The present invention relates to a recording or playback device of the type in which a rotating record carrier foil is held, by a flowing gaseous medium disposed between a stabilization surface and the foil, in the vicinity of the stabilization surface, and in which a unit for recording or playing back is movable above the foil transverse to its direction of advancement.
Picture record systems are known which operate according to the pressure scanning principle, where information is stored on an electrically nonconductive record carrier foil in the form of deformations, or undulations, disposed along a spiral track. In order to play back the signal recording, the foil is moved beneath a pressure pickup and the pickup moves so that its motion combined with that of the foil causes the pickup to follow the spiral track. Fluctuations in the pressure force on the pickup caused by the deformations in the foil are converted into corresponding electrical signals by a transducer. In order to be able to produce a perfect signal reproduction, it is necessary that physical oscillations and vertical wobble of the foil be substantially prevented.
It is known to produce a smooth, vibration-free movement of the foil by rotating the foil at high speed at a slight distance above a stationary stabilization surface, which can be a planar plate. A thin cushion of air forms between the foil and the stabilization surface and air in the cushion exhibits a net outward flow so that the foil is held at a slight distance above the stabilization surface. With a given rate of rotation and appropriate design of the portion of the stabilization surface which determines the quantity of air passing outwardly, it is possible to counteract local changes in shape of the foil by corresponding local increases or decreases in pressure in the cushion of air. The foil and the cushion of air are in a stable equilibrium.
With such arrangements, it sometimes occurs, on occasion periodically, that the foil brushes against the stabilization surface or even that the foil adheres to the stabilization surface.
This interference with the steady movement of the foil is caused mainly by the effect of electrostatic charges which may form on the foil and on the stabilization surface. They not only interfere with the steady movement but also lead to discharges through the pressure pickup which produce interference in the playback of the recorded picture.
U.S. Pat. No. 3,767,865 issued to Schuller et al, discloses a manner of solving this problem by providing means which substantially prevent the formation of an influence charge on the surface of the stabilization surface when the record carrier is electrostatically charged and thus prevent the formation, within the air cushion, of an electric field which could cause the carrier to adhere to the stabilization surface. This can be effected either by providing the stabilization surface with a sufficiently thick layer of an insulating material on its side facing the underside of the record carrier or by providing a record carrier foil which moves above a metallic stabilization surface with a coating of conductive material on its bottom surface.
In the latter case no electrical field can form in the cushion of air between the stabilization surface and the underside of the foil if the stabilization surface is conductively connected with the underside of the foil. While this can be considered to be the ideal arrangement, it is difficult to realize in practice, since, inter alia, a conductive foil coating has an adverse influence on the foil flexibility characteristics required for high quality signal reproduction by the pressure scanning process. The application of such a coating would also increase the cost of manufacturing the record carrier foils.
A picture record player operating with a record carrier foil without a conductive coating operates satisfactorily only if neither the foil nor the stabilization surface becomes strongly electrostatically charged, if no impurities such as dust are present on either part, and if the foil is free of dents.