Patent GB-1091170-A (GB)
Title: Improvements in or relating to colour television cameras
Inventor: CLARK WILLIAM JAMES ROWLEY
IPC: H04N9/093
Filed: 1965-05-07
Abstract: 1,091,170. Colour television cameras. MARCONI CO. Ltd. March 1, 1966 [May 7, 1965], No. 19415/65. Heading H4F. In a multi-tube colour television camera automatic control of the position and size of the scanned raster in each tube is effected with the aid of optical mask means, Fig. 2, which surrounds the picture area P and incorporates reference areas a-h, whereby during the scanning of the raster area F there are generated pulse signals as illustrated in Fig. 3 for lines in the raster occurring at positions (I)-(VII). The pulses are separated from the picture representative components by gate circuits controlled by line and field blanking signals and are then compared in time with pulses derived from the camera line and field drive pulses to produce error signals, in dependence upon one difference in time, which are employed for the automatic control of the scanning circuits shift and amplitude. The line shift of the raster is controlled with the aid of reference areas b and g which are located midway across the picture area. The resulting pulses are compared with pulses which are derived from a train of drive pulses at twice line frequency and timed to occur exactly at the middle of a correctly scanned line. The field shift of the raster is controlled with the aid of areas d and e which are located midway down the picture area. The pulses are used to produce a pulse lasting from the onset of the first until the onset of the second, and hence of the duration of approximately one scanning line, and this is compared with a pulse of like duration which is derived by counting down line frequency drive pulses and arranged to occur half-way through a correctly scanned raster. The line size of the raster is controlled with aid of reference area a, the resulting pulse being compared with a pulse derived from the line drive pulse and briefly delayed to allow for the slight inset of the area with respect to the edge of the raster. The field size of the raster is controlled with the aid of areas a and c. The pulses are used to produce a pulse lasting from the onset of the first to the onset of the second, and hence of the duration of approximately one line, and this is compared with a pulse of like duration which is derived from the line drive pulses and timed to occur during the fifth line of a field when areas a and c are scanned. The pulse comparisons are made by feeding the pulses to be compared to the inputs of a differential amplifier which feeds a pair of linear ramp voltage generators whose maximum outputs are detected. By this means there is produced an error signal of amplitude and sign determined by the magnitude and direction of any timing error. Four such arrangements are required for the four controls which are required for the raster of one pick-up tube. A group of four arrangements may be provided for each tube, or one group may control the tubes in succession, the scanning circuits being arranged to retain the adjustments imparted by the error voltages between the periods of control. The error voltages may control the scanning circuits electromechanically, e.g. by adjustment of resistance elements, or wholly electrically. Detailed arrangements for carrying out the invention are described (in block diagram form only) with reference to Figs. 4 and 5 (not shown). The invention is mainly described with reference to three-tube cameras, although reference is made to a two-tube camera. The mask shown in Fig. 2 may be included in a common part of the optical path to the camera tubes so as from the picture area of all the tubes. Alternatively, a separate mask may be provided for each tube. Although not all the reference areas are utilized in the arrangements described above the remaining areas are included so as to provide a symmetrical system which will function if the directions of scan are reversed. The reference areas are located outside the picture area so that the pulse signals occur in the line and field blanking intervals. Alternatively, they may be located within the picture area in the peripheral areas which are masked in reproducer tubes. As a further alternative, a pattern of reference areas occupying the picture area may be exposed to the camera by a shutter during fields which are well spaced in time, e.g. at 50 field intervals, so as not to be particularly noticeable to viewers.