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T325: Technologies for digital media

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  1. T325:Technologies for digital media Second semester – 2011/2012Tutorial 2 – Information Storage Arab Open University – Spring 2012 1

  2. Rotating media • Optical media • Solid-state memory Outline Arab Open University – Spring 2012

  3. Rotating Media Arab Open University – Spring 2012

  4. Why are we still using CDs and DVDs despite the huge competition of semiconductor memories? Question Arab Open University – Spring 2012

  5. Semi-conductor vs. Rotating media Arab Open University – Spring 2012

  6. Both spellings are common • Disk for magnetic media (Hard Disk) • Disc for optical media (Blue-ray disc) Disc or Disk ? Arab Open University – Spring 2012

  7. Magnetic media have a ferromagnetic surface on which areas can be magnetised • Some metals, such as iron and its alloys, may be magnetised(permanent magnetism) • Information may be encoded as a pattern of magnetism • This pattern can subsequently be detected and the information retrieved • Non-volatile medium • Patterns persist until the material is demagnetised or re-magnetised • Does not depend on a continual supply of power Magnetic disks Arab Open University – Spring 2012

  8. Magnetic disks Arab Open University – Spring 2012

  9. How read/write operations are performed in a magnetic disk drive? • a write head converts electrical signals to magnetised areas on the disk surface, or changes an already magnetised area. • a read head produces an electrical signal in response to a magnetic field. • The two heads are constructed as a single assembly and mounted on an arm which can move radially across the spinning disk, so that any point on the recording surface may be reached. • Data is recorded in concentric tracks, with each track being written or read with the heads at a certain nominal radius Magnetic disks Arab Open University – Spring 2012

  10. Reading and writing operation in Magnetic disks rely on the relation between electricity and magnetism  What is this relation? Magnetic disks Arab Open University – Spring 2012

  11. Any electrical conductor carrying a current is associated with it a magnetic field • A compass needle was deflected by a nearby wire carrying an electric current connection between magnetism and electricity (Hans Christian Oersted(1777-1851)) • Two current carrying wires attracted or repelled each other (depending on the direction of the currents) due to their magnetic fields(André-Marie Ampère (1775-1836)) • Electric currents could be produced by a magnetic field as long as the field was varying with time (Michael Faraday (1791-1867)) Relationship between electricity and magnetismExperiments Arab Open University – Spring 2012

  12. How can we apply these experiments results on the HDD R/W operations? • A write head is an electromagnet constructed so as to concentrate the magnetic field over a very small area of the surface of the disk. • If there is sufficient current through the write head, the magnetic field is intense enough to magnetise this area permanently (or until the next write). • By varying the current as the disk rotates under the head, a pattern of magnetism is built up along the track, corresponding to a stream of bits. Rotating media Arab Open University – Spring 2012

  13. Does the converse of the operation work to read out the stored information?  Faraday’s law Question Arab Open University – Spring 2012

  14. Answer: Yes, but there is a subtle and important difference! • Faraday’s law of magnetic induction: Changingmagnetic field DOES generate a current • As the disk rotates under the head, the magnetic field it encounters does indeed change frequently, with consecutive areas being magnetised in either of two directions. • A head of this type does produce a voltage signal • This signal represents the transitions between magnetic states rather than the states themselves. Rotating media Arab Open University – Spring 2012

  15. The presented R/W scheme is how almost all magnetic recording was done up until the 1990s  BUT! • Growth of demand for larger data storage capacities • Greater areal density • Smaller diameter disks  Magnetic bits become smaller (radially and along the track) • This constitutes a limitation for the original R/W scheme  Smaller magnetic bits means smaller voltage signal (difficult to be detected against the background noise) Solution Giant MagnetoResistance Limitation of the original R/W scheme Arab Open University – Spring 2012

