Optical Recording and Optical Recording and and tilt it just right, - PDF document

Optical Recording and Communications 1 Optical Recording and Communications 2 Introductory Question Introductory Question When you submerge a digital watch in water When you submerge a digital watch in water � Optical Recording and Optical Recording and and tilt it just right, the watch’s face appears to and tilt it just right, the watch’s face appears to be a perfect mirror. This mirror reflection is be a perfect mirror. This mirror reflection is Communications Communications from from from from the outer (front) surface of the watch face the outer (front) surface of the watch face A. A. the inner (back) surface of the watch face the inner (back) surface of the watch face B. B. Optical Recording and Communications 3 Optical Recording and Communications 4 Observations about Optical Observations about Optical 5 Questions about Optical Recording 5 Questions about Optical Recording Recording and Communications Recording and Communications and Communication and Communication � Optical disks can store lots of audio or video Optical disks can store lots of audio or video � How is information represented digitally? How is information represented digitally? � That audio or video is of the highest quality That audio or video is of the highest quality � How is information recorded on an optical disk? How is information recorded on an optical disk? � Optical disks continue to play perfectly for years Optical disks continue to play perfectly for years � How is information read from an optical disk? How is information read from an optical disk? � Playback of optical disks involves lasers Playback of optical disks involves lasers � How can light carry information long distances? How can light carry information long distances? � Lasers and fibers are used in communication Lasers and fibers are used in communication � Why does light follow an optical fiber’s bends? Why does light follow an optical fiber’s bends? Optical Recording and Communications 5 Optical Recording and Communications 6 Question 1 Question 1 Review of Digital Representation Review of Digital Representation � Audio or video info is a sequence of numbers Audio or video info is a sequence of numbers � How is information represented digitally? How is information represented digitally? � � Each number can be represented digitally Each number can be represented digitally � by putting specific symbols in a set of digits. by putting specific symbols in a set of digits. � Digital representations often involve binary digits, � Digital representations often involve binary digits Digital representations often involve binary digits Digital representations often involve binary digits, � which can each hold only two symbols: which can each hold only two symbols: 0 0 and and 1 1. . � Each symbol is a discrete value of a physical quantity Each symbol is a discrete value of a physical quantity � Digital representations have Digital representations have � good noise good noise- -immunity immunity � and permit error correction (elimination of noise). and permit error correction (elimination of noise). 1

Optical Recording and Communications 7 Optical Recording and Communications 8 Clicker Question Clicker Question Digital Audio Digital Audio � The Napoleonic semaphore telegraph sent The Napoleonic semaphore telegraph sent � The air pressure in sound The air pressure in sound information using towers with two long vanes information using towers with two long vanes is measured thousands of is measured thousands of which could have 7 different orientation. A which could have 7 different orientation. A times per second times per second chain of telegraph towers and operators with chain of telegraph towers and operators with chain of telegraph towers and operators with chain of telegraph towers and operators with � Each measurement is � Each meas rement is Each meas rement is Each measurement is telescopes passed information quickly across telescopes passed information quickly across represented digitally using represented digitally using France. Was this digital optical communication? France. Was this digital optical communication? about 16 binary digits, about 16 binary digits, each a each a 0 0 or a or a 1 1. . A. Yes Yes A. B. No No B. Optical Recording and Communications 9 Optical Recording and Communications 10 Question 2 Question 2 Structure of CDs and DVDs Structure of CDs and DVDs � How is information recorded on an optical disk? How is information recorded on an optical disk? � These disks have spiral tracks in reflective layers These disks have spiral tracks in reflective layers � Each track contains pits and flats Each track contains pits and flats � Digital symbols consist of the Digital symbols consist of the l l lengths of the pits and flats lengths of the pits and flats h h f h f h i i d fl d fl � The track structure is made as The track structure is made as small as can be detected by the small as can be detected by the playback system, so as to maximize playback system, so as to maximize the density of information. the density of information. Optical Recording and Communications 11 Optical Recording and Communications 12 Question 3 Question 3 Playback Techniques Playback Techniques � How is information read from an optical disk? How is information read from an optical disk? � Laser light is focused on the shiny layer Laser light is focused on the shiny layer � Reflection is weaker from a pit than from a flat Reflection is weaker from a pit than from a flat � Reflected light is directed to photodiodes Reflected light is directed to photodiodes � Reflected light intensity Reflected light intensity indicates pits or flats indicates pits or flats 2

Optical Recording and Communications 13 Optical Recording and Communications 14 Playback Issues Playback Issues Clicker Question Clicker Question � Light must hit pits perfectly Light must hit pits perfectly � Light is used to construct computer chips and, Light is used to construct computer chips and, as the features on those chips get smaller, the as the features on those chips get smaller, the � Feedback optimizes the position of the light spot Feedback optimizes the position of the light spot wavelength of the light must get shorter because wavelength of the light must get shorter because � Light must hit only one pit Light must hit only one pit shorter wavelength light shorter wavelength light shorter wavelength light shorter wavelength light � Use laser light U U Use laser light—a single wave l l li h li h a single wave i i l l � Use a converging lens to focus that wave to a spot Use a converging lens to focus that wave to a spot A. is brighter. is brighter. � The wave forms a “waist” The wave forms a “waist”— —its minimum width its minimum width A. B. is dimmer. is dimmer. � Wave limits on focusing are known as diffraction Wave limits on focusing are known as diffraction B. � Waist can’t be much smaller than a wavelength Waist can’t be much smaller than a wavelength C. carries more energy per photon. carries more energy per photon. C. � so pit size can’t be much smaller than a wavelength. so pit size can’t be much smaller than a wavelength. D. can be focused to smaller spots. can be focused to smaller spots. D. Optical Recording and Communications 15 Optical Recording and Communications 16 Advantages of Digital Recording Advantages of Digital Recording Question 4 Question 4 � Freedom from noise and media damage issues Freedom from noise and media damage issues � How can light carry information long distances? How can light carry information long distances? � Digital representation avoids information loss Digital representation avoids information loss � Error correction ensures clean information Error correction ensures clean information � Surface contamination doesn’t matter (much) � S rf S rf Surface contamination doesn’t matter (much) nt min ti n d nt min ti n d n’t m tt r (m n’t m tt r (m h) h) � High information density High information density � Data compression is possible Data compression is possible � Perfect, loss Perfect, loss- -less copies are possible less copies are possible Optical Recording and Communications 17 Optical Recording and Communications 18 Optical Communication Optical Communication Transmission Techniques Transmission Techniques � Both analog and digital representations possible Both analog and digital representations possible � Light in air: direct line Light in air: direct line- -of of- -sight sight � � Analog representation Analog representation � Infrared remote controls Infrared remote controls � � Infrared computer links Infrared computer links � often involves AM modulation of the light often involves AM modulation of the light � Light in optical fibers: arbitrary paths Li h i Li h i Light in optical fibers: arbitrary paths i i l fib l fib bi bi h h � and is often used for remote process monitoring. and is often used for remote process monitoring. d i d i f f d f d f i i i i � Digital representation Digital representation � Optical cables and networks Optical cables and networks � often uses pulses with discrete amplitudes as symbols often uses pulses with discrete amplitudes as symbols � and provides noise and provides noise- -immunity, error correction, immunity, error correction, compression, and channel- compression, and channel -sharing. sharing. 3