Patent GB-1142980-A (GB)
Title: Colour television signal generating apparatus
IPC: H04N9/04
Filed: 1966-08-26
Abstract: 1,142,980. Colour television; selecting largest signal. RADIO CORPORATION OF AMERICA. 26 Aug., 1966 [15 Sept., 1965], No. 38509/66. Headings H3T and H4F. In a colour television camera a dichroic element 14, Fig. 1, transmits green (G) image light to a first pick-up tube 17 and reflects red (R) and blue (B) light to a second pick-up tube 23 the optical path to which includes a strip filter 21 which may comprise either alternate strips transmitting R and B light and having a three-to-one width ratio (Fig. 3, not shown) or, alternate strips transmitting R and purple (=R+B) light and of equal width (Fig. 5, not shown) the strips of each filter being equally transmissive for white light, i.e. when white light is derived from the subject 11, equal quantities of red and blue light are transmitted through the filter strips. The output signal from pick-up tube 23 thus includes an alternating component modulated to represent the R and B colours and the circuit means 24, 25 are provided for combining a signal representative of the peak to peak amplitude with a signal representative of the average value of this output signal to derive the R and B representative signals separately. Further circuit means 26, 27 are also provided for deriving the I and Q signals and a brightness signal M according to the NTSC specification. When the former filter (i.e. that of Fig. 3) is employed the circuit means 24, 25 comprises, as shown in Fig. 7, a polarity reversing amplifier to the input 44 of which the output signal (which has the waveform depicted in Fig. 4, when subject 11 comprises a standard colour-bar pattern) is supplied. Following amplifier 45 the circuit has four branches 46 having respectively zero delay, and delays of one-quarter (line 51), onehalf (line 52) and three-quarters (line 53) of the scanning time for one set of colour strips (about 53 microseconds for a filter of 100 sets) so that one of the output branches is always at a peak level of signal. Each branch is connected to a respective driver transistor 54-57 each of which supplies a respective pair of positive and negative peak detectors 58, 59 . . . 65, 66 to derive positive peaks across resistor 67 and negative peaks across resistor 68. These signals are then combined via various resistors (i.e. 72, 73 and 74, 75) to produce a signal X = 0À5B+0À5R (at point E) and a signal x/3 (at point E<SP>1</SP>) which when combined (at the same points E, E<SP>1</SP>) via networks 84, 85, 86 and 87, 88, 89 with the average signal, derived by combining the outputs of the four branches of 46 via resistors 76... 79 to provide a signal Y = 0À25B+0À75R at the input to a polarity reversing amplifier 81, 82, in suitable amplitude and polarity, provides at the outputs of a non-reversing amplifier 94, 95 and a polarity reversing amplifier 91, 93, the red (R) and blue (B) signals R = 2Y - X and B = 3X - 2Y respectively. These signals are then utilized in a matrix 26 (Fig. 1) together with the green (G) signal output of tube 17 to provide the I and Q signals and the Y signal (from terminal P) is combined with the G signal in a matrix 27 to provide a luminance signal M= 0À4Y + 0À6G = 0À1B+0À3R+0À6G. When the filter of Fig. 5 (not shown) is employed, producing an output signal from tube 23 having the form depicted in Fig. 6 (not shown), in response to light from a standard colour bar pattern as subject 11, the circuit means comprising the detector 24 of Fig. 1, utilizes a single delay line and employs diodes as positive and negative peak detectors (Fig. 8, not shown). The various waveforms appearing at particular points in both circuits are depicted in Figs. 9a . . . 9f (not shown). Instead of the optical arrangement of Fig. 1, the filter 21 may be included in the tube 23, e.g. by being incorporated in the face plate or placed directly in contact with the photosensitive electrode of the tube (Fig. 2, not shown).