GB1218293A - Improvements relating to television systems - Google Patents

Abstract

1,218,293. Television picture shift control. FERRANTI Ltd. 20 Aug., 1969 [22 Aug., 1968], No. 40301/68. Heading H4F. Horizontal or vertical picture shift on a television display tube 21 as compared with the image position on the camera 20 is achieved by varying the time interval #T between the leading edges of the camera drive pulses SD and of the display synchronizing pulses S, selecting one of the two pulse trains and deriving a control signal operative only between the leading edges of each pulse in the selected train and of the ensuing pulse in the other train, and using this control signal to operate a gate 19 allowing video signals to pass from the camera 20 to the display 21 only in the absence of the control signal. Vertical shift may be achieved by using a frame-shift arrangement, but the embodiment shown employs line-shift to obtain a horizontal picture shift. The time interval #T between the pulses S and SD is achieved by means of three monostable devices 11, 12, 14, two of which, 11, 12, are voltage controlled to produce variable delays #T] , #T 2 which together total to #T. A further monostable device 13 provides blanking pulses of duration A 1 to the camera 20. The control voltage for the devices 11, 12 is also applied to a comparator 18 where it is compared with a reference voltage. Fig. 3 illustrates the case when the control voltage is less than the reference voltage, in which circumstances #T is arranged to be less than the line interval and the comparator 18 provides a "0" signal to NOR gate 16 and a "1" signal to NOR gate 17. The output of gate 16 thus depends upon the state of a bi-stable circuit 15, which is set and reset respectively by the leading edges of successive pulses SD and S. The video gate 19 is consequently closed during the corresponding time interval, with the result that the right-hand part of the scan of camera 20 coincides with the left-hand part of the scan of display 21; i.e. the picture is shifted to the left. Picture shift to the right is similarly achieved by making the control voltage greater than the reference voltage, resulting in a delay #T greater than the line interval up to a maximum of twice the line interval, and reversing the "0" and "1" signals from the comparator 18 to the gates 16 and 17. When the control voltage approximately equals the reference voltage no shift is obtained. The invention may be used with interlaced or radial scan. As shown it is used in a closed-circuit system, but may be adapted for broadcasting, the display 21 then being a monitor.