CDs, DVDs, MP3s and iPods

1
 CDs, DVDs, MP3s and iPods

• Michael N. Tralli6th Grade Foothills Middle
  SchoolArcadia Unified School DistrictArcadia,
  CA 91007
• 2005 IEEE Aerospace ConferenceJunior Engineering
  Conference
• Big Sky, Montana
• March 5-12, 2005

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Outline

• Compact Discs (CDs)
• Digital Versatile (formerly Video) Discs (DVDs)
• MP3s
• iPods
• Summary

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What is a CD?

• A CD consists of an injection-molded piece of
  clear polycarbonate plastic, about 4/100 of an
  inch (1.2 mm) thick.
• During manufacturing, this plastic is impressed
  with microscopic bumps arranged as a single,
  continuous long spiral track of data.
• The elongated bumps that make up the track are
  each 0.5 microns wide, a minimum of 0.83 microns
  long and 125 nanometers high.
• If you could lift the data track off a CD and
  stretch it out into a straight line, it would be
  almost 3.5 miles (5 km) long!
• A thin, reflective aluminum layer then is
  sputtered onto the disc, covering the bumps.
• A thin acrylic layer is sprayed over the aluminum
  to protect it.

4
CD Data Storage

• A CD can store up to 74 minutes of music, so the
  total amount of digital data that must be stored
  on a CD is
• 44,100 samples/channel/second x 2 bytes/sample x
  2 channels x 74 minutes x 60 seconds/minute
  783,216,000 bytes.
• Music is sampled 44,100 times per second. The
  samples are 2 bytes (16 bits) long.
• Separate samples are taken for the left and right
  stereo channels.
• A CD stores a huge number of bits for each second
  of music
• 44,100 samples/second 16 bits/sample 2
  channels 1,411,200 bits per second
• 1.4 million bits per second equals 176,000 bytes
  per second (next slide). If an average song is
  three minutes long, then the average song on a CD
  uses about 32 million bytes of space.
• That's a lot of space for one song, and it's
  especially large when you consider that over a
  56K modem, it would take close to two hours to
  download that one song (later slide, MP3).

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Bits and Bytes

• Bits are bundled together into 8-bit collections
  called bytes.
• 8 bits in a byte can represent 256 values ranging
  from 0 to 255
• 0 00000000 1 00000001 2 00000010
• ...254 11111110 255 11111111
• A CD uses 2 bytes, or 16 bits, per sample.
• That gives each sample a range from 0 to 65,535,
  like this
• 0 0000000000000000 1 0000000000000001
  
• 2 0000000000000010 ...65534
  1111111111111110
• 65535 1111111111111111

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Sampling
An audio CD contains a sample audio at 44,100 per
second (Hz) with bandwidth samples at 16 bits.
The figure left at the top shows an analog
signal, the right shows the same signal converted
in digital signal at 44,100 Hz and bandwidth of
16. The following figures show two possible
cases sampling frequency halved at 22,000 Hz
with a width of 16 bits and a sampling frequency
at 44,100 Hz and width of 8 bits. In both cases
there is a loss of information in comparison to
the digital signal at 44,100 Hz and a width of 16
bits, but you have smaller audio files Ref.
MP3News.
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CD Player Components

• The CD player finds and reads the data stored as
  bumps.
• The drive consists of three fundamental
  components
• A drive motor spins the disc.
• This drive motor is precisely controlled to
  rotate between 200 and 500 rpm depending on which
  track is being read.
• A laser and a lens system focus in on and read
  the bumps.
• A tracking mechanism moves the laser assembly so
  that the laser's beam can follow the spiral
  track.
• The tracking system has to be able to move the
  laser at micron resolutions.

8
CD Player Laser Focus

• The laser beam passes through the polycarbonate
  layer, reflects off the aluminum layer and hits
  an opto-electronic device that detects changes in
  light.
• Bumps reflect light differently, and the
  opto-electronic sensor detects that change in
  reflectivity.
• The electronics in the drive interpret the
  changes in reflectivity in order to read the bits
  that make up the bytes.
• By interpreting "perfect mirror" as a "1," and
  "bump" as a "0," it is easy to store digital
  information on a CD.

9
CD Player Tracking

• The laser beam must stay centered on the data
  track.
• This centering is done by the tracking system.
• As the CD plays, it has to continually move the
  laser outward.
• As the laser moves outward from the center of the
  disc, the bumps move past the laser faster
• This happens because the linear, or tangential,
  speed of the bumps is equal to the radius times
  the speed at which the disc is revolving (rpm).
• As the laser moves outward, the spindle motor
  must slow the speed of the CD.
• Bumps travel past the laser at a constant speed,
  and data comes off the disc at a constant rate.

10
CD Data Encoding

• Since the laser is tracking the spiral of data
  using the bumps, there cannot be extended gaps
  where there are no bumps in the data track.
• To solve this problem, data is encoded using EFM
  (eight-fourteen modulation). In EFM, 8-bit bytes
  are converted to 14 bits, and it is guaranteed by
  EFM that some of those bits will be 1s.
• Since the laser wants to be able to move between
  songs, data needs to be encoded into the music
  telling the drive "where it is" on the disc.
• This problem is solved using what is known as
  subcode data. Subcode data can encode the
  absolute and relative position of the laser in
  the track, and can also encode such things as
  song titles.
• Since laser may misread a bump, there need to be
  error-correcting codes to handle single-bit
  errors.
• To solve this problem, extra data bits are added
  that allow the drive to detect single-bit errors
  and correct them.

