Patent GB-1272970-A (GB)
Title: Improvements in or relating to colour television cameras
Inventor: RYLEY DEREK VERNON
IPC: H04N9/093
Filed: 1969-07-29
Abstract: 1,272,970. Colour television camera. MARCONI CO. Ltd. 15 July, 1970 [29 July, 1969], No. 37994/69. Heading H4F. In a colour television camera of the type having a plurality, e.g. three, camera tubes, registration between pairs of the cameras is obtained by detecting in each video camera output signal "edge" sections having amplitudes above a predetermined datum and average slopes greater than or equal to a predetermined amount, producing signals representative of these sections, comparing the time occurrance of corresponding pairs of these signals produced from pairs of the video signals and producing a registration correction signal from each comparison for one of the video signals. In the threecamera arrangement of Fig. 1, cameras 2 and 3 are brought into registration with camera 1 in a TDM manner. Considering the video signal (a) Fig. 2 from camera 1, this is fed to two level detectors 111, 112, fed at their reference inputs by reference voltages R 1  and R 2 , spaced symmetrically around the mid-grey level MG. When the video signal rises above the lower level R 2  the output (c) of detector 112 changes from 0 to 1 and stays at 1 until the video signal falls below level R 2 . Detector 111 similarly gives a digital output (b) having a 1 valve when the video signal rises above level R 1 . The digital outputs are time quantized by means of bi-stable devices 121 and 122 switched by clock pulses (d) at 5MH 3  such that the outputs (e) and (f) of the bistable devices 121 and 122 take up, at the leading edge of each pulse, and for the enduring repetition period, the value of the digital output signal at the time of said leading edge. The quantized outputs (e) and (f) are fed to an "edge recognition" circuit 130 (Figs. 6a, 6b, not shown) where four types of edges, Fig. 4 are recognized and distinguished, viz the PF or positive going fast rise edge, where the detector outputs (b) and (c) have leading edges closer than one clock pulse width and the leading edges of the quantized outputs (e) and (f) are coincident, the PS or positive going slow rise edge, where the detector outputs have leading edges so spaced that the leading edges of the quantized outputs (e) and (f) are spaced by on clock period, and the two corresponding negative going edges NF fast rise and NS slow rise. Circuit 130 can exist in ten states and changes from one to the next according to a sequence dependent on the type of edge, and shown at (g) Fig. 4. Thus state (1) indicates a 0, state (2) indicates that a leading (positive going) edge has been detected and states (3)-(4) and (5) indicate that the edge is slow and fast respectively. Corresponding states for the trailing edge are (6), (7), (8)-(9) and (10). The outputs of detectors 211/212 and 311/312 for camera 2 and 3 respectively are fed to a switch 240 which is switched at  field frequency such that the detector outputs are fed in TDM manner to bi-stable devices 221, 222 for quantization and thence to edge recognition circuit 230. Cameras 2 and 3 are thus alternately registered with camera 1. Thus the outputs of the edge recognition circuits are fed to a comparator 50 which compares the times of corresponding types of edges and produces a corresponding registration correction signal or scan shift signal for camera 2 or 3. The area of the television field over which the above comparison taken place is shown in Fig. 3 and comprises a central section SH of one line to produce horizontal correction signals and a vertical line SV consisting of aligned parts of consecutive lines, to produce a vertical correction signals. The clock signals at 5MH 3  thus only occur for said sections SH, and for line SV, additional clock pulses are provided occurring at line intervals at the same time in each line. If camera 1 gives luminance signals and camera 2 gives chrominance signals, the signals may contain edges which although roughly aligned are not associated in the original. On the basis of a comparison of these two edges only, a large but erroneous correction signal could be produced. To overcome this, comparator 50 only gives an output when successive positive and negative edges are detected with the same sign of registration error. Extensive use is made of Boolean algebra analysis and embodiments of circuits 130 and 50 are derived and described by said use.