CD’s (Cont.)
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Fixing Mistakes
• CDs have a lot of ECC so that errors can be fixed.
• If the data for a given sample cannot be recovered
using ECC, one can interpolate. Assume the bad
sample is halfway between the previous and the
next sample.
• Interpolation is not available to CDs used to store
data, so they have even more space devoted to
ECC.
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CD-DA Recap
• The CD-DA standard as set out in the “Red
Book,” defines a sector or block of 3234
bytes.
• Of those 3234 bytes,
– 2352 are actual audio data
– 98 are control (analog of servo information,
synchronization and location)
– 784 are EDC/ECC
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CD-DA Percentages
• Each CD-DA block or sector is
– 72.7% actual audio data
– 24.2% EDC/ECC
– 3% control data
• For every 8 bits of data, there are 3 more
bits of control/error data.
• If a CD-DA error cannot be corrected, one
can interpolate.
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CD-ROM
• The red-book CD-DA standards were
adapted by the “Yellow Book” standards to
handle non-audio data, i.e. regular data
files, programs, etc.
• These standards are for CD-ROMs,
compact disc – read only memory –
information written by the manufacturer and
not changed by the user.
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CD-ROM Mode 1
• CD-ROM Mode 1 starts with the basic CD-DA
sector division
3234 = 98 (control) + 784 (error)
+ 2352 (data)
and devotes some of the data portion to additional
error code and control yielding
3234 = 98 (control) + 784 (error)
+ 304 (more error/control)
+ 2048 (data).
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CD-ROM Mode 1 Percentages
• Each CD-ROM block or sector is
– 63.3% actual data
– 33.6% EDC/ECC
– 3% control data
• More than one-third error detection and
error correction code
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Less Data/Fewer Errors/More Control
• The CD-ROM standards impose more
control because one must be able to locate
data with more precision.
• The CD-ROM standard imposes more error
detection and error correction because by
the nature of the data it stores it does not
have available the interpolation approach
to dealing with errors that CD-DA does.
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Speeds
• The 150 KB/s is the base data transfer rate for CD-ROM.
• Higher speeds are reported as multiplicative factors of this
base: 2, 3, etc.
• Recall that as disc speeds got higher, they switched from
CLV (constant data rate) to CAV (variable data rate).
• The latter data rates may be reported with the term “Max”
to indicate that the reported data rate is the maximum of
the range (i.e. When one is on the outer edge of the CD).
• Such drives must still support CLV at lower speeds so that
they can play audio CDs.
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The Other Speed Factor
• The data transfer rate is the rate for reading
consecutive data. Another speed factor is
average access time which is the typical
time required to locate a “random” address.
• As with hard disks, this involves the radial
positioning of the head (seek time) and the
angular spinning of the disk (latency).
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CD-R
• As typical files grew larger and larger, the
limitations of the floppy disk as a software/data
delivery mechanism became evident.
• The CD-ROM had hundreds of times the capacity
of a floppy, but was limited in that it was “read
only.” The standards essentially required a
manufacturing process to write data.
• So the CD-R standards were introduced. They are
laid out in the orange book mainly by Phillips in
1990.
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CD-R
• CD-R (compact disc, recordable) are also
sometimes known as CD-WORM (write once read
many) or just CD-WO (write once).
• While the Yellow Book, Green Book and White
Book outline variations in the logical overlay of
the physical specifications laid out in the Red
Book, the Orange book must outline new
physical requirements that allow a user to
record a CD.
• Basically the same logical overlay as CD-ROM
will be maintained for compatibility.
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Changing Medium
• Recall that a CD-ROM is stamped from a master.
The stamping provides the pits. When reading,
the pits and distinguished from the lands because
the lands yield specular reflection (clean,
organized) while the pits yield diffuse reflection
(scattered).
– Think of light reflecting off a mirror versus light
reflecting a rippling body of water (pool, lake, etc.)
• So all that is needed is to produce this change in
the way light is reflected.
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Before Recording (Burning)
• A CD-R starts off with a plastic substrate. The
substrate is not flat, rather it has a wobbly spiral
groove.
– Think of it as lined paper, the lines guide us as to where
we will write the information.
• On top of the plastic is a photosensitive dye.
• On top of that is the reflective layer (gold, silver
or aluminum alloy).
