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Peripherals and Storage
Looking at:
Scanners
Printers
 Why do we need storage devices anyway?
 What are magnetic disks?
 How do magnetic disks physically store data?
 What are optical storage media?
 How is data stored on optical media?
 What other options is there for storage?
What Is a Scanner for?
Two main tasks
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Converting images (e.g. Drawings and photographs)
into graphical files
As part of an optical character recognition system
converting printed pages into text files
Forms of Scanners
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Flat-bed scanner - object is placed over the glass
plate.
Hand-held scanner - scanner is moved across what
ever is to be scanned
How a Scanner Works
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A light source is shines down on to the surface, and
is moved along the object being scanned
Light is reflected off the surface of the object
Narrow strip of the reflected light hits a row of
detectors (usually a charge coupled device(CCD))
The different levels of light intensity are detected,
converted to numbers, and passed to the computer
Resolution
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There are a fixed number of sensors, so image quality
is defined by the number of sensors, and measured in
pixels (picture elements) the smallest dot that will
appear on the scanned image
Quality of a scanned image is usually given in dots per
inch (dpi)
Optical Resolution
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Optical resolution - the previous slide explained
horizontal resolution, and is dependent on the actual
detector. There is vertical resolution which is
dependent on by how much the scanning head (light
and sensors) moves. A vertical resolution of 600dpi
means that the head moves 1/600th of an inch each
time
The horizontal and vertical resolution both give the
optical resolution, e.g 600 x 600 dpi
Interpolated Resolution
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This higher than the optical resolution for the same
equipment
This performed by software
Adjacent pixels are compared and an estimate of what
the pixel would be is produced
Ink-jet Printers
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Ink is in a cartridge, which as a circuit board on the
outside, which takes signals form the computer to holes
in the cartridge. Ink is attracted through the holes that
the have a signal set to them. The goes through the
holes and sprays onto the paper, forming the required
pattern
Colours can be produced by mixing three primary
coloured inks
Typical resolution is 600dpi, quality approaches that of
a laser printer, at lower cost
Laser Printer
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An image is formed on a rotating drum, by a laser in effect
drawing an invisible picture on the drum. The laser
changes the charge on the drum on the image is formed
in this way
Toner (powder) is attracted to where the charge is, and
not where there is no charge. Toner is then transferred to
the paper from the drum
600dpi or better
Capable of high quality images and text
More expensive than the other printers
Why do we need storage devices
anyway?

We often want to store data or program files that are
not in use at a particular time, but we may want to use
them later. Memory such as RAM could do this but
when the power is turned off what was stored is lost.
Therefore, we need storage methods that do not lose
information when the power is turned off.

What is common to all forms of storage is they store
data.
Magnetic
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All magnetic disk storage allows direct access to the
data. When data is written to or read from disk, a
particular part of the disk can be identified and use by
its address.
A disk address uses sector and track. To access a
location on disk:
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the read/write heads have to move to the appropriate track
Wait for correct sector to appear under the head.
Disk can usually be read from or write on both the top
and bottom of the disk.
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The data on a disk is stored in a binary form
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as magnetic dots along the tracks,
as one polarity for a 1 and the other polarity for 0.
The read/write head (R/W head) detects the polarity,
because there are only two polarities, there are only
two states. The pattern of pulses produced by the R/W
head in relation to the drive timing is translated into
data and set to the processor.
Questions
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Does the Read/Write Head touch the disk?
No
Does putting a faster processor directly effect the rate
of getting data of the disk?
Not directly.
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To use the disk has be formatted, the r/w head does
under the control of the operating system.
During formatting tracks are created on all the area of
the disk used for storage,
a track can be thought of as being concentric bands on
the disk, similar idea to tree rings.
Each track into sectors, the closer to the centre of the
disk a track is the less sectors per track.
Hard disk have more than one disk stacked vertically,
so if data exists on the different platters but at the
same head position it is said to be in the same cylinder
so all the data in that cylinder can be read without
moving the heads.
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Because hard disks have the ability to store lots of data
with fast access times, at an affordable cost, they are
still the main method for storage, holding applications,
data and usually the operating system.
Types of magnetic media
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Name three types of magnetic media?
Optical media
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Magnetism is not the only way data can be stored,
another option is to use optical media.
The disk has holes burnt in it by a laser to record the
binary code. A hole or pit on the disk reflects less light
than the surface of the disk.
By detecting the difference in reflected light in relation
to the time of the drive, a pattern of pulse is produced
and this is translated into data sent to the CPU.
Compact Disks
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CDs are optical media storing approximately 650MB of
data, but are slower than a hard disk to access. The
head used for reading from a CD has two parts a laser
that shines on a small part of the disk and a light
sensor to measure the reflected light.
Digital versatile disk (DVD)
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A DVD uses the same basic principle as a CD.
the data is packed more densely
– Smaller pit widths and lengths.
It has a larger capacity (4.7GB) as compared to a CDs
650MB, it also has a quicker transfer rate than a CD.
Compression algorithms help to improve the capacity.
Alternatives
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Can you think of any alternatives to those mentioned?
Summary
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The main reason we often want to store data or
program files that are not in use at a particular time, but
we may want to use them later. We need storage
methods that do not lose information when the power
is turned off.
The disk address uses sector and track. The data on a
disk is stored in a binary form as magnetic dots along
the tracks
Floppy disk: A floppy disk is a small magnetic disk it is
cheap and a floppy drive is still standard on a PC.
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Scanner - shining light on to an object,detecting a
narrow band of reflected light.
The detector and light source are moved along the
object
Quality/resolution of the image is dependent on the
number of detectors and is measured in dpi
Interpolated resolution gives a higher resolution than
the optical resolutionThree printers - dot matrix, ink-jet
and laser
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Hard Disk:
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A typical PC has a hard disk with a gigabytes (GB)
capacity.
Because hard disks have the ability to store lots of
data with fast access times, at an affordable cost,
they are still the main method of storage, holding
applications, data and usually the operating
system.
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Dot matrix- worked by pins pushing ink on to paper in a
particular pattern. Quality low but it is cheap
Ink-jet - works by spraying ink on to paper. Reasonable
quality
Laser printer- works image being formed on a rotating
drum and transferring toner on the paper
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Compact Disk (CD)
– Optical media where the disk has holes burnt in it
by a laser to record the binary code, and a laser
detects the presence of holes and converts it into
a binary code.
– storing approximately 650MB of data,
– Slower than a hard disk to access.
Digital versatile disk (DVD)
– A DVD uses the same basic principle as a CD,
– data is packed more densely than CD
– Quicker transfer rate than a CD.
Sources for further reading
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Chalk et al (2004) pages 144-150
Dick (2002) PC support handbook pages 237 –
249, 260-271, 282-293 and 305-307