Video Disc: Capacitive Recording

The next approach to be tried was to store the information as variations of the electrical capacitance of the surface of the disc. By making both the disc and the pickup part of an electrical circuit, these variations can then be read back. The capacitance of the surface will depend upon the thickness of a top layer, allowing discs to be produced simply by stamping them out.

Two systems were developed using capacitive recording, SelectaVision / CED from RCA, and VHD / AHD from JVC.

SelectaVision / CED

This format, as CED, was on sale in the UK, so is covered in its own exhibit. To summarise, it used 30cm vinyl discs, read by a sapphire stylus running in a groove, giving up to an hour on each side and with stereo sound.

VHD / AHD

JVC also developed a capacitive format, called VHD / AHD (Video High Density / Audio High Density). As the name suggests, it was intended to be a carrier for both video and high-quality audio.

Unlike CED/SelectaVision, no groove was used; the 25cm disc was flat, and the stylus followed the tracks electronically. Although this was more complex, it made trick-play functions less prone to damaging the disc - though as the stylus was still in contact with the surface of the disc, some wear was inevitable. As with CED, a sleeve or caddy was used to protect the discs from casual wear and prying fingers.

The VHD / AHD format was first demonstrated in 1978, and eventually released in April 1983. It was only sold in Japan, and appears to have sunk like a stone, despite all the usual promises. A re-launch for school use was apparently attempted, and some modern Karaoke bar machines apparently use VHD discs.

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