11
Encoding Interleaving

• Because a scratch or a speck on the CD might
  cause a whole packet of bytes to be misread
  (known as a burst error), the drive needs to be
  able to recover from such an event.
• This problem is solved by actually interleaving
  the data on the disc, so that it is stored
  non-sequentially around one of the disc's
  circuits.
• The drive actually reads data one revolution at a
  time, and un-interleaves the data in order to
  play it.
• If a few bytes are misread in music, the worst
  thing that can happen is a little fuzz during
  playback.
• When data is stored on a CD, however, any data
  error is catastrophic. Therefore, additional
  error correction codes are used when storing data
  on a CD-ROM.

12
Burning CDs

• To create a writeable CD (CD-R), you need to
  modify the surface of a CD so you can burn data
  onto it, turning it into a CD-R.
• There are no bumps on a new CD-R.
• A clear dye layer covers the CD's mirror.
• A write laser heats up the dye layer enough to
  make it opaque.
• The read laser in a CD player senses the
  difference between clear dye and opaque dye the
  same way it senses bumps -- it picks up on the
  difference in reflectivity.
• To create a rewriteable CD (CD-RW), you need a
  dye layer that can be changed back and forth
  between opaque and transparent.
• The material changes its transparency depending
  on temperature.
• By changing the power (and therefore the
  temperature) of the writing laser, the data on
  the CD can be changed, or "rewritten."
• A CD-R can be read by just about any CD player.
  CD-RW discs are not so versatile -- lots of older
  CD players cannot read them.

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How Does a DVD Work?

• A DVD (Digital Versatile Disc) works exactly the
  same way as a CD, but it can hold a lot more
  information -- about 4.7 gigabytes (about seven
  times as much as a CD).
• DVDs can hold more data than CDs because the
  bumps are smaller and the tracks are closer
  together
• Up to 133 minutes of high-resolution video in
  letterbox or pan-and-scan format, at 720 dots of
  horizontal resolution (The video compression
  ratio is typically 401 under MPEG-2.)
• Soundtrack presented in up to eight languages
  using 5.1 channel Dolby digital surround sound
• Subtitles in up to 32 languages
• You can also use DVDs to store music.
• You can store almost eight hours of music per
  side!  

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DVD Compression / MP3

• MPEG is the acronym for Moving Picture Experts
  Group. This group has developed compression
  systems used for video data.
• DVD movies, HDTV broadcasts and DSS use MPEG
  compression to fit video and movie data into
  smaller spaces.
• The MPEG compression system includes a subsystem
  to compress sound, called MPEG audio Layer-3.
• We know it by its abbreviation, MP3.

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MP3 Format Ripping

• The MP3 format therefore is a compression system
  for music.
• A technique called perceptual noise shaping is
  used. It is "perceptual" partly because the MP3
  format uses characteristics of the human ear to
  design the compression algorithm.
• There are certain sounds that the human ear
  cannot hear.
• There are certain sounds that the human ear hears
  better than others.
• If there are two sounds playing simultaneously,
  we hear the louder one but cannot hear the softer
  one.
• Using facts like these, certain parts of a song
  can be eliminated without significantly hurting
  the quality of the song for the listener.
• Compressing the rest of the song with well-known
  compression techniques shrinks the song further -
  by a factor of 10 to 14.
• When you are done creating (ripping) an MP3 file,
  you have a "near CD quality" song.

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About iPods

• iPods are like MP3 players but they were started
  by Apple and now also HP.
• iPods also support AAC audio compression
• Short for Advanced Audio Coding, one of the audio
  compression formats defined by the MPEG-2
  standard.
• MPEG Advanced Audio Coding (AAC), also known as
  MPEG-2 NBC (Non-Backward Compatible) represents
  the actual state of the art in natural audio
  coding.
• The MPEG-4 standard adds some new tools in AAC,
  in order to improve coding quality at low bit
  rates
• Perceptual Noise Substitution allows to replace
  coding of noise-like parts of the signal by some
  noise generated on the decoder side.
• Long Term Prediction requires less computation
  power than the one used in MPEG-2 AAC, while
  providing comparable coding performance.

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MP3 Players vs. iPods

• MP3 players support MP3, WMA (Windows) and .WAV
  formats
• iPods also support AAC (iTunes) and other Apple
  compression formats
• AAC provides higher quality audio reproduction
  than MP3 and requires 30 less data to do so.
• AAC is able to handle more channels than MP3
• 48 full audio channels and 16 low frequency
  enhancement channels compared to 5 full audio
  channels and 1 low frequency enhancement
• Can handle higher sampling frequencies than MP3
  (up to 96kHz compared to 48kHz).
• MPEG tests demonstrate that for 2 channels it is
  able to provide slightly better audio quality at
  96 kb/s than MP3 at 128 kb/s.
• According to Apple, AAC is worthy of replacing
  MP3 as the new Internet audio standard.
• Also, improved decoding efficiency, requiring
  less processing power

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Summary

• Whats better, CD or DVD?
• Whats better, iPods or MP3 players, iTunes or
  MP3 files?
• Whats better, to burn music or rip music?
• And
• Whats better, snowboarding or skiing?
• Ref http//computer.howstuffworks.com