Output devices: output is anything that comes out a
computer. Output can be meaningful information or gibberish and it can
appear in a variety of forms – as binary numbers, as characters, as
pictures, and as printed pages. An output device is any machine capable
of representing information from a computer. Output devices include
display screens, loudspeaker, printers, plotters, etc.
....
Computer Monitors
The computer monitor is an output device that is part
of your computer's display system. A cable connects the monitor to a
video adapter (video card) that is installed in an expansion slot on
your computer’s motherboard. This system converts signals into text and
pictures and displays them on a TV-like screen (the monitor).
The computer sends a signal to the video adapter,
telling it what character, image or graphic to display. The video
adapter converts that signal to a set of instructions that tell the
display device (monitor) how to draw the image on the screen.
Cathode Ray Tube (CRT)
The CRT, or Cathode Ray Tube, is the "picture tube"
of your monitor. Although it is a large vacuum tube, it's shaped more
like a bottle. The tube tapers near the back where there's a negatively
charged cathode, or "electron gun". The electron gun shoots electrons at
the back of the positvely charged screen, which is coated with a
phosphorous chemical. This excites the phosphors causing them to glow as
individual dots called pixels (picture elements). The image you see on
the monitor's screen is made up of thousands of tiny dots (pixels). If
you've ever seen a child's LiteBrite toy, then you have a good idea of
the concept. The distance between the pixels has a lot to do with the
quality of the image. If the distance between pixels on a monitor screen
is too great, the picture will appear "fuzzy", or grainy. The closer
together the pixels are, the sharper the image on screen. The distance
between pixels on a computer monitor screen is called its dot pitch and
is measured in millimeters. (see sidebar). You should try to get a
monitor with a dot pitch of .28 mm or less.
Note: From an environmental point of view, the
monitor is the most difficult computer peripheral to dispose of because
of the lead it contains.
There are a couple of electromagnets (yokes) around
the collar of the tube that actually bend the beam of electrons. The
beam scans (is bent) across the monitor from left to right and top to
bottom to create, or draw the image, line by line. The number of times
in one second that the electron gun redraws the entire image is called
the refresh rate and is measured in Hertz (Hz).
If the scanning beam hits each and every line of
pixels, in succession, on each pass, then the monitor is known as a
non-interlaced monitor. A non-interlaced monitor is preferred over an
interlaced monitor. The electron beam on an interlaced monitor scans the
odd numbered lines on one pass, then scans the even lines on the second
pass. This results in an almost imperceivable flicker that can cause
eye-strain.
This type of eye-strain can result in blurred vision,
sore eyes, headaches and even nausea. Don't buy an interlaced monitor,
they can be a real pain in the ... ask your optometrist.
Interlaced computer monitors are getting harder to
find (good!), but they are still out there, so keep that in mind when
purchasing a monitor and watch out for that "steal of a deal".
Video Technologies
Video technologies differ in many different ways.
However, the major 2 differences are resolution and the number of colors
it can produce at those resolutions.
Resolution
Resolution is the number of pixels that are used to
draw an image on the screen. If you could count the pixels in one
horizontal row across the top of the screen, and the number of pixels in
one vertical column down the side, that would properly describe the
resolution that the monitor is displaying. It’s given as two numbers. If
there were 800 pixels across and 600 pixels down the side, then the
resolution would be 800 X 600. Multiply 800 times 600 and you’ll get the
number of pixels used to draw the image (480,000 pixels in this
example). A monitor must be matched with the video card in the system.
The monitor has to be capable of displaying the resolutions and colors
that the adapter can produce. It works the other way around too. If your
monitor is capable of displaying a resolution of 1,024 X 768 but your
adapter can only produce 640 X 480, then that’s all you’re going to get.
When we talk about the different technologies, we’re
talking about the video card and monitor that make up that display
system. Also, standards describe the basic number of colors and
resolutions for each technology, but individual manufacturers always
take liberties, providing options and enhancements that are designed to
make their product more appealing to the end user. This is, of course,
how new standards come about.
Monochrome : Monochrome monitors are very basic
displays that produce only one color. The basic text mode in DOS is 80
characters across and 25 down. When graphics were first introduced, they
were fairly rough by todays standards, and you had to manually type in a
command to change from text mode to graphics mode. A company called
Hercules Graphics developed a video adapter that could do this for you.
Not only could it change from text to graphics, but it could do it on
the fly whenever the application required it. Today’s adapters still
basically use the same methods.
CGA/EGA : The Color Graphics Adapter (CGA) introduced
color to the personal computer. In APA mode it can produce a resolution
of 320 X 200 and has a palette of 16 colors but can only display 4 at a
time. With the introduction of the IBM Enhanced Graphics Adapter (EGA),
the proper monitor was capable of a resolution of 640 X 350 pixels and
could display 16 colors from a palette of 64.
