Optical driving

The acceptance of optical data storage iato the mass storage market, which is as yet exclusively dominated by magnetic systems, will be fundamentally boosted if optical drives and media are subject to uniform standards and become fully compatible, and multiuser drives are offered which enable the user to employ alternatively CD-ROM and EOD disks, and maybe WORM disks as well (and CD-R disks, respectively). A prerequisite, however, will be whether rewritable optical memories will use the MOR or the PCR process. This accord especially will be hard to reach. 

There are many additional types of storage available for PCs today. However, most of them are not covered on the A+ exam, so we ll just discuss them briefly here. Among the other types of storage are Zip drives, tape backup devices, and optical drives. 

The final type of storage is the optical drive. Optical drives work by using a laser rather than magnetism to change the characteristics of the storage medium (typically an aluminum-coated plastic disk). Optical drives look similar to and are used for the same applications as Zip drives (e.g. archival storage and large file transport). However, optical drives can store more information and have slower access times than Zip drives. 

Another type of removable media drive is the CD-ROM drive (see Figure 4.5). These drives are slightly different from other storage media in several ways. First of all, they have a different way of reading information than magnetic media disk drives do. Because CD-ROM drives use laser light to read the information from the media, they are described as optical drives. 

In contrast to the Einstein constraints for optical driving, transition coefficients for chemical driving obey microscopic reversibility, where for any transition i = j the ratio of forward to backward transition constants is given by... 

Computer Applications. Almost all computers, including basic laptops, contain one or more optical drives, most of which house a read-write drive. The drive is used to load computer programs onto a hard drive, data storage and retrieval, and entertainment. The inclusion of Blu-ray readers as well as writers (burners) is increasing this is a result of significant drops in price of these devices as well as blank media since their introduction to the marketplace. Most internal drives for computers are designed to fit in a... 

The basic layout of the optical head of a magneto-optic drive is shown in Fig. 14.12. The laser is mounted in a heat sink designed to achieve an athermal response. The output from the laser diode is collimated by lens 1. A prismhke optical element called a circularizer is then used to reduce the elhpticity of the laser beam. The beam then passes through a polarizing beamspHtter, which reflects part (30%) of the beam toward a detector and transmits the rest toward the disk. 

In addition to tracking and seeking, the laser spot in an optical drive must be kept in perfect focus on the disk regardless of the motion of the disk (there can be quite a lot of vertical motion if the disk has tilt or is slightly warped). To do this, the objective lens must be constantly adjusted to correct for the axial motion of the disk surface as the media spins. The lens position is controlled by a focus servomechanism. 

Figure 14.13 shows a block diagram of an optical drive servocontrol system (combined tracking and focusing). The return beam contains information on the focus and position of the spot, which is processed... 

The ability to accurately foUow the radial and axial motions of the spinning disk results directly from the quality of the focus and tracking actuators. To reject the errors due to shock, vibration, and media runout, the servosystem must have high bandwidth. The limitation to achieving high bandwidth is usually the resonance modes of the actuator. As the actuators are reduced in size, the frequencies of the resonances become higher and the achievable bandwidth of the system rises. Servosystems in optical drives face additional challenges because they have to handle removable media, which has variations between different disks (such as media tilt). 

A schematic block diagram of the functions in an optical drive is shown in Fig. 14.15. The SCSI controller handles the flow of information to and from the host (including commands). The optical disk controller... 

Error correction and control (ECC) bytes are added to each block of data. Optical drives use Reed-Solomon codes, which are able to reduce the error rate from lE-5 to about lE-13 (Golomb, 1986). After the addition of ECC information, the data are encoded with run length limited (RLE) modulation codes (Tarzaiski, 1983 Treves and Bloomberg, 1986) in order to increase efficiency and improve detection. Special characters are inserted in the bit stream such as a synchronization character to tell where the data begins. All in all, the overhead required to store customer data is about 20%. 

The earliest writable optical drives were, in fact, WORM drives, and although the rewritable drive is more popular for daily storage needs, the WORM technology has a clear place in data storage because it allows permanent archiving capability. 

Optical drives can be extended to automated storage systems, which are essentially jukeboxes consisting of one or more optical drives and a large number of optical disk cartridges. Optical Hbraries can provide on-line, direct-access, high-capacity storage. 

Optical drives and libraries, like any high-technology products, will see two types of improvement processes. The first is an incremental improvement process in which quality and functionality will continuously improve. The second is a more dramatic improvement process in which disk capacity and drive performance will improve in distinct steps every few years. 

FIGURE 14.25 Technological directions to achieve performance and capacity improvements in optical drives. 

In terms of radical improvements, there is a considerable amount of technical growth that is possible for optical drives. The two primary directions for future work on optical drives are (1) increasing capacity and (2) improving performance specifications (such as data rate and seek time). The techniques to achieve these are shown schematically in Fig. 14.25. 

In optical disk products, higher capacities can only be achieved by higher areal densities since the size of the disk cannot increase from presently estabhshed standard sizes. There are a number of techniques that are being considered for improving the storage capacity of optical drives. These include... 

Data block size The data that is to be recorded on an optical disk is formatted into minimum block sizes. The standard block size, which defines a single sector of information, is 512 byte. Many DOS/windows programs expect this block size. If the block size can be made larger, storage usage becomes more efficient. The next jump in block size is 1024 byte, often used in Unix appHcations. Device driver This is a piece of software that enables the host computer to talk to the optical drive. Without this piece of software, you cannot attach an optical drive to a PC and expect it to work. As optical... 

Some network backup software and hardware work with only certain network operating systems. Other network backup software will work with only the hardware device with which it is sold. The interaction between hardware devices and software such as operating systems or network operating systems is often controlled by specialized software programs (drivers). It is essential to make sure that the necessary drivers are supplied by either the tape backup device vendor or software vendor in order to ensure the operability of the tape backup device. Hardware devices may be various types of tape subsystems, or optical drives. Key differences among hardware devices include... 

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