The Audio-Visual Revolution
- a site about the Audio-Visual Revolution.
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CDs are used primarily for audio reproduction. Music CDs have raw audio data in digital form recorded on them. This produces a sound that is extremely close to the original sound quality of the recording, yet there is still a small amount of data loss. CDs are read and written to via a laser. The laser reads the bumps on the CD, which unlike records (which are a smooth recording of an analogue signal) the CD’s data surface is made up of small grooves, which are averaged to closely represent the bumps on a record track. The bumps that make up the track are 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high (nanometers being a billionth of a meter).
| Specification |
CD |
DVD |
| Track Pitch |
1600 nanometers |
740 nanometers |
Minimum Pit Length
(single-layer DVD) |
830 nanometers |
400 nanometers |
Minimum Pit Length
(double-layer DVD) |
830 nanometers |
440 nanometers |
Compact disks store information as a set of 1s and 0s (known as binary data). These are recorded on compact disks as a set of raised "bumps" and sunken "pits". The Junction between a bump and a pit represents a 1 whereas in the middle of a bump or a pit represents a 0.
An infrared laser is used to read the data on the disk. When the laser is shone onto the disk, the coherent monochromatic light reflects off the surface and the reflected light is detected by a photodiode.
If the light from the laser is shone onto the disk and the high intensity beam of light reaches the photodiode it is read as a 0. However, when the laser reflects off of the junction between a pit and a bump, the beam splits into two rays. One of the rays travels 1/2 a wavelength further than the other (as the pit is 1/4 of a wavelength deep). When the two rays reach the photodiode, they destructively interfere. This low intensity beam is read as a 1.
A diffraction grating is also employed to aid the laser in retaining its position on the CD's track. The diffraction grating is used to split the laser into three separate beams (first orders and the center beam). These beams are detected by a photodiode array. Which determines which direction the laser needs to move in order to correct it's path. The centre light ray from the diffraction grating is used to read the data off of the CD. The correction mechanism operates by determining the strength of the light rays on each of the first orders from the diffraction of the laser, then attempting to keep the center ray lined up with the data on the CD, hence reading the data from the CD correctly.