GB997134A - Circuit arrangement for cross-fading television signals - Google Patents

Abstract

997,134. Television. FERNSEH G.m.b.H. Oct. 3, 1963 [Oct. 3, 1962], No. 38945/63. Heading H4F. To suppress test line signals contained in television signals which are cross-faded so that the automatic gain control system at the television transmitter does not increase the gain to offset the falling level of the white impulse in the test line due to the fading operation, the test line is either gated-out by a gating impulse generated in the cross-fade unit or the signals are re-blanked, the duration of the vertical re-blanking impulses being such as to exclude the test line signals except during the cross-fade operation. As shown, television signals are applied at 1 and 2, Fig. 1, and after traversing respective amplifier stages 3 and 4 the signals pass into an electronic change-over device 5. Under control of switching impulses applied to device 5 either the first or the second signal is passed to device 6 in which the synchronizing component of the signal is clipped off and the test line signal is gated out when necessary, the gating signal being produced in device 7 (see later). The picture component then passes into a fader stage 8 in which its amplitude is varied under the control of a variable potential applied by way of a change-over switch 9. When switch 9 is set to its right-hand position the fading operation is controlled by a manually operated potentiometer 10, and when set to its left-hand position the fading operation is automatically controlled by means of device 11. In device 12 the synchronizing component is separated from the signal and after traversing a limited stage 14 is added at its proper level to the output signal from fader 8 in a mixer stage 13. The vertical synchronizing impulse is separated from the synchronizing signal in integrating stage 16 and applied to the change-over switch device 5 to ensure that the change-over takes place during the vertical blanking interval, and also by way of delay device 17 to a device 7 in which the test line gating pulses are produced. When manual control of the cross-fade is employed the switching on of the test line gating pulses produced in device 7 to gating stage 6 is indicated by a mechanical contact on the manual fader 10 as indicated by broken line 19 connecting manual control 10 to switch 18. Alternatively the switch 18 is actuated by the current passed by the controlled amplifying device in fading stage 8, as indicated by chain line 20. Fig. 2 shows a block diagram of apparatus for cross-fading and regenerating television signals containing test line signals in which the signals are applied at terminals 1 and 2, as in Fig. 1. In device 21 the signal is re-blanked to a black level controlled by control 22 and the signal (from which the synchronizing components have been removed) is passed to a fader stage 8. The signal selected by switch device 5 also passes to a separator stage 23 in which the horizontal and vertical synchronizing components H and V are separated and from which they are applied firstly to a device 24 in which the regenerated synchronizing signal is formed and then added to the signal in output stage 13. The H and V signals from separator 23 are also applied to stage 26 in which the regenerated blanking impulses necessary to operate the stage 21 are produced. The change-over switch 27 associated with stage 26 determines the duration of the vertical blanking impulses produced, e.g. 21 lines (for a 625-line signal) during the fading operation so that the test line is blanked out, and 17 lines at other times so that the test line is not blanked out. The switch 27 is actuated in the same manner as described with reference to Fig. 1.