How do rewriteable CDs work?
February 15, 2007 12:23am CST
15 Feb 07
A CD recorder, CD writer or CD burner is a compact disc drive that can be used to produce discs readable in other CD-ROM drives and audio CD players. A DVD recorder produces DVD discs playable in stand-alone video players or DVD-ROM drives. A Blu-ray Disc recorder produces BD discs playable in BD-ROM drives. They are generally used for small-scale archival or data exchange, being slower and more materially expensive than the moulding process used to mass-manufacture pressed discs. Optical disc recorders, in one form or another, have in many places displaced floppy disks and tape backup as data transfer and storage formats alongside flash memory, and are also widely used to create custom entertainment products such as mix CDs and home movie collections.A recorder encodes (or burns) data onto a recordable CD-R, DVD-R, DVD+R, BD-R or HD DVD-R disc (called a blank) by selectively heating parts of an organic dye layer in the disc with a laser in its write head. This changes the reflectivity of the dye, thereby creating marks that can be read as with the "pits" and "lands" on pressed discs. The process is permanent and the media can be written to only once. For rewriteable CD-RW, DVD-RW, DVD+RW, DVD-RAM, BD-RE, BD-R DL, BD-RE DL, BD-RAM, HD-DVD-R, HD DVD-RW HD-DVD-R DL, HD-DVD-RW DL, HD-DVD-RAM and HD DVD-RAM media, the laser is used to melt a crystalline metal alloy in the recording layer of the disc. Depending on the amount of power applied, the substance may be allowed to melt back into crystalline form or left in an amorphous form, enabling marks of varying reflectivity to be created. Most rewriteable media is rated by manufacturers at up to 1000 write/erase cycles. The competing DVD+R and DVD-R disc formats use very similar dye-based media, but differ mainly in the way timing hints for the write head are laid out on the disc surface. This is also the case with DVD+RW and DVD-RW. Most internal CD recorders for personal computers, server systems and workstations are designed to fit in a standard 5.25" drive bay and connect to their host via an ATA, SATA or SCSI bus. External CD recorders usually have USB, FireWire or SCSI interfaces. Some portable versions for laptop use power themselves off batteries or off their interface bus. SCSI recorders are less common and tend to be more expensive because of the cost of their interface chipsets and more complex SCSI connectors. Some drives support Hewlett-Packard's LightScribe photothermal printing technology, using specially coated discs.The recording speed of a drive is determined by the speed at which the spiral groove of the disc passes under its recording head. This is its linear velocity. The rate at which the disc spins is its angular velocity. Early-model recorders were CLV (constant linear velocity) drives. The recording speed on such drives was rated in multiples of 150 KiB/s; a 4X drive, for instance, would write steadily at around 600 KiB/s. The transfer rate was kept constant by having the spindle motor in the drive vary in speed and run about 2.5 times as fast when recording at the inner rim of the disc as on the outer rim. There are mechanical limits to how quickly a disc can be spun. Beyond a certain rate of rotation, tensile stress will cause the disc plastic to creep and possibly shatter. This limits the maximum reading and writing speeds for CDs to about 52x at the outer edge of the disc. Modern 52x drives spin the disc at slightly over 10000 RPM. According to some sources (Mythbusters), however, an alternate value for 52x is actually around 30000 rpm. Higher reading speeds may be achieved by using multiple lens assemblies or by reading several consecutive tracks simultaneously, but such drives are expensive to manufacture and are uncommon. To keep the rotational speed of the disc safely low, more recent high-speed recorders tend to use the Z-CLV (zoned constant linear velocity) scheme. This divides the disc into stepped zones, each of which has its own constant linear velocity. A Z-CLV recorder rated at "52X", for example, would write at 20X on the innermost zone and then progressively step up to 52X at the outer rim. Some drives also limit the maximum read speed to lower values such as 40x. The reasoning is that it is safe to assume that a blank CD fresh off the spindle will be clear of any structural damage, but the same assumption will not hold true for every disc inserted for reading. In the late 1990s, buffer underruns became a very common problem as high-speed CD recorders began to appear in home and office computers, which—for a variety of reasons—often could not muster the I/O performance to produce a data stream to keep the recorder steadily fed. The recorder, should it run short, would be forced to halt the recording process, leaving a truncated track that often renders the disc useless. In response, manufacturers of CD recorders began shipping drives with "buffer underrun protection" (under various trade names, such as Sanyo's "BURN-Proof", Ricoh's "JustLink" and Yamaha's "Lossless Link"); these can suspend and resume the recording process in such a way that the gap the stoppage produces can be dealt with by the error-correcting logic built into CD players and CD-ROM drives. The first of these drives were rated at 12x and 16x CD-R burn speeds. The DVD+R and DVD+RW disc formats were designed with this kind of discontinuous recording in mind because they were expected to be widely used in digital video recorders. Many such DVRs used variable-rate video compression schemes which required them to record in short bursts; some allowed simultaneous playback and recording by alternating quickly between recording to the tail of the disc whilst reading from elsewhere.
• United States
15 Feb 07
first of all, you are talking about the CD - RW, right? you can re-write over and over. lets say you copy a CD from a friend and you burn that CD, when you get bored with that music, and you like to burn another CD your friend has, you can use that same CD - RW. you can put that written CD - RW back in your burner and you get delete the songs you burn first and then you can put the new songs off your friends new CD, get me?
15 Feb 07
in detail i dont know how it really works but i know that with cd-rw u can delete or edit ur cd contain which u cannot do when u use cd-r.. so u can reuse ur cd many times... its good for data transfer from a computer to another computer because u'll need only one cd-rw compare to cd-r.. usually the cd-rw will cost u more ut who cares since u can use it not only once...