VGA: Up until VGA, colors were produced digitally.
Each electron beam could be either on or off. There were three electron
guns, one for each color, red, green and blue (RGB). This combination
could produce 8 colors. By cutting the intensity of the beam in half,
you could get 8 more colors for a total of 16. IBM came up with the idea
of developing an analog display system that could produce 64 different
levels of intensity. Their new Video Graphics Array adapter was capable
of a resolution of 640 X 480 pixels and could display up to 256 colors
from a palette of over 260,000. This technology soon became the standard
for almost every video card and monitor being developed.
SVGA : Once again, manufacturers began to develop
video adapters that added features and enhancements to the VGA standard.
Super-VGA is based on VGA standards and describes display systems with
several different resolutions and a varied numberof colors. When SVGA
first came out it could be defined as having capabilities of 800 X 600
with 256 colors or 1024 X 768 with 16 colors. However, these cards and
monitors are now capable of resolutions up to 1280 X 1024 with a palette
of more than 16 million colors.
XGA : Extended Graphics Array was developed by IBM.
It improved upon the VGA standard (also developed by IBM) but was a
proprietary adapter for use in Micro Channel Architecture expansion
slots. It had its own coprocessor and bus-mastering ability, which means
that it had the ability to execute instructions independent of the CPU.
It was also a 32-bit adapter capable of increased data transfer speeds.
XGA allowed for better performance, could provide higher resolution and
more colors than the VGA and SVGA cards at the time. However, it was
only available for IBM machines. Many of these features were later
incorporated by other video card manufacturers.
Printer (computing) : In computing, a printer is a
peripheral which produces a text or graphics of documents stored in
electronic form, usually on physical print media such as paper or
transparencies. Many printers are primarily used as local peripherals,
and are attached by a printer cable or, in most new printers, a USB
cable to a computer which serves as a document source. Some printers,
commonly known as network printers, have built-in network interfaces,
typically wireless or Ethernet based, and can serve as a hard copy
device for any user on the network. Individual printers are often
designed to support both local and network connected users at the same
time. In addition, a few modern printers can directly interface to
electronic media such as memory cards, or to image capture devices such
as digital cameras and scanners; some printers are combined with
scanners or fax machines in a single unit, and can function as
photocopiers. Printers that include non-printing features are sometimes
called multifunction printers (MFP), multi-function devices (MFD), or
all-in-one (AIO) printers. Most MFPs include printing, scanning, and
copying among their many features.
Printing technology : Printers are routinely
classified by the printer technology they employ; numerous such
technologies have been developed over the years. The choice of engine
has a substantial effect on what jobs a printer is suitable for, as
different technologies are capable of different levels of image or text
quality, print speed, cost, and noise. Some printer technologies don't
work with certain types of physical media, such as carbon paper or
transparencies.
A second aspect of printer technology that is often
forgotten is resistance to alteration: liquid ink, such as from an
inkjet head or fabric ribbon, becomes absorbed by the paper fibers, so
documents printed with liquid ink are more difficult to alter than
documents printed with toner or solid inks, which do not penetrate below
the paper surface.
Cheques should either be printed with liquid ink or
on special cheque paper with toner anchorage.[2] For similar reasons
carbon film ribbons for IBM Selectric typewriters bore labels warning
against using them to type negotiable instruments such as cheques. The
machine-readable lower portion of a cheque, however, must be printed
using MICR toner or ink. Banks and other clearing houses employ
automation equipment that relies on the magnetic flux from these
specially printed characters to function properly.
Modern print technology : The following printing technologies are routinely found in modern printers:
.....
Toner-based printers
A laser printer rapidly produces high quality text
and graphics. As with digital photocopiers and multifunction printers
(MFPs), laser printers employ a xerographic printing process but differ
from analog photocopiers in that the image is produced by the direct
scanning of a laser beam across the printer's photoreceptor.
Another toner-based printer is the LED printer which
uses an array of LEDs instead of a laser to cause toner adhesion to the
print drum.
.....
Liquid inkjet printers
Inkjet printers operate by propelling variably-sized
droplets of liquid ink onto almost any sized page. They are the most
common type of computer printer used by consumers. Today's photo-quality
ink jet printers can typically print with a resolution of 1200 to 4800
dots per inch. They will give acceptable quality photo prints of images
with 140-200 pixels per inch (PPI) resolution, and high quality prints
of images with 200-300 ppi resolution.
