Differences between current version and revision by previous author of CompactDisc.
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| Newer page: | version 9 | Last edited on Saturday, January 31, 2009 12:52:59 pm | by LawrenceDoliveiro | |
| Older page: | version 5 | Last edited on Wednesday, May 10, 2006 6:52:14 pm | by CraigBox | Revert |
@@ -1,36 +1,41 @@
-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.
+Standard discs hold 650[MB] or there abouts, although 700MB discs have now become more
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
.
-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
.
+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
.
-So be
on the lookout for things
that are ''
not'' CompactDisc~s
which are labelled illegally while shopping
.
+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 buying 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)::
<verbatim>
# mount /path/to/image.iso /place/to/mount -o loop
</verbatim>
-There are two competing standards to allow longer filenames and a few other things [ISO]9660 does not provide. The earlier one, originating from the [Unix] environment, is called Rock Ridge. The other one, made up my
MicrosoftCorporation, is called Joliet. (Note that some characters that are valid on [Unix] FileSystems are not allowed on Joliet discs.)
+There are two competing standards to allow longer filenames and a few other things [ISO]9660 does not provide. The earlier one, originating from the [Unix] environment, is called Rock Ridge. The other one, made up by
MicrosoftCorporation, is called Joliet. (Note that some characters that are valid on [Unix] FileSystems are not allowed on Joliet discs.)
!!! Compact Discs and your computer
For reading audio discs or writing CDs, you need access to the [CDROM] drive raw device. For a data disc, this raw device should be [mountable|mount(8)] onto the filesystem.
-Under a [Linux] OperatingSystem, these raw devices are called <tt>/dev/hd''x''
</tt> for [IDE
] drives and
<tt>/dev/scd''n''</tt> for [SCSI
] drives. Under [FreeBSD] 5 and later, they're called <tt>/dev/acd''n''</tt> for [IDE
] drives and ''??? (AddToMe)'' for [SCSI] drives.
+Under a [Linux] OperatingSystem, these raw devices are called <tt>/dev/hd</tt>''xn''
for [PATA
] drives,
<tt>/dev/scd</tt>
''n'' for [SCSI] drives and <tt>/dev/sr
</tt>''n''
for [SATA
] drives. Under [FreeBSD] 5 and later, they're called <tt>/dev/acd''n''</tt> for [PATA
] drives and ''??? (AddToMe)'' for [SCSI] drives.
See also:
* [CDRippingNotes]
* [CDWritingNotes]
