Materials

The materials used for the data (recording) and metal (reflecting) layers are different for different kinds of optical discs.

CD	DVD	Type	Data Layer	Metal Layer
CD-ROM (Audio/video, PC software)	DVD-ROM (Video/audio, PC use)	Read Only	Molded	Aluminum (Also silicon, silver, or gold in double-layered DVDs)
CD-R	DVD-R DVD+R	Recordable (once!)	Organic dye	Silver, gold, silver alloy
CD-RW	DVD-RW DVD+RW DVD+RAM	Rewritable (write, erase, write again)	Phase-changing metal alloy film	Aluminum

Read Only:

CD-ROM stands for Compact Disc - Read Only Memory. It looks just like an audio CD but the recording format is quite different. CD-ROM discs are used for computer software. DVD used to stand for Digital Video Device or Digital Versatile Device, but now it doesn't really stand for anything at all! DVDs are used for recording movies and large amounts of data or large programs.
The CDs and DVDs that are commercially produced are of the Write Once Read Many (WORM) variety. They can't be changed once they are created.

The data layer is physically molded into the polycarbonate. Pits (depressions) and lands (surfaces) form the digital data. A metal coating (usually aluminum) reflects the laser light back to the sensor.

CD-ROM and DVD-ROM discs should be readable for many, many years (100? 200?), but only if you treat them with respect.

Lost data: Oxygen can seep into the disc through the edges or scratches, especially in high temperatures and high humidity. This corrodes the aluminum, making it too dull to reflect the laser correctly. You will have errors or complete failure when trying to read the disc.

Write Once:

The optical discs that you can record on your own computer are CD-R, DVD-R, and DVD+R discs, called writable or recordable discs. The metal and data layers are separate. The metal layer can be gold, silver, or a silver alloy.

The data layer in this type of disc is an organic dye that the writing laser changes. Once the laser modifies the dye, it cannot be changed again. Write Once!

Lost data: Ultraviolet light and heat can degrade the organic dye. Sulfur dioxide from the air can seep in and corrode silver over time.

Go for the Gold: Gold layers are best because gold does not corrode. Naturally, the best is more expensive.

Manufacturers say that these discs have a shelf-life of 5 - 10 years before they are used for recording. There is no testing yet about how long the data will last after you record it. Humph!

A writable disc is useful as a backup medium when you need long-term storage of your data. It is less efficient for data that changes often since you must make a new recording each time you save your data. Pricing of the discs will be important to your decision to use writable discs.

Rewrite:

An option for backup storage of changing data is rewritable discs, CD-RW, DVD-RW, DVD+RW, DVD+RAM.

The data layer for these discs uses a phase-changing metal alloy film. This film can be melted by the laser's heat to level out the marks made by the laser and then lasered again to record new data.

In theory you can erase and write on these discs as many as 1000 times, for CD-RW, and even 100,000 times for the DVD-RW types.

Дата добавления: 2014-12-23