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Understanding the compact disc format

What is a "real" or "silver" cd?
What is a "cdr?"
How do cdrs differ from real cds?

"Real" cds, or "silver" discs, are those physically produced and musically encoded in a cd manufacturing facility. A clear, round piece of die-cut plastic is heated and then pressed (from the top) with a die that has microscopic bumps (lans & pits) on its surface. After the disc has been pressed, a hot metallic liquid (usually silver in color) is poured on top of the clear disc, allowed to cool, and is then sealed with a light epoxy. The bumps are the data to be read by the laser in cd players. The laser beam is emitted from beneath the disc. It passes up through the clear disc, through the bumps while reading them, and then reflects off the silver layer, sending the data to be converted into sound.

"CDR's" are recordable cds. They are made blank, without any music/data, in a manufacturing facility. Their production is different than that of a silver cd. The clear plastic disc is completely smooth, without any bumps being pressed into its surface. On top of the clear disc, a dense layer of hot colored dye is poured and allowed to dry. A thin layer of reflective metal foil is applied over the dye and then is sealed with an epoxy.
To record to a blank cdr, the recorder emits a laser beam from beneath the disc and burns microscopic holes into the dye to encode the disc with data. The dye needs to be colored so it will absorb the light from the laser beam to burn/encode. If the dye were clear, the laser beam would pass through and be reflected by the foil layer, without encoding.

The thickness of a cd/cdr is 99% clear plastic. The data is stored in a layer at the top of the disc, just underneath the epoxy seal. If a cd is scratched badly enough on the bottom side, the laser will be deflected and won't be able to communicate any data. Scratches from the bottom can be repaired. If a cd is scratched or dented from the top side deeply enough to remove a piece of the reflective layer, the laser will never be reflected back to for interpretation. When the reflective layer is damaged, the bumps are usually destroyed too. This type of damage is permanent.

The differences between cdrs and real cds can be few or many.

CDR Appearance (disc only):
The color of the undersides of the discs can range from any shade of blue, green, or yellow.
The topside of cdrs come in a variety of colors and textures.

CDR Performance (disc only):
Properly made cdrs will perform exactly like real silver discs most of the time.
However, certain players have difficulty acknowledging cdrs. Sometimes manipulation, such as trying to advance the track immediately after loading the cdr into the player, will help the player to read the disc.
Poorly made cdrs can have a variety of problems:
The first cdrs produced have a life expectancy of about 5 years before the dye layer deteriorates and becomes unreadable. The cdrs that came after these are expected to last 25 years. Cdrs produced after 1998 should last 100 years. Future cdrs will last longer due to technological changes to the dye's chemical properties.

Real cds are fully professional in appearance and performance. They are rarely faulty.
The life of a silver disc is fully dependent on its' handlers.

Live cds (real "silver" discs) retail for an average price of $25/disc due to the following risks of exposure:

Cdrs should only be used as an inexpensive alternative to live silver discs. They are for keeps, so make sure you get the best audio available. There's no market for used cdrs. Many people on the internet will trade cdrs for free. If you can't trade and have to buy, make sure you don't pay anything close to the retail price of a real silver disc.