What is a CD (Compact Disk)?

CDs or Compact Disks are optic readable media. CDs are the replacement of the phonograph disc. The main material of the CD is plastic. The shape of the plastic is circular and one side of the circular plastic is coated with the reflecting metal coating, usually aluminum. Data can be stored much more densely in optic media than in magnetic media, like Hard disk. Optic media have a much longer life span. Millions of bits are burned into this coating.


CDs can store many types of data, like audio, video, games, any documents, etc. The data are scanned by a laser beam with a CD driver to visualize the data. The only thing that is less in CD is storage capacity is very less compared to HDD or DVD and the read-write speed is also very less. The storage capacity of a CD is 700 MB only.


At first, as CD invented in the year 1960 by the American physicist James Russell. The compact disk (CD) was introduced by Philips and Sony in 1980 and released in 1982. One day he listened to music and he recognizes that the audio quality is very poor and he tried to improve the record player. But he realized that the poor quality of the music happens due to the recorder due to the contact of the needle with the recorder. Then he thought this can be avoided by using a light to read the music without physically touching the disk. Then he started working on digital data with optic media and found a way to record media onto a photosensitive platter with tiny “bits” of light and dark of size 1-micron in diameter. Then he filed a patent application in 1966, and the patent was granted in 1970. After this Sony and Philips licensed with his patents for recording using optic readable media. After all this finally, in 1982 Sony released a CD player name CDP-101. This is how CDs are introduced.


The standard diameter of a CD is 120mm and thickness is 1.2mm. The main body of the CD is made of hard plastic known as polycarbonate with a reflective metallic layer. This layer is coated with metallic acrylic plastic. This metal is aluminum. In this layer, the data are stored using the laser light which reflects the coated layer for reading and writing the data. The data is read in the form of pits, each pit is of 0.83-micrometre and the data is arranged as spiral track from the disc’s inner hole to its outer edge, because the CD is of circular shape.

The hole is in the center of the CD, which diameter is 15mm. The area closest to the hole is known as the clamping area, no data is stored in this area because there is no metallic layer. This area is nearly equal to 26 mm to 33 mm in radius.

The radius of the usable area, that is where the data can be store is of 38mm.

Types of Compact Disks

There are three types of CDs available:

  • CD-R (Compact Disc – Recordable): This type of compact disc can be written only once and can not ereased.
  • CD-ROM (Compact Disk-Read Only Memory): This disks are read once, after read it is use as a ROM, that is cannot be updated later.
  • CD-RW (Compact Disc-ReWritable): This type of compact disc can be written multiple times and can be erased also, like Pendrive.


  • Cost of CD is cheap.
  • It can be carried out easily. Its portable
  • It uses a laser to burn data into the disk, so that no noice in the data..
  • In that time, an entire software package can be stored on one CD, so its very reliable for the software industries in the days.
  • In that days CD is only thing which has much storage upto 700 MB than other available storage devices.
  • CD provides random data access.
  • CD is durable for long time.


  • Easy to scratch on CD.
  • Storage capacity is lower than a hard drive or DVD, nowadays CD is not used at all.
  • Read-write speed is slower.
  • To burn CD we need software, so burning is not much easy as hard disk or pendrive.

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