Modifying a television signal to inhibit recording/reproduction

Abstract

A composite television signal is modified to inhibit the reproduction of an unauthorized recording thereof by conventional video recorders but enable the display of a video picture therefrom on a television receiver. The length of a frame (more particularly, the length of each field in a frame) is increased or decreased from standard length, either by changing the time duration of the respective horizontal line intervals included in each frame while keeping a constant, standard (e.g. 525) number of lines per frame, or by changing the number of horizontal line intervals which constitute a frame while maintaining the standard duration of each line interval (e.g. 63.5 microseconds). A profile pattern representing the variation of the video frame duration, or vertical period, (whether by changing the horizontal line durations or the number of lines in a frame) with respect to time is adjustable to correspondingly control the rate at which the vertical period (i.e. frame length) changes, the maximum and minimum vertical period and the ratio of increased to decreased vertical periods. Accordingly, substantially the same electronics responsive to the profile pattern may be used to modify the television signal; and the profile pattern itself may be changed easily, such as by simple software modifications, to accommodate different circumstances and to satisfy different conditions.