12 August 2009
Laser-diode recording process
The invention: Video and audio playback system that uses a lowpower laser to decode information digitally stored on reflective disks. The organization behind the invention: The Philips Corporation, a Dutch electronics firm The Development of Digital Systems Since the advent of the computer age, it has been the goal of many equipment manufacturers to provide reliable digital systems for the storage and retrieval of video and audio programs. A need for such devices was perceived for several reasons. Existing storage media (movie film and 12-inch, vinyl, long-playing records) were relatively large and cumbersome to manipulate and were prone to degradation, breakage, and unwanted noise. Thus, during the late 1960’s, two different methods for storing video programs on disc were invented. A mechanical system was demonstrated by the Telefunken Company, while the Radio Corporation of America (RCA) introduced an electrostatic device (a device that used static electricity). The first commercially successful system, however, was developed during the mid-1970’s by the Philips Corporation. Philips devoted considerable resources to creating a digital video system, read by light beams, which could reproduce an entire feature- length film from one 12-inch videodisc. An integral part of this innovation was the fabrication of a device small enough and fast enough to read the vast amounts of greatly compacted data stored on the 12-inch disc without introducing unwanted noise. Although Philips was aware of the other formats, the company opted to use an optical scanner with a small “semiconductor laser diode” to retrieve the digital information. The laser diode is only a fraction of a millimeter in size, operates quite efficiently with high amplitude and relatively low power (0.1 watt), and can be used continuously. Because this configuration operates at a high frequency, its informationcarrying capacity is quite large.Although the digital videodisc system (called “laservision”) works well, the low level of noise and the clear images offered by this system were masked by the low quality of the conventional television monitors on which they were viewed. Furthermore, the high price of the playback systems and the discs made them noncompetitive with the videocassette recorders (VCRs) that were then capturing the market for home systems. VCRs had the additional advantage that programs could be recorded or copied easily. The Philips Corporation turned its attention to utilizing this technology in an area where low noise levels and high quality would be more readily apparent— audio disc systems. By 1979, they had perfected the basic compact disc (CD) system, which soon revolutionized the world of stereophonic home systems. Reading Digital Discs with Laser Light Digital signals (signals composed of numbers) are stored on discs as “pits” impressed into the plastic disc and then coated with a thin reflective layer of aluminum. A laser beam, manipulated by delicate, fast-moving mirrors, tracks and reads the digital information as changes in light intensity. These data are then converted to a varying electrical signal that contains the video or audio information. The data are then recovered by means of a sophisticated pickup that consists of the semiconductor laser diode, a polarizing beam splitter, an objective lens, a collective lens system, and a photodiode receiver. The beam from the laser diode is focused by a collimator lens (a lens that collects and focuses light) and then passes through the polarizing beam splitter (PBS). This device acts like a one-way mirror mounted at 45 degrees to the light path. Light from the laser passes through the PBS as if it were a window, but the light emerges in a polarized state (which means that the vibration of the light takes place in only one plane). For the beam reflected from the CD surface, however, the PBS acts like a mirror, since the reflected beam has an opposite polarization. The light is thus deflected toward the photodiode detector. The objective lens is needed to focus the light onto the disc surface. On the outer surface of the transparent disc, the main spot of light has a diameter of 0.8 millimeter, which narrows to only 0.0017 millimeter at the reflective surface. At the surface, the spot is about three times the size of the microscopic pits (0.0005 millimeter). The data encoded on the disc determine the relative intensity of the reflected light, on the basis of the presence or absence of pits. When the reflected laser beam enters the photodiode, a modulated light beam is changed into a digital signal that becomes an analog (continuous) audio signal after several stages of signal processing and error correction. Consequences The development of the semiconductor laser diode and associated circuitry for reading stored information has made CD audio systems practical and affordable. These systems can offer the quality of a live musical performance with a clarity that is undisturbed by noise and distortion. Digital systems also offer several other significant advantages over analog devices. The dynamic range (the difference between the softest and the loudest signals that can be stored and reproduced) is considerably greater in digital systems. In addition, digital systems can be copied precisely; the signal is not degraded by copying, as is the case with analog systems. Finally, error-correcting codes can be used to detect and correct errors in transmitted or reproduced digital signals, allowing greater precision and a higher-quality output sound. Besides laser video systems, there are many other applications for laser-read CDs. Compact disc read-only memory (CD-ROM) is used to store computer text. One standard CD can store 500 megabytes of information, which is about twenty times the storage of a hard-disk drive on a typical home computer. Compact disc systems can also be integrated with conventional televisions (called CD-V) to present twenty minutes of sound and five minutes of sound with picture. Finally, CD systems connected with a computer (CD-I) mix audio, video, and computer programming. These devices allow the user to stop at any point in the program, request more information, and receive that information as sound with graphics, film clips, or as text on the screen.