• On top of that is a protective layer of plastic.
• On top of that is the label.
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Before Burning
From http://entertainment.howstuffworks.com/cd-burner2.htm
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Photosensitive Dye
• The chemical between the plastic and the metal is
photosensitive, meaning that it changes its
properties when exposed to light. (like film)
– The process is known as burning.
• In this case the light is laser light, the changes are
permanent, and the important aspect here is that
the changed property affects the way light is
reflected.
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After Burning
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All at once?
• The recording is permanent but it does not
necessarily have to be done all at once.
• If the recording is allowed to be done in
more than one sitting, then the recording is
said to be multi-session as opposed to
single-session.
• Another term is Track-At-Once (TAO)
versus Disc-At-Once (DAO).
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Table of Contents
• A CD-ROM has a table of contents (TOC) at the
beginning. It serves a similar purpose to the file
allocation table and root directory found on a hard
disk, it allows the files to be found.
– A.k.a. index of the disc.
• Multi-session CDs allow data to be written in
various sessions, that is for data to be appended
(not overwritten) at a later time.
– Such CDs have a table of contents for each session.
– The new TOC contains the old info plus the new.
– Such CDs cannot be read by ordinary drives unless they
are “finalized”.
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Single Session vs. Multiple
Session
• With single-session CDs, the TOC is easily
located by the drive. Multi-session CDs
drives need to be able to find the latest
TOC.
• Many older CD drives do not have this
capability.
• Reading a CD-RW requires a multi-session
capability, so it is becoming standard.
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CD-R Drive
• The burning of a CD requires a special laser,
different from the one in an ordinary CD-ROM
drive used for reading.
• CD-Rs can typically handle many of the various
logical formats (CD-DA, CD-ROM, CD-I, etc.)
• A CD-R drive can also read. Typically it reads at a
higher speed than it writes.
• Writing a CD requires a steady flow of data and
can be demanding.
– Early on, simple CD-ROM drives were often
IDE/ATAPI but CD-R drives were more likely to be
SCSI, which tend to perform better and allow other
things to occur simultaneously.
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Keep that data flowing
• To keep the flow of data steady, a CD-R
drive may use a buffer.
• Another approach is to make an image of
the disc to be burned on the hard drive
(collect all the files from their various
locations, add the error code, etc.) and then
have a nice, steady, fast continuous read of
consecutive data.
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Usefulness of CD-R
• CD-Rs had floppies beat on capacity by a
(multiplicative) factor of several hundred,
but floppies could be written and re-written.
• For files that are in the editing/updating
process, the ability to rewrite is crucial.
– CD-Rs have come down in price, but they’re
too expensive to be thrown away regularly – let
alone the impact that would have on the
environment.
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CD-RW
• CD-Rs are “write once” because the photosensitive dye is permanently changed when it is
written/burned.
• In CD-RW this material is replaced with a phasechange layer.
– A phase is a state that a material can be in. For
example, H2O can be in one of three phases: ice (solid),
water (liquid), stream (gas).
– Materials may have several different solid phases.
• A phase is stable (the material can remain in a
given phase indefinitely), but a phase change is
reversible (heating for example may return the
material to its original state).
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Phase change
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Dr Jekyll and Mr Hyde
• The two phases in question are a matter of how
“organized” the material is.
• In one phase, the material is a very orderly crystal
that reflects light specularly.
• In the other phase, the material is disorderly and
reflects light diffusely.
• Heating the material a little can give it a little bit
of movement and flexibility so it can organize.
Heating it up a lot throws it into disorder, but it
cools quickly (quenches) and remains in the
disorderly state.
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The price one pays
• The differences in these phases is more
subtle than the differences in CD-ROMs
and CD-Rs.
• Thus a drive must have a more sensitive
photo-detector in order to read a CD-RW.
• Many older (“legacy”) drives do not have
such a sensitive detector and cannot read
CD-RWs.
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Digital Versatile/Video Disk
(DVD)
• The size of software and data files continues
to grow, so the CD with its 650 MB
capacity is becoming too limited.
• A newer, higher capacity alternative is the
DVD which stands for either digital video
disk or digital versatile disk.
– Although some will now say DVD doesn’t
stand for anything.
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DVDs
• A DVD is just a variation on a CD, information is
read from the disc by reflecting light from its
surface.