  16. The effect of magneto-resistance is a change in the electrical resistance of a conductor when it is placed in a magnetic field • Relates the voltage across a conductor to the current flowing through it in accordance with Ohm’s law: V = I x R • R can be measured by applying a known voltage to the conductor and measuring the current through it Magneto-resistance Arab Open University – Spring 2012

  17. How do you think magnetoresistance can be applied in the HDD read head? • Changes in resistance are, in principle, rather easier to detect than the tiny amounts of energy that the magnetic bits of a high density disk can generate by induction. • The power used for the measurement now comes from an external voltage/Current source. • The voltage and current can be made as large as necessary for accurate results • Limits: excessive I or V could overheat or damage the head • Note:magnetoresistanceis alsoknown as Anisotropic Magneto-Resistance (AMR) Magnetoresistance for a read head mechanism Arab Open University – Spring 2012

  18. Although magnetoresistance was observed by William Thomson (Lord Kelvin) 150 years ago, the change in resistance of below 1% was too small to be practically useful. • The real breakthrough came with the discovery of Giant magnetoresistance (GMR), when huge resistance changes (e.g. 50%) were reported. • Discovered by Albert Fertin France and Peter Grϋnbergin Germany, working independently. • They jointly received the Nobel Prize in Physics in October 2007 for their discovery. • GMR read heads were one of the first products of nanotechnology! Giant MagnetoResistance (GMR) Arab Open University – Spring 2012

  19. What is nanotechnology? • The term nanotechnology covers a wide variety of ideas and techniques, but its defining characteristic is that it deals with objects with dimensions in the order of a nanometer (10-9m, or a millionth of a millimeter) • Many of the techniques used in nanotechnology were developed for the production of microelectronics • Thousands and millions of transistors are built on silicon chips Nanotechnology Arab Open University – Spring 2012

  20. Purpose: to increase areal densities • In conventional recording, or longitudinal magnetic recording, the magnetic bits may be thought of as tiny bar magnets which point along the track. • In perpendicular recording, the bar magnets are aligned at right angles to the disk and point in and out of the disk surface. Perpendicular Magnetic Recording (PMR) Arab Open University – Spring 2012

  21. Longitudinal recording Perpendicular recording Perpendicular Magnetic Recording (PMR) Arab Open University – Spring 2012

  22. In Perpendicular recording, much stronger magnetic field is required for writing than in case of longitudinal recording • Materials used for the recording surface should have a much higher coercivity • Coercivity is a measure of how difficult a material is to magnetize or demagnetize • High coercivity material difficult to magnetize but also difficult to demagnetize Coercivity Arab Open University – Spring 2012

  23. Optical Media Arab Open University – Spring 2012

  24. Data is written on a spiral track as a series of ‘pits’of various lengths • Track pitch: Distance between adjacent tracks Pits and Lands  Optical Media - Introduction Arab Open University – Spring 2012 http://www.sonic.net/~ideas/graphics/mma_cd.gif

  25. E • Four layers • A polycarbonate disc layer has the data encoded by using bumps. • A shiny layer reflects the laser. • A layer of lacquer protects the shiny layer. • Artwork is screen printed on the top of the disc • A laser beam reads the CD and is reflected back to a sensor, which converts it into electronic data Optical Media - Introduction Arab Open University – Spring 2012

  26. A laser beam is focused onto the track and reflected back • The reflected beam is analyzed to detect the pits. • Feedback mechanisms (servomechanisms, or ‘servos’ for short) involving optics, mechanics and electronics ensure that the beam is kept focused on the track and does not drift away from the track. • They compensate effectively for minor warping or eccentricity and for some vibration. • The data surface is at the opposite side of a transparent polycarbonate substrate from the laser reader. • The laser light penetrates the transparent surface before it reaches a point of focus, so it is spread over a relatively wide area and, therefore, is less affected by minor scratches and dust particles. Optical Media – Reading Data Arab Open University – Spring 2012

