Capacitance Electronic Disc
The Capacitance Electronic Disc is an analog video disc playback system developed by Radio Corporation of America, in which video and audio could be played back on a TV set using a special stylus and high-density groove system similar to phonograph records.
First conceived in 1964, the CED system was widely seen as a technological success which was able to increase the density of a long-playing record by two orders of magnitude. Despite that achievement, the CED system fell victim to poor planning, various conflicts with RCA management, and several technical difficulties. That slowed development and stalled production of the system for 17 years, until 1981, by which time it had been made obsolete by laser videodisc as well as Betamax and VHS video cassette formats. Sales for the system were nowhere near projected estimates. In the spring of 1984, RCA announced it was discontinuing player production, but continued the production of videodiscs until 1986, losing an estimated $650 million in the process. RCA had initially intended to release the SKT425 CED player with its high-end Dimensia system in late 1984, but cancelled CED player production prior to the system's release.
The format was commonly known as videodisc", leading to confusion with the contemporaneous LaserDisc format. LaserDiscs are read optically with a laser beam, whereas CED discs were read physically with a stylus, similar to a conventional phonograph record. The two systems were incompatible.
RCA used the brand name "SelectaVision" for the CED system, a name also used for some early RCA brand VCRs, and other experimental projects at RCA. The Video High Density system is similar to that of CED.
History
Beginnings and release
RCA began developing the videodisc system in 1964, in an attempt to produce a phonograph-like method of reproducing video under the name 'Discpix'. Research and development was slow in the early years, as the RCA CED team originally consisted of only four men, but by 1972, the CED team had produced a disc capable of holding ten minutes of color video.The first CED prototype discs were multi-layered, consisting of a vinyl substrate, nickel conductive layer, glow-discharge insulating layer and silicone lubricant top layer. Failure to fully solve the stylus/disc wear and manufacturing complexity forced RCA to seek simpler construction of the disc. The final disc was crafted using PVC blended with carbon to make the disc conductive. To preserve stylus and groove life, a thin layer of silicone or methyl alkyl siloxane was applied to the disc as a lubricant.
CED videodiscs were originally conceived as being housed in jackets and handled by hand similar to LP records, but during testing it was shown that exposure to dust caused skipped grooves. If dust was allowed to settle on the discs, the dust would absorb moisture from the air and cement the dust particle to the disc surface, causing the stylus to jump back in a locked groove situation. Thus, an idea was developed in which the disc would be stored and handled in a plastic caddy from which the CED would be extracted by the player so that exposure to dust would be minimized.
After 17 years of research and development, the first CED player went on sale on March 22, 1981. A catalog of approximately 50 videodisc titles was released at the same time. The first title to be manufactured was Race for Your Life, Charlie Brown. Fifteen months later, RCA released the SGT200 and SGT250 players, both with stereo sound while the SGT-250 was also the first CED player model to include a wireless remote control. Models with random access were introduced in 1983.
Demise
Several problems doomed the RCA CED videodisc system long before it was even announced. The introduction of VCRs and home videotape in the mid 1970s—with their longer storage capacity and recording capabilities—posed a major threat to the system. However, development of CED continued. When the forthcoming system was formally announced in late 1979, RCA had projected annual sales of between five and six million players and 200 to 500 million videodiscs. The company had expected to sell 200,000 players by the end of 1981, but only about half that number had been sold, and there was little improvement in sales throughout 1982 and 1983.| "...Machiavelli noted that '..there is nothing more difficult to take in hand, more perilous to conduct, or more uncertain in its success, than to take the lead in the introduction of a new order of things...' At videodisc, I believe these words had special significance..." |
| Dr. Jay J. Brandinger, Vice President, RCA SelectaVision Videodisc Operations, June 27, 1986. |
The extremely long period of development—caused in part by political turmoil and a great deal of turnover in the high management of RCA—also contributed to the demise of the CED system. RCA had originally slated 1977 as the release date for the videodisc system; at that point, discs were not able to hold more than 30 minutes of video per side and the nickel-like compound used for manufacturing the discs was not sturdy enough. Signal degradation was also a problem, as handling the discs was causing them to deteriorate more rapidly than expected, baffling engineers.
60 minutes per side rendered it impossible for most movies over 120 minutes to be released on one CED disc. Many popular films such as some of the James Bond series, Mary Poppins, Star Trek: The Motion Picture and Return of the Jedi had to be released on two CED discs. All three of these examples were typically available on one VHS/Betamax cassette.
RCA had projected that by 1985, CED players would be in nearly 50% of American homes, but the sales of players continued to drop. RCA cut the prices of CED players and offered incentives to consumers such as rebates and free discs, but sales only slightly improved. RCA management realized that the system would never be profitable and on April 4, 1984, announced the discontinuation of production of CED players. Remaining stocks of players were sold by dealers and liquidation retailers for as little as $20 each. Unexpectedly, demand for the videodiscs themselves suddenly became high immediately after the announcement; RCA alerted dealers
and customers that videodiscs would continue to be manufactured and new titles released for at least another three years after the discontinuation of players. Less than a year after this announcement, the sale of discs began to decline, prompting RCA to abandon videodisc production in 1986, after only two years.
