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CD-ROM: A Versatile and Compact Digital Storage Solution

Learn about the history and structure of CD-ROMs, the difference between optical and magnetic storage media, and the process of mass production.

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CD-ROM: A Versatile and Compact Digital Storage Solution

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  1. BY 03CP616 VIJAY 03CP627 SHETUL COMPACT DISC DIGITAL VERSETILE DISC ULTRA DENSITY OPTICAL OPTICAL STORAGE

  2. What is a CD-ROM? • A cd rom most resembles the vinyl long play records still dear to the hearts of audio fanatics. On a record, a single spiral in the vinyl winds from the outside to the inside. • 74 minutes = 682MB • 80 minutes = 737MB • Size: 120mm (4.72 inch) diameter , 1.2mm (0.047 inch) thick plastic disc.

  3. CDs: A Brief History • In 1979 the phillips and sony corporation joined forces to co-produce the CD-DA (Digital Audio) standard. • Phillips contributed most of physical design, with has similar to the laser disc format. It had previously created with regards to using pits and lands on the disc that are read by laser. • Sony contributed the digital to analog circuitry and digital encoding and error correction code design.

  4. In 1980 the companies announced the CD-DA standard which has since been refer to as the red book format. • The Red book included the specification for recording , sampling, and 120mm diameter physical format. This size was chosen because it could contain all the 70 minutes of recording without interrupt. • October 1,1982,sony has developed the world’s first commercial CD recording- Billy Joel’s 52ndstreet album.

  5. What is the difference in optical and magnetic storage media? • The tracks in the magnetic media is in the discrete, concentric manner means one track after another ,while in the optical storage media the tracks are in spiral manner. • Optical storage mostly have data only on one side >continue

  6. while in the magnetic storage the data may be stored on both the sides. • example : • Magnetic storage: Hard-disks, floppy disks. • Optical storage: Compact disc

  7. Why CD ROM invented? • The industry needed high-capacity, removable medium for multimedia and software distribution, and the consumer electronics industry had developed an inexpensive, high-capacity digital technology for distributing music. • The window 98 operating system would require more than 75 floppy disk while if we use CD only 1 is required.

  8. Structure of the compact disk: • Track pitch : It is radial distance between the two track spirals that is of 1.6 microns. • The pits themselves are oval or sausage-shaped depressions 0.15 micron deep and 0.5 micron wide. • The length of the pit varies from 0.834 to 3 microns.

  9. Because a CD is read by a beam of light instead of by an electromagnetic head, the tracks can be much closer together than the magnetic media. • The track density of a CD is 16000 tracks per inch (tpi) ,as compared to 96 tpi for a floppy disk and an average of 400 tpi for a hard disk.

  10. CD ROM Technology • A CD is made of a polycarbonate wafer, 1.2mm thick, with a 15mm hole in the centre. • This wafer base is stamped or molded with a single physical track in a spiral configuration starting from the inside of the disk and spiraling outward.

  11. When viewed from the reading side the disk rotates in counterclockwise. • If you examined the spiral track under a microscope, you would be able to see the pits and lands. • Here rised bump is called pits, and the area between two pits are known as land. • The pits are actually depression made in the plastic.

  12. The stamped surface is coated with a reflective layer of metal (Aluminum) to make it reflective. • The aluminum is coated with a thin protective layer of acrylic lacquer and finally a label or printing is added.

  13. CAUTION: • Do not write with pen with pressure on the disc, don’t use the marker which are more chemically reactive. • Use only those markers which are specially made for the CD’s.

  14. Mass production of CD-ROMs Commercial mass produced CDs are stamped or pressed and not burned by a laser as many people believe. Using a laser to directly burn disk would be impractical for the thousands of copies.

  15. Steps of manufacturing the CDs • Photo resist coating • Laser recording • Master development • Electroforming • Master separation • Disc stamping operation • Metallization • Protective coating • Finished product

  16. Photo resist coating: A circular 240mm diameter piece of polished glass 6mm thick is spin coated with a photo resist layer about 150microns thick and then hardened by backing at 80°c. • Laser recording: A laser beam fires pulses of blue laser light to expose and shorten portion of the photo resist layer on the glass master.

  17. Master development: The NaCl solution is spun over the exposed glass, which then dissolves the areas exposed to the laser, thus etching pits in the photo resist. • Electroforming: The developed master is then coated with a layer of nickel alloy. This creates a metal master called a father.

  18. MASTER SEPARATION: The metal master father is then separated from the glass master. The father is a metal master that can be used to stamp discs, and for short runs, it may in fact be used that way. However , because the glass master is damaged when the father is separated, and because of troformed to create, several reverse image mothers. These mothers are then subsequently electroformed to create the actual stampers.

  19. Disc stamping operation: A metal stamper is used in an injection molding machine to press the data image into approximately 18 gms of molten polycarbonate plastic with a force of about 20,000psi.Normally 1 disc can be pressed in 2-3 sec. • Metallization: The clear stamp disc base is then sputter-coated with a thin layer of aluminum to make the surface reflective.

