Electrokinetica The Electro-mechanical Museum

Media Centre - the interactive video department

Video started to take over from 16mm film for training and promotional purposes with the advent of open-reel video tape recorders in the 1960s and 70s. By the 1980s, video tape had become standardised and streamlined into a number of cassette formats, while at the same time exciting new disc-based formats were beginning to surface.

Thorn VHD commercial video playback system

VHD player

VHD player

VHD stands for Video High Density, which was one of three competing playback-only video disc systems of its era that failed to achieve a sustainable market share. The format was developed by JVC and uses double-sided discs carrying one hour of video per side including two audio channels. Released in Japan in 1983 as a truly interactive video recording format, VHD attempted to overcome some of the shortcomings of its predecessors; RCA’s basic, consumer-oriented Selectavision format (a.k.a. CED, capacitance electronic disc) released in 1981 and Philips’ technologically advanced but costly LaserVision of 1982. Taken up in the UK by Thorn, VHD was targeted at educational and industrial users who could benefit from the interactive and microcomputer-control options at a time when interactive TV was uncharted territory. Its facilities made it suitable for arcade games, expert systems, video jukeboxes and point-of-sale installations; it had greater run-time than LaserVision could achieve in interactive mode, and better facilities and performance than Selectavision. Yet it was heading for extinction within a year of its UK debut, a victim of the inter-system rivalry, some inherent technical limitations, and the explosive growth of the videotape sector at ever-increasing performance / cost ratio. VHD found a niche market in Japan for karaoke but never hit the mainstream.

The VHD vinyl disc format and player

Inside VHD player

Inside VHD player

VHD demo disc

VHD demo disc

The recording medium is a double-sided conductive vinyl disc 25cm in diameter, made in a standard record press, on which a spiral of microscopic pits is impressed into the plastic by stamper, which in turn is derived from a laser-generated master. The information is encoded on the disc in analogue form, by modulating the length and spacing of the pits with the video and audio signals. A pickup stylus much larger than the pits (and therefore unable to fall into them) runs on the disc’s otherwise smooth surface, carrying on its trailing edge a thin conductive electrode connected to the playback circuitry. The capacitance between the electrode and the disc varies as the pit walls pass beneath it, allowing the player to recover an electrical signal directly from the relative motion of the disc and stylus. The earlier CED system used a physical groove in the surface of the disc to keep its stylus aligned over the data track. In VHD this function is carried out electronically under guidance from additional tracking signals encoded as pits between adjacent data tracks. The servo system monitors the tracking signals recovered by the pickup and responds by deflecting the stylus electromagnetically on its compliant mountings. Five actuator coils apply minute forces to the stylus carrier, manipulating it in two dimensions to follow the spiral form of the track and any eccentricity in the disc, while simultaneously advancing or retarding it along the track where necessary to minimise timing errors caused by fluctuations in the angular velocity. The stylus can also be lifted clear of the surface during fast scan. The pickup and its deflection system are carried across the radius of the disc on a motor-driven sled, just like the optical pickup in a modern DVD player. Compared to CED, the grooveless tracking system avoids wear to disc and pickup when searching and skipping across the recorded surface. The pickup is a consumable part nonetheless, with an expected life of 2000 hours. It was designed to be user-replaceable in the same way as the stylus or cartridge on an audio record player, by opening an access panel in the top of the machine.

VHD stylus

VHD stylus

Pickup deflection coils

Pickup deflection coils

Stylus access hatch

Stylus access hatch

Making the system robust

Even microscopic scratches and dirt particles can affect the signal playback quality by disrupting the smooth running of the pickup, therefore every disc is protected in a caddy that the user inserts and then immediately withdraws from the player during loading. The disc is left inside, suspended on the transport mechanism, whilst a drive hub pops up into its central hole. The player then completes the loading cycle by clamping the disc onto a platter without any part of the mechanism touching either of the recorded surfaces. The caddy can be inserted either way up according to which side of the disc is to be played. A mechanical interlock ensures that the disc can only be collected with the caddy the same way up as it was when the disc was inserted, preserving the relationship between the sides of the disc and the labels on the caddy.