Solid ink printers
Solid ink printers, also known as phase-change
printers, are a type of thermal transfer printer. They use solid sticks
of CMYK-coloured ink, similar in consistency to candle wax, which are
melted and fed into a piezo crystal operated print-head. The print head
sprays the ink on a rotating, oil coated drum. The paper then passes
over the print drum, at which time the image is immediately transferred,
or transfixed, to the page. Solid ink printers are most commonly used
as colour office printers, and are excellent at printing on
transparencies and other non-porous media. Solid ink printers can
produce excellent results. Acquisition and operating costs are similar
to laser printers. Drawbacks of the technology include high energy
consumption and long warm-up times from a cold state. Also, some users
complain that the resulting prints are difficult to write on, as the wax
tends to repel inks from pens, and are difficult to feed through
automatic document feeders, but these traits have been significantly
reduced in later models. In addition, this type of printer is only
available from one manufacturer, Xerox, manufactured as part of their
Xerox Phaser office printer line. Previously, solid ink printers were
manufactured by Tektronix, but Tek sold the printing business to Xerox
in 2001.
Dye-sublimation printers
A dye-sublimation printer (or dye-sub printer) is a
printer which employs a printing process that uses heat to transfer dye
to a medium such as a plastic card, paper or canvas. The process is
usually to lay one colour at a time using a ribbon that has colour
panels. Dye-sub printers are intended primarily for high-quality colour
applications, including colour photography; and are less well-suited for
text. While once the province of high-end print shops, dye-sublimation
printers are now increasingly used as dedicated consumer photo printers.
Ink-less printers, Thermal printers
Thermal printers work by selectively heating regions
of special heat-sensitive paper. Monochrome thermal printers are used in
cash registers, ATMs, gasoline dispensers and some older inexpensive
fax machines. Colours can be achieved with special papers and different
temperatures and heating rates for different colours; these coloured
sheets are not required in black-and-white output. One example is the
ZINK technology.
UV Printers
Xerox is working on an inkless printer which will use
a special reusable paper coated with a few micrometres of UV light
sensitive chemicals. The printer will use a special UV light bar which
will be able to write and erase the paper. As of early 2007 this
technology is still in development and the text on the printed pages can
only last between 16–24 hours before fading.
The following technologies are either obsolete, or
limited to special applications though most were, at one time, in
widespread use.
Impact printers rely on a forcible impact to transfer
ink to the media, similar to the action of a typewriter. The impact
printer uses a print head that hits the surface of the ink ribbon, which
presses the ink onto the paper. All but the dot matrix printer rely on
the use of formed characters, letterforms that represent each of the
characters that the printer was capable of printing. In addition, most
of these printers were limited to monochrome printing in a single
typeface at one time, although bolding and underlining of text could be
done by "over striking", that is, printing two or more impressions in
the same character position. Impact printers varieties include,
typewriter-derived printers, teletypewriter-derived printers, daisy
wheel printers, dot matrix printers and line printers. Dot matrix
printers remain in common use in businesses where multi-part forms are
printed, such as car rental services. An overview of impact printing [4]
contains a detailed description of many of the technologies used.
Pen-based plotters were an alternate printing
technology once common in engineering and architectural firms. Pen-based
plotters rely on contact with the paper (but not impact, per se) and
special purpose pens that are mechanically run over the paper to create
text and images.
Typewriter-derived
printers
Several different computer printers were simply
computer-controllable versions of existing electric typewriters. The
Friden Flexowriter and IBM Selectric typewriter were the most-common
examples. The Flexowriter printed with a conventional typebar mechanism
while the Selectric used IBM's well-known "golf ball" printing
mechanism. In either case, the letter form then struck a ribbon which
was pressed against the paper, printing one character at a time. The
maximum speed of the Selectric printer (the faster of the two) was 15.5
characters per second.
Teletypewriter-derived printers
The common teleprinter could easily be interfaced to
the computer and became very popular except for those computers
manufactured by IBM. Some models used a "typebox" that was positioned,
in the X- and Y-axes, by a mechanism and the selected letter form was
struck by a hammer. Others used a type cylinder in a similar way as the
Selectric typewriters used their type ball. In either case, the letter
form then struck a ribbon to print the letterform. Most tele printers
operated at ten characters per second although a few achieved 15 CPS.
....
Daisy wheel printers
Daisy-wheel printers operate in much the same fashion
as a typewriter. A hammer strikes a wheel with petals, the "daisy
wheel", each petal containing a letter form at its tip. The letter form
strikes a ribbon of ink, depositing the ink on the page and thus
printing a character. By rotating the daisy wheel, different characters
are selected for printing. These printers were also referred to as
letter-quality printers because, during their heyday, they could produce
text which was as clear and crisp as a typewriter, though they were
nowhere near the quality of printing presses. The fastest letter-quality
printers printed at 30 characters per second.
Dot-matrix printers
In the general sense many printers rely on a matrix
of pixels, or dots, that together form the larger image. However, the
term dot matrix printer is specifically used for impact printers that
use a matrix of small pins to create precise dots. The advantage of
dot-matrix over other impact printers is that they can produce graphical
images in addition to text; however the text is generally of poorer
quality than impact printers that use letterforms (type).