  20. Protective coating: The metallization disc is then spin coated with a thin layer of acrylic lacquer, which is then cured (covered) with UV light. This protects the aluminum from oxidation. • Finished product : Finally a label is affixed or printing is screen-printed on the disc and cured with UV light.

  21. Pits and lands • Reading the information back from a disc is a matter of bouncing a low-powered laser beam off the reflective layer in the disc. • Photo sensitive receptor detects when the light is reflected back. • When the light hits a land then the light will be reflected back otherwise no light will be reflected back.

  22. Pits on CD have 0.125 micron and 0.6 micron wide. • Both the pits and lands vary in land from about 0.9 micron at their shortest and 3.3 micron at their largest. • The track is spiral with 1.6 micron between adjacent turns. • The pit height is exactly ¼ of the wavelength of the laser light used to read the disc. • So, the light striking a land travel ½ of the wavelength of light further than the light striking the top of a pit ( ¼ + ¼ = ½ ).

  23. This means the light reflected from a pit is ½ wavelength out of phase with the rest of the light being reflected from a disc. • The out of phase wave cancel each other out, dramatically reducing the light that is reflecting back and making the pit appear dark at photo detector.

  24. The read laser in the CD drive is a 780nm wavelength laser of about 1 milliwatt in power. • The polycarbonate plastic has a reflective index of 1.55, so light travels through the plastic 1.55 times more slowly then air. • So, that 780nm light wave are now compressed to 780/1.55 =500nm. • ¼ of 500 nm is 125 nm, which is 0.125 microns - the specific height of the pit.

  25. Tray motor • Ejects and pulls in CD tray • Read head • Shines light on CD and reads reflection • Spin motor • Spins the CD at various speeds

  26. Laser shines light on disk Pits and lands reflect light differently Reflected light is picked up by sensor Sensor changes light to electrical signal for the computer Motor moves the head to read all parts of the spinning CD.

  27. Drive mechanical operation • The laser diode emits a low energy infrared beam towards a reflecting mirror. • The servo motor, on command from the microprocessor, position the beam onto the correct track on the CD rom by moving the reflective mirror. • When the beam hits a disc, its reflected light is gathered and focused through the first lens and sent toward the beam splitter.

  28. The beam splitter detects the returning laser light toward another focusing lens. • The last lens detects the light beam to a photo detector that converts the light into electric pulses. • This incoming pulses are decoded by the microprocessor and send the analog to the host computer as a data.

  29. Tracks and sectors: • The disc is divided into 6 main area : • Hub clamping area : The hub clamping area is just that a part of a disc where the hub mechanism in the drive can grip the disc. No data and information is stored in that area. • Power calibration area: This is found only on CD-R/RW (writable) disc and is used only by recordable drive to determine the laser power necessary to perform and optimum burn.

  30. Program memory area: This is area where the TOC (table of content) is temporary return until a recording session is closed. After the session is closed the TOC information is written to the lead in area. • Lead-in: • The lead-in area contains the disc (or session) TOC in the Q subcode channel. The TOC contains the start address and length of all tracks (songs or data), the total length of the program area, and information about the individual recorded sessions.

  31. A single Lead-in area exists on a disc recorded all at once ,or a Lead-in area starts each session on the multi-session disc. The Lead-in takes up 4500 sectors on the disc (1 minute if measured in time, or about 9.2MB worth of data).

  32. Program area: This area of the disc starts at a radius of 25 mm from the centre. • Lead-out: The Lead-out marks end of the program area or the end of the recording session on a multi session disc. No actual data is written in a Lead-out, it is simply marker. Generally in single session disc has 6750 sectors long Lead-out.

  33. A typical disc ( 74minutes) - each second contain 74 block of 2048 bytes each. By calculating the storage capacity 681,984,000 bytes- rounded as a 650MB. CD capacity :

  34. B = byte (8 bits) • KB = kilobyte (1000 bytes) • Kib = kibibyte(1024 bytes)

  35. Data encoding on the disc: • After all 98 frame (audio or data) composed for sector then the information is run through a final encoding process called EFM (eight to fourteen modulation). • This scheme takes 8 bits and converts into the 14 bits. • These 14 bits conversion codes are designed so that there are never less than 2 or more than 10 adjacent 0 bits . • This is formed of Run Length Limited (RLL) enconding called RLL 2,10 .

  36. This is designed to prevent long string of 0s which could more easily misread as well as to limit the minimum and maximum frequency of transition actually placed on recording media. • EFM codes start and end with a one or more than 5 zeros.

  37. Sampling : • When music is recorded on CD it is sampled at a rate of 44,100 times per second (Hz). • Each sample have right & left channel (stereo). • Each channel component is digitally converted into a 16 bit number. This allows for a resolution of 65,536 possible values which represent the amplitude of the sound wave for that channel. • The sampling rate determine the range of the audio frequency that can be represented in the digital recording.

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