Here you go. This is the message I was referring to. ----- Original Message ----- From: Thomas N. Chan To: [EMAIL PROTECTED] Sent: Friday, January 20, 2006 10:33 AM Subject: RE: Storage expert warns of short life span for burned CDs - Computerworld
The material are basically solid inside the Disc, nothing inside which its soft or chemical which can be move by turning it around or spin it. Imagine this, if you think that the chemical is moveable by putting on certain position as what you quoted, then everytime you put it on the CD player or DVD player, all the chemical should have spun to the external ring of the Disc and random it, unplayerable.. The CD or the lens need to track them as well so it's a problem. Here's how CD works I found A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic . During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. We'll return to the bumps in a moment. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic. A CD has a single spiral track of data, circling from the inside of the disc to the outside. The fact that the spiral track starts at the center means that the CD can be smaller than 4.8 inches (12 cm) if desired, and in fact there are now plastic baseball cards and business cards that you can put in a CD player. CD business cards hold about 2 MB of data before the size and shape of the card cuts off the spiral. What the picture on the right does not even begin to impress upon you is how incredibly small the data track is -- it is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. (A micron is a millionth of a meter.) And the The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.) Looking through the polycarbonate layer at the bumps, they look something like this: You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps. The incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long! To read something this small you need an incredibly precise disc-reading mechanism. CD Player Components The CD player has the job of finding and reading the data stored as bumps on the CD. Considering how small the bumps are, the CD player is an exceptionally precise piece of equipment. The drive consists of three fundamental components: A drive motor spins the disc. This drive motor is precisely controlled to rotate between 200 and 500 rpm depending on which track is being read. A laser and a lens system focus in on and read the bumps. A tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral track. The tracking system has to be able to move the laser at micron resolutions. Inside the CD player, there is a good bit of computer technology involved in forming the data into understandable data blocks and sending them either to the DAC (in the case of an audio CD) or to the computer (in the case of a CD-ROM drive ). The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light . The bumps reflect light differently than the "lands" (the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the Bytes What the CD Player Does: Tracking The hardest part is keeping the laser beam centered on the data track. This centering is the job of the tracking system . The tracking system, as it plays the CD, has to continually move the laser outward. As the laser moves outward from the center of the disc, the bumps move past the laser faster -- this happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving (rpm). Therefore, as the laser moves outward, the spindle motor must slow the speed of the CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate. You can read the rest at http://electronics.howstuffworks.com/cd7.htm ------------------------------ regards Thomas N. Chan -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Bill Powers Sent: Friday, January 20, 2006 12:16 PM To: [EMAIL PROTECTED] Subject: Re: Storage expert warns of short life span for burned CDs - Computerworld Dana, >From what I understand, the reason for storing vertically instead of horizontally has to do with settling of the chemicals in the underside of the CD. By storing them horizontally, the theory is that gravity will force a quicker instance of too much of the chemicals to the very bottom of the CD thus hastening decomposition of the media thus making the CD unplayable. Again, this is only speculation and I have not read any hard data yet actually proving this. But I do store all my CDs vertically out of habit and caution. This might be especially true if you make a label to place on top of the CD which has a certain amount of glue on the underside to stick to the CD. That glue will eventually break down and theoretically seep through the CD, and likewise the ink in magic markers and some ink pens will do the same. I've also found that vinyl stores better vertically, BUT it must be straight up, not leaning one way or the other on a record shelf. Laying the record down and pressing other albums on top of it is not a good idea because dirt can get so deeply lodged into the grooves that it may never come out. Bill Powers BlindTech is a service of MosenExplosion.com. To find out about the other e-mail lists we run, please visit us on the web at http://www.MosenExplosion.com ________________________________ YAHOO! GROUPS LINKS * Visit your group "blindtech <http://groups.yahoo.com/group/blindtech> " on the web. * To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> * Your use of Yahoo! Groups is subject to the Yahoo! 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