Dot-matrix printers can be broadly divided into two major classes:
Ballistic wire printers
Stored energy printers
Dot matrix printers can either be character-based or
line-based (that is, a single horizontal series of pixels across the
page), referring to the configuration of the print head.
At one time, dot matrix printers were one of the more common types of
printers used for general use, such as for home and small office use.
Such printers would have either 9 or 24 pins on the print head. 24-pin
print heads were able to print at a higher quality. Once the price of
inkjet printers dropped to the point where they were competitive with
dot matrix printers, dot matrix printers began to fall out of favor for
general use.
Some dot matrix printers, such as the NEC P6300, can
be upgraded to print in colour. This is achieved through the use of a
four-colour ribbon mounted on a mechanism (provided in an upgrade kit
that replaces the standard black ribbon mechanism after installation)
that raises and lowers the ribbons as needed. Colour graphics are
generally printed in four passes at standard resolution, thus slowing
down printing considerably. As a result, colour graphics can take up to
four times longer to print than standard monochrome graphics, or up to
8-16 times as long at high resolution mode.
Dot matrix printers are still commonly used in
low-cost, low-quality applications like cash registers, or in demanding,
very high volume applications like invoice printing. The fact that they
use an impact printing method allows them to be used to print
multi-part documents using carbonless copy paper, like sales invoices
and credit card receipts, whereas other printing methods are unusable
with paper of this type. Dot-matrix printers are now (as of 2005)
rapidly being superseded even as receipt printers.
Line printers
Line printers, as the name implies, print an entire line of text at a
time. Three principal designs existed. In drum printers, a drum carries
the entire character set of the printer repeated in each column that is
to be printed. In chain printers, also known as train printers, the
character set is arranged multiple times around a chain that travels
horizontally past the print line. In either case, to print a line,
precisely timed hammers strike against the back of the paper at the
exact moment that the correct character to be printed is passing in
front of the paper. The paper presses forward against a ribbon which
then presses against the character form and the impression of the
character form is printed onto the paper.
Comb printers, also called line matrix printers,
represent the third major design. These printers were a hybrid of dot
matrix printing and line printing. In these printers, a comb of hammers
printed a portion of a row of pixels at one time, such as every eighth
pixel. By shifting the comb back and forth slightly, the entire pixel
row could be printed, continuing the example, in just eight cycles. The
paper then advanced and the next pixel row was printed. Because far less
motion was involved than in a conventional dot matrix printer, these
printers were very fast compared to dot matrix printers and were
competitive in speed with formed-character line printers while also
being able to print dot matrix graphics.
Line printers were the fastest of all impact printers
and were used for bulk printing in large computer centres. They were
virtually never used with personal computers and have now been replaced
by high-speed laser printers. The legacy of line printers lives on in
many computer operating systems, which use the abbreviations "lp",
"lpr", or "LPT" to refer to printers.
Pen-based plotters
A plotter is a vector graphics printing device which
operates by moving a pen over the surface of paper. Plotters have been
used in applications such as computer-aided design, though they are
rarely used now and are being replaced with wide-format conventional
printers, which nowadays have sufficient resolution to render
high-quality vector graphics using a rasterized print engine. It is
commonplace to refer to such wide-format printers as "plotters", even
though such usage is technically incorrect. There are two types of
plotters, flat bed and drum.
Printing mode
The data received by a printer may be:
A string of characters
A bitmapped image
A vector image
Some printers can process all three types of data, others not.
Character printers, such as daisy wheel printers, can handle only plain text data or rather simple point plots.
Pen plotters typically process vector images. Inkjet based plotters can adequately reproduce all three.
Modern printing technology, such as laser printers
and inkjet printers, can adequately reproduce all three. This is
especially true of printers equipped with support for PostScript orPCL,
which includes the vast majority of printers produced today.
Today it is common to print everything (even plain
text) by sending ready bitmapped images to the printer, because it
allows better control over formatting. Many printer drivers do not use
the text mode at all, even if the printer is capable of it.
Monochrome, colour and photo printers : A monochrome
printer can only produce an image consisting of one colour, usually
black. A monochrome printer may also be able to produce various tones of
that color, such as a grey-scale. A colour printer can produce images
of multiple colours. A photo printer is a colour printer that can
produce images that mimic the colour range (gamut) and resolution of
prints made from photographic film. Many can be used on a standalone
basis without a computer, using a memory card or USB connector.
Printing speed : The speed of early printers was
measured in units of characters per second. More modern printers are
measured in pages per minute. These measures are used primarily as a
marketing tool, and are not as well standardised as toner yields.
Usually pages per minute refers to sparse monochrome office documents,
rather than dense pictures which usually print much more slowly,
especially colour images. PPM are most of the time referring to A4 paper
in Europe and letter paper in the United States, resulting in a 5-10%
difference.
