with all due honor to Tim, I'm afraid Liliana is essentially right, at least about commercially produced cd's. I have tried to repair a number of cd players and I got my tutorial on the web from a scientist and electronics engineer [can't recal the URL right now] who goes into excruciating microscopic detail about how the players work, how the cd's are first mastered, then molded and pressed. Liliana is right. the label side of the disc is the sisde the inforamation pitting is pressed into. the shiny rainbow diffraction side is actually much thicker, relatively speaking, in terms of comparison to the inforamation layer, than the labeled side.
In reading up about the focus servo and the tracking servo, I came to understand that the focal length of the laser is set to penetrate to a particular depth thru the rainbow side and self correct that depth until it settles itself comfortably right at the correct focal length to read the pitting that represents the information deposits . the metal coating on the label side is for high reflectivity so the laser can acheive optimal diffraction inbouncing the infrared light back to the mirror and into the sensors that read the binary data. the metal is applied in a vapor vaccum chamber, in a layer measured in molecules. the label side is by far the more vulnerable side of the disc. the servos that position the lens , and the error correction coding incorporated into the reading of the data stream, have something like 8 levels of ways to filter out things that it can tell shouldnt be there before it gets to the output port. its fascinating reading. scratches on the rainbow side are far less likely to affect the disc's ability to be read, among other reasons, because the lens isn't tuning itself to focus at that plane of reading. another reason isthat if the scratch is radial [like a clock hand], it only interrupts the laser for an instant and the correcting code recognizes the inconsistency from the data before and after it and knows to edit it out, supress it, or insert continuity that logically follows what it reads before that or after it. it fills in the logical blanks,. a deep scratch that goes concentrically alters and distorts more data in a continuous stream and much more is lost. this is one reason why they tell you to clean your discs from the center out and not spin them to wipe them clean. but damage to the rainbow side is minor, compared to a scratch on the label side that results in your being able to hold the disc up to light and see light coming thru it, where the metal foil layer is gone. that data cannot be recovered. it's ruined forever. that has gone into the information layer where the pitting is deposited, and permanently altered it-- and there is no cure for it. you have to go out and get or make another dics. I have read that DVD's differ from cd's in that the rainbowing is on both sides, and the protection clear layer is on both sides, so that the information metal layer is in the center. the disc can be read and played on both sides. would that all cd's were made like that. maybe a new way to protect our single sided cd's would be to invent or demand a new kind of protective additional label, say, a clear vinyl donut shape. just like the present cd labels you can buy for RW's and cdr's--but in clear vinyl to go over our labels to put another tough, resilient layer of protection on the metalfoil side, to head off the disasters like lilliana is talking about. myself, I always regarded cd's like they were fragile, and i gasp whenever i see someone carelessly leaving them tossed around laying out, naked., and I immediatley put them away in their protective cases. but familiarity breeds contempt, and ignorance breeds carelessness. our kids have never known a time when cd's didnt exist. to them, its part of the landscape and they will get tossed around like anything else, unless you try to teach them otherwise. but i share liliana's frustration and disgust that a simple step like putting on clear labels on the data side just hasn't occured to any manufacturer to protect the side that matters. we have no end of polishing kits and devices, but its the other side that needs the attention!! anybody wanna make a quick fortune in data protection? should be simple enough to go into business making these. janet http://community.webtv.net/mensabrains/BADCODE -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | -- Sonnet & PowerLogix Upgrades - start at $169 | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> PCI-PowerMacs list info: <http://lowendmac.com/lists/pci-powermacs.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/pci-powermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
