> The data on a cd is written in one long track. When you record your cd
> (burn a disk) you lay the data down and when you are ready to finish, you
> "close" the end of the recording.
Correct - each track or recording is called a session.
> In both cdr and cdrw this finishes the
> cd and it is now possible to read from it.
... in a CD-ROM drive. CD-R and CD-RW drives can read and append to
unclosed sessions.
> With a cdr the cd is no longer
> writeable and is ROM.
Incorrect. You can write another session to the CD-R - the disc becomes a
multi-session disc, and the vast majority of drives can read them. Kodak
PhotoCDs are multi-session, with each session being a new roll of film.
> In the case of a cdrw the cd may be written to over and again, though
there
> is a limit of how often, something like 50 times before the media wears
> out.
Yep. CD-RW is a MO (magneto-optical) system - you have a phase-change
material in the disc substrate, and the laser is used to heat this material.
A magnetic field (or perhaps it's another laser) then flips the phase of the
warm area, changing it from dark to light or vice versa. Minidiscs and PD
use the same system.
> As my friend explained, and the school, when you wish to rewrite the
> cd you must first format it to wipe out the previous single track and then
> begin again as a blank cd.
Depends on how the disc is used. You can wipe it out and start again, and
that's more efficient in terms of media - it's like buying a new pad of
paper. You can, however, mark a session (or part of a packet, see later) as
erased, and write new data at the end of the disc - akin to crossing out
what you've written and starting a new page. Or you can just bung another
session at the end of the disc.
> If you have experience with drives that can create readable cdrw that
> aren't closed and can be written into without first being reformatted, you
> have a most wondrous technology which is not yet common knowledge.
The technology is actually pretty common, and most drives ship with the
required software and drivers. It's known as packet writing or packet
formatting, and basically the entire disc is formatted to hold data. When
you write the the drive, the drivers plonk the data down and update the
table of contents, just as it does with hard drives and floppies. However,
when you overwrite or erase, it simply blocks out the data to be changed and
creates a new section. I think these discs can then be read in normal
drives, but I'm not too sure - a friend has a CD-RW of various files which
can be read in most CD-ROM drives, and may be packet-formatted.
Regards,
Ben A L Jemmett.
(http://web.ukonline.co.uk/ben.jemmett/, http://www.deltasoft.com/)
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