Note on Memory (ii)

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Optical Disk

At the present time, optical disk technology is well known and most common among the people. It is a removable disk on which data is written and read through the use of laser beams. A laser beam is a concentrated narrow beam of light focused and directed on a particular location to read or write data. It is used as backup memory. Its advantages are :

. figure: optical disk

Advantages of Optical Disk

  • It has very high storage capacity and is relatively cheaper.
  • Cost-per-bit of storage for the optical disk is very low because of their low cost and high density.
  • Use of single spiral track makes optical disk an ideal storage or crash into large blocks of sequential data such as audio or video.
  • It does not have any mechanical read/ write heads to rub against or crash into the disk surface. This makes optical disk more reliable storage medium than magnetic tape or magnetic disk.
  • Since data once stored on this disk becomes permanent, the danger of stored data getting inadvertently erased or overwritten is not there.
  • Due to its compact size and lightweight, it is easy to handle, store and carry from one place to another.

Limitation of Optical Disk

  • CD-ROM and WORM are the read-only storage medium. Data once recorded cannot be erased. Hence, they cannot be reused.
  • Data access speed for the optical disk is slower than magnetic disk.
  • It requires more complicated driver mechanism than magnetic disk due to the need to have laser generating source and detection lens that requires precision engineering and careful handling.
  • Since it is a removable media, prone to scratches, dust, it should be handled carefully.


1) CD-ROM (Compact Disk Read Only Memory)

The best-known type of optical disk is the CD-ROM. It is an optical disk that is used to store text, graphics, sound and video. Read only memory (ROM) means the disk cannot be written on or erased by the user. The users can only read the data imprinted by the disk’s manufacturer. So, it is suitable for storing the data which the user cannot change.

It is non-volatile optical data medium and readable by a computer with a CD-ROM drive. It consists of aluminum coated plastic, which reflects light differently for lands and pits. A land reflects the laser light into the sensor and pit scatter the light. The light reflected from the land is detected by a photocell that converts them into electronic signals.

To read stored information, the CD players passes a laser beam over the tracks. When the laser passes over a flat area in the track, the beam is reflected directly to an optical sensor on the laser assembly. The CD player interprets this as 1. When the beam passes over a pit, the light is bounced away from the optical sensor. The CD player recognizes as 0.Reading the information back of reflecting a lower powered laser off the aluminum film receiver or a light receptor notes where light is strongly reflected or where it is absent or diffused. Diffused to absent light is caused by the pits made on the CD. Strongly reflected light indicates no pit that is called land. The individual pit on a DC are 0.12 microns deep and 0.6 microns wide.

2) CD-R (Compact Disk Recordable)

CD-R is a CD format that allows you to write data into a specially manufactured disk, thenwhich can be read by a standard CD-ROM driver. It is once writable CD, the change to CD-ROM. It is coated with a photosensitive organic dye that has the reflective properties. The photosensitive organic dye within this disc changes from a reflective state to non-reflective state when it is exposed to the laser recording beam of a CD-R drive. The reflective state acts as land and Non-reflective state acts as a pit in a CD-ROM.

3) CD-RW (CD-Rewriteable)

The CD-RW disc can be refused by erasing the content of the CD and again writing data on it. Using a CD-RW drive, the user can write data onto special rewritable compact disk (CD-RW disk), then overwrite it with new data. In other words, the user can change the contents of this disk in the same manner as a floppy disk. It has the same capacity as a standard compact disk, and most can be overwritten up to 100 times. however, it may not play on every CD-ROM drive.

4) DVD (Digital Versatile / Video Disk) ROM

DVD is the most popular optical disk storage technology at present. It’s essentially a high capacity, faster CD that can hold video as well as audio and computer data. It aims to encompass home entertainment, computers and business information with a single digital format, eventually replacing audio CD, videotape, laserdisc, CD-ROM and perhaps even video data, it was mistakenly taken to stand for Digital Video Disc. This in not so; as indicated above, DVD stands for Digital Versatile Disc.

5) DVD’s can be Double-Sided and Dual Layered

The DVD specification allows multiple configurations of data layers. All of this helps to increase the capacity,The basic configuration of Single-Single Layer gives a capacity of 4.7 GB.

The single-Sided, Dual Layer configuration increases the capacity by 3.8 GB to 8.5 GB on one-sided by using the second layer. The laser has to read through the outer layer to the inner layer. To reduce the possibility of interference between layers, the minimum pit length of both layers is increased from .4 um to .44 um. Also, the reference scanning velocity is slightly faster from 3.49 m/s for a single layer disc to 3.84 m/s. These longer pits, being spaced further apart, are easier to read correctly. However, their increased length means fewer pitper revolution; hence the reduced capacity per layer and the reason why a Single-Sided, Dual-Layer disc is only 8.5 GB and not 2 x 4.7=9.4 GB. This is more than 13 times the capacity of a CD-ROM.

Fig: Single- sided, dual layer DVD land and pit pattern


Fig: Double-sided, Single DVD land and pit pattern


The Double-sided, Single configuration gives a capacity of 9.4 GB with 4.7 GB on each side. However, as data is on both side, the disc must be either ‘flipped-over’ or used in a driver that is capable of reading both sides.

