Patent GB-1207442-A (GB)
Title: Apparatus for cross-fading between two television signals
Inventor: LEGLER ERNST
IPC: H04N5/265
Filed: 1968-01-04
Abstract: 1,207,442. Television. FERNSEH G.m.b.H. 4 Jan., 1968 [4 Jan., 1967], No. 704/68. Heading H4F. Apparatus for cross-fading between two television signals, preferably two colour television signals FBASI and FBASII, Fig. 1, comprises an electronic change-over switch 3, the output of which is applied directly and via a device 7 for extracting the synchronizing information, including the colour synchronizing signals i.e. bursts, to the two inputs of a cross-fader device 5 which is so controlled that as the amplitude of the applied television signal (picture signal component and synchronizing information) is varied between zero and the standard level the amplitude of the synchronizing information is correspondingly but oppositely varied between the standard level and zero, so that during cross-fading when the amplitude of the picture signal component of the first television signal is reduced from a standard level to zero, the amplitude of the synchronizing information remains constant, and after changeover (preferably during the period of a vertical synchronizing impulse) when the synchronizing information of the first is changed for that of the second signal and when the amplitude of the picture signal component of the second signal is increased from zero to the standard level, the amplitude of the synchronizing information is again maintained constant. Cross-fader device 5 comprises two adjustable amplifier stages, of which the outputs are connected in parallel, gain control being automatically effected, by means of a control voltage, between unity and zero in such a manner that the sum of relative gains is constant and equal to unity. Two embodiments for deriving the synchronizing information from a colour television signal are described. In the first embodiment, Fig. 2, the synchronizing information comprising the synchronizing signal and the colour synchronizing signal i.e. burst, of the colour television signal FBASI or FBASII at input 11 are gated out by means of gating impulses applied to gate 12. These impulses are derived by suppressing the chrominance component of the colour television signal in trap 14 comprising a low-pass filter, separating the synchronizing signal in an amplitude separator 15, deriving therefrom the horizontal synchronizing pulses in stage 16, advancing these pulses in time in stage 17 so that their leading edges appear somewhat earlier than the leading edges of the horizontal synchronizing pulses, and shaping these pulses in shaper 18 so that their trailing edges occur later than the ends of the colour synchronizing signals. To these pulses is then added in OR gate 19 the equalizing impulses and the vertical synchronizing signal from separator 15. In the second embodiment, Fig. 3, the colour television signal is first separated in frequency into the luminance signal and the chrominance signal in cross-over filter 32 (a combination of high-pass and low-pass filters). From the luminance signal the synchronizing signal S is separated in clipper 33 and from this signal gating pulses are derived in device 34 which are used to separate the colour bursts from the chrominance signal in gating stage 35. The complete synchronizing information is thus obtained from adding stage 36 and applied to the input 8 of the fader device 5 of Fig. 1.