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- The first disc that could be written and read by optical means (using
light as a medium) was developed by James T. Russell. In the late 1960s,
Russell created a system that recorded, stored, and played audio/video
data using light rather than the traditional contact methods, which
could easily damage the disks during playback.
- Russell developed a photosensitive disc that stored data as 1
micron-wide dots of light and dark. The dots were read by a laser,
converted to an electrical signal, and then to audio or visual display
for playback.
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3
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- Russell's own company manufactured the first disc player in 1980,
although the technology never reached the marketplace until Philips and
Sony developed the technology.
- In late 1982, Philips and Sony released the first of the compact disc
(CD) formats, which they then called CD-DA (digital audio).
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4
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- CDs and DVDs are types of optical storage media. Optical storage media
are written and read with an extremely fine, precisely aimed laser beam.
Data storage consists of millions of indentations burnt into a
reflective metallic surface.
- The indentations refract (change the direction of) the light, reducing
the intensity of the reflection. When a laser beam is aimed at the disc
surface through a two-way mirror, sensors register the difference in
reflection intensity as binary data - zeros and ones.
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5
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- Digital Versatile disc (DVD) had its beginning in 1994, when two
formats, Super disc (SD) and Multimedia CD (MMCD) were introduced.
- Promoters of the competing technologies failed to reach an agreement on
a single standard until 1996, when DVD was selected as a convergence
format.
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- DVD-R
- Digital Versatile disc - Recordable (DVD-R) is a type of write once,
read many (WORM) DVD format that allows the user to record a single time
on a DVD disk.
- DVD-RW
- Digital Versatile disc -
Rewritable (DVD-RW) is a DVD format that allows the user to record and
erase multiple times on a single DVD disk.
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- DVD-5
- Single-sided, single-layered disc with 4.7Gb capacity
- DVD-9
- Single-sided, double-layered with 8.5Gb capacity
- DVD-10
- Double-sided, single-layered disc with 9.4Gb capacity
- DVD-18
- Double-sided, double-layered disc with 17Gb capacity
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- Digital Versatile disc - Read Only Memory (DVD-ROM) is a DVD format with
technology similar to the familiar DVD video disk, but with a more
computer-friendly file structure.
- The DVD-ROM format was designed to store the same type of computer data
typical of a CD-ROM.DVD-ROMs have seven times the storage capacity of
CD-ROMs.
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10
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- CD-R disks usually hold the standard 74 minutes (650MB) of data,
although some can hold up to 80 minutes (700MB).
- CD-RW disks usually hold 74 minutes (650MB) of data, although some can
hold up to 80 minutes (700MB) and, according to some reports, can be
rewritten as many as 1000 times.
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11
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12
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- 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.
- Looking through the polycarbonate layer at the bumps, they look
something like the above illustration
- NOTE: A nanometer is a billionth
of a meter!
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- 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.
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15
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16
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- Data is encoded in the CD's polycarbonate layer in a continuous spiral
track, from the inside to the outside, about 1.6 to 2.2 microns wide (a
micron is 1/1000 of a millimeter), with pits about 0.6 microns wide
scored into the track. The thin metallic layer conforms to the contours
of the substrate, and the outer acrylic layer over it is impressed with
the CD label.
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- Based on its length, each pit is interpreted as a sequence of zeroes,
and, based on its length, each land is interpreted as a sequence of
ones. digital-to-analog conversion translates the binary data into audio
signals for reproduction.
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- When a CD is placed in a CD player, the recorded track is scanned by a
low-intensity infrared laser. To enable a consistent scanning rate (from
the smaller center to the larger outside of the disk), the rotation rate
slows from 500 to 200 rpm (revolutions per minute) as the laser beam
spirals 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).
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- Two additional lasers are sometimes used to help control the focus of
the primary laser and the rotation of the disk. The pits and smooth
areas (the smooth areas are called lands) are read by a laser when the
disc is played.
- Pits and lands reflect the light from the laser differently, and that
difference is encoded as binary data: the light hitting a land reflects
back directly to a photodiode, which generates an electrical pulse,
while the light hitting a pit is refracted (deflected from a straight
path, or scattered), and, consequently, reduced below the level needed
to activate the photodiode.
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- CD-R (for Compact Disk, recordable) is a type of write once, read many (worm)
CD format that allows one-time recording on a disk. The CD-R (as well as
the CD-RW) format was introduced by Philips and Sony in their 1988
specification document, the Orange Book.
- CD-Rs are composed of a polycarbonate plastic substrate, a thin
reflective metal coating, and a protective outer coating. However, in a
CD-R, a layer of organic polymer dye between the polycarbonate and metal
layers serves as the recording medium.
- The composition of the dye is permanently transformed by exposure to a
specific frequency of light. Some CD-Rs have an additional protective
layer to make them less vulnerable to damage from scratches, since the
data - unlike that on a regular CD - is closer to the label side of the
disk.
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- The composition of the dye is permanently transformed by exposure to a
specific frequency of light. Some CD-Rs have an additional protective
layer to make them less vulnerable to damage from scratches, since the
data - unlike that on a regular CD - is closer to the label side of the
disk.
- A pregrooved spiral track helps to guide the laser for recording data,
which is encoded from the inside to the outside of the disc in a single
continuous spiral.
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- The laser creates marks in the dye layer that mimic the reflective
properties of the pits and lands (lower and higher areas) of the
traditional CD.
- The distinct differences in the way the areas reflect light register as
binary data that is then un-encoded for playback.
