A System for Reducing Scratch in Record Reproduction

The reduction of record scratch is shown to be possible by taking advantage of the masking characteristics of the ear. High frequency background noise can be masked only by high frequency sounds; therefore, to reduce scratch, the high frequency response should be suppressed except during the intervals when high frequency sounds are transmitted. Most typical high frequency sounds are instantaneously accompanied by related sounds of mid-range frequency. The control of high frequency response may be accomplished either as a function of the level of such mid-range signals, or of the level of high frequency signals. The former is preferable since the latter distinction may be obliterated by the great variation in the level of scratch. Electrical circuits are shown by which high frequency response is suppressed when no mid-range signals are present and instantly restored when such signals occur. These circuits must be arranged to provide substantially instantaneous restoration of highs, but the rate of suppression must be relatively slow in order to avoid interfering with the natural rhythm and accent of music or speech.