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The Compact Disc.

Extracts from this document...

Introduction

The Compact Disc

   The compact disc if by far one of the most revolutionary mediums to have been invented in recent times. It has completely changed society in a numbers of ways and has benefited everyone. In this coursework I aim to discover how a CD is produced, written on and how it is read inside a CD-ROM. I will also be looking at the difference between a CD-R and a CD-RW. The average CD-R or RW can hold about 6 billion bits of binary data. This is about 780 megabytes of data, and at 2000 characters per page an average CD can store up to 275,000 pages of text. A CD can also hold about 74 minutes of audible music as it samples at 44.1kHz.

    The first aspect I chose to look at was how a CD is produced at a manufacturing plant and what materials and components go into making a CD.

   A CD is made up of a number of components. Its base material is polycarbonate; it is what makes the CD strong and provides a surface for other layers to be applied to. A reflective layer is then applied to the surface of the polycarbonate using a process called sputtering. This is a shiny layer that is used to bounce the laser beam back to its original source. This means that this layer must have a very high integrity so that it remains in shape and not break apart. This reflective layer is usually made up of silver, but on occasions it is made up of gold or platinum.

...read more.

Middle

The first of these three pictures shows two images of an arrow in the calcite crystal. The second shows under polarization only the ordinary arrow is transmitted. However under polarization that has been rotated 900 the calcite only shows the extraordinary arrow. This is why calcite is so readily used in a number of different polarizing prisms.

The light that has been emitted from the laser then travels into a diffraction grating. This is shown on the picture on the next page.

image10.png

This picture shows how the light is split in the grating into a number of different tracking beams. Once this light has been split it then passes into a polarizing prism. The polarizing prism is actually made up of two prisms. The angle at which these prisms is cut is very accurate and means that the plane of polarization parallel to the surface undergoes total internal reflection, however any light which is perpendicular to the surface passes straight through the prism. This processes is used to get the quarter wave plate, which is at 450 degrees. This is done so that when the light travels up and hits the bumps and pits the beam returning from them will be polarized parallel to the surface and will be reflected 900 degrees towards the photodiode detector. This idea is used so that the light that strikes the land travels ¼ + ¼ = ½ of a wavelength further than light striking the top of the pit.

...read more.

Conclusion

ly choosing parts of the disc to darken and leaving other areas translucent, the CD-WRITER creates a digital pattern that can be read by a CD-ROM. The light from the laser only reflects off the areas of the CD that are left completely translucent.

 However, the light that hits the areas that have been darkened does not reflect back to the photodiode. So even though the CD does not actually have any bumps and pits pressed into it, it still works in exactly the same way as a normal CD.

The laser beam that is used to burn onto the CD can heat up to a very high intensity. It has enough energy to reach Curie temperature instantly. (Curie temperature is 300 degrees Celsius, the level at which the magnetic domain loses its characteristic as a magnet). This is much higher than just the read only laser.

The table below shows all the average statistics for a normal CD.

DISC

Playing time:

74 minutes, 33 seconds maximum

Rotation:

Counter-clockwise when viewed from readout surface

Rotational speed:

1.2 - 1.4 m/sec. (constant linear velocity)

Track pitch:

1.6 µm

Diameter:

120 mm

Thickness:

1.2 mm

Centre hole diameter:

15 mm

Recording area:

46 mm - 117 mm

Signal area:

50 mm - 116 mm

Material:

Any acceptable medium with a refraction index of 1.55

Minimum pit length:

0.833 µm (1.2 m/sec) to 0.972 µm (1.4 m/sec)

Maximum pit length:

3.05 µm (1.2 m/sec) to 3.56 µm (1.4 m/sec)

Pit depth:

~0.11 µm

Pit width:

~0.5 µm

OPTICAL SYSTEM

Standard wavelength:

780 nm (7,800 Å)

Focal depth:

± 2 µm

SIGNAL FORMAT

Number of channels:

2 channels (4 channel recording possible)

Quantization:

16-bit linear

Quantizing timing:

Concurrent for all channels

Sampling frequency:

44.1 kHz

Channel bit rate:

4.3218 Mb/sec

Data bit rate:

2.0338 Mb/sec

Data-to-channel bit ratio:

8:17

Error correction code:

Cross Interleave Reed-Solomon Code (with 25% redundancy)

Modulation system:

Eight-to-fourteen Modulation (EFM)

...read more.

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