the monitor and receive data from the CPU at the same
time. Fast and expensive.
PC Projectors
More common now to use software to create presentations directly to the
screen. A pc projector plugs into one of the computer’s ports and
projects the video output onto an external surface. Most pc projectors
use LCD technology to create images. Room needs to be darkened and
display is blurry.
Newer technology à Digital light processing (DLP) displays brighter
crisper images. DLP devices use a special microchip called a digital micro
mirror device that uses mirrors to control the image display.
Sound Systems
Speakers and their associated technology are now important output
devices. Speakers attached to a pc system are similar to those on a
stereo only on a smaller scale and they contain their own amplifiers. More
complicated part à the sound card.
The sound card translates digital sound to electric current to be sent to
the speakers
1. Electric current is sent to the speakers.
2. Electromagnet receives the signal.
3. Magnet vibrates due to the signal.
4. Wave created à Sound.
Printers
Overview:
2 categories:
1. Impact
2. Non-impact
Impact
Creates an image by pressing an inked ribbon against paper using
pins or hammers to shape the image e.g. typewriter.
Most common type of impact printer à Dot matrix.
Slow and limited to the kinds of images that it can produce.
Non-impact
Use other means to create an image.
Most popular types à Inkjet printers, Laser printers.
Considerations when buying/comparing printers
1.
2.
3.
4.
Image quality
Speed
Initial Cost
Cost of Operation.
Image Quality
Also known as printer resolution and is usually measured in dots per inch
(dpi). The more dpi a printer can produce the higher the quality.
E.g. Medium quality inkjet or lasers print 300 – 600 dpi. Professional
quality printers offer resolutions of 1800 dpi or higher.
Speed
Speed is measured in the number of pages per minute (ppm) that the
device can print. Different ppm rates for text and graphics. As speed
increases so does cost.
Consumer level lasers offer speeds of 6 – 8 ppm while high volume
professional printers can exceed 20 ppm.
Initial Cost
Cost of printers has fallen dramatically while capabilities and speed have
improved in the last few years.
Good quality inkjet ~$100. Low-end laser ~$250. Professional systems
range from $1000 to $10,000’s.
Cost of Operation
Cost of ink or toner and maintenance varies with the type of printer.
Type of paper can affect the cost of operation.
Dot Matrix Printer
Commonly used in workplaces where physical impact with the paper is
important, such as when the user is printing to carbon copy or pressure
sensitive forms. Can produce sheets of plain text very quickly. Used to
print very wide sheets.
Creates an image using a mechanism called a print head, which contains a
cluster of short pins arranged in one or more columns. On receiving
instructions from the pc the printer can push any of the pins out in any
combination creating alphanumerics. Protruding pins’ ends strike a ribbon
pressing ink onto a page.
Where a single pin hits the ribbon a dot of ink is printed onto the page
hence the name dot matrix.
More pins à higher resolution.
Lowest resolution à 9 pins
Highest resolution à 24 pins.
Speed of dot matrix printers not ppm but characters per second (cps)
Slowest à 50 – 70 cps
Fastest à 500 cps.
Dot matrix printers use tractor feed paper (also called continuous feed
paper). Sheets of paper are joined end to end with perforations between
the sheets. Rows of holes run down both long edges of each page and a
tractor feed mechanism pulls the paper through.
Other types of impact printers
Line Printers
Works like a dot matrix printer but prints an entire line at a time. Not
very high resolution but very quick – approx 3000 lines of text per min.
Band Printers
Features a rotating band that is embossed with alphanumeric characters.
Very fast and robust – approx 2000 lines of text per min.
Daisy wheel printers
Almost obsolete. Spinning wheel with characters embossed around its
edge. Creates clean text but no graphics and is very slow.
Inkjet Printers
Creates an image directly onto the paper by spraying ink through tiny
nozzles. Good inkjet printers typically attain print resolutions of at least
360 dpi and can print from 2 – 4 ppm.
Compared to laser printers operating costs are relatively low. Most
inkjets use separate cartridges for colour and black and white printing.
This saves money by reserving coloured ink only for coloured printing.
Another improvement
photocopying paper.
à
paper
required,
can
now
use
standard
Very cosy effective way to print in colour. 4 ink nozzles à cyan (blue),
magenta (red), yellow and black. These 4 colours can combine to create
any colour in the visible spectrum and are sometimes called the
subtractive colours. Colour printing is sometimes called 4-colour printing.
Laser Printers
More expensive than inkjet printers but offer a higher print quality and
are faster. A separate CPU and memory are built into the printer to
interpret the data received from the computer and to control the laser.
Technology is similar to that of a photocopier.
Laser can aim at any point on a drum creating an electrical charge. Toner,
which is composed of tiny particles of oppositely charged ink sticks to
the drum in the places that the laser has charged. Pressure and heat
transfer the toner from the drum to the paper.
Amount of memory that laser printers contain determine the speed at
which documents are printed.
A colour laser works like a single colour model except that the process is
repeated 4 times and a different toner colour is used for each pass. The
same 4 colours as in an inkjet – cyan, magenta, yellow and black.
Single colour laser printers typically produce 4 –16 ppm of text, printing
graphics is slower.
Most common resolutions 300 – 600 dpi both
horizontally and vertically. High end models have resolutions of 1200 –
1800 dpi. The higher resolution is much more noticeable in graphics
reproduction.
Advantages
• Convenience
Disadvantages
• Price
• Cost of Operation.
Prices increase dramatically with speed and resolution.
cartridges every few thousand pages.
New toner
Snapshot Printers
Small format printers that use special glossy paper to create medium
resolution prints of 150 – 300 dpi. Best snapshot printers can create
images that look nearly as good as a photo.
Very slow – printout takes between 2 and 4 mins on average – and
generally creates prints no larger than standard 4x6 inch snapshot. Can
take several mins to dry à smearing can be a problem.
Typical price range $400 - $500. Cost per print ranges from 50c - $1.
Other High Quality Printers
Special purpose printers often used by publishers or small print shops to
create high quality output.
Thermal wax printers
Used primarily for presentation graphics and handouts. Create bold
colours and have a low percentage cost. Produces vivid colours because
the inks do not bleed into each other or soak the specially coated paper.
Ribbon coated with panels of coloured wax that melts and adheres to
plain paper as coloured dots when passed over a focused heat source.
Dye – sub printers
Dye sublimation printers have ribbons containing panels of colours which
are moved across a focused heat source capable of subtle temperature
variations. Heated dyes evaporate from the ribbon and diffuse on
specially coated paper creating extremely sharp images but are very slow
and costly.
Fiery printers
Special purpose computer – fiery print server – that transmits
documents to a digital colour copier where they are printed. These
printers are used in print shops as an alternative to press printing.
IRIS printers
Used by print shops to produce high resolution presentation graphics and
colour proofs resembling images. Individual sheets of paper mounted
onto a drum, nozzles on the printing head pass from one end of the
spinning drum to the other spraying minute drops of coloured ink to form
an image. Image resolutions of 1800 dpi.
Plotters
Special kind of output device, like a printer because it produces images on
paper but typically used to print large format images.
•
•
•
Table plotters (or flatbed) use 2 robotic arms. Complex, large and
slow.
Roller plotters (or drum plotters) uses 1 drawing arm but moves
the paper instead of holding it flat and stationary.
Mechanical plotters have been displaced by thermal, electrostatic
and inkjet plotters as well as large format dye – sub printers
which are all faster and cheaper.
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