Video High Density
Video High Density was an analog video disc format storing up to 60 minutes per side, predominantly marketed by JVC in Japan. In contrast to the optical LaserDisc format, the VHD format was read with a physical conductive stylus. Facing numerous competing formats both in and outside of Japan, the reach of VHD remained very limited.
History
VHD was first demonstrated in 1978 and was eventually released in Japan on April 21, 1983. Despite demonstrating the player at several Consumer Electronics Shows, JVC opted not to release VHD as a consumer product in North America.In the UK, Thorn EMI, which was the leading consumer provider of the VHS tape system, saw VHD as the next step in the market and committed to the system. In 1981 it invested in a factory to press discs and a production unit to develop a catalogue of "interactive" titles to support a planned 1984 launch, but canceled the investment in late 1983. VHD remained on the market in the UK primarily as an educational and training tool, usually linked to a computer, but attracted few customers.
It found its main niche as a karaoke system, and was also used in video games and interactive training systems. Commercial versions were available in the UK for training, demonstration and fault diagnosis.
A stereoscopic system was also sold in Japan. These 3D VHD systems achieved the 3D effect using double-speed discs with alternate-eye images and LCS glasses to pass the correct view to each eye.
While discs continued to be manufactured in the 1980s, the format saw its last new release in 1990.
Technology
VHD discs are in diameter, and store up to 60 minutes of video per side. As with CED, each disc is stored in a caddy: the user never handles the disc directly. The entire caddy is inserted into the player, and then withdrawn, leaving the disc inside where it will be loaded and start playing.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 CED 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. This means less wear, though there is still physical contact so some wear would still occur.
The discs contain two frames per revolution, and play in CAV mode, which makes play simple, as all frames start at the same place on the disc, but having two frames per rotation means that true freeze frame is not possible. There would be some "jitter" on moving sequences.
The VHD system had advantages over both CED and LaserDisc. In active mode it had a greater capacity than LaserDisc. It also had the ability to carry 99 randomly accessible 'chapters' and had the same autostop capability. These two functions had to be programmed into the master tape from which the videodisc was made, along with the two-frame freeze frame function. This made videotape master editing a highly specialized operation requiring precise insertion of vertical interval codes into the video signal, and field accurate editing.
VHD was always intended as a highly interactive format, and many non-linear 'trick-play' features were supported, directly by the players or via an optional VHDpc computer interface for the MSX and Sharp X1 computers.