• The differences between DVDs and CDs is a
matter of speed and capacity (DVDs are better on
both counts) and logical format.
• As with CDs there are various formats and one has
to be careful about compatibility.
– Many DVD standards are maintained by ECMA.
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ECMA
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DVD-R Standards Page
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DVD-R Standards Page (cont.)
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Viva la difference
• A different laser
– A DVD laser has a wavelength of 636 nm or 650 nm
compared to a CD laser having a wavelength of 780
nm.
• The nm stands for nanometer, that’s 10-9 m, a billionth of a
meter.
– With a smaller wavelength, one can “resolve”
(distinguish) smaller/closer objects. In this case, the
smaller wavelength allows the pits and the lands to be
smaller and closer together on a DVD than on a CD.
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Waves with different
wavelengths


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Electromagnetic spectrum
←Wavelength getting smaller
Red light has a smaller wavelength than IR (infrared). Blue
light (used by Blu-Ray) has an even smaller wavelength.
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Smaller Pits  Higher
Density/Capacity
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DVD Pits
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Another Difference
• The Error Correction Code (ECC) used in
DVDs is more efficient.
– As with CDs the amount and type of ECC will
vary between DVDs used for multimedia and
DVDs used for “regular” files.
• DVDs also use a somewhat larger area for
recording.
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Result: Higher Capacity
• The previous factors result in a DVD having
several times the capacity of a CD.
– DVD capacity is approximately 4.7 GB
(minimum) compared to CD-ROM capacity of
650 MB – about seven times larger.
– Caution: In DVD standards GB means 109
(1,000,000,000) instead of 230 (1,073,741,824).
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Higher Speeds
• DVDs have a higher standard data transfer rate –
1.32 MB/s compared to 150 KB/s for CD-DA –
about nine times faster.
– This speed specification is for video viewing. When
DVDs are used for non-video data, they can be
operated at higher speeds, which are reported as
multiplicative factors of the standard: 2X, 3X etc.
– Older DVD-ROM drives used CLV, now they tend to
use CAV. In the latter case, the maximum
multiplicative factor is reported, e.g. 16X max.
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Sizes, Sides and Layers
• DVDs come in two standard sizes: diameter 80
mm or diameter 120 mm (standard CD size).
• DVDs can be single-sided (SS) or doubled-sided
(DS), effectively two DVDs glued together.
• A DVD can be single layer (SL) or double layer
(DL).
– The upper layer is semi-transparent, so one can see
through to the second layer of data.
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Single Versus Double Layer
The laser is focused on the different layers at different
times.
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Numbers and Capacities
•
•
•
•
•
DVD-5: single-sided, single-layered 4.7GB
DVD-9: single-sided, double-layered 8.5GB
DVD-10: double-sided, single-layered 9.4GB
DVD-14: double-sided, mixed layers 13.2GB
DVD-18 double-sided, double-layered 17GB
– Still uncommon
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Layers and Sides
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UDF
• There was some attempt to avoid the pitfalls of the
CD situation (so many formats) and out of that
came UDF.
– Of course, they didn’t succeed, there a number of DVD
formats.
• UDF stands for Universal Disk Format.
• “UDF, defined by the Optical Technology Storage
Association (OTSA), is a subset of ISO 13346, an
interchange standard for non-sequential recording
of data.”
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UDF (Cont.)
• It is a file system used for DVDs and optical
media in general.
– Some CDs can use it.
• It is “universal” in that all of the major DVD
vendors have agreed to use it.
• It is also “universal” in that data files and
multimedia files are not treated separately.
• It allows the operating system to understand what
is on a DVD.
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Various Formats
• In addition to the different physical formats,
there are a number of different application
formats and the accompanying
compatibility issues.
– DVD+R, DVD+RW, DVD-RAM, DVD-R,
DVD-RW, DVD-ROM
• DVD drives are usually “backward
compatible” in that they can read most CD
formats.
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DVD-Video and DVD-ROM
• Analogous to CD-DA (for music) and CDROM (for data) are DVD-Video (for
movies) and DVD-ROM (for data).
– An DVD player may only be able to read DVDVideo, whereas a DVD drive will be able to
read DVD-ROM as well as DVD-Video.
• These standards are read only. The
information is stamped or pressed onto the
disc by a manufacturer.
