Capacitance Electronic Disc - How CEDs Work

How CEDs Work

CEDs are conductive vinyl platters that are 30.0 cm (11.8 in) 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 12 miles (19 km). The discs rotate at a constant speed during playback (450 rpm for NTSC, 500 rpm for PAL) and each rotation contains several full frames (four frames for NTSC, three for PAL). This meant that freeze frame was impossible on players without an expensive electronic frame store facility.

A keel-shaped needle with a titanium electrode layer rides in the groove with extremely light tracking force (65 mg), and an electronic circuit is formed through the disc and stylus. Like a turn table, the stylus reads the disc, 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 amount of air pressure between the stylus tip and the undulations in the groove under it 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 (the phono pickup) to an electrical signal, the CED stylus normally does not vibrate and moves only to track the CED groove (and the disc surface—out-of-plane), 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 tilts 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 tone arm angle, which in turn depends on the radial position on the record of the stylus. Whereas for a phonograph, where the stylus has a pinpoint tip, 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 (i.e. fine spacing of adjacent revolutions of the spiral,) 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 (actually square on the outside edge) 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 lowers the disc onto the turntable (which is actually just a hub); the spine is also lowered with it. To start playing the disc, the player spins it up and moves the stylus onto the disc surface.

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.