COMPACT DISC (CD) SYSTEM:
A compact disc (CD) may hold a wide range of data. Compact disc signals are stored in a high-density digital format. As a result, the signals captured are a replica of the original audio stream. Text, picture images, audio, video, and software are all stored on compact discs. The compact disc was formerly the standard for storing audio recordings. The typical CD measures 12 cm (5 inches) in diameter. It is made up of three layers.
1. Transparent Substrate:
A polycarbonate wafer [plastic disc] makes up this layer.
2. Thin metallic Layer:
A thin metallic coating of aluminium alloy is applied to the wafer base. This metallic layer has a lengthy series of pits with a diameter of around 0.5 micrometres. The CD drive generally reads the reflective aluminium film part of the disc.
3. Outer Layer of Protective Acrylic:
The underlying data is further protected by a plastic coating called a protective lacquer layer, which is applied to the aluminium sheet. The CD's layout is seen in Figure.
Fig: Layout CD-ROM Disc
When a CD is made, a glass disc is covered with a photoresist that is sensitive to laser light. The Laser light exposes a precise helical pattern on the photoresist material during recording. Typically, the track width is 0.5m. As each 1 happens, the Laser beam is switched ON and OFF alternately, recording the 1s in the digital data onto the photoresist surface.
No 0's are written to the disc; instead, they are regenerated by the CD player. The photo-resist surface of the disc is exposed during recording 1, resulting in a sequence of pits and lands. Pits appear on the surface of the CD as a result of recording the digital data ‘1'.
Similarly, there is no change in the surface of the digital data 0 when it is recorded. As a result, the pits have a data of 1 and the other areas have a data of 0. The period or distance between 1's determines the length of the pit or land.
1. CD RECORDING :
Using a sample and hold circuit and an ADC, the signal to be recorded on CD is first amplified and then transformed into a digital signal. The output of the ADC is also used by the Laser Beam Generator. The control circuit and the servo system are both controlled by the signal from the crystal oscillator and Laser beam generator.
The servo system, which is controlled by a motor, regulates the disc rotation as well as the track and focus of the Laser beam generator. The picture depicts a block schematic of a CD recording system.
The unexposed photoresist material is chemically removed after recording, leaving a helical pattern across the glass disc's surface. This becomes the glass master for mass-production CDs.
Figure: Block Diagram of CD Recording
2.
CD REPRODUCTION :
The data retrieval system is made up of the phases listed below.
1. A servomechanism, which spins the CD.
2. A laser head that moves in a radial pattern. The laser head can both emit and detect a 70nm laser beam.
When the disc spins, the laser beam is focused onto the playing surface, where it is reflected by the ‘lands' and scattered by the ‘pits,' resulting in a change in the quantity of light reflected whenever there is a pit-to-land or land-to-pit change. As a result, the pit borders are detected by a laser beam.
A pit border is a ‘1', whereas its absence is a ‘0.' No pit, i.e. land, is indicated by a strong reflection of the light. A light receptor receiver determines whether light is highly reflected, absent, or dispersed. The recorded holes on the CD produce a non-existent or diffused light reflection. The picture depicts the block diagram of a CD replication mechanism.
Figure: Block Diagram of CD Reproduction System
A crystal oscillator signal is compared to the clock signal acquired from the disc. The servo system is controlled by the control circuit's output. The servo system regulates the motor speed as well as the optical mirror and lens system's track correction and focuses adjustments.
The photodetector receives the reflected beam from the disc. The light fluctuations are converted into a digital signal. The digital output is then amplified and transformed to an analogue signal using DAC. As a result, the original modulating signal may be obtained.
The control circuit receives the clock signal from the disc as well as the signal from the crystal oscillator. The servo system is controlled by the control circuit utilizing the two signals. The servo system is in charge of controlling the motor's speed. The optical mirror and lens system, as well as the ray focusing, are all controlled by the servo system. There are two techniques for encoding data on CDs.
They are:
1. CIRC – Cross -Interleave Reed – Solomon Code
2. EFM – Eight to Fourteen Modulations
Advantages of Compact Disc:
1. Certain codes can be used to detect errors on the disc.
2. Because the CD is protected by transparent plastic, it is not damaged by dust, oil, or scratches.
3. The sound reproduction is of high quality, with minimal background noise.
4. There is a high signal-to-noise ratio.
5. The stereo effect is effectively retained.
6. Excellent Channel separation.
7. Small in size.
8. Broad frequency range.
9. There is very little distortion.
10. Flutter does not exist.
11. A compact disc allows for programme selection.