23 Mar
2013
23 Mar
'13
1:57 p.m.
On Mar 23, 2013, at 15:31, "Jerome H. Fine" <jhfinedp3k at compsys.to>
wrote:
Burned or pressed, the DVD drive seems to read
the DVD
equally well.
What about the physical properties of the media? I tend to
assume that the media of a CD is different from the media
for a CD. Does anyone have the physical specifications?
For a mass-production DVD or CD, it's just a flat layer of
aluminum between two polycarbonate sides. The aluminum
takes an impression from the master before the very thin
top layer is added.
I don't know what the second layer of a dual-layer DVD is.
I imagine the Internet does, though.
Mouse wrote:
I do
agree that current DVD players are still able to read the data
from any original DVD media burned when they first became available.
Were the
first DVDs burned? I've always assumed they were pressed,
more or less like CDs. Am I wrong? Since a DVD drive can read both a CD and a DVD (as
well
as, for many drives, but not all) burn both a CD and a DVD,
the media can't be that different. However, since my DVD
drive is able to distinguish between a DVD blank and a CD
blank, I assume that the media must be somewhat different.
The DVD has a finer pitch in both track width and bit spacing
(not unlike a high-density floppy). My assumption is that
the data is self-clocking and that the DVD drive will read
both just fine to begin with, but that the disc header tells it
what kind of disc it is (much like the header tells a CD/DVD
burner what kind of media a writable disc is so it can
determine the capacity and burn timing strategy). I don't think
the laser has the ability to determine different types of
media on its own; the receiver generally just sees ones and
zeroes (and if you have a drive that wasn't meant to be
used with writable discs, or has been intentionally tuned
to work poorly with them, like the PlayStation or the
original XBox, it doesn't even see those so well).
The file structure is probably the most important
aspect. Do
a DVD and a CD have essentially the same file structure? Since
the DVD typically holds ten times the storage capacity as a CD,
a 32 bit file structure would seem just about correct for the file
structure for both the CD and the DVD.
ISO9660 is the standard for CD filesystems, though its
limitations often force the use of extensions to get useful
behavior out of it (long file names, more hierarchy, etc.). Early
Mac CDs were just HFS volumes, but ISO9660 is fairly
standard for OS X. I seem to recall that the VMS distribution
CDs are just Files-11 volumes. Audio CDs have their own
format ("Red Book", named after the standards document
itself), which is a simple TOC followed by raw audio tracks
(with forward error correction, of course). You can
put any filesystem you'd like on a CD/DVD (for example,
UDF was a promising favorite for DVDs, since it has
fewer limitations than stock ISO9660), you just have to
have something that can read it.
Further complicating matters is the fact that lots of consumer
drives don't behave well with stuff that's perfectly standard,
but not common outside of the PC world. For example,
lots of VAXen don't like to boot off SCSI CD drives that won't
put out 512-byte blocks, but there are plenty of drives like
that out there because most common CD formats use
2k or 4k blocks (or 2k plus a bit for the expansion due to
the FEC).
At one point, I burned a CD with both a CD file
structure starting
at CD sector 16 and an RT-11 file structure starting at CD sector
zero. Where the RT-11 partitions started (every 16384 sectors
of 2048 bytes or 65536 blocks of 512 bytes), I placed spacer
files with originally all zeros and filled in the RT-11 file structures
after the CD was burned with the CD file structure and converted
to an ISO image on the hard drive. It took a couple of attempts
to figure out everything, but after converting the first CD image to
an ISO file, I used RT-11 to locate all the files from the CD. The
first RT-11 file became the file structure of the CD.
As for all types of storage, the CD and now the DVD, seems to be
about the most stable and unchanging of all storage types. However,
I understand that many current PCs these days no longer have DVD
drives. Is that the trend?
It's getting there. Just like floppies. Like floppy drives, though,
external ones will probably be available for a long time (at
least for those of us willing to deal with USB or whatever
comes after).
- Dave