[Ham-Computers] RE: DVD Media Question

[Hsu, Aaron (NBC Universal)]

Ronald KA4INM Youvan write:

"I think you are referring to the UDF format, but many people use DVD-RW's
and CD-RW's just like a DVD-R or a CD-R.  If it doesn't compare favorably,
we erase it and write it again, possibly at a slower speed.  Why avoid
*-RW's?"

-----------------------
My Reply:

Don't even get me started on the UDF format.  That's a whole can of worms in
and of itself!  I *always* create CD/DVD's in ISO format...no "drag-n-drop"
for me.

Any optical "Recordable" format is inheirently unstable.  It is this
"designed" instability that allows a laser to reconfigure the physical
properties of the "dye" on the media to either absorb or reflect the laser
beam (one of two "states", e.g. zero or one).  For write-once media, the dye
can be reconfigured once.  The "blank" media is in one state and the write
process changes certain areas to the other state (note that those areas not
changed in state can still be changed by exposure to a suitable laser or
light source).  Realize that because it's unstable, the dye can degrade
slowly over time changing state on it's own (especially if exposed to
certain lighting conditions such as sunlight).  In fact, there was a recent
discussion on a CD-R reflector where media that was purchased several years
ago is now generating a greater number of "coasters" than when first bought.
The discs were sent to an analysis company (they have a presence on the CD-R
reflector) and were found to be "degrading" - the dye was already changing
states.  Imagine recording data on media where in certain areas the dye is
already in a "partial" state change.  These "partial" state changes are
"fuzzy" to the drive and it has to determine if that "fuzzy" bit is a one or
zero.  CRC correction can handle a certain number of bit errors per X number
of bytes (soft errors), but beyond that you run into uncorrectable "hard"
errors.  Over time, as the dye degrades and more bits become fuzzy, hard
errors manifest themselves.  It is these "hard" errors (developing over time
on any CD/DVD recordable media) that determine the archival and shelf life
of any particular media.

The "RW" format is designed to be even more unstable.  The dye needs the
ability to change states on demand.  Unfortunately, current dyes can't
change states 100% from one to the other.  As you can tell, at some point in
time, the dye will be "fixed" in some fuzzy or "pseudo" state, theoretically
near 50%.  This is why RW discs eventually need to be tossed.  Formatting
the disc, in this case, will not help as it's the physical properties of the
disc that are compromised.  The most I've ever been able to re-write a disc
is about 15 times, and this was with a disc of reputable manufacture.  I
typically only use RW's when recording temporary VCD's to watch on a DVD
player (I have a PC setup as a PVR, but not currently hooked up to a TV).
After so many writes on RW media, I have to toss the disc.  For this
purpose, it's still a bit more economical that write once media, but that
$$$ gap is closing fast.

So for WA5CAB's purpose of "archival" backups, RW is definitely not the
right choice of media.  Even "generic" write-once media is not suitable for
archives.  Discs for archival use should be of top grade and purchased from
a reliable source.  Most discs bought "off the shelf" is of lower quality
than bulk media bought from a distributor who handles higher quality media.
As I mentioned before, those who really want or need higher quality media
"know" where to buy it.  Retail stores cator to the typical consumer who may
not know (or care) about the quality...just the price.  As such, media
vendors sell their lowest cost product to the retail outlets.  Add this
delima to the inheirent unstability of RW media and you'll see why I choose
not to use RW's.

And, as usual, I've ranted long enough.  Sorry for the length of my
messages...I generally like to be thorough in my explainations and that
*can* get annoying sometimes!

73,

  - Aaron Hsu, NN6O (ex-KD6DAE)
    {nn6o}@arrl.net
    {athsu}@nbcuni.com
    No-QRO Int'l #1,000,006
    . -..- - .-. .-   ".... . .- ...- -.--"