Control Mechanisms in the Philips ‘VLP’ Record Player

The playing time and the bandwidth of the recorded video signal on the Philips ‘VLP’ record are such that the optical scanning system must be able to perceive extremely fine details when the record is payeld. This requires an optical system with a large aperture and consequently a small depth of focus. It is also essential that the beam of light used for scanning should be centred very accurately on the information-carrying track on the record. The requirements specified here are about 1000 times more exacting than is usual in mechanical engineering. For example, the record surface may differ by 100-500 μm from a true plane while the depth of focus of the lens used to centre the scanning beam on the record is approximately 1 μm. The eccentricity of the track may amount to 100 μm, while the beam of light has to be kept centred on the track with an accuracy of 0.2 μm to avoid cross-talk [1] . To ensure continuous tracking of the information contained in a spiral of pits pressed in the surface of the record, the entire optical system has to be displaced on average by 2 μm radially for each revolution. Finally, a high-quality playback of the detected colour-television signal is only obtained if the speed at which the record rotates is constant to within at least 1 part in 10 3