Peripherals and Storage

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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?

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What Is a Scanner for?

• Two main tasks
• Converting images (e.g. Drawings and photographs)
  into graphical files
• As part of an optical character recognition
  system converting printed pages into text files

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Forms of Scanners

• Flat-bed scanner - object is placed over the
  glass plate.
• Hand-held scanner - scanner is moved across what
  ever is to be scanned

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How a Scanner Works

• 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

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Resolution

• 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)

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Optical Resolution

• 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

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Interpolated Resolution

• 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

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Ink-jet Printers

• 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

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Laser Printer

• 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

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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.

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Magnetic

• 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
• 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
• 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.

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Questions

• 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.

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Types of magnetic media

• Name three types of magnetic media?

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Optical media

• 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.

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Compact Disks

• 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.

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Digital versatile disk (DVD)

• 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.

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Alternatives

• Can you think of any alternatives to those
  mentioned?

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Summary

• 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
• 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.

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Sources for further reading

• Chalk et al (2004) pages 144-150
• Dick (2002) PC support handbook pages 237 249,
  260-271, 282-293 and 305-307