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Optical Disc Technology

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Optical Disc Technology

Introduction

 Optical disc technology is a growing force in home video which became popular with the introduction of the audio Compact Disc. Today, various formats exist, which besides sound can also carry data, graphics and full motion video parts. First we will take a look at the basics of the optical disc technology, and then at some of the existing formats of optical discs and accompanying hardware.

                                                                           

Digital Technology

Optical disc technology makes use of digital signal processing, contrary to the analogue audio and video carriers, such as the gramophone record and the magnetic video tape. But why digital? In analogue transmission any imperfection during the registration, storage or reproduction phases of recording will  decrease the quality of the audio and/or video signal.

From Analogue to Digital


In optical disc technology, analogue signals are converted into digital signals (with the exception of the video information on laserdiscs, as will be explained later). During this process, the analogue signal (audio and/or video) is measured in parts and converted into a series of values, called sampling. One can envision a waveform representing an audio or video signal, being measured with intervals. The signal strength and polarity at these intervals can be expressed in decimal numbers (1,2,3, etcetera) and express the signal strength and polarity (+ or -) from point to point. The frequency of how often the signal strength is measured in a waveform, determines the accuracy of the registration of the original waveform. These steps must therefore be very small. The measurements are in the order of tenths of millivolts.



An analogue-to-digital (A/D) converter translates these decimal values into binary notation (bits). Bits are made up of  '1's and '0's only, and by combining these ones and zeros in different combinations, decimal numbers can be expressed in binary notation.

Thus the analogue signal becomes a digital signal which is now a series of pulses: pulses for the '1's, and non-pulses for the '0's. For optical discs these pulse series are recorded on the surface of the disc as microscopically small pits and lands, with the help of a fine laser beam. Pits stand for '0's and lands stand for '1's.In most recordings every value (44,100 per second) is converted into a string of 16 bits. This totals over one million bits per second. A 16-bit number of '1's and '0's can indicate no less than 65,536 different values (2 values possible for each bit = 216 = 65,536 possibilities).  

Scanning The Disc


Like gramophone records, the information on optical discs is recorded on a spiral track. The laser starts 'reading' the disc from the inside and ends at the outside. When played back, a laser beam shines on the pits and lands. When the beam strikes a land, the beam is reflected onto a photo-electrical cell. When it strikes a pit, the photo cell will receive only a weak reflection. Thus the photo-electrical cell receives series of light pulses corresponding to the pits and lands in the disc. A D/A-converter (digital to analogue converter: DAC) converts the series of pulses back to binary coding, and then to decimal values. Now the original analogue signal can be rebuilt.

Cutaway view of the laser pickup. Depending on whether the laserbeam hits a pit or a land, the laserbeam is reflected and received by the photo-electrical cell.

CIRC

Thanks to this optical scanning system, there is no friction between the laser beam and the disc. As a result, the discs do not wear, however often they are played. However, they must to be treated carefully, as scratches, grease stains and dust might intercept or diffract the light, causing whole series of pulses to be skipped or distorted. This problem can be solved, as during the recording the Cross Interleaved Reed Solomon Code (CIRC) is added, which is an error correction system that automatically inserts any lost or damaged information, by making a number of mathematical calculations. Without this error correction system optical disc players would not have existed, as even the slightest vibration of the floor would cause sound and image distortions.


Compact Disc player mechanism. The laser pickup reads the disc from below.

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