Types of Optical Disks: CD-ROM, CD-R, CD-RW, and DVDs

 

Types of Optical Disks:

 

All optical disks are round platters. They come in different sizes and capacities. Commonly used types of optical disk are CD-ROM, WORM (CD-R), CD-RW and DVD disks.

 

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

 

CD-ROM stands for Compact Disk Read Only Memory. It is a spin-off of audio CD technology and works much like audio CDs used in music systems. CD ROM is a shiny, silver colour metal disk usually of 5 ¼ inch (12 cm) diameter. It is made of polycarbonate plastic and thin layer of pure aluminium is applied to make the surface reflective. A thin layer of lacquer protects it.

 

It has a storage capacity of about 650 MB or 700 MB in newer ones. It is so called because of its large storage capacity on a compact size disk and because it is a read-only storage medium. These disks come pre-recorded and information stored on them cannot be altered.

 

Since an optical disk is read from the centre to outer edge, several sizes are possible. Apart from the most common 120 mm 74/80 minute disk having storage capacity of 650 MB to 700 MB, 80mm 21 minute disk called Mini CD having 184 MB storage is also available. They can be used with the disk drive of a 120 mm disk. Mini CDs are designed in various fancy shapes like round, oval, triangle, etc., while ensuring that the storage area is not affected.

  

WORM Disk / CD - Recordable (CD-R) Disks:

 

WORM stands for Write Once, Read Many. WORM disks allow users to create their own CD-ROM disks by using a CD-R recordable drive attached to a computer as a regular peripheral device. WORM disks look like a standard CD-ROM disks, are purchased blank, and later encoded using a CD-R drive.

 

The information recorded on a WORM disk by a CD-R drive can be read by any ordinary CD-ROM drive. As the name implies, data can be written only once on a WORM disk but can be read many times. That is, like CD-ROM disk, once data has been etched on the surface of a WORM disk, it becomes permanent and can be read but never altered. However, all the data to be recorded on a WORM disk can be written on its surface in multiple recording sessions.

 

The sessions after the first one are always additive and cannot alter the etched information of earlier sessions. The information added in a situation can be hidden in a subsequent session by creating the File Allocation Table (FAT) at a new location but the etchings on the surface cannot be removed. Such disk is called multi-session disk. Laser beam technology is used for data recording/reading.

  

CD-RW (CD Read/Write) Disks:

 

A CD Read/Write disk is a very similar to a WORM disk with the exception that you can erase the previous content and write on it multiple times. Such disks use metallic alloys layer. Laser beam changes the chemical property during writing (or burn process) changing reflectivity at desired places. The land-pit difference on CD- RW is not significant and hence CD drives have to be compatible to read such disk.

 

A CD-RW disk usually has a lifetime of 100 or more erase-write cycle. A disk written once can be erased by changing the chemical property again and then it can be written on to afresh. CD-RW drives have such erase capability. CD-RW disks are little expensive than CD-R disk but are a great cost saver because they can be reused many times due to their erase capability.

  

DVD (Digital Video/Versatile) Disks:

 

DVD was designed primarily to store and distribute movies. However, it is fast becoming a mainstream optical disk as prices are reducing and a need for large capacity storage is increasing.

 

It is similar to CD-ROM in principle but is denser in recording data. It follows Eight-to-Fourteen Modulation Plus (EFMPlus) Encoding as compared to Eight-to-Fourteen Modulation (EFM) Encoding used by CD-ROM.

There are two variants of DVD - single layer disk and double layer disk. Single layer disk has a storage capacity of 4.7 GB, whereas, double layer disk has a storage capacity of 8.5 GB.

 

Like CD-ROM, DVD also has many types - DVD-R/RW, DVD+R/RW, DVD Video and DVD Audio. DVD-Video is now the most dominant movie storage format used. It allows storage of video in 4:3 or 16:9 aspect ratios in MPEG-2 (Moving Pictures Expert Group – 2) video format using NTSC (National Television System Committee) or PAL (Phase Alternate Line) resolution.

 

NTSC has a resolution of 720x480 and PAL has resolution of 720x576. The audio is usually Dolby Digital (AC-3 i.e. Audio Compression - 3) or Digital Theatre system (DTS) and can be either monaural or 5.1 Surround Sound.

  

DVD has enough space to store movie and support multilingual subtitles, multilingual audio,  multiple camera angles, etc. It also supports region marking to protect against piracy and use of DVD from one region to another. It also supports Content Protection for Pre-recorded Media (CPPM) security technique to safeguard against copying, etc.