  27. For read-only CDs, the pits are actual indentationsin the data surface • For rewritable media they are simply areas of different reflectivity from the ‘land’ that surrounds them. • These discs use a material that can exist in one of two physical states or phases:crystalline (where the molecules are arranged in regular patterns) and amorphous (where there is a lack of order). • The phase depends on how the material is heated and cooled, and can be changed from one to the other and back again by heat from the laser. • This is called phase-change recording. Read Only vs. Rewritable CDs Arab Open University – Spring 2012

  28. Blu-ray disc (BD) use blue lasers with a wavelength of 405 nm (shorter than CD and DVD) • Shorter wavelength means: • laser beam can be focused to a smaller spot on the data surface. • pits and track pitch (the spacing between tracks) could be reduced in size, increasing the data capacity achievable. • Data in Blu-ray is recorded in a similar way to previous rewritable optical discs, although the details differ. • The recording surface uses a phase-change material, and the pits are areas of contrasting reflectivity. Blu-ray Arab Open University – Spring 2012

  29. Optical media Arab Open University – Spring 2012

  30. Optical media - Comparison Arab Open University – Spring 2012

  31. One important design decision was the thickness of the transparent layer covering the data surface. • 1.2 mm thickness in CDs • 0.6 mm thickness in DVDs • There are optical advantages to be gained by having a much thinner transparent cover layer. • If the disc tilts at all, then a thick layer could deflect and defocus the beam. Thickness of the transparent layer Arab Open University – Spring 2012

  32. In the Original CD, the pits were laid in a spiral pattern on a flat surface • Blu-ray discs have a physical spiral track on which the pits are recorded. • This track is a spiral groove • The preformed groove is there to guide the beam so that it deposits pits in the correct spiral pattern Blu-ray Disc Arab Open University – Spring 2012

  33. Optical media Arab Open University – Spring 2012

  34. Blu-ray disc Arab Open University – Spring 2012

  35. Any optical disc player uses a servomechanism to stay on track. • This is a form of closed-loop control where the position of the beam on the track is constantly monitored. • This signal is fed back to the motors that position the beam in such a way as to correct the departure. • The alternative, open-loop control, does not use monitoring and feedback, but simply relies on very precise mechanics and favorable operating conditions to keep errors to a minimum. Servomechanism in Optical Media Arab Open University – Spring 2012

  36. As the disc rotates, the servomechanism ensures that the laser follows the groove • Servomechanism will be moving the laser from side to side to follow the wobble. • The rate at which it moves from side to side will be determined by the length of the wobbles (the wobble period – the distance between peaks) and the speed of rotation. • The length of the wobbles is predefined, so the equipment can determine the speed of rotation from the rate at which the laser is moving from side to side. • A feedback loop is used to control the speed of rotation, based on measuring the rate at which the servomechanism is having to move the laser from side to side. Optical media Arab Open University – Spring 2012

  37. Blu-ray data is held as a pattern of pits along a track. • The laser beam reading the data is kept on the track and in focus by a servomechanism. • The track also produces a wobble signal. • The wobble information is used in locating blocks of data. • Small dust particles may corrupt only a few bytes, but marks or scratches might extend for some distance along the track and cause errors in a long sequence of data. • When data is recorded it is encoded with an error-correcting code (Reed-Solomon (or RS) code). • This allows a large proportion of the errors to be corrected. Blue-ray Disc : Conclusion of Important features Arab Open University – Spring 2012

  38. Solid State Memory Arab Open University – Spring 2012

  39. E • Flash memory development : storage capacity and cost • Variety of usages: memory keys, memory cards, built-in telephone memory Introduction Arab Open University – Spring 2012

  40. Flash memory is an example of memory that is described as ‘solid-state’ What do you hear by Solid inthe term : “Solid State Memory” ? • In science ‘the solid state’ means solid as distinct from liquid, gas or plasma. • Transistor (Solid-state electronics ) vs. thermionic valve (electrons in a vacuum tube) • “Solid state” vs. “Rotating Media” • “solid” seems in contrast with the moving mechanical parts of the hard drive. Solid-state memory Arab Open University – Spring 2012

