Differences between version 6 and predecessor to the previous major change of CompactDisc.
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Newer page: | version 6 | Last edited on Sunday, April 29, 2007 2:43:42 pm | by LawrenceDoliveiro | Revert |
Older page: | version 5 | Last edited on Wednesday, May 10, 2006 6:52:14 pm | by CraigBox | Revert |
@@ -1,21 +1,26 @@
-Round shiny things made of aluminium and plastic.
+Round shiny things made of aluminium and plastic. Data is encoded onto them in the form of tiny indentations called "pits" (for the 1-bits) and gaps between called "lands" (for the -bits). The bits are read using an infrared laser.
+
+Note that the data is encoded in a very thin layer just underneath the ''label'' side of the disc; any damage to this side can cause __irreversible loss of data__. When you look at the ''shiny'' side, you are actually looking through the layer of transparent plastic which makes up most of the thickness of the disc, towards the data layer under the label. Scratches to this plastic layer can be tolerated to some extent, and there are even ways (though not necessarily reliable ones?) to repair scratches by smoothing them out
.
Often used for copying LinuxDistribution~s onto and distributing at InstallFest~s.
Standard discs hold 650[MB] or there abouts, although 700MB discs are also common.
!!! Audio Discs
-For
audio,
the logo
"Compact Disc Digital Audio" is actually owned
by Philips and licensed for use
by manufacturers who meet
the "Red Book
" standard for digital
audio. Many forms of CopyControl degrade the audio quality or alter the data structures on the plastic wafer in an attempt to prevent customers using the data. By thus degrading their product, the manufacturers may no longer meet the "Red Book" standard, meaning they cannot ''legally'' be called CompactDisc~s.
+The physical format of a CD and the representation of
audio tracks is defined in a specification called
the "Red Book". Discs and players that are certified to this specification carry the familiar
"Compact Disc Digital Audio" logo, which
is controlled
by Philips.
+
+The raw error rate of reading the zeroes
and ones on the disc is about 1 in 1000; in audio terms, that would translate to a signal-to-noise ratio of 60dB, which is about comparable with the old audiocassettes (i.e. it would not be considered "hi-fi"). However,
by the use of error-correcting codes (specifically,
"Reed-Solomon Cross-Interleave Redundancy Checking
"), this error rate drops to 1 in 1000000, which translates in
audio terms to a signal-to-noise ratio best described as "fantastic". However, in computer terms, that is still not reliable enough for storing computer data
.
+
+
Many forms of CopyControl degrade the audio quality or alter the data structures on the plastic wafer in an attempt to prevent customers using the data. By thus degrading their product, the manufacturers may no longer meet the "Red Book" standard, meaning they cannot ''legally'' be called CompactDisc~s.
Since consumers might notice that plastic wafers which do not work in their equipment are missing the "CD" logo and so stop bying such CopyControl~led wafers, there is an incentive for manufacturers to mislabel their discs.
So be on the lookout for things that are ''not'' CompactDisc~s which are labelled illegally while shopping.
-
!!! Data Discs
-The normal filesystem used on a data disc is [ISO]9660. Hence the common use of "[ISO]" to refer to disc images and of the extension <tt>.iso</tt> for their filename.
+The "Yellow Book" specification takes the original audio CD, adds some more error-correcting codes to make things reliable enough to store computer files, to produce a disc format called "CD-ROM" ("Compact Disc Read-Only Memory").
The normal filesystem used on a data disc is [ISO]9660. Hence the common use of "[ISO]" to refer to disc images and of the extension <tt>.iso</tt> for their filename.
Images can be created using mkisofs(8) and inspected using isoinfo(8) or isodump(8). You do not need root permissions or any special devices to use these commands (although you do need normal read/write file permissions). Alternatively, you can mount an ISO image like a regular device using the [Kernel]'s loopback support (but note that this requires SuperUser privileges)::
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