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- The patterns created are less distinct than those of other CD formats,
requiring a more sensitive device for playback. Only drives designated
as "MultiRead" are able to read CD-RW reliably.
- The alloy phase-change recording layer, which is commonly a mix of
silver, indium, antimony and tellurium, is sandwiched between two
dielectric layers that draw excess heat from the recording layer.
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- After heating to one particular temperature, the alloy will become
crystalline when it is cooled; after heating to a higher temperature it
will become amorphous (won't hold its shape) when it is cooled.
- By controlling the temperature of the laser, crystalline areas and
non-crystalline areas are formed. The crystalline areas will reflect the
laser, while the other areas will absorb it. The differences will
register as binary data that can be un-encoded for playback.
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- To erase or write-over recorded data, the higher temperature laser is
used, which results in the non-crystalline form, which can then be
reformed by the lower temperature laser.
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- CD-i (Compact disc - interactive) is a multimedia CD format specified in
1986, in the Green Book.
- CD-i was detailed as a whole system, comprising not just a disc and data
format, but an entire hardware and software system, a variety of special
compression methods for audio and visual data, and a method of
interleaving audio, video, and text data.
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- Optical Media Drive Speed
- Maximum Data Transfer Rate
- RPMs (revolutions per minute)
- 1X CD-ROM
- 150 KB/sec
- 200 - 530
- 2X CD-ROM
- 300 KB/sec
- 400 1060
- 4X CD-ROM
- 600 KB/sec
- 800 2120
- 8X 12X CD-ROM
- 1.2 MB/sec
- 1600 4240
- 24X 50X
- 1.8 - 6 MB/sec
- 2400 6360 approximately
- 1X DVD-ROM
- 1.25 MB/sec
- No exact data, but much slower than 1X CD-ROM
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- The DVDs data layer is in the middle of the disc thickness, in order to
accommodate double-sided disks, whereas data is recorded near the
surface of the CD-ROM disk.
- The laser on the DVD drive has a pair of lenses on a swivel: one to
focus the beam on the correct DVD data layer, and one for reading CD-ROM
disks.
- Example: Data Transfer Rates
- 1X DVD-ROM drive transfers data at 1,250KBps.
- 1X CD-ROM drive transfers data at only 150KBps.
- Multispeed DVD-ROM drives transfer at 2,700 KBps
- Presently, DVD drives are capable of a 6X speed (8,100 KBps)
- for DVD media, and 32X speed for reading CD-ROMs.
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- Although DVD-ROM drives have a much lower RPM (revolutions per minute)
value, data transfer rates are substantially higher than a CD-ROM drive
at equivalent RPMs, because the data is compressed by the use of a
greater number of smaller data pits and a smaller track pitch (the
distance between tracks).
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- For example, a 1X DVD-ROM drive transfers data at 1,250KBps, whereas a
1X CD-ROM drive transfers data at only 150KBps. By 1998, multispeed
DVD-ROM drives became available that were capable of reading DVD media
at double-speed, resulting in a transfer rate of 2,700 KBps, and of
rotating CDs at 24X.
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- When movies are put onto DVDs, they are encoded in MPEG-2 format and
then stored on the disc. This compression format is a widely accepted
international standard. Your DVD player contains an MPEG-2 decoder,
which can uncompress this data as quickly as you can watch it.
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- HD-ROM (High-Density - Read Only Memory) is a high capacity storage
technology developed at Norsam Technologies, in conjunction with an IBM
research group, that enables the discs to store hundreds of times as
much information as a CD-ROM.
- HD-ROM uses a very narrow, finely focused particle beam (consisting of
charged gallium ions) to write data. HD-ROM technology can be used to
write data on different types of media, such as metal or other durable
materials, to create virtually indestructible storage.
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- The key technology behind both HD DVD and Blu-ray is the blue laser, an
evolutionary development of the infrared and red lasers used in CD and
DVD players.
- The wavelength of the light coming from the blue lasers in both high-def
disc formats is 405 nanometers (billionths of a meter), which is shorter
than the DVD wavelength of 650 nanometers (a pure red) and nearly half
the CD wavelength of 780 nanometers (in the near-visible infrared).
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- Data Rate: 36 megabits per second (Mbps) for both BD and HD DVD compared
with 11 Mbps for standard DVD
- Capacities: While a single-sided, single-layer DVD can hold 4.7
gigabytes (GB), a single-sided, single-layer HD DVD can hold 15 GB, and
a comparable Blu-ray Disc can hold 25 GB
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- This requires an expensive changeover to all-new disc-pressing and
manufacturing processes and machines.
- Also, DVD optical-scanning mechanisms are optimized for focusing on the
deeper data layers in DVDs and HD DVDs. The laser-pickup mechanisms in
Blu-ray players and recorders also need to be able to read CDs and DVDs
as well, and possibly even HD DVDs.
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- So laser pickups that can handle a number of disc formats are likely to
be more expensive than ones optimized just for BD.
- Bringing the data layer closer to the disc surface also increases
Blu-ray's sensitivity to surface defects like scratches, dust, and
fingerprints, and it will be
important to keep BDs as clean and scratch-free as possible (not a bad
idea for any optical format).
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- With CDs, DVDs, and HD DVDs, the defects are out of focus by the time
the laser spot hits the data layer inside the disc, minimizing
interference.
- The first Blu-ray prototype discs had disc cartridges to guard against
surface contamination, but recent prototypes have had a “hard-coat”
layer (which TDK uses on some of its recordable DVDs) that reduces
data-corrupting disc damage.
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Notes
