| At first, the main drawback of the CD was the impossibility of writing data to it at home. Users are accustomed themselves to copying music from records to tapes, to expecting similar opportunities and from optical
discs. Soon, their expectations were met - first appeared singly recordable CD-R (CD-Recordable), and then a
device for rewritable CD-RW (CD-ReWritable). Although externally recordable
discs are very similar to the usual CD-ROM, their internal structure and method of recording the information signal significantly different. Plastic base for CD-R does not carry useful information - instead of pits in it are printed but empty track (they are needed to guide the drive heads). Top covered with a thin film of organic molecules, and then the disk is covered with a layer of reflective metal. Used by organic molecules (cyanines, phthalocyanines) are able to permanently change their optical properties when heated. During the recording laser beam heats selected points of the
device, they cease to transmit light to a reflective layer, forming regions similar to pits. Thus, with appropriate hardware and software you can record a variety of \
discs, including audio and then use them in traditional CD-drive or domestic players. In repeatedly rewritable CD-RW
discs use different technology. Instead of a layer of organic molecules using a metal film (or rather, an alloy of rare earth metals), which can reversibly change their phase state under laser irradiation. When heated, the laser above the critical temperature corresponding portion of the metal transforms into an amorphous state and remains there after cooling (fast enough). Since the amorphous areas is worse than crystal to reflect light, they look darker and can thus serve as the pits on the
device. Re-heating to a temperature well below the critical restores the original crystalline state, thereby erasing the recorded information. Thus, in contrast to CD-R, allowing only one-time recording information on a CD-RW can be overwritten repeatedly: a standard disc maintains the order of thousands of write cycles. A
device capable of recording (and, of course, read) data to CDs, called CD-recorder. Modern CD-recorders can work with both drives CD-R and a CD-RW. Today, despite the huge popularity drives CD-ROM/R/RW it is believed that a standard CD is hopelessly outdated. For many modern applications capacity of 600-700 MB is not enough. In particular, this applies to digital video. Advances in digital technology come at a frantic pace. Every year the need for increasingly sophisticated and high-capacity storage media is increasing. |