<This is an outgrowth of the Multiformat CD>
As CDs are now, the substrate is solid, clear plastic, then a layer of aluminum that holds the data, and usually a painted on label. If the bottom / shiny side of the CD gets scratched, it is possible to buff the scratch out, since the data is untouched. Some brilliant person here at work decided to scratch the serial number of several thousands of dollars worth of software into the CDs, through the label...and thus, through the data. Surprisingly, they didn't work after...
This suggestion is to sandwich the data-carrying aluminum between two disks of plastic, thus making it scratch resistant on both sides. Each layer would be thinner, but the scratches that kill CDs are not often very deep. Possibly by having a solid bottom, plate the aluminum on, then cast a layer on top of that, rather than trying to glue two thin CD blanks together.
This way, a scratch on EITHER side would not necessarily kill the CD, and could be buffed out. This might also allow double-sided CDs, although those may exist and I just haven't seen them. Another thin plastic layer to give the second data layer a smooth surface, then another data layer, then the cast top. Flip it over and double the space.-- StarChaser, Feb 09 2002 Multiformat CD http://www.halfbake...ea/MultiFormat_20CDWhat gave me the idea, when I annotated it.... [StarChaser, Feb 09 2002, last modified Oct 05 2004] CD Media World http://www.cdmediaw...ware/cdrom/cd.shtmlThe skinny on CD-Rs, their manufacturers, and anticipated longevity. [acb, Feb 11 2002, last modified Oct 05 2004] what position does "brilliant person" now hold? just curious?-- po, Feb 09 2002 I really have no idea if this would be a problem or not, but the optics in a cd player may have trouble focusing on the data if it is half a cd's thickness closer. It should be possible to test if this would be a problem: buff out a CD until it is about half as thick, then try it in a whole bunch of CD players to see if you get any problems. Do this at your own risk, of course, if you make a little hairline crack while doing all that buffing, the CD could fly apart like shrapnel when you try and play it.-- JakePatterson, Feb 10 2002 My soul is in agony, ¯StarChaser. Compared to your brilliant person, I'm not such a dumbass. Sharpie™ always.-- reensure, Feb 10 2002 Like [JakePatterson], my only concern is the focus of the laser. I shouldn't think it would be that big of a deal to add a bit of plastic to the top. Making the CD thicker by a bit should still allow interoperability.-- phoenix, Feb 10 2002 yeah, i once put a cd on top of another cd in my player while half asleep and the one that was already in there played fine. sure confused me for a while though-- cuba, Feb 10 2002 Po: The same position, software librarian.
Hadn't considered the focus of the laser, that's a good point. I'd never tried the two CD thing, but if it works, then you could just stick another CD blank on top.-- StarChaser, Feb 10 2002 [reensure]: I too, use Sharpie™ pens to label CDs and CD-Rs but have recently been told that the solvent in that ink can eventually penetrate the reflective layer and cause problems. Given the 80 gazillion CDs I have labeled this way, it worries me a bit, yes.-- bristolz, Feb 10 2002 [bristolz] By the time that happens you'll be telling your grandchildren about how you used to wait 15 minutes to archive ~600MB of data.-- phoenix, Feb 10 2002 Hope so. I hear an uncorrupted CD-R should be expected to survive about 50 years under normal storage conditions.-- reensure, Feb 10 2002 Longevity of written CDs depends on the quality of the disk; good-quality gold disks are said to last the longest, and the cheap ones shouldn't be trusted with anything you want your grandchildren to be able to read.-- acb, Feb 11 2002 Stacking 2 CD's doesn't tell you anything about laser focus. The laser is a fixed distance from the bottom disc, regardless of how many more you put on top of it.-- quarterbaker, Feb 11 2002 Hmmm. My company's IT dept. cautions us that CD-Rs only have a 2-5 year life span, but, hey, what do they know, anyway? I mean, if they really knew something, they'd be doing something else, no?-- bristolz, Feb 11 2002 [bristolz] If you use them every day, they'll wear out due to scratching, bad CD-ROM drives, etc. Burn one and set it on the shelf in the closet and it'll last 50 years. Assuming there are no fires in the closet.
[quarterbaker] That's the point. Since the laser shines up from underneath, it would be possible to add a plastic layer to the top in order to protect the data layer. By building up the top, you're not affecting the laser's (current) focus.-- phoenix, Feb 11 2002 phoenix... oops, I mean, [phoenix], isn't there some sort of oxidizing going on 'in' the burn process of a CD-R that would continue to ever-so-slowly work away over the years? I recall reading that somewhere, of course that could have been eons ago.-- waugsqueke, Feb 11 2002 Any Software Librarian jokes circulating?-- thumbwax, Feb 11 2002 <bristles at Bristolz> Hey!-- StarChaser, Feb 11 2002 [waugsqueke] No. Once the manufacturing process is complete (and assuming no scratches on the upper layer of lacquer) the only portion of the reflective layer exposed to air is along the circumference... and no data is written there. The burn process for CD-Rs is just that: It creates a pit in the reflective layer that doesn't reflect light (which explains why the focus is important).-- phoenix, Feb 11 2002 [phoenix] A twice-as-thick CD may work in some, or even most, CD players, but I doubt it would work in all of them, particularly slot loading ones.-- JakePatterson, Feb 11 2002 I've not once suggested they should be twice as thick or that two should be glued together. I've only suggested the varnish/lacquer be replaced with a layer of plastic. Making a CD half again as thick shouldn't cause any serious problems.-- phoenix, Feb 11 2002 DVDs players already support 2-sided media that is built exactly as [StarChaser] describes. The 2-sided discs use two half-thickness substrates with the metal layers (and a thin separating film) sandwiched between them. The laser focus is indeed affected. Player hardware must detect whether single- or dual- sided media is present and adjust its optics accordingly.
I think there is also some dual-layer technology where both layers are bottom-readable. The lower layer is semi-transparent to allow the top layer to be read. The player can choose between layers by varying the focus of the laser. Optical sensitivity becomes a concern, though, because both layers exhibit reduced reflectivity.
Probably the biggest reason for not applying this technology to CDs is economics. CDs are mass-market driven and the vast majority of players are cheap low-end hardware. There's no room in the budget for multi-format support and variable optics. DVDs are still part of the high-end market. There's more room in the budget for variable optics and multi-format support. There is also a need for higher storage capacity, where no such need exists for typical CD applications.-- BigBrother, Feb 12 2002 DVDs come in the following configurations: Single Sided/Single Layer Single Sided/Double Layer Double Sided/Single Layer Double Sided/Double Layer-- phoenix, Feb 12 2002 You know there is a much easier way to protect both sides of a CD and DVD's for that matter. They make disposable plastic shields…not to be confused with panty shields…that will protect both sides. Then if either get “scratched” just replace. I know cyberguys.com has them.-- zguy, Oct 15 2003 random, halfbakery