Disc Manufacturing

JIM JOHNSTON – VHD Manager, Swindon


The THORN EMI Videodisc factory at Swindon was purpose built to master and press VHD Videodiscs.

This is where the new technology takes over. Videodisc manufacture involves a blend of old and new technologies — and from the old demands higher standards than ever before.

The factory itself is a building within a building. The mastering and manufacturing processes all take place in an environmentally controlled series of clean rooms. The standard demanded throughout is not less than Class 10,000 and in the most sensitive areas is better than Class 100 — fewer than 100 microscopic particles in every cubic foot of air.


Programme material from virtually any source can be considered for videodisc. The final stage of post-production is to edit all the material onto a broadcast standard 1″ C format videotape in the order in which it is required on the final disc.

All audio must also be recorded in the correct places on the correct tracks.

During this process the exact location of each of the special VHD control signals is determined. These signals identify chapters, time code, page addresses, autostops, and automatic audio switching.

The finished videotape is called the PREMASTER TAPE.

Videodisc Mastering

In the VHD videodisc plant the Glass Master is prepared. This is an optically perfect glass disc which has been cleaned, coated with a photo-resist solution and baked to harden the coating. After careful checking it is placed in the Laser Cutter.

The Premaster Tape is played back to the Laser Cutter which encodes the signals into a form which drives the powerful laser. A complex series of optics condense and shape the laser beam and focus it onto the Glass Master to create the latent image of the microscopic pits in the surface.

The Glass Master is then developed, checked for flaws and after thorough cleaning is ready for the next stage in the process.

Swindon U.K.

Stamper Production

The cleaned Glass Master is sprayed with silver nitrate and a reducing solution to create a thin coat of silver. It is then electro-plated with high grade nickel in a two stage process. This gives the metal form rigidity, and it can then be removed from the Glass Master. This is the Metal Master, a negative image.

After careful cleaning the Metal Master is used to ‘grow’ positive masters, or Mothers. This is another electro-forming process. A number of mothers can be made from one Metal Master.

Again after careful cleaning the positive Mother is used to create the Stampers – the final negative image from which the videodiscs are pressed.

Disc Pressing

A pair of Stampers (Side 1 and Side 2) are placed in the press. The electro-conductive plastic compound from which the disc is made is extruded in a semi-liquid state into a carefully regulated form, known as a Patty. This is transferred to the space between the stampers, and as the press closes. steam is injected.

When the Patty has spread out cold water replaces the steam — the compound solidifies, and a videodisc emerges.

The disc is edge-trimmed, proceeds through quality assurance, and is then placed into its protective plastic disc case.

The VHD Videodisc is housed in a plastic case. The VHD player removes the disc when the case is inserted and replaces it after use.