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MPEG2
• MPEG2 is part of the DVD-Video standard. It
describes how multimedia files can be compressed
(50-to-1).
• MPEG stands for Moving Picture Experts
Group.
– MPEG is part of ISO.
• There are three major MPEG standards: MPEG-1,
MPEG-2 and MPEG-4.
• The decoding can be done by software or
hardware.
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MPEG
MPEG Uses
1. Discrete Cosine Transform (DCT).
2. Quantization, selectively throwing away information
that won’t be missed (“lossy compression”).
3. Huffman coding, a lossless compression technique that
uses code encoding frequently occurring data with short
codes and infrequently occurring data with long codes.
4. Motion compensated predictive coding: encode
changes from one frame to the next rather than
encoding each new frame in its entirety.
5. Bi-directional prediction like interpolation discussed
in CD-DA.
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Recordable
• As with CD-R, making a recordable DVD
requires some changes in the physical
medium.
– The writing is not done by pressing but by
exposing the DVD-R’s photo-sensitive dye
layer to light and “burning” the information on
it.
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Rewritable
• As with CDs, to achieve rewritability, one must
replace the photosensitive material which is
permanently burned with “phase change” material.
– The phase change material in DVD RW means that
they do not reflect as strongly as DVD-ROM or DVDR. This makes them harder to read.
– Sometime drives have trouble distinguishing between
double layered DVDs and DVD-RW.
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The Competition
• There are three competing technologies for
rewritable DVDs
– DVD-R
– DVD+R
– DVD-RAM
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DVD-RW
• A.k.a. DVD-ER and DVD-R/W
• Can be written about 1000 times
• There is a distinction with DVD-RW discs
– 1.1 do not support CPRM (Content Protection
for Removable Media)
– 1.1B do support CPRM
• Apple and Compaq
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CPRM
• Content Protection for Removable Media
(CPRM) enforces copy protection
restrictions using a mechanism built into the
storage medium itself.
• Based on broadcast encryption, the CPRM
system would incorporate “tags” into
storage media.
• Controversial, other alternative pursued.
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DMCA
• Stands for Digital Millennium Copyright Act (a
update of copyright law law from 1998).
– “It is a crime to circumvent anti-piracy measures that
are built into commercial software.”
– “It is a crime to manufacture, sell or distribute codecracking devices that illegally copy software. However,
it is not a crime to crack copyright protection devices in
order to conduct encryption research, assess product
interoperability or test the security of computer
systems.”
– Etc.
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DVD-RAM
• Better access speeds
• Can be written 100,000 times
• Uses a cartridge (for better medium
protection)
• Standard DVD drives don’t support the
cartridge. You may lose rewritability if you
remove them from the cartridge.
• Hitachi, Panasonic and Toshiba
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DVD+RW
• Written to 1,000 times
• No cartridge
• Not officially a standard of the DVD Forum
but backed by Sony and HP.
• Records video well.
• DVD+R is a write-once technology that is
compatible with DVD+RW.
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DVD-Audio versus Audio CD
Sampling Rate
Number of Levels
CD
44,100 Hz
216 = 65,536
DVD
192,000 Hz
224 = 16,777,216
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Is DVD-Audio overkill?
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Blu-Ray
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Viva la difference (Again)
• A different laser
– Blu-Ray uses a laser diode with wavelength 405 nm
compared to a DVD laser of 636 nm or 650 nm
compared to a CD laser of 780 nm.
• The nm stands for nanometer, that’s 10-9 m, a billionth of a
meter.
– With a smaller wavelength, one can “resolve”
(distinguish) smaller/closer objects. In this case, the
smaller wavelength allows the pits and the lands to be
smaller and closer together on a Blu-Ray than on a
DVD.
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Hard-coating technology
• Blu-ray data is closer to the surface of the
disc (compared to the DVD)
– More vulnerable to scratches.
– At first they came in cartridges for protection.
– Newer polymer coating technology makes the
cartridges unnecessary.
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Recording speed
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Region Codes
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References
• PC Hardware in a Nutshell, Thompson and
Thompson
• http://www.pctechguide.com/10dvd.htm
• http://www.webopedia.com
• http://www.pcguide.com
• http://entertainment.howstuffworks.com/cdburner2.htm
• http://en.wikipedia.org/wiki/Blu-ray_Disc
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