Video High Density

Video High Density (VHD) is a videodisc format which was marketed predominantly in Japan by JVC. There was also an audio-only variant, Audio High Density (AHD).

Technology

VHD discs are 25cm (9 5/6 inches) in diameter, though the user never sees them as they are stored in caddies—like the rival SelectaVision system from RCA, also known as CED. The entire caddy is inserted into the player, and then withdrawn, leaving the disc inside where it will be loaded and start playing. At the end of the side the disc must be removed, turned over and re-inserted.

Like the RCA system, the signal is recorded on the discs as variations in capacitance, a conductive coating on the disc itself forming part of a resonant circuit. A diamond stylus reads the signal, though unlike CED there are no actual grooves—the stylus follows the tracks electronically, like a compact disc. Naturally this means less wear, though there is still physical contact (unlike LaserDisc) so some wear would still occur.

The discs contain two frames (four fields) per revolution, and play in CAV mode. This makes trick play simple, as all frames start at the same place on the disc, but having two frames per rotation means that true freeze frame was not possible—there would be some "jitter" on moving sequences. (Discs which were designed to be used a frame at a time—such as picture catalogues—doubled up the frames, to get true stills at the cost of reduced capacity).

VHD was always intended as a highly interactive format, and many trick-play and non-linear features were supported, directly by the players or via an optional computer interface. Applications included interactive adventure games, and car engine diagnostic tools.

History

VHD was first demonstrated in 1978, and after numerous advertisements in 1981/1982 National Geographic magazines; it was eventually released in Japan and in the United States in 1983. By this time, both LaserDisc and CED were already suffering from the onslaught of VHS and Betamax VCRs.

It found its main niche as a karaoke system, and was also used in anime video games and interactive training systems. Commercial versions were available in the UK (and probably the US) for training, demonstration and fault diagnosis.

A stereoscopic system was also sold in Japan, using double-speed discs with alternate-eye images and LCD glasses to pass the correct view to each eye.

VHD today

VHD was not a great success as a consumer product, but the system sold well for the applications discussed above. VHD has something of a cult following in Japan, and discs and players are both still collected.

External links

* [http://www.totalrewind.org Total Rewind - The Virtual Museum of Vintage Video]
* [http://disclord.tripod.com/vhddiscworld/index.html VHD DiscWorld]
* [http://www.LDDb.com The LaserDisc Database - LD/CED/VHD discs, profiling and marketplace]