Patent GB-969880-A (GB)
Title: Improvements in colour television systems
IPC: H04N9/47
Filed: 1963-04-04
Abstract: 969,880. Colour television; semi-conductor circuits. COMPAGNIE FRANCAISE DE TELEVISION. April 4, 1963 [April 5, 1962; June 6, 1962; Jan. 7, 1963; March 14, 1963], No. 13530/63. Headings H3T and H4F. Colour Television.-In a colour television system in which two colour signals are transmitted alternately at line frequency with the aid of a switch and at a receiver the two signals are directed into appropriate channels also with the aid of a switch, checking signals are transmitted with one or both colour signals during at least certain of the vertical blanking intervals for the purpose of permitting the receiver to establish its switch operation in the same phase as that of the transmitter. The invention is described as applied to a system inwhich the two colour signals, which are of the form K 1 (R-Y) and K 2 (B-Y), are transmitted as frequency modulations of a subcarrier wave and combined with a luminous signal Y. In carrying out the invention rectangular trapezoidal signals, Fig. 2, each of the duration of one line, are added to the colour signals during a part D of the vertical blanking interval. Fig. 1. The signals are added after an interval C following the actual synchronizing interval A and may persist throughout the remainder of the blanking interval, Fig. 1a, or for only part of the interval, Fig. 1 The checking signals are preferably added to each colour signal (although the Specification briefly discusses operation with one signal) and are added with opposite polarity to the two signals. The signals are of an amplitude corresponding to the maximum allowable modulation level. Colour television transmitter.-A transmitter arrangement is described in block diagram form with reference to Fig. 3 (not shown). The checking signals are added via the colour signal matrix or subsequently, and the combined signals applied to modulate a sub-carrier wave via a switch which is changed over at line rate. The coefficient K 1  for the R-Y signal may be positive or negative. When negative, which is the preferred arrangement, both the colour signal and the checking signal are of opposite polarity to the B-Y signal and its checking signal. The transmitter includes means for applying high-frequency pre-emphasis to the colour signals, and means for suppressing the colour sub-carrier during horizontal blanking intervals and those parts of the vertical blanking intervals not occupied by the checking signals. The regular alternation of the transmitter switch may be discontinued during the vertical blanking interval, before the beginning of the checking signals, preferably at the beginning of the blanking interval, if it is desired to modify the regular alternation of the signals. Colour television receivers-At a receiver, Fig. 4 (not shown), the colour signals whilst still modulated on the sub-carrier wave are directed into separate R-Y and B-Y channels by means of a switch which is changed over at line rate under the control of a bi-stable multivibrator triggered by received line synchronizing pulses. The switch is in the form of a doublepole change-over switch and receives one input directly and the other via a one-line-duration delay circuit. The effect of this arrangement is to provide continuous trains of R-Y and B-Y signals each comprised alternately of direct and delayed signals. The switch outputs are frequency demodulated and then applied through de-emphasis filters (complementing those at the transmitter) to a matrix circuit which produces a G-Y output together with R-Y and B-Y outputs. When the coefficient K 1  for the R-Y signal is chosen to be negative, the frequency demodulator in the R-Y channel is arranged to invert the signal polarity. The matrix circuit functions to produce outputs during the checking signals. If the phase of the receiver switch operation is correct the outputs in response to the checking signals are of one polarity, whereas if the phase is incorrect the outputs reverse in polarity due to the changed polarity of the checking signals. By reason of the form of the matrix equation for deriving G-Y, the polarity change at the G-Y output of the matrix is opposite to that at the R-Y and B-Y outputs. In carrying out the invention, a selected one of the matrix outputs is employed as an indication of the phase of the receiver switch operation. If the phase becomes incorrect the changed polarity is employed to alter the switch operation by causing either an extra trigger pulse to be applied to the switch bistable multivibrator or by suppressing one of the normal line synchronizing trigger pulses. The selected matrix output is integrated and where the output in response to the checking signals is sufficiently distinguished in amplitude from that due to the normal picture