Fig: Double-sided, Dual layer DVD land and pit pattern


The Double-Sided, Dual Layer configuration gives 17 GB with 8.5 GB on each side. Again that must be either ‘flopped-over’ or used in a driver that is capable of reading both sides.


A DVD-RW (DVD-Rewritable) disc is a rewritable optical disc with equal storage capacity to a DVD-R, typically 4.7 GB. The format was developed by Pioneer in November 1999 and has been approved by the DVD forum. Unlike DVD-ROM, it is playable in about 75% of conventional DVD players. The smaller Mini DVD-RW holds 1.46 GB, with a diameter of 8 cm.

The primary advantage of DVD-RW over DVD-R is the ability to erase and rewrite to a DVD-RW disc. According to Pioneer, DVD-RW disc may be written to about 1,000 times before needing replacement, making them comparable with the CD-RW standard. DVD -RW disc is commonly used for non-volatile data, such as backups or collections of files. They are also widely used for home DVD video records. One benefits of using a rewritable disc are if there are writing errors when recording data, the disc is not ruined and can still store data by erasing the faulty data.

7)Blue-Ray Disk (BD)

BD is the name of the new generation optical disk. It was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing a large amount of data. This disk offers more than five times the storage capacity of present DVDs. It can hold up to 25GB on a single-layered disk and 50GB on a dual layered of the present disk. The current optical disk technology such as DVD-R, DVD-ROM and DVD-RW used a red laser to read and write data. the new disk, BD uses a blue-violet laser beam, hence the name of Blu-ray used. The benefit of using a blue-violet laser is that it has a shorter wavelength than a red laser, which makes it possible to focus the laser with even greater precision. This allows data to be packed more tightly and in less space, so it is possible to fit more data on the disc even though it has the same size as a CD/DVD. Similar to CD and DVD, it also has different variations. Such as, BD-R (Blu- ray disk recordable), BD-ROM (Blu-ray read only memory) and BD-RW (Blu-ray disk re-writable).


Winchester Disk

Winchester technology disk was developed by IBM in order to overcome some problem associated with established disk technology. It is now made by many different manufacturers.

. Fig: Winchester Disk

Winchester disk is fixed disk in hermetically sealed disk units and has rebutted mechanical features. It has toughened surface and read-write heads that move even closer to the disk’s surface when the disk finally stops. It can operate in adverse environments that are dusty or humid, has greater reliability and also has greater reliability and greater storage capacities in comparison with the earlier technology disks of the same size.

Zip Disk

A Zip disk is a small, portable disk used primarily for backing up and archiving personal computer files. The trademarked Zip drive was developed and is sold by Iomega Corporation. Its drivers and disks come in two sizes.

. Fig: Zip Disk

The 100-megabyte size actually holds data equivalent to 70 floppy diskettes. These are also a 250-megabyte drive and disk. The Iomega Zip drive comes with a software utility that lets the user copy the entire contents of the hard drive to one or more Zip disk.

Jaz Disk

. Fig: Jaz Disk

It is a removable hard disk drive developed by Iomega Corporation in late 1995. Jaz was fairly popular in its role and a 2GB driver was introduced in 1992. In 2003, Iomega stopped production of new drivers but continued to provide support.

Pen Drive

The pen drive is a portable USB flash memory devices that can be used to quickly transfer audio, video and data files from the hard drive of one computer to another. With a construction that is small enough to fit into a pocket, the pen drives drivers name from the fact that many of these USB drive devices resemble a small pen or pencil in size and shapes.

. Fig:Pen Drive

It is equipped with a large amount of memory capacity. It is considered to be an improvement on both the older floppy driver disk and the more modern compact disks that are often used from to copy data the files on a various storage device. Even a pen drive with a relatively low storage capacity tends to provide plenty of space for a number of files.

Utilizing a pen drive is a simple task; one end of the driver is equipped with a USB connector. The connector is inserted into the USB port of the computer. Once the pen drive places, it is possible to drop and drag files into the memory of the driver or forward the files to the driver. The process is no more difficult than attaching files to an email or copying files onto disk. double-sided, dual-layer land, and pit pattern both sides.


Shrestha, Prachanda Ram, et al. Computer Essentials. Kathmandu: Asmita's Publication, 2014

Dilli , S. P., Singh, k. E., Khadka , D., Bhatta, K. D., Baral, N., Saud, R. S., . . Dangi, R. (2015). Fundamental of Computer Science. Kathmandu: KEC publication and Distribution.

  1. CD-R is a CD format that allows you to write data onto a specially manufactured disk, which can then be read by a standard CD-ROM driver. 
  2. A DVD-RW (DVD-Rewritable) disc is a rewritable optical disc with equal storage capacity to a DVD-R, typically 4.7 GB.
  3. A Zip disk is a small, portable disk used primarily for backing up and archiving personal computer files.
  4. Jaz disk is a removable hard disk drive developed by Iomega Corporation in late 1995.
  5. The pen drive is a portable USB flash memory devices that can be used to quickly transfer audio, video and data files from the hard drive of one computer to another.

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