So What Does DVD Really Mean? DVD originally stood for Digital Video Disc. Since it spanned both the movie video and computer worlds, it was later called Digital Versatile Disc. Today, the acronym has become its name. DVD stands for D-V-D.
The DVD specification supports disks with capacities of from 4.7GB to 17GB and access rates of 600KB/sec. to 1.3 MB/sec. / 2.7 MB/sec.. One of the best features of DVD drives is that they are backward-compatible with CD-ROMs. This means that DVD players can play old CD-ROMs, CD-I disks, and video CDs, as well as new DVD-ROMs. DVD 1X players CANNOT, however, READ CD-R disks.
DVD 1X Transfer rate = 1.38 MB/Sec. DVD 2X Transfer rate = 2.76 MB/Sec. ( equivalent to about a 20X CD-ROM drive. )
The next-generation video CD and high-capacity CD-ROM : The MultiMedia CD (MMCD) from Sony and Philips and the Super Density (SD) format from Toshiba, Time Warner and others were merged into one standard.
The disc is the same diameter as a CD-ROM, but can be recorded on both sides. Each side holds 4.7GB, equivalent to seven CD-ROMs, or 14 CD-ROMs, if both sides are used. A dual-layer version is also planned that yields 8.5GB per side, or the equivalent of 28 CD-ROMs if both sides are used. DVD players are compatible with audio CDs, CD-ROMs, CD-I discs and Video CDs, but not first-generation CD-R disks.
DVD discs use MPEG-2 compression and provide approximately 135 minutes of video per side, which approaches the quality of an analog LaserDisc. This is not a guaranteed length, because the data rate of compressed video varies depending on how much action takes place in each frame.
Both the computer and movie industries have worked on DVD development, and it is expected to become the next CD-ROM and primary digital movie medium, superseding Video CDs and analog LaserDiscs. It is also expected to become the VHS tape of the 21st century as recordable versions (DVD-RAM) become available in the 1997-1998 timeframe.
There are several flavors of DVD. The DVD movie player (DVD-Video) became available at the end of 1996. It attaches to a TV and plays DVD movies. It supports AC-3 Dolby surround sound, which provides five channels of CD-quality audio plus a subwoofer (5.1 channel).
DVD-ROM drives allow for interaction as well as audio and video and require a DVD-ROM drive installed in a computer. DVD-R and DVD-RAM are write once and re-writable versions of the DVD-ROM, which are expected in the 1997-1998 timeframe. The music industry may not embrace the DVD format right away, if at all, because the audio CD is a digital medium that has sufficient room for the traditional number of songs offered in an album. However some may lobby for a new audio format with more digital information, which the DVD could accommodate. Many audio purists still yearn for the analog LP days and believe that higher digital sampling of audio material is necessary to truly recreate the total sound spectrum.
Short for Compact Disk-Recordable drive, a type of disk drive that can create CD-ROMs and audio CDs. This allows users to "master" a CD-ROM or audio CD for publishing. Until recently, CD-R drives were quite expensive, but prices have dropped dramatically.
A feature of many CD-R drives, called multisession recording, enables you to keep adding data to a CD-ROM over time. This is extremely important if you want to use the CD-R drive to create backup CD-ROMs.
To create CD-ROMs and audio CDs, you'll need not only a CD-R drive, but also a CD-R software package. Often, it is the software package, not the drive itself, that determines how easy or difficult it is to create CD-ROMs.
CD-R drives can also read CD-ROMs and play audio CDs.
CD-R media addresses this issue by doing away with pits and lands entirely and using a different kind of media. CD-R media starts with a polycarbonate substrate, just like regular CDs do. Instead of physical etching this substrate, it is stamped with a spiral pre-groove, similar to the spiral found on a regular CD except that it is intentionally "wobbled". This groove is what the CD-R drive uses to follow the data path of the disk during recording.
The key to the media is the dye layer (and the special laser used in the drives.) It is chosen so that it has the property that when light from a specific type and intensity of laser is applied to it, it heats up rapidly and changes its chemical composition. (When folks talk about creating a CD-R as "burning" a disk, they're basically correct.) As a result of this change in chemical composition, the area "burned" reflects less light than the areas that do not have the laser applied. This system is designed to mimic the way light reflects cleanly off a "land" on a regular CD, but is scattered by a "pit", so an entire disk is created from burned and non-burned areas, just like how a regular CD is created from pits and lands. The result is that the created CD media will play in regular CD players as if it were a regular CD, in most cases.
Since the media is being physically altered by a process of heat and chemistry, the change is permanent and irreversible. Once any part of the CD has been written, the data is there forever. Some drives allow you to record some information in one sitting, and then more information later on, if the disk is not yet full. This is called multi-session recording, and requires a CD player capable of recognizing multi-session disks in order to use the burned disk.
CD-R media are generally 74 minutes in length. There are some disks with less capacity, and some with more, but all 74-minute disks have the same capacity, about 650 MB. Some media seems to work better than others in some drives.
Just as CD-R requires the use of special media, it also of course requires the use of a special CD-R drive. This drive is very different than a standard CD player because it must include a special laser. The laser is the key component from the drive's perspective, in that it is what burns the image into the CD-R media's dye layer.
CD-RW media are formed in the same basic way that CD-R media are; they start with a polycarbonate base and a molded spiral pre-groove to provide a base for recording. There are several layers applied to the surface of the disk, with one of them being the recording layer where ones and zeroes are encoded. The recording layer for CD-RW is different of course than it is for CD-R. The problem with CD-R is that the dye layer used is permanently changed during the writing process, which prevents rewriting.
CD-RW media replaces this dye with a special phase-change recording layer, comprised of a specific chemical compound that can change states when energy is applied to it, and can also change back again. Much the way water can change to steam, or ice, depending on its temperature, there are some types of chemicals that can not only change their state after having heat or other conditions applied, but even retain that state when the heat is removed. They can later be returned to their original state through another, different process.
The material used in CD-RW disks has the property that when it is heated to one temperature and then cooled, it will crystallize, while if it is heated to a higher temperature and then cools, it will form a non-crystalline structure when cooled.
CD-RW media have one very important drawback: they don't emulate the pits and lands of a regular CD as well as the dye layer of a regular CD-R, and therefore, they are not backward compatible to all regular audio CD players and CD-ROM drives. Also, the fact that they are written multiple times means that they are multi-session disks by definition, and so are not compatible with non-multi-session-capable drives. CD-R disks made in a CD-RW drive can be read in any drive that can read CD-R media. In other words, the compatibility problem is with the CD-RW media, not the CD-RW drives.
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