An introduction to Optical Discs

In the Information Age, storage, retrieval, transmission and distribution of data has taken on unprecedented importance. The medium of storage and distribution has been revolutionized by the birth and emergence of Optical Discs

Invented by Philips in 1977, the Compact Disc (CD) was first commercially manufactured in 1982. The production process was further refined and by 1987, the introduction of the first inline machine heralded a new era of CD production.

Better and cost-effective production systems, a greater supply than demand, mushrooming of production units around the world rapidly brought down the costs of CDs making them affordable and increasing their demand many fold.

Towards increasing the storage capacity of Optical Disks, Philips, Sony and SD alliance announced their intention of creating the Digital Video Disc in 1995. Today, both CDs and DVDs have become industry standards in storage, archival, transmission and distribution of data, software and entertainment.

Types of optical discs

  1. Pre-recorded: These are discs which are manufactured with the data they are to contain. They can be read-from but no additional data can be added to them. Examples: CD, DVD
  2. Recordable: These discs, when manufactured, are blank and contain no data. These are “Write Once Read Many” [WORM] media and data can be written on to these discs only once. In other words, data once recorded can neither be erased nor over-written. Examples: CD-R, DVD-R, +R
  3. Re-writable: Manufactured as blanks, these discs allow data to be written, erased and overwritten many times. Examples: CD-R, DVD-R, +RW.

Besides many differences in technical specifications and the way data is accessed from these media, the similar-looking CDs and DVDs have two major functional differences – capacity and access speed.

While a CD can store up to 680 MB of data that can be accessed at a maximum speed of 1.23 MB per second, a DVD can store up to 4.7 GB (4,700 MB) of data on each layer that can be accessed at a maximum speed of 11.08 MB per second.

Compact Disc

A compact disc is a pre recorded disc where information is stored as PITS and Lands . the pits and lands are read by the light spot made by the laser beam of the player .

Now how does this work . An audio or a movie is given to the replicator in a Analog format or now a days in digital format also like a cd or DAT ,Beta tape etc by the customer , from this master the replicator has to make cd and print them and deliver it to the customer. This information in the input source is converted in to PITS AND LANDS AND RECORDED on a Nickel master called as Stamper. From this stamper the replication process starts .

This stamper is put in the injection molding machines and a polycarbonate copy disc is made on which a reflective layer of aluminium is put by a process called as sputtering . this layer is required for the laser beam of the player to read the pits and land on the replicated disc and come back to the player . the difference in the phase of the beam is taken as 1and 0′s.



Compact Discs Recordable

CD-R is the Record able format in the family of Optical discs like CD-audio, VCD, CD-Rom etc and was developed by Phillips , The Netherlands in 1992.

Compact Disc Record able or CD-R is a one time record able disc i.e. it can be written only once . It is also called as WORM Disc (Write Once Read Many ).

After recording it behaves like CD audio ,VCD, CD-Rom depending upon the data recorded and can be played on Normal CD-player, VCD player or CD-Rom drive on the computers depending upon the data recorded

All kinds of data like Music, Movies, Software, Photo’s Etc can be recorded on this disc.

Writing Speeds:
The cd-r is recorded or Written from 1x speed to 52x speeds . But now days most of the cd-r are written at 48x-52x speeds. The writng speed depends upon the dye process and the Laser power of the Writer .

The DVDR is written on 1-16 X speeds but these writing speeds depends upon the process used , the MID codes and the player .

Digital Video Discs Recordable

DVDR is manufactured in the same way as cdr ,only difference being that DVDR has 2 halves of 0.6mm each called as active and dummy layer , molded on 2 Injection molding machines. After the Sputtering step (Silver deposition) instead of lacquering, the two halves a joined together by Bonding process.

Then these 2 bonded discs are cured under the u.v light and scanned for further testing .

DVDR has 2 formats

  1. DVD-R : Most common format in India .This was the first of the DVD formats to be launched in the world . After the Disc are made some information has to be pre written before the disc Can be recorded by the customer .So an additional pre writer is required to be installed.
  2. DVD+R: A newer format where in no prewriting is required. Can be directly recorded like CDR.

DVDR has capacity of 4.5 GB /135 Mins. written and recorded on one side . the other side is used for Printing purposes.