Analog and Digital Optical Recording : the LaserDisc and Compact Disc Systems
The availability to the domestic market of planar rotating optical media for recording audio and video dates to 1978. Players for such media, now transformed into general data-storage systems, may still be the single biggest use of lasers.
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Since there is no practicable way of transmitting the value of every point in the picture all at once, the points are sampled successively over the course of one frame period. Until recently, the most common way of doing this was to trace an electron beam across a light-sensitive or light-emitting screen, using electric or magnetic fields. By precise control of timing, the image as seen at the receiver is a good duplicate of that at the transmitter. Because the eye is more sensitive to flicker than to motion, the picture only needs to be scanned about 24 times each second, but dividing it into two interdigitated sets of lines scanned in succession 48 or more times a second reduces the sensation of flicker to tolerable levels.
The interlaced scanning structure provides the possibility of economizing on bandwidth by frequency interleaving. The sampled, digitized Pulse-Code-Modulation audio signal in the Compact Disc format is shuffled into data blocks to reduce the deleterious effect of "drop-outs" caused by imperfections in the disc, and other playback errors. In all cases, the physical motion of the disc is correlated to the signal parameters.
Constant Angular Velocity recording, with a specified number of events per revolution (eg, one video frame), allows easy access to events. Constant Linear Velocity recording, in which each event occupies a fixed track length regardless of angular position, allows longer recording time.
"Bumps" or "pits" in the reflective surface, of dimensions corresponding to the wavelengths of visible light, cause changes in the amount of light received by a photodiode, in the manner of a phase-contrast microscope. This pattern of changes can correspond to digital data, or to the zero-crossings of an analog modulated waveform.
The eight-to-fourteen modulation schema, which takes data words and translates them into codes with specified intervals between zero crossings and no nett DC component, makes the Compact Disc practicable. Before it was introduced, the bits were often represented by light and dark areas in a synthetic video signal.
By the use of mathematical means of error-detection and -correction, it is possible to recover the original digital data perfectly, despite the imperfections of the medium and reproducing equipment.