  41. Our main interest is in memory which stores digital media files: audio, and still and moving images. • Main memory is comprised of integrated circuits • Two sorts of Main memory: ROM and RAM. • ROM stands for Read-Only Memory • ROM (and secondary storage media) are non-volatile • ROM is used for the programs needed on start-up. • RAM stands for random-access memory. • Random Access: a data word in any location in the whole memory can be accessed just as quickly as a data word in any other location. • RAM is volatile: all the programs and associated data stored in it are lost when the power supply to the RAM is switched off. Memory Basics Arab Open University – Spring 2012

  42. PROM is ‘programmable’ ROM. • Non-programmable ROM has the data put into it when it is manufactured. • PROM, by contrast, is manufactured with no data stored in it and can be programmed by the user (the computer manufacturer for example) • Special equipment is needed to program PROM, and it cannot be changed once it is programmed (it cannot be erased). • Why using the term programming rather than writing ? • because it is all done at once ‘up front’, rather than writing individual bits or bytes when needed, although for some devices the distinction is not so clear-cut. Programmable ROM (PROM) Arab Open University – Spring 2012

  43. EPROMis ‘Erasable’ PROM • Erasable PROM can be erased and reprogrammed • EPROM is used specifically for a device that can be erased by exposure to ultra-violet light • EEPROMis Electrically Erasable PROM • EEPROMis erased with an electrical signal rather than needing UV light. Erasable PROM (EPROM) Electrically EPROM (EEPROM) Arab Open University – Spring 2012

  44. EPROM EEPROM EEPROM can be erased one bit or one byte at a time. EEPROM is more complex costs more and it is not possible to fit as much memory on a single integrated circuit. • Erasing EPROM has to be done to the whole of the device (UV light is shone onto the whole of the ‘chip’) EPROM vs. EEPROM Arab Open University – Spring 2012

  45. What is the meaning of the term flash in Flash Memory ? Question Arab Open University – Spring 2012

  46. Flash memory is a variation ofEEPROM • The word ‘flash’ is used in reference to the way it is erased, which is done either on the whole device at once or on a block of data in the device and can be done quickly, ‘in a flash’. • “Offering the electrical erase capability, traditionally featured by the expensive EEPROM, at cost and density comparable to EPROM, Flash memories not only have taken a big portion of their progenitor’s markets, but in addition they have greatly expanded the fields of application of non-volatile memories.” (Source: Bez and Cappelletti, 2005, p. 84) Flash Memory Arab Open University – Spring 2012

  47. Memory cell: Part of the memory that can store a single bit • Cell must be able to exist in each of two states: • one state that represents a data 0 • one state that represents a data 1 • Operations: Reading, Erasing, and Writing • Reading a cell:detecting which of the two states it is in. • Erasing a block of memory:setting all the cells in a block to the same known state, that representing a data 1 • Writing to the cells (programming the memory):setting some of the cells to the other state, that which represents a data 0. • Since the remaining cells are already in the state representing a data 1, there is no need to do anything to them Flash Memory Cell Arab Open University – Spring 2012

  48. Solid-state memory Arab Open University – Spring 2012

  49. Flash cells use semiconductor technology • Constructed using the same or similar semiconductor materials and manufacturing processes to those used in microelectronics such as microprocessors. • The basic building block of microelectronics is the transistor • In flash memory, a memory cell uses a single transistor. In some other types of semiconductor memory, several transistors are needed for a single memory cell • This is one of the reasons that Flash memory allows high storage densities Semiconductor Technology Arab Open University – Spring 2012

  50. A transistor can be thought of as a controlled switch • A signal on the gate connects the source to the drain Refer to block 1 – pages 79 - 83 for more reading about transistors Solid-state memory Arab Open University – Spring 2012