Multimedia
Blogs
Forum
Affordable HD
Whitepapers
Newsletters
Events
Advertisers
Blogcast
MillimeterMore Eggs in One Basket: Will Blu-ray and HD-DVD Be Archival?
Oct 13, 2005 2:51 PM, D.W. Leitner
At the upcoming annual Optical Storage Symposium (www.osta.org/oss/) September 28 in San Francisco, topics include “Beyond DVD: What's in Store for the Future of Archival Optical Media,” and “Blue Laser Optical Storage: A Technology Waiting to Happen.” A side session exploring commercial optical storage will feature, “The Search for an Archival Medium--the 100-Year Optical Disc.”
So it’s not just you and me wondering how long our homemade DVD-Rs are going to last.
Over the course of several of these columns, we’ve discussed concerns about the archival reliability of DVDs. We’ve looked at the vulnerability of the polycarbonate substrate to moisture and airborne acids, the susceptibility of the aluminum reflective layer to oxidation, and the inevitability of the fading of the photosensitive organic dyes used for burning data into DVD-Rs and DVD+Rs.
We’ve examined the architecture of CDs and DVDs to point out the superior physical protection afforded DVDs by sandwiching the data layer safely between two 0.6 millimeter-thick polycarbonate substrates, in contrast to locating the data layer near the surface as do CDs, where pin pricks are a very real danger.
Lastly, we’ve noted the advantages of inert 24-karat gold as a reflective layer and taken heart at the special Phthalocyanine organic dyes recently brought to market for long-term data-layer stability.
Now’s lets speculate about how HD-DVD and Blu-ray fit into the archivability picture.
What HD-DVD and Blu-ray have in common are a 405-nanometer (nm) blue-violet laser and a data density several orders greater than DVDs.
The shorter 405nm wavelength of blue-violet light compared to 780nm and 650nm of red lasers, respectively, in CDs and DVDs is key to both HD-DVD’s and Blu-ray’s reduced pit size, narrower track pitch, and thinner laser beam diameter.
A look at data densities per square inch tells the whole story: CDs pack in a modest .4 GB/in2; DVDs, 2.2 GB/in2; HD-DVDs, 7.5 GB/in2; and Blu-rays, 12.5 GB/in2. Translated into single-layer storage capacity, a recordable CD can offer 700 MB, a DVD, 4.7 GB; an HD-DVD, 15 GB; and a Blu-ray disc 25 GB.
Blu-ray wins, right?
Not so fast, cowboy.
While Blu-ray and HD-DVD both exploit 405nm lasers, they situate their data layers at different positions. HD-DVD uses exactly the same architecture as DVDs, sandwiching its data layer between bonded 0.6 millimeter-thick polycarbonate substrates.
(That’s the basis of Toshiba’s and NEC’s argument for the advantages of low-cost HD-DVD disc manufacture: DVD manufacturing equipment currently in wide use can immediately be used to make HD-DVDs. Sure enough, several weeks ago Amoi Electronics and Sichuan Changhong Electric, top Chinese manufacturers of low-cost DVD players, threw their hats in the HD-DVD ring.)
Blu-ray, on the other hand, takes a page from the architecture of hard disks, whose magnetic layer lies on a surface immediately below the read heads. Blu-ray’s data layer is positioned on the disc surface closest to the blue-violet laser, shielded by a minimal 0.1 mm protective coating. (In CDs, by comparison, the data layer resides near the surface opposite the laser, which must penetrate the entire 1.2 mm polycarbonate disc to read the data.)
Thinking among the 143 members of the Blu-ray Disc Association—including industry heavyweights Sony, Matshushita, Thomson, and Samsung—is that use of an ultra-thin 0.1 mm protective layer eliminates any optical distortions of the laser’s beam caused by irregularities in the clear polycarbonate substrate or tilting of the spinning disc.
Considering HD-DVD’s 200nm pit size and 400nm track pitch as an example, it’s easy to see why optical distortions due to 0.6mm of spinning plastic substrate would be an issue. In placing the laser virtually directly against the data layer, Blu-ray technology achieves less beam scatter and therefore the smaller pit size and track pitch (320nm) necessary for higher recording capacity.
But everything comes at a price. Sony’s XDCAM Professional Disc media used in XDCAM and XDCAM HD camcorders is an adaptation of Blu-ray mounted in a cartridge with a mechanical shutter, which Sony says ensures that discs are “resistant to dust, shock and scratches.”
While HD DVDs with their bonded plastic sides seem OK as “bare” media, the possibility of damage to a Blu-ray disc from scratches would have to be of significant concern to any archivist. For this reason, although Hollywood studios will soon mass market films in HD using bare Blu-ray discs, professional versions of Blu-ray for data storage have already been introduced with cartridges.
Of course this means two types of Blu-ray drives, those that accept cartridges and those that don’t or can’t, like thin laptop disc drives. (It has already been reported that some library vendors remove Blu-ray discs from cartridges to sidestep the costs of redesigning their existing bare media DVD libraries.)
So the Blu-ray trade-off against HD-DVD is greater capacity for greater exposure of the data layer to dings and scratches. (Will next year’s Sony’s PlayStation 3 use cartridges to kid-proof its Blu-ray discs?) At the moment, blank media for both are predicted to sell to consumers in the $25-35 range. Plus the added cost of cartridges to professional Blu-ray discs.
Incidentally, in June, Maxell, Verbatim, and Toshiba announced development of an HD-DVD-R disc using a new photosensitive organic dye for blue-laser which can be produced at high volume on standard DVD-Recordable production lines. No word yet on stability of the dyes over time.
One last thought: as DVDs are to blue-laser discs, blue-laser discs are to upcoming holographic discs. Many, many more eggs in one basket. In other words, what happens when you consolidate the contents of many, many DVDs onto a single holographic disc, and someday that holographic disc won’t play?
Which brings up the question: maybe fast-spinning media like DVDs and HD-DVDs and Blu-ray discs which require mechanical integrity over time are not the ultimate archival choice? Could mechanism-free media like solid-state flash memory, rapidly gaining on optical media in cost and storage capacity, offer better data stability over the long run?
Something to think about.
In my last DVD column, I promised to discuss whether highly compressed interframe MPEG-2 compression used by DVDs is a smart archival choice for video. Stay tuned.
Continue the discussion on “Crosstalk” the Millimeter Forum.
