Why CD reading fails? Fact File 15

Two main reasons are scratch damage and heat damage. Compact Disc is a 1.2 mm thick Polycarbonate transparent disc used for digital writing with the help of Laser beam. The poly carbonate plastic used in CD is Polymethyle Meta Acrylic. The surface of the CD is coated with a thin layer of aluminium to make it reflective, and is protected by a film of lacquer on top of the reflective layer, on which the label print is applied .In the CD, from the center outward there are areas like center (spindle) hole, the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the information (data) area, and the rim. A standard CD has 120mm diameter and 15-20 gm weight. It can hold 80 minutes uncompressed (700 MB) data.

The writing side of the CD is polished on which the data is recorded as Bits (lands) and Pits (holes). Each pit is 100nm deep and 500nm wide and the pits are arranged spirally from the centre of the CD to its rim. Pits are formed during CD writing by burning the plastic with the help of pencil beam Laser. Pits are much closer to the label side of a disc, so that defects and dirt on the clear side can make the beam out of focus during playback. CD suffers more scratch damage on the label side. If the CD is exposed to heat, the lacquer coating melts and label detaches from the CD leaving small air gaps between the CD and label. This severely affects its working because the Laser beam passes out from the CD. Due to over use the dye’s physical characteristics may change, causing read errors and data loss.

CD players use laser technology to read the optically recorded data in the form of Bits and Pits on a CD. About 20000 or more tracks are found in a CD’s recording surface. The distance between the tracks, the pitch, is 1.6 µm. A CD is read by focusing a 780 nm wavelength (near infrared) semiconductor laser through the bottom of the polycarbonate layer.

The change in height between pits and lands results in a difference in intensity in the light reflected. By measuring the intensity change with a photodiode, the data can be read from the disc. The digital information is defined as the length of pits and distance between them. The pits and reflective surface represents logic 0 and logic 1. The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead, Non-return-to-zero, inverted (NRZI) encoding is used: a change from pit to land or land to pit indicates a one, while no change indicates a series of zeros. There must be at least two and no more than ten zeros between each one, which is defined by the length of the pit.

Some Do’s and Don’t’s

1. Always purchase a good quality CD

2. If the CD is used to store important data, make one more copy and keep this as reserve.

3. Always keep the CD in the pouch. Soft pouch is ideal.

4. Keep the CD in the pouch after its use

5. Don’t keep the CD in the CD writer after its use.

6. Don’t keep the CD on the table with the data side downwards

7. Remove the CD from the CD player after its use.

8. Don’t keep the CD in the dash board of car or other hot surface.

9. Clean the polished surface before and after the use with tissue paper.

10. Clean the lens of the CD player with Lens cleaner fluid, if a good CD fails to work.


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