The last titles released were The Jewel of the Nile by CBS/Fox Video, and Memories of VideoDisc, a commemorative CED given to many RCA employees involved with the CED project, both in 1986.
Technology
CEDs are conductive vinyl platters that are in diameter. To avoid metric names they are usually called "12 inch discs". A CED has a spiral groove on both sides. The groove is 657 nm wide and has a length of up to. The discs rotate at a constant angular speed during playback and each rotation contains eight interlaced fields, or four full frames of video. These appear as spokes on the disc surface, with the gap between each field clearly visible under certain light. This meant that freeze frame was impossible on players without an expensive electronic frame store facility.A keel-shaped stylus with a titanium electrode layer rides in the groove with extremely light tracking force and an electronic circuit is formed through the disc and stylus. Like an audio turntable, the stylus reads the disc, starting at the outer edge and going towards the center. The video and audio signals are stored on the Videodiscs in a composite analog signal which is encoded into vertical undulations in the bottom of the groove, somewhat like pits. These undulations have a shorter wavelength than the length of the stylus tip in the groove, and the stylus rides over them; the varying distance between the stylus tip and the conductive surface due to the depth of the undulations in the groove under the stylus directly controls the capacitance between the stylus and the conductive carbon-loaded PVC disc. This varying capacitance in turn alters the frequency of a resonant circuit, producing an FM electrical signal, which is then decoded into video and audio signals by the player's electronics.
The capacitive stylus pickup system which gives the CED its name can be contrasted with the technology of the conventional phonograph. Whereas the phonograph stylus physically vibrates with the variations in the record groove, and those vibrations are converted by a mechanical transducer to an electrical signal, the CED stylus normally does not vibrate and moves only to track the CED groove, while the signal from the stylus is natively obtained as an electrical signal. This more sophisticated system, combined with a high revolution rate, is necessary to enable the encoding of video signals with bandwidth of a few megahertz, compared to a maximum of 20 kilohertz for an audio-only signal—a difference of two orders of magnitude. Also, while the undulations in the bottom of the groove may be likened to pits, it is important to note that the spacing of vertical wave crests and troughs in a CED groove is continuously variable, as the CED is an analog medium. Usually, the term "pits", when used in the context of information media, refers to features with sharply defined edges and discrete lengths and depths, such as the pits on digital optical media such as CDs and DVDs.
In order to maintain an extremely light tracking force, the stylus arm is surrounded by coils, which sense deflection, and a circuit in the player responds to the signals from these coils by moving the stylus head carriage in steps as the groove pulls the stylus across the disc. Other coils are used to deflect the stylus, to finely adjust tracking. This system is very similar to—yet predates—the one used in Compact Disc players to follow the spiral optical track, where typically a servo motor moves the optical pickup in steps for coarse tracking and a set of coils shifts the laser lens for fine tracking, both guided by an optical sensing device, which is the analogue of CED stylus-deflection sensing coils. For the CED player, this tracking arrangement has the additional benefit that the stylus drag angle remains uniformly tangent to the groove, unlike the case for a phonograph tonearm, in which the stylus drag angle and consequently the stylus side force varies with the tonearm angle, which in turn depends on the radial position on the record of the stylus. For phonographs with pinpoint tip styli, linear tracking is merely ideal to reduce wear of records and styli and to maximize tracking stability; for a CED player linear tracking is a necessity for the keel-shaped stylus, which must always stay tangent to the groove. Furthermore, the achievement of an extremely light tracking force on the CED stylus enables the use of a fine groove pitch, necessary to provide a long playing time at the required high rotational speed, while also limiting the rate of disc and stylus wear.
The disc is stored inside a caddy, from which the player extracts it when it is loaded. The disc itself is surrounded by a "spine", a plastic ring with a thick, straight rim-like edge, which extends outside of, and latches into, the caddy. When a person inserts a caddy containing a disc into the player, the player captures the spine, and both the disc and the spine are left in the player as the person pulls the caddy out. The inner edges of the opening of the caddy have felt strips designed to catch any dust or other debris that could be on the disc as it is extracted. Once the caddy has been withdrawn by the person, the player loads the disc onto the turntable, either manually with all SFT and most SGT prefix RCA players or automatically with the RCA SGT-250 and all other models and brands of players. When playback has been started, the player spins the disc up to speed while moving the pickup arm over the disc surface and lowering the stylus onto the beginning of the disc.
When Stop is pressed, the stylus is lifted from the disc and returned to its parking location, and the disc and spine are lifted up again to align with the caddy slot. When ready, the slot is unlocked, and the caddy can be inserted and withdrawn by a person, now with the disc back inside.