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Annotated edit history of CompactDisc version 9, including all changes. View license author blame.
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6 LawrenceDoliveiro 1 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 0-bits). The bits are read using an infrared laser.
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3 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.
3 AristotlePagaltzis 4
5 Often used for copying LinuxDistribution~s onto and distributing at InstallFest~s.
2 GerardSharp 6
7 LawrenceDoliveiro 7 Standard discs hold 650[MB] or there abouts, although 700MB discs have now become more common.
4 JohnMcPherson 8
9 !!! Audio Discs
6 LawrenceDoliveiro 10 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.
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12 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.
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14 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.
7 LawrenceDoliveiro 15 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.
2 GerardSharp 16
17 So be on the lookout for things that are ''not'' CompactDisc~s which are labelled illegally while shopping.
4 JohnMcPherson 18
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20 !!! Data Discs
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6 LawrenceDoliveiro 22 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.
4 JohnMcPherson 23
24 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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26 <verbatim>
27 # mount /path/to/image.iso /place/to/mount -o loop
28 </verbatim>
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7 LawrenceDoliveiro 30 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.)
4 JohnMcPherson 31
32
33 !!! Compact Discs and your computer
34 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.
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9 LawrenceDoliveiro 36 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.
4 JohnMcPherson 37
38 See also:
5 CraigBox 39
40 * [CDRippingNotes]
4 JohnMcPherson 41 * [CDWritingNotes]

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