From: "Glenn Meadows" 

Then you get into the newer standards that are starting to allow a Red Book
and DVD to be glued together in a single sided dual layer disk, as well as
the hybrid SACD disks.  The "deep layer", is the redbook CD, so
that the lasers of a CD player can read it properly (and ignore the upper
layer which is the DVD), and a DVD player can see both.  I believe the DVD
players that can read the now "legal" format (originally it was
an undefined standard, and while technically possible, most players would
never get to the DVD layer because the DVD player would say..."Ah,
it's an audio CD", and never look for the second layer), will now look
for a second layer and if there, provide a visual menu from the disk that
will allow you to pick what you want to see/hear.

The disks that are Audio on one side, and DVD on the other, are also
originally an "un-defined" format, although there was no reason
not to do them, they technically could not carry the DVD or CD logos, since
neither fully met the spec (the spec didn't allow for a dual sided disk for
an audio CD, nor did the DVD allow legally an audio disk on the back.) 
Putting the logos for the DVD/CD on the disks are important marketing items
for the big boys, as is "compliance" within the specific
standards.

The SACD standard, allows for dual layer of either SACD (DVD style signal,
but all the bits are used for audio), as well as the hybrid disks.  ALL of
the Rolling Stones Box Set re-issues were SACD Hybrid disks, although Sony
didn't make big noise about it, as they didn't want to scare off audio cd
purchasers who might think it as an SACD only release.  Those that have an
SACD player, can really hear what the masters truly sounded like, as both
layers are the same mastering, just different resolutions.

--
Glenn M.


"Don Hills"  wrote in message
news:vpkvBtgaXuoS092yn{at}attglobal.net...
> In article , "Glenn Meadows"

wrote:
> >                                                   ...  Seeing a true
> >Digital master vs. the DVD finished disk on a split screen is very eye
> >opening as to what's lost compared to what's actually there.
>
> Thanks, Glenn. Nice post. While we're on the subject, here's something I
> wrote in answer to a similar query a while ago:
>
> There are 4 different types of DVD disc, known in the industry as DVD-5,
> DVD-9, DVD-10, and DVD-18. The numbers refer to the approximate disc
> capacity in GB. All of them are the same thickness (1.2 mm).
>
> DVD-5 is single sided, single layer. The data layer is moulded into one
side
> of a 0.6 mm disc and covered with an opaque reflective coating. A blank
0.6
> mm disc is then glued over the data layer, forming a 1.2mm disc
"sandwich"
> with the data in the middle.
>
> DVD-9 is single sided, dual layer. The first data layer is moulded into
one
> side of a disc as before, but this time covered with a coating that is
only
> partly reflective. The second data layer is moulded into one side of a
> second disc, and covered with a fully reflective coating. The two data
> layers are glued together facing each other, so that the data layers are
in
> the centre of the disc as before. Because it's harder for the laser to
"see"
> through the semi-reflective coating to the layer behind it, the data
density
> is reduced by about 10% to make reading easier (a bit like using bigger
> print in a book to make it easier to read by people with poor eyesight).
>
> DVD-10 is double sided, single layer. It is made the same way as the DVD-9
> disc, except that both data layers are covered with fully reflective
> coatings. Hence, each layer has to be read through its corresponding
"side".
> Because there's no semi-reflective coating to make reading difficult, the
> capacity reduction for dual-layer is not required.
>
> DVD-18 is double sided, dual layer. It is the hard one to manufacture.
> The first layers are moulded as before on each "half disc" and covered
with
> a semi-reflective coating, then a layer of clear polymer is applied and
the
> second data layer for each half disc is pressed into the polymer. A fully
> reflective coating is then applied to each disc and they are glued
together
> to form the final sandwich as before.
>
> One may well ask, "how does the player read the different layers on a dual
> layer disc?" Good question... the glue between the layers is quite thin,
but
> does provide enough gap so that one layer is slightly out of focus to the
> laser while the other is being read. But the main difference is that the
> signal level from the partly reflective layer is lower than that from the
> fully reflective layer. So, a weak signal is identified as the "front"
layer
> and a strong signal is identified as the "back" layer. The
signal from the
> layer being focussed on at the time will also be "sharper"
than that from
> the other layer.
>
>
> --
> Don Hills    (dmhills at attglobaldotnet)     Wellington, New Zealand
> It's ironic that people who are too smart to engage in politics
> are governed by people who are not as smart.

--- BBBS/NT v4.01 Flag-5