Optical Memory

Introduction of Optical Memory

• Despite the rise of alternative modern and advanced storage technologies such as cloud storage and solid-state drives, optical memory continues to be used in specific industries and applications where its unique characteristics are advantageous.

Definition of Optical Memory

• Optical memory has been widely used for archival, backup, and distribution purposes due to its high capacity, durability, and non-volatile nature.
• An Optical Memory is a Secondary, External or Removable, Cheap, non-volatile/permanent, and alternate storage devices or memory of a computer.

Features of Optical Memory

• Optical storage devices have a higher storage density at a lower cost.
• Optical memory is commonly associated with various types of optical discs, such as CDs (Compact Discs), DVDs (Digital Versatile Discs), and Blu-ray discs. These discs use different laser wavelengths and recording formats to store digital data.
• A CD/DVD-ROM can be used as a main and common optical storage device.
• Information is written to or read from an optical disk using a laser beam.
• An optical disk has a very high storage capacity, up to 20 GB/more.
• An optical disk is relatively inexpensive and has a long life of at least 15-20 years.
• Optical memories or Optical disks are alternate mass storage devices with huge capacity (up to 20 GB/more).
• Optical discs offer different levels of data storage capacity depending on the format and type of disc. For example, CDs typically have a storage capacity of up to 700 MB, DVDs can store up to 4.7 GB (single-layer) or 8.5 GB (dual-layer), and Blu-ray discs can store up to 25 GB (single-layer) or 50 GB (dual-layer).
• Optical discs are known for their durability and resistance to physical damage, such as scratches and fingerprints, compared to other storage media like magnetic tapes or hard drives. Properly stored optical discs can have a long lifespan, making them suitable for archival purposes.

Structure of Optical Memory

Advantages of Optical Memory

• These are portable & medium storage devices.
• These are external and removable storage devices.
• They are inexpensive.
• Some optical disks (CD-RW/DVD-RW) can be reused many times i.e. rewritable disks.
• In an optical disk, the read/write head does not touch the disk surface, there is no problem of disk wear or head crash.

Disadvantages/Drawback of Optical Memory

• The main drawback of the optical disk system is its slow average access time.
• Only one surface of an optical disk is used to store data.
• Some older computers cannot read CD/DVD-RW media.
• In CD/DVD-R discs, new or additional data cannot be added after writing once.
• While optical memory offers several advantages, it also has some limitations, such as slower data transfer rates compared to solid-state drives (SSDs) or hard disk drives (HDDs), limited rewrite cycles for writeable discs, and susceptibility to environmental factors like heat and humidity.

Use/Applications of Optical Memory

• Optical memory has been widely used for archival, backup, and distribution purposes due to its high capacity, durability, and non-volatile nature.
• Many software companies provide both operating systems and application or driver software on CD/DVD-ROMs today.
• This optical memory technology has been acting as the main catalyst for the development of multimedia fields in computing because it is used broadly in storing multimedia contents as removable/external devices.
• Optical memory is non-volatile, meaning that data stored on optical discs remains intact even when the power is turned off. This makes optical discs suitable for long-term storage and backup solutions.
• Optical memory has been widely used for various applications, including data backup, software distribution, multimedia content storage (e.g., movies, music), archival storage of important documents and records, and medical imaging.

Types of Optical Memory

• Optical discs can be either read-only (ROM) or writeable (RW). ROM discs are pre-recorded with data during manufacturing and cannot be modified or erased. Writeable discs, on the other hand, allow users to write data onto the disc using a compatible optical disc drive.
• Optical disks are of four types :
  1. CD-ROM (Compact-Disk Read Only Memory)
  2. WORM (Write Once Read Many) or CD-R (CD-Recordable).
  3. Erasable Optical Disk(EOD)/CD-RW(CD-Rewritable).
  4. DVD-ROM, DVD-R, and DVD-RAM

CD-ROM

Introduction

Features

• • • Each disk can store up to 650/700MB of data.
    • A laser light records data by burning process on a permanent microscopic pit present in a spiral track to represent 1.
    • CD-ROM disk drives use a laser device to read the binary codes formed by those pits. For reading the data a laser beam of lower intensity is employed. A laser system needs 25mW for writing whereas only 5mW is needed for reading.

Structure

• • • CD-ROM is a 12-centimeter (4.7-inch) compact disk (CD).
    • CD-ROMs use long spiral tracks to store data serially.
    • Here, the track is divided into blocks of the same size.
    • A CD-ROM disk rotates at a variable speed so that the pits are read by the laser at a constant linear speed. The speed of the disk is adjusted in such a way that the track passes under the read/write head at a constant linear velocity.

Advantages

• • • CD-ROM has high storage capacity.
    • The making cost per bit of storage is cheaper than the other types of memory devices.
    • It is a removable storage device, so suitable for archival storage.

Disadvantages

• • • A CD-ROM stores only 650/700 MB of data.
    • It takes longer access time to reach the data.
    • The major limitation of CD-ROM disk is that recorded data can not be erased i.e. information can not be updated after burning once because it is a read-only memory and stores data permanently.

WORM/CR-R

• • CD-R (compact disk recordable) is an optical disk.
  • In this, the user can record (write) their data once on a CD-R in a CD-R disk drive unit. After this recording user can read the data as many times as desired.
  • CD-R is suitable for storing data, information, and files permanently for maintaining records that are not to be changed.

Advantages :

• • • It has a high storage capacity.
    • It has better reliability and a longer life.
    • It has a greater access time as compared to a hard disk.

Limitations :

• • • The major limitation of CD-R disks is that recorded data can not be erased.

EOD/CD-RW

• • CD-RW (CD-rewritable) is an optical disk that records and erases data many times by using a laser ray to heat a microscopic point on the disk’s surface.
  • Advantages :
    • It has a very high storage capacity. A 5.25-inch optical disk can store about 650/700 MB of data.
    • It is more reliable and has a long life.
  • Limitations :
    • It has a longer access time as compared to that of a hard disk.

DVD-ROM

• • DVD stands for ‘Digital Video Disks’ or ‘Digital Versatile Disks’.
  • A DVD stores much more data than a CD-ROM.
  • Its capacities are 4.7GB, 8.5GB, and 20GB, etc.
  • The capacity of a DVD depends on whether it is a single layer, double layer; single-sided or double-sided disk.
  • DVD uses a laser beam of shorter wavelength than CD-ROM and therefore more tracks are available.
  • The working mechanism of DVD disks is similar to those of a CD-ROM, CD-R, or CD-RW.
  • The Speed of CD-ROM or DVD-ROM is given in terms of nX, where n is an integer. For example 24X. In the case of CD, X=150 KB/s, so 24X=24×150=4.8 MB/s. In the case of DVD, X=1.38 MB/s.
  • DVD-R: It is a recordable DVD, the same as a CD-R disk. The user can write data once on a DVD-R, and then read the data as many times as required.
  • DVD-RAM: It is a rewritable DVD, the same as a CD-RW disk. DVD-RAM uses a phase change technology to write, read, and erase data.