• Introduction
• History
• The Compact Disk
• The CD player
• Sampling analogue data
• The Floppy Disk
• The CD-R
• The CD-RW

Introduction

The Compact Disk (CD) over the last 10 years has become one of the most convenient, and most used ways to store music, and also many other types of information. It has been a major development in newer technologies like the DVD and has changed the way we deal with information.

In this report I will cover the History of the Compact Disk, the way a compact disk is made, the parts of a compact disk player and the way a CD works with the CD player.

 

History

1877 - Thomas A. Edison invented the phonograph, one of the first machines to reproduce sound. The phonograph played tinfoil cylinders rather than discs, but it would give others in the future, ideas to create similar devices.

1884 - Emil Berliner, a German developer working in Washington, D.C. developed the first usable disk recording technology that could "press" records. This was a big step forward as previously sound that was recorded could not be copied.

1917 Albert Einstein created theories about the process that makes lasers possible called "Stimulated Emission" (a very complex process where One photon interacting with an excited atom results in two photons being emitted.)

1947 - Magnetic tape recorders entered the US Market

1958 - The laser was invented, based on theory by Albert Einstein

1969 - The idea of creating a CD was conceived by Klass Compaan, a physicist that worked at Phillips Research. The main part of the idea, a lot like the record, was being able to use a master template as a mould to create disks for photographic or video images. This could be done in less time than it took to create other types of media and the resulting disk could hold much more information.

1970 - Klass Compaan became partners with another Phillips researcher, Pete Kramer. They spent the next two years investigating how practical such a system was.

1972 - Two years after starting to investigate how practical the system was, they completed a glass disk prototype and determined that a laser will be needed to read the information.

Later that year and in the following years, Phillips developers took this idea and expanded on it with the idea that an audio disk done by similar means. Phillips soon developed the first audio compact disk prototype.

1978 - Phillips soon ran into problems with the development of the compact disk and decided to collaborate on the idea with Sony. There were soon suggestions that world wide standards should be made for similar compact disk like prototypes.

Latter that year, more decisions about the Compact Disk were made, with PolyGram (a division of Phillips) deciding that polycarbonate would be the best material for Compact Disks, that the data should start on the inside of the disk and spiral out, and that the Diameter should be 115 mm. The type of laser to be used was also selected.

1979 - Prototype Compact Disk systems were soon demonstrated in Europe and Japan and with the Phillips and Sony collaboration going ahead, more standards were made. Phillips and Sony decide to set the standard sampling rate of the CD to 44.1 kHz, to have 16 bit audio, and the maximum playing time to be just over 74 minutes.

As the current disk diameter did not allow enough room for these specifications, the disk diameter was changed to 120 mm.

1981 - With the announcement of Phillips and Sony's CD standard, they decided to end their collaboration so they can work on their individual products. Also during 1981, Sharp started to mass produce semiconductor lasers.

1982 - Sony and Phillips with in one year had a product ready to go, and soon released the CD to the public

In the years after 1982, the Compact Disk continued to climb in popularity. Soon there were many devices that used the CD. CD-ROMs were used to store computer files, Cars were fitted with CD players, and portable CD players were sold.

In the last ten years, the idea of the Compact Disk has been taken further with recordable CDs. Even the new "DVDs" use technology that was originally developed for CDs.

 

The Compact Disk

 The largest part of Compact Disk is the clear polycarbonate base of the plastic disk. When a typical Compact Disk is being made, a polycarbonate plastic is injected into a pre made mould. This mould is used to create very small bumps over the top surface of the CD. As the CD is read with a laser from the bottom, the bumps appear to the CD player as pits.

When the plastic base is set, a thin aluminium layer is spread to cover the top of the CD. This aluminium layer is used to reflect the light when the light hits a pit or flat region (where there is no pit).

The next layer that is applied to the top of the Compact Disk is a thin acrylic layer to protect the disk, followed by a label.

The advantage that the method of creating Compact Disks has over previous methods of creating audio storage is that the injection (or stamping) of the plastic also puts the data onto the disk. Previous, tape based devices had to be recorded onto once the tape was made.

The series of pits that were placed on the CD when it was created, spiral from the inside to the outside in once continuous track. The spiralling track is about 1.6 microns away from itself.

A typical pit that is on this data track is about 0.83 to 3.56 microns long, about 125 microns deep and 0.5 microns wide.  

To give an indication on how small these bumps are, there are one million microns in a meter and a human hair is about 75 microns wide. Because they are so small, they refract the light and give the CD a rainbow effect.

 

The CD player

The drive motor is the actual motor that spins the disk. This motor must be precise and be able to spin the CD between 500 and 200 RPMs. The reason that it needs to be able to vary the CDs spin speed is that the track must pass over the laser at a constant speed. When laser moves towards the edge, the length of track that it takes to go around the CD increases, meaning a lower speed is needed. A laser track mechanism is used to move the laser from the inside of the disk to the outside. It is precise enough to move the laser over the track and make micron sized adjustments.

A laser and lens are used to focus light onto the CD. When the laser is focused onto the pits, the beam goes through the polycarbonate layer and reflects off the aluminium layer.

The pits reflect the light in such a way the electronics equipment can pick up the difference between a pit and an area on the disk without a pit. This series of pits and non-pits can be interpreted by light sensitive equipment and translated in a series of ones and zero called binary.

 

Sampling analogue data

The problem with putting analogue data onto a digital storage medium like a CD is that when the data is read from the CD, the information is in binary. Unlike a normal analogue sound signals that at any one time can be a multitude of values, binary is a one or a zero. It is because of this that it would not be possible to transfer all the analogue information on a CD this way. Instead, at a steady rate, the value of the analogue signal has been stored to the CD. This rate is called the sampling rate and in the case of the CD, it is 44.1 kHz.

When this sampled value that has been stored on the CD is read, it still only has a sampled value. With the use of a Digital to analogue converter, the sampled rate is converted back in an analogue signal. As not all the information was sampled, the resulting line becomes less curved. As the sampling rate is increased, the rate of error or lost information is reduced.

 

The Floppy Disk

Basically, a floppy disk drive reads and writes data to a small, circular piece of metal-coated plastic

Although floppy disks have been used for over 20 years, a normal floppy disk is about 1.44MB in size. This is 450 times smaller than a CD, and it is because of this that the floppy disk is nearly unused now.

Also because of their design, dust can stop the disk operating and it is because of this that CDs are more reliable than floppy disks.

 

The CD-R

CD recordable or CD-R is similar to the CD, but can be written to once by a CD-R drive. This means that more people can write CDs and larger files can be swapped between computers.

 

The CD-RW

The CD-RW is similar to the CD-R, but as well as being able to write to the disk multiple times, the CD can also be wiped. This means that files can be corrected and changed on the disk.

The amount of times that a CD-RW can be wiped, depends on the quality and can vary greatly.