Patent GB-1318041-A (GB)
Title: Television camera registration
IPC: H04N9/093
Filed: 1969-11-24
Abstract: 1318041 Colour television BRITISH BROADCASTING CORP 22 Sept 1970 [24 Nov 1969 24 Feb 1970] 57451/69 and 7598/70 Heading H4F Misregistration of two pick-up tubes of a colour television camera is detected by subtracting the output signals of one tube from those of the other tube and differentiating the result (or differentiating both output signals first and then subtracting or performing both operations simultaneously), rectifying the resultant signal, integrating the rectified signal and detecting a minimum in the integrated signal. Departure of the signal from the minimum value is then employed to restore correct registration by producing a correction "shift" in one of the tubes. In order to make the arrangement "directionsensitive" two responses are created by delaying each of the output signals A, B by suitable period T to produce delayed signals A T  and B B  which are then combined as shown in Fig. 2 to produce the two responses. Substraction of the latter then results in the direction sensitive error signal shown in Fig. 3 the frequency of maximum output of which is 1/2#t where #t is an incremental delay quantity. When this frequency is greater than the maximum frequency of the signal to be differentiated a sufficiently close approximation to the true derivative may be realized as (S-S# t ) where S is the signal to be differentiated and S# t  is the signal S delayed by the time increment t Applying this technique to the signals A and B and making the delay period T also equal to #t the derivatives of the signals (A-B T ) and (A T -B) become A-B# t -A# t +B 2 t and A# t -B-A 2 # t  + B# t  respectively. Figs. 4 and 5, show an embodiment utilizing the above technique for detecting misregistration in the line scan direction in which the output signals from the two tubes are supplied via respective L.P. filters 11A, 11B having a sine-squared response and a cut-off frequency below 1/2# t  producing signals A and B and thence to the various delay networks the delayed output signals from which are combined with the undelayed signals in the circuit of Fig. 5, which includes summing resistors 21, differential amplifiers 22, 23, full wave rectifiers 24 and integrating networks 25, 26 the signals produced by which are amplified by a high-gain differential amplifier 27 producing the waveform shown in Fig. 3 which may be employed, e.g. via two threshold detectors, to effect the required compensation by introducing an appropriate "shift" components into the line scanning circuit of one of the tubes. For producing registration correction in the field scan direction a simulated line scan is produced by sampling a short length of each line scan (e.g. about 4À5 Ásec.) at the same position throughout a field seam and employing a further sampling technique to produce the required delays, which in this embodiment is made equal to one line period the delayed and undelayed signals being combined in the same manner as described in connection with Fig. 5 (Figs. 7, and 8, not shown). In order to avoid spurious responses and to produce a misregistration response sensitive to the whole of the field the (vertical) line of samples is shifted in the line direction during successive field scans such that the process repeats once every twelve fields.