Loading VHD disc

Loading VHD disc

VHD electronics

VHD electronics

Playing VHD disc

Playing VHD disc

Interactive features

Disc detail

Disc detail

The random-access capability was advertised as one of the principal advantages of the VHD system. Users could skip or search to any chapter or time using the remote control handset, or the controlling microcomputer could command the player to do so. This was intended to allow interactive navigation of branching paths within the recorded material, for applications such as troubleshooting guides and video games. Still images could be freely intermixed with video, during which the player automatically stops on a menu page with options for the viewer to choose. Getting a satisfactory still image by pausing during a segment of video footage was more troublesome for a specific technical reason that turned out to be a limitation of VHD. In any recording format, tradeoffs must be made between maximum recording duration, quality and player technological performance. One of the most critical parameters for a disc system is its rotational speed. Modern digital systems tend to vary the disc speed automatically, to maintain a constant linear velocity of the data track past the pickup. With VHD's analogue format, the disc must rotate at a fixed speed locked to the frame rate of the TV system, so that the frames all lie in a radial pattern. This system, termed constant angular velocity, is easier to implement and brings certain advantages such as allowing the stylus to navigate from track to track during the vertical interval (where no picture information needs to be recovered) for instance during still playback, because all the vertical intervals are aligned radially. Its chief disadvantage is the variation in image quality that occurs between tracks recorded near the outside of the disc (where the linear velocity is highest and the bandwidth widest) and the inside, a difference that is noticeable on VHD although not objectionable. It plays from outside inwards, so that a partially filled disc uses the best part of the media.

Interactive features - the disadvantages

Given that the period of one disc rotation must be an integer multiple of the period of one TV frame, only a few speeds are technically feasible. For European 25-frame standards, in view of the tradeoffs mentioned above, the speed chosen for VHD was 750 rpm or 12.5 revolutions per second, thus storing two frames per rotation. This makes it physically impossible to display a single still frame by following the same track on the disc repeatedly; two consecutive frames will be played alternately. This two-frame GOP (group of pictures) is the smallest individual element of material that the player can address. If there are significant differences between the two frames, for example a fast moving object or rapid change in brightness, the resulting alternation blurs the image or makes it flicker, rendering the ‘still’ unviewable. To counter this, where an interactive programme required a still to be available from a fast-changing image, a special track was added containing a repeat of the frame that would display correctly. On the plus side, the still image quality is identical to that of normal playback, unlike the typically poor stills performance of contemporary analogue videotape in which stopping the linear motion of the tape caused impaired playback by affecting the relative geometries of head-path and track layout. Compared with VHD, LaserVision could boast true still-pause functionality in interactive mode by running the disc at frame rate (1500 rpm), although at the expense of halving the recording time. LV also permitted unlimited still playback duration, while VHD inevitably suffered localised disc wear as the stylus remained in contact with one track, in the same way that magnetic coating wear can occur during videotape still-pause.

From VHD to AHD

VHD computer interface

VHD computer interface

VHD's audio configuration supported both stereo and independent dual-mono use of its two audio tracks with automatic mode selection recorded on the disc. Scenes with musical content could be presented in stereo, for example, while monaural recordings of commentaries could be given in two languages for the viewer to select. Another suggested use for the dual-mono mode was within training programmes, where the same block of video content could be used as either a test or a lecture, according to whether the ‘questions’ or ‘answers’ audio was chosen. Cleverly, the demonstration disc has a chapter consisting of a school science programme, in which one audio track is the actual programme soundtrack whilst the other gives a commentary relating how the programme makes use of VHD’s features as it proceeds. An additional audio-only format using the same discs was proposed under the name AHD or Audio High Density, intended to offer discrete 4-channel 16-bit digital recording, or three channels and a sequence of still images. Audio AHD never took off, so the acronym was recycled as Advanced High Density meaning VHD under computer control.

Watch VHD

For this video clip we took the top cover off the player to reveal the disc and mechanism during playback. A user would never normally see the exposed disc as it is enclosed in its caddy until inserted into the player - a necessary precaution against damage by dirt and scratching. The brief insert of programme material is from the opening chapter of the demonstration disc.

Watch video: VHD player loads and plays a disc

The following clip touting the advantages of VHD is taken directly from the introduction on the demonstration disc, prior to the main chapter selection menu.

Watch video: Sample from VHD demo disc

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