Science: Video in the Round

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Can current television sets be equipped to play recorded programs that could be purchased and stored like so many hi-fi discs? Manufacturers have been competing for a decade to be first to provide a practical answer. Trouble is, both videotapes and films are too expensive to produce for the mass market. Standard long-playing records, which can be stamped out by the millions, cannot carry all of the information necessary to produce TV images and sounds; millions of electrical signals are required every second to create a TV picture.

Major Movies. Almost simultaneously, scientists in the U.S. and Europe have finally overcome the problem by combining the information capacity of videotape with the low cost of phonograph records. Their electronic hybrid is called the video disc—a relatively inexpensive record that can be played through a TV set to show everything from major Hollywood movies to educational and cultural features. By the end of 1976, both RCA and a team composed of the N.V. Philips Co. in The Netherlands and MCA, the California entertainment conglomerate, plan to market their systems in the U.S.

At first glance, RCA's SelectaVision and Philips-MCA's Disco-Vision look virtually identical. Both systems use 12-in. LP-sized discs that play for 30 minutes* on a side on high-speed turntables. Each is connected to the standard TV set by simply attaching a pair of wires to the antenna leads. There the similarities end. Behind the systems are entirely different technologies; the records used by one cannot be played on the turntables of the other.

RCA's engineers opted for a relatively simple turntable. A major innovation is the metal-coated record, which is covered with a spiral groove only 0.00018 in. wide—less than a tenth as thick as a human hair. In ordinary LPs, the groove encodes the sound; as the pickup needle runs over its "hills and dales," the needle is forced to vibrate at the same frequencies as the recorded sound. Translated into electrical pulses and amplified, the vibrations drive the loudspeaker. By contrast, RCA's SelectaVision does not depend on mechanical vibrations. The disc's groove serves only to guide a sapphire stylus over a series of irregularly spaced slots in the groove. The slots are so small (up to 84,000 per in.) that they must be etched into the master disc by an extremely fine, high-powered beam of electrons. Yet variations in the width and spacing of the slots contain all the information necessary to reproduce a program in color (with stereo sound if desired). As the record spins at 450 r.p.m., a metallic strip on the back of the stylus "reads" the constantly changing electrical capacitance —a measure of the capacity to hold an electrical charge—between the stylus and the disc's slotted surface. The varying capacitance is electronically transformed into picture and sound.

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