Video High Density
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Girl With a Suitcase" | |
Media type | video playback media |
---|---|
Capacity | 60 minutes NTSC video per side |
Read mechanism | stylus |
Usage | Home video |
Released | 1983 |
Video High Density (VHD) is an
.History
VHD was first demonstrated in 1978, and after numerous advertisements in 1981/1982
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 (in Swindon) and a production unit to develop a catalogue of "interactive" titles—a unique step—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 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
VHD became obsolete after 1987. Discs continued to be manufactured up until the early 1990s. The last new release on the format was in late 1990.
Technology
VHD discs are 25 centimetres (9.8 in) in diameter, and store up to 60 minutes of video per side.[1] 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 The entire caddy is inserted into the player, and then withdrawn, leaving the disc inside where it will be loaded and start playing.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 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 (unlike LaserDisc) so some wear would still occur.
The discs contain two
The VHD system had advantages over both CED and LaserDisc. In active (CAV) mode (not available in CED) it had a greater capacity than LaserDisc. It also had the ability to carry 99 randomly accessible 'chapters' (more than LaserDisc) 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 specialist operation requiring precise insertion of vertical interval codes into the video signal, and field accurate editing (most videotape editing required only frame accuracy).
VHD was always intended as a highly interactive format, and many
Uses
Applications included interactive adventure games, and car engine diagnostic tools. Constructing an interactive disc required a lot of planning as well as the specialist video master editing. The costs entailed in discovering these complexities and solving the problems, as well as recognising that the video post-production technology of the time was being pushed to its limits probably contributed to the decision to withdraw the system.
Offshoots
There was also a digital audio-only variant, Audio High Density (AHD; not released/canceled).[citation needed]