"Looking" at Optical Disks..

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"Looking" at Optical Disks..

Postby Karlsweldt » Tue Mar 07, 2006 8:28 am

The standard CD disk is 120 mm in diameter, with a 1.2 mm thickness. The center hole is 15 mm in diameter. The reflective layer in the disk sandwich is electro-deposited aluminum. There is a dye layer just under this which is the actual data storage area. The laser "burner" etches the dye layer, and the aluminum layer reflects through the burned "holes" in the dye layer. There are no cylindrical tracks as in a hard-drive design. There is only one "track" on an optical disk.. it starts from the center hole, and spirals out to the rim of the disk. The reflective layer is ony about 6~7 microns from the top surface.
If you write on the disk's label with a ball-point pen, you can deform the reflective layer and render the disk's data unreadable. Even felt-tip pens can cause discoloration in the reflective layer, too.

By standard design, the width of the track is .6 microns.. and the spacing between tracks is 1.6 microns, center-to-center. That leaves only 1.0 micron "dead" space! The pits and bumps are normally .9 microns short minimum to 3.3 microns long maximum. There can be anywhere from .5 microns to 5.0 microns spacing between the pits and bumps on the track.
The laser beam is generated from a laser diode source, and is directed through a prismatic mirror setup. The beam is emitted from the lens, and returns in its same path.. but the prismatic mirrors redirect the reflected beam to another location, the photo sensor.
With a DVD disk that has double-layer, the lower layer of reflective material is thiner than the top layer. Different strengths of laser beam are used to read the differing layers.

When you purchase a disk in a store, it is not "burned" in the same manner as you would create one. It is actually a stamped copy from a master disk, exactly the same manner in which vinyl records are made. Only so many guaranteed replicas of the master can be made, before the master must be destroyed and a new master used. In this process, a clear poly disk is hot-stamped with the data master, then electro-coated with aluminum. Then the protective top layer is added, and finally the printed label. ACtually, the original master is laser-burned on a bare metallic surface on a glass disk, then an electro-casting of metal is formed on this that becomes a "Father" master. This then becomes the actual stamping die to make future copies. The hot-stamping begins with a molten blob of polycarbonate that is flattened by the master, and then cools to a permanent image. The protective top layers are then added for the finished product.

Brief history: In 1979, Philips and Sony collaborated to create a standard for the optical disk. This was known as the CD-DA, or the Compact Disk- Digital Audio. Sony had already produced digital video disks, over 12" in diameter. They were proprietary to their own design. In 1980, a newer standard was created, for the presently known item we use for all general purposes. These design standards were issued in a book known as the "Red Book" due to its red cover. There is a legend about the design of the optical audio disk.. so intended that it could include the 70-minute entirety of Bethoven's Ninth Symphony without interruption!
In 1984, a new standard was issued regarding the expanded uses of the optical format. The book had a yellow cover, and was known as the "Yellow Book".
For the read speed of any optical disk, the best data read speed is less than 15 Mb/s, or 10x. The rated speed on any unit is its maximum [/b]seek[b] speed, from file-source tables to data track location. A DVD can speed up the reading of data to give images fast movement.. but the normal read speed for data, esp. that of audio files, is 150 Kb/s. That is the 1x original standard.

Many problems associated with reading files from an optical disk are due to the laser lens being cloudy or dusty. Use of one of the laser lens cleaners is very easy and quick. The emitted laser beam must be "crisp" or very narrow and focused, to be reflected back from the data layer of the disk and return to the sensor. If the beam is distorted on leaving the lens, it is further distorted on returning.. and errors appear. If the beam is wider than the desired track, it cannot read the data pits and bumps.. and is just reflected "noise".
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Postby scottla » Tue Mar 07, 2006 10:01 pm

Jinkies, that's a lot of deep info on these things I never knew. Thanks for the post.

As an aside, perhaps you'd know this as well: I've always wondered whether there's some redundant data or partity bits of something placed on a CD to help it continue functioning even if damaged. I've got a few discs that are quite noticeably scratched, yet continue to perform without a hiccup...
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Postby Karlsweldt » Thu Mar 09, 2006 9:58 am

As noted, the laser beam is reflected back in its own path, to reduce the chances of a secondary reflection from scratches or irregularies, as would happen in a triangulated beam.. such as a mirrored reflection of a laser from an angle. The beam is "focused" by the laser carriage's electro-magnetic bearing, which places it within a thousandths of an inch of its best focus (intensity). The hard drive has its data address tables or FAT at the outer rim of the platters. The optical disk is designed the opposite.. with the FAT or data access tables nearest the hub. Also, if the beam were triangulated, there would be a reflection from the surface of the disk, and a secondary prismatic refraction from the material used. This would compete with the desired separate reflection wanted from the data layer.
If an optical disk has harsh scratches, there are repair kits to effect a decent cure and enable the disk to be continued in use. Such brands as "Disk Doctor" and "Disk Wizard" do a decent job, and are not too costly. Usually less than $20.00 USD. Specific solvents are used to smoth the surface without causing ripples or "orange peel" effects.
Old-time furniture had a real varnish coating. There was a product called "crack eradicator" that softened the varnish, and it appeared new again. Acetone is a very powerful solvent for most plastics.. but also very flammable. It may work on some blems of optical disks, but is not recommended. So too is not recommended the "hot plate" repair to CDs. It could warp the data layer, and render the disk useless. Only burnishing the surface using a mild solvent will restore its smoothness.
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