Burned vs pressed CDs

[Henry Spencer]

On Sun, 5 Dec 2004, David J Patrick wrote:
> ...other folks whose CDrom drives would only worked with "pressed" 
> CDs, not "burned" CDs. What's the difference ?

Well, the fast answer is that are *different media* and they can look
different to the drive.  Especially if that "CDROM drive" is not really a
CDROM drive, but a DVD drive that also reads CDROMs, as is now common.

Pressed CDROMs have actual microscopic pits pressed into a reflective
aluminum layer, and then covered over with transparent plastic.  (For
those who remember vinyl records, the production process is similar, to
the point that many vinyl-record plants were converted to CD production.)

CD-R and CD-RW media are essentially a blank CDROM with a heat-sensitive
layer on top of the reflective layer:  in a CD-R it's organic dye that can
be (more or less literally) burned by heat from the writing laser; in a
CD-RW it's a complex metal alloy which can be in one of two states, which
reflect different amounts of light, depending on how hot it got the last
time it was heated. 

The thing to understand is that these aren't optically equivalent, not at
all.  The aluminum layer in a CDROM is highly reflective over a very wide
range of light wavelengths, and generally is a very favorable case for
reader hardware.

The dye layer in a CD-R is optimized for the laser wavelength used in
standard CDROM drives, and many early or cheap DVD drives have trouble
with it.  DVDs use a shorter wavelength for higher resolution, and that's
very badly matched to CD-R dyes, to the point that the good DVD drives
don't even try to use the DVD laser on CD-Rs -- they simply have two
lasers, one for each wavelength. 

CD-RW disks have a different problem:  they're very dark and have poor
contrast, relative to CDROMs.  The reflective and nonreflective states of
the metal alloy are both fairly poor reflectors, and moreover they don't
differ nearly as much as pit vs. smooth aluminum in a CDROM or clear vs.
burned dye in a CD-R.  This again can make trouble in marginal cases.

                                                          Henry Spencer
                                                       henry-lqW1N6Cllo0sV2N9l4h3zg at public.gmane.org

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