CDs and DVDs

Blank CDs on display

CD is short for compact disc. DVD initially stood for digital video disc, then digital versatile disc, but today the term DVD is often used without referring to a specific set of words. Both CDs and DVDs are optical media, meaning media that use light technology (more specif­ically, laser light) for data retrieval. A disc drive focuses a laser light beam into the CD or DVD to “read” the bits (data) in the disc. The drive can also “write” bits by focusing the laser beam into recordable CDs or DVDs. The laser reads and writes data starting from the center of the disc and proceeding in a spiral direction toward the outer edge. A pre-groove is stamped in all blank recordable and rewritable CDs and DVDs to guide the laser as it writes.

 

Usability

Optical discs are differentially identified to designate specific features such as recordability, rewritability, and accessibility. For example, CD-R, DVD-R, and DVD+R discs are dye-based record­able (write-once) discs—i.e., recordable but not erasable. CD-RW, DVD-RW, and DVD+RW discs are phase-change based, recordable, (rewritable) discs, or discs that permit the erasing of earlier informa­tion and the recording of new material in the same location on the disc. DVD-RAM discs are phase-change based, recordable (rewrit­able) discs formatted for random access, much like a computer hard drive. CD-ROM and DVD-ROM discs are pressed and molded, non­recordable, read-only discs.

Used DVDs on display.

CDs and DVDs consist of the same basic materials and layers but are manufactured differently. A DVD is actually like two thin CDs glued together. A CD is read from and written to (by laser) on one side only; a DVD can be read from or written to on one or both sides, depending on how the disc was manufactured. Recordable DVDs (DVD-R, DVD-RW, DVD-RAM) can be manufactured with one re­cording layer on each side. Prerecorded DVDs (DVD-ROM) can be manufactured with one or two recorded layers on each side.

CD-R, CD-RW, DVD-R, DVD+R, DVD-RW. DVD+RW, and DVD-RAM discs can become unusable in a matter of days. If such a disc is left in an environment that allows direct sunlight and extreme heat buildup (e.g., on the dashboard of a car in summertime, or next to a heater by a window), the organic dye or phase-changing film that holds the data will degrade quickly, causing the disc to become unreadable. A disc is not protected from the effects of heat buildup if left in a case that is exposed to direct sunlight or

other sources of heat. Extreme heat buildup can also cause warping of the disc.

Direct sunlight to R discs is harmful for two reasons: The sunlight’s ultraviolet photons (the higher frequency of the sunlight spectrum) have enough energy to produce a photochemical reaction, altering the optical properties of the dye (recording layer) molecules. The broad spectrum of unfiltered sunlight, infrared to ultraviolet (low frequency to high), can impart heat to the disc. The increased temperature generated by sunlight will accelerate the degradation or breakdown of the dye layer (recording layer) of the disc. The combination of high temperature and high relative humidity will further accelerate that degradation.

Scratches generally cross data lines or tracks on the disc, and how bad (deep and wide) they are will determine the extent of interfer­ence with laser focus on the data. Small or occasional scratches will likely have little or no effect on the ability of the laser to read the disc, because the data are far enough below the surface of the disc that the laser is focused beyond the scratch.

Choosing the best writing speed

Choosing a higher writing speed will result in a faster disc burn, but the optical quality may be lower (less reflective). If the reflectivity is too low to be accurately read when the CD is played back, it may skip or contain unwanted audio artifacts such as squeaking and clicking sounds. For optimal results burn at the media's rated speed. 

New age DVDs; the HD-DVD and Blue-ray Disc

New age DVDs

DVD is still very successful, and will definitely be around for some time. With the introduction of HD-DVD and Blue-ray Discs and their respective media, large media content of upto 25GB on single-layer, and 50GB dual-layer can be stored. Blu-ray and HD-DVD both employ Blue Laser technology which has a much shorter wavelength than the red laser technology used in current DVD). Blu-ray and HD-DVD enables a disc the size of a current DVD disc but, which has much greater storage capacity than a standard DVD, to hold an entire film at HDTV resolution or allow the consumer to record two hours of high definition video content.