CD/DVD AND HIFI SYSTEM

A Little History The history of the compact disk (CD) started in the 1970’s with the videodisk in the form of Video Long Play (VLP) read-only systems. The videodisk did not become a commercial success, even after write-once optical disks of different formats and sizes were introduced. These were analog systems. In 1982 the CD-DA (compact disk-digital audio) was introduced to the market jointly by Phillips and Sony. It stored a high-quality stereo audio signal in a digital format. These systems became a huge success. In 1985, the technology was extended to computer storage, again in a collaboration between Phillips and Sony. This was called a CD-ROM (compact disk-read only memory). Early in 1995, two major groups were competing to develop the next generation of high-density compact disks. Under the partnership of Philips and Sony, there began the development of one such format. Concurrently, a group led by Toshiba and Time Warner was working on another format. In September of 1995 the two camps agreed to develop a single standard for a high-density compact disk. The first DVD-video players were sold in Tokyo in November’96, followed by their US introduction in August’97.

OPTICAL RECORDING ON DISC

When the modulated laser strike the master disc When the modulated laser strike the master disc. It reacts with the photo-resist. The disc is now develop by a process skin to photography. This result in microscopic sized pattern of pits and flats. The developed master disc is coated with silver to make it electrically conductive. FATHER DISC :- The next step is nickel plating. After plating the nickel is peeled off the master disc and then it is called ‘ father disc ‘ . It is negative replica of the glass master disc. MOTHER DISC :- the father disc is again plated and removal of the plating produces a mother disc which is identical in form with the master disc. Generally ten mother discs are obtained from a single master disc. Mothers are inverted and cannot be used for producing final discs. SON DISC: - The mother disc are plated and the plating when removed gives a son disc is identical to father disc. A son disc is also called a negative nickel-plated stamper. FINAL DISC :- Consumer discs for playback are obtained by pressing on the stamper son disc. These disc are positive disc . Recording is done from center to edges.

Playback Techniques Laser light is focused on disc aluminum layer Reflection is weaker from ridge than flat Reflected light is directed to photodiodes Light intensity indicates ridges or flats Demonstration: Show Internal Components of CD Reader

Playback Issues Light must hit ridges perfectly Feedback optimizes position of light spot Light must hit only one ridge Use laser light Focuses laser to diffraction limit Feedback focuses laser on layer Ridge must be large enough to detect Ridge can’t be much smaller than light wavelength

Advantages of Digital Recording Freedom from noise and media damage issues Digital representation avoids information loss Error correction ensures clean information Surface contamination doesn’t matter (much) High information density Optical density greatly exceeds mechanical density Data compression is possible Perfect, loss-less copies are possible

HIGH FIDELITY SYSTEM

Hi-fi means reproduction of sound with high degree of similarity to the original sound. For high fidelity the following requirement should be fulfilled:- Signal to noise ratio should be better than 50dB. Frequency should be flat within 1dB over frequency of 40 to 15000 Hz. Non-linear distortion should not more than 1% Dynamic range should be atleast 80dB Stereophonic effect should be present. Enviromental condition pertaining to noise and reverberation should not be worse than that at the source of sound. Noises in the reproduced sound are hum, hise, cross talk, self oscillation etc. all these noises reduce signal to noise ratio. Low frequency response of an audio and mechanical system is affected by series capcitance or mechanical complainces and high frequency response by shunt capacitance.

Non – linear distortion can be reduced by properly selecting the operational point, bais and using negative feedback in amplifiers. Disc and magnetic recorders presents difficulties about dynamic range. Processing of signal before recording and equlaising in the playback remove this problem. For directional effect sterophonic system of sound is adopted. Enviromental conditions are improved by using suitable absorbers and instulators.