1. Explain CD (COMPACT DISC) TECHNOLOGY A compact disc, also known as a CD, is a plastic optical disc with a metalized surface that is used for digital data storage.. This format was later adapted for storage of data (CD-ROM), write-once audio and data storage (CD-R), rewritable media (CD-RW), Video Compact Discs (VCD), Super Video Compact Discs (SVCD), Photo CD, Picture CD, CD-i, and Enhanced CD. When a compact disc is played, the information is read by a laser and converted into sound that represents an original audio source. The CD's storage capabilities have expanded alongside its technology to read other data like CD-ROM for computers or DVD and Blu-ray for video. A standard CD typically holds 74 to 79 minutes of audio. The CD debuted in 1982 under Philips Electronics and Sony Corporation. The basic compact disc is simple in appearance, but consists of multiple layers. The base layer is polycarbonate plastic which holds the digital data. This layer is topped by an aluminium coating that serves to reflect the laser that reads the disc's information. (In rare instances, silver or gold may be used in place of aluminium). A clear layer of shiny acrylic protects the aluminium. The standard CD has a 12 cm diameter and a 1.2 mm thickness. From its centre outward, it consists of a spindle, a clamping ring, a stack ring, a mirror band, an information area and the rim. The CD's data layer is comprised of billions of tiny indentations called pits that are invisible to the human eye. These pits are encoded with binary data (0's and 1's) that maintain the disc's speed and sound. They also serve to control the disc player's laser. The patterns of pits rest along tightly coiled spiral tracks followed by a laser. The reflected laser beam hits a photodiode that converts the binary data into an electrical signal that is heard like the original audio. Cross section view of a compact disk Understanding the CD: The Spiral 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 centre 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.) Upper view of a CD. Different types of CDs: 1. CD-ROM: CD-ROM stands for Compact Disc Read-Only Memory. It functions as a CD that stores computer data of graphics, text and audio. They are popular for software and other multimedia applications. CD-ROMs commonly store up to 700MB of information. This data comes pre-stamped by the manufacturer so it cannot be erased nor edited. 2. CD-R: CD-R, or compact disc-recordable is a type of compact disc that can be written by a consumer using disc recording hardware, thus following the format acronym WORM (write once, read many). CD-R's were engineered by the inventors of the compact disc, Philips and Sony, and first available in 1988. A CD-R stores digital video, images, music, document imaging and data archiving. In the beginning, there was a price gap due to copyright issues that made music CD-Rs more expensive than data CD-Rs. 3. CD-RW: CD-RW, or Compact Disc-ReWritable, is a CD format that provides flexible recording options. This format permits optical data to be written, re-written, read and erased multiple times. At its conception, it was known as CD-E (CD- Erasable) but was introduced as an extension of the Orange Book standard under the moniker CD-RW in 1997. CD-RWs are designed for computer storage and backup since they can be re-written, but their re-writable format results in a disc with a smaller storage capacity than a write-once CD-R disc. 4. PRINTABLE CD: A printable CD takes media to a higher level of creativity and customization. These discs are designed with a printable surface area to display artwork without interfering with the function of the CD. These discs are available in a variety of formats to suit the needs of specified printers as well as suit an artistic vision. CD-R is the most popular format of printable CDs available. The less frequently used CD-RWs are also available on the market. 2. Explain DVD (DIGITAL VERSATILE DISC) TECHNOLOGY The term DVD is an acronym for Digital Versatile Disc or Digital Video Disc, and it refers to a type of optical disc used for storing data and video content. The capacity of a DVD is at least seven times greater than a compact disc and provides enough room for a full-length feature film. A DVD will support standard as well as widescreen television title views in 4:3 and 16:9 aspect ratios. The DVD was introduced to a test-marketed U.S. in 1997 and developed by Toshiba, Philips, Sony and Time Warner. DVDs are a consumer-friendly format primarily used for home entertainment for all mediums, as well as for business needs, storage backup and computer software. DVDs took a strong reign over the global audio and video market by replacing videotapes, video cartridges and laserdiscs as the mainstream format within a decade of its debut. DVD technology is akin to the next generation of the compact disc, as it mirrors its technology. Both discs share the same dimensions of 1.2mm thickness and 120mm diameter. The DVD contains microscopic pits of binary data on its polycarbonate layer like the CD, as well as substrate, adhesive, reflective and lacquered layers, but the DVD's construction has some singular exceptions. The DVD's pits of data are smaller and rest closer together than those of a compact disc. This higher density of pits allows a shorter laser wavelength that in turn allows more data to be stored per track. The result is more efficiency in error correction and channel bit jitters. In addition the disc has a thinner outer layer to let the reading laser pass through the extra layers with more ease. There are four basic constructions of the DVD depending on one's optical storage requirements: single-sided single layer, single-sided double layer, double-sided single layer and double-sided double layer. A shift by the drive's laser is all that is needed to read the next layer which therefore allows more storage per disc while eliminating the need to switch discs between projects or media entertainment. Storage capacity does not completely double with the addition of another layer. Rather, the pits on the second layer must be constructed longer and less dense in order to combat obstruction and errors between layers. Dual layer and single layer DVD drives have comparable costs; however the recordable media itself still has a market separation. DVDs are of the same diameter and thickness as CDs, and they are made using some of the same materials and manufacturing methods. Like a CD, the data on a DVD is encoded in the form of small pits and bumps in the track of the disc. A DVD is composed of several layers of plastic, totalling about 1.2 millimetres thick. Each layer is created by injection moulding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data. More on the bumps later. Once the clear pieces of polycarbonate are formed, a thin reflective layer is sputtered onto the disc, covering the bumps. Aluminium is used behind the inner layers, but a semi-reflective gold layer is used for the outer layers, allowing the laser to focus through the outer and onto the inner layers. After all of the layers are made, each one is coated with lacquer, squeezed together and cured under infrared light. For single-sided discs, the label is silk-screened onto the no readable side. Double-sided discs are printed only on the no readable area near the hole in the middle. Cross sections of the various types of completed DVDs (not to scale) look like this: Data tracks on a DVD Each writable layer of a DVD has a spiral track of data. On single-layer DVDs, the track always circles from the inside of the disc to the outside. That the spiral track starts at the centre means that a single-layer DVD can be smaller than 12 centimetres if desired. What the image to the right cannot impress upon you is how incredibly tiny the data track is - - just 740 nanometres separate one track from the next (a nanometre is a billionth of a meter). And the elongated bumps that make up the track are each 320 nanometres wide, a minimum of 400 nanometres long and 120 nanometres high. The following figure illustrates looking through the polycarbonate layer at the bumps. We will often read about "pits" on a DVD instead of bumps. They appear as pits on the aluminium side, but on the side that the laser reads from, they are bumps.