Technology: Disc maker explains pressing problems

日期:2019-02-28 02:10:12 作者:嵇殂 阅读:

By BARRY FOX BRITAIN’s only manufacturer of compact video discs – the video version of CDs – has broken eight years of silence to talk about the problems it has encountered in pressing discs at its factory at Blackburn in Lancashire. PdO, a company which was set up as a joint venture between Philips in the Netherlands and Du Pont, the American chemicals giant, has improved the standards of production at the factory. At the same time, there have been changes in the technical standards for playing the discs, which are known in the trade as CDV discs. Pressing faults are not a problem on audio CDs as they are with CDV discs. This is because the error-correction codes included in the digital sound signal effectively conceal them. There is no error correction for the video signals, which are analogue. So even the slightest defect shows up on the screen, usually as white or black ‘blips’. PdO decided to talk openly about its problems partly because of recent criticisms from David Fine, the president of the record company Polygram, a subsidiary of Philips. Fine blamed the failure of CDV discs to sell in Europe on the poor quality of pressings. The industry first announced CDV discs in 1987 but it did not launch them until a year of delays had passed. Engineers at the PdO factory now believe that they have finally cracked the technical problems which left them unable to press enough acceptable CDV discs reliably. The main breakthrough was a change in the technical standard for playing CDV discs; this has made the discs easier to press. Dave Wilson, the company’s manager of marketing services, admits: ‘No one anticipated the level of control that would be needed. The problem was not the size of the disc; it was the need to put digital sound on the disc along with analogue video of PAL standard (the standard for European TV).’ The disc rotates at a speed which varies continually, in order to keep the relative speed of the read-out laser over the surface constant. Because the speed of rotation is always changing, the relationship between successive TV pictures recorded in the spiral of pits on the disc is not simple. If the laser beam in the player is not perfectly focused on one turn of the spiral, it will read different parts of several pictures at the same time. To meet the new standard, the disc’s speed is now varied in small steps, as many as 10 in each rotation. The result is that similar parts of successive pictures in adjacent turns of the spiral align neatly. If the laser’s spot of light overlaps them, there is less risk of interference on the screen. Joint research between PdO and Philips’ Central Laboratories in Eindhoven showed how the shape of the pits critically affects the interference between the analogue video signal, which contains the picture information, and the digital sound signal. The signals have to be mixed together before recording and unscrambled on playback; interference produces a fuzzy picture. The shape of the pits depends on a string of variables: the intensity of the laser beam used to cut the pits in a layer of light-sensitive material on the glass master disc; the degree of focus of the beam; the chemical nature of the light-sensitive layer; and the chemical development of this layer. All these parameters, Blackburn found, were ‘organic’ and varied unpredictably from day to day. ‘We would get good pressings one week, but then not the next,’ said Wilson. The factory in Blackburn has now tightened control on all stages of the production process. Artists making recordings and other important visitors from record companies are no longer allowed to go inside the clean rooms where vital processes are carried out. They must look through glass windows instead. Like all plants around the world that press CDV discs, Blackburn still faces one problem. Each master disc, which is made of glass, can produce only one electroplated-nickel stamper, which is used in the injection moulds to press the finished duplicate discs. When this stamper fails, usually after four or five thousand pressings,