signals it may control directly a gate circuit through which the synchronizing pulses pass to trigger the multivibrator. The integrator circuit is associated with a diode so that the gate is opened only in response to a signal of the appropriate polarity. The gate may be formed as a valve circuit, Fig. 11 (not shown), or a transistor circuit, Fig. 12 (see below). In another arrangement, Fig. 7 (see below), the operation is controlled by horizontal flyback pulses so that the matrix output is effective only during the interval occupied by the checking signals. In yet a further arrangement, Fig. 13 (not shown), and Fig. 15 (see below), the system for checking and correcting the phase of the switch operation is combined with a " colour killer " system. A bi-stable circuit, which controls the blocking of the colour channels, is set to the " unblock " state at the beginning of each vertical blanking interval (unless it is already in that state) and is thereafter set in the " block " state unless a checking signal of the correct polarity is obtained from an output of the matrix. Resetting the circuit from the " unblock " to the " block " state is also arranged to produce a pulse which is applied as an extra pulse to the switch bi-stable multivibrator. The effect of the arrangement is (1) to cause automatic blocking of the colour channels during black-and-white operation, when no colour signals and hence no checking signals are received; (2) to cause unblocking of the colour channels when colour signals are received and the phase of the switch operation is correct; and (3) to cause correction of the switch phase when wrong and blocking of the colour channels for the first field following-the detection of an incorrect checking signal. Semi-conductor circuits.-In Fig. 12 line synchronizing pulses are applied from terminal 44 via transistor 360 to trigger the switch bistable multivibrator (see above) which is connected to terminal 65, and the transistor is blocked, so as to suppress the pulses, should the checking pulses (see above), which are applied at terminal 46 and are negative going during correct operation, become positive going. Network 349, 350 constitutes a low-pass filter for the checking pulses, which are at line frequency. Diode 347 prevents operation to negative pulses, whereas positive pulses are passed and integrated in network 342, 352 to produce a positive base bias which blocks the transistor. In Fig. 7 transistor 78 is connected as a normally inoperative blocking oscillator and is controlled by both the checking pulses, which are applied at terminal 46 and become positive when the switch phase is incorrect, and vertical flyback pulses, Fig. 8a, applied at terminal 45. The oscillator is triggered only in response to the simultaneous application of positive base bias due to checking pulses integrated in network Ci, Rb and a negative emitter potential in the form of the first negative half cycle of an oscillation, Fig. 8b, induced in a resonant circuit Le, Ce, by the vertical retrace pulse. The circuit constants are selected so that the negative half cycle persists throughout the interval occupied by the checking pulses. The triggering of the blocking oscillator causes an extra pulse to be added to the train of line synchronizing pulses which are applied from terminal 44 via one winding 75 of the oscillator transformer to the switch bistable multivibrator 65. Reference is also made to the suppression of a pulse. In Fig. 15 a Schmitt trigger circuit 201, 202 is controlled by vertical flyback pulses, Fig. 16, applied at terminal 45 and checking signals, which in this case become positive going for incorrect switch phase, applied at terminal 46. A flyback pulse induces a current, Fig. 17, in a critically damped primary circuit 124, 125, 127 of a transformer 123 and thereby creates a voltage wave in the secondary winding 126 as shown in Fig. 14a. The wave exhibits negative and positive portions each extending beyond the trigger levels A41, A40 of circuit 201, 202, whilst the positive portion is arranged to embrace that part T2-T5 of the vertical blanking interval which includes the checking signal. The voltage wave is applied to the base of transistor 201 via capacitor 121 and its effect would normally be to trigger circuit 201, 202 to one state (i.e. the state when it unblocks the colour killersee above) during the negative portion and then to reset the circuit immediately to the other (the " block ") state during the positive portion. The trigger operation is however also affected by a potential, Fig. 14b, created across capacitor 121 by integration of the checking pulses. The solid line shows the potential produced during correct phase operation: the dotted line shows the potential during incorrect operation. During correct operation the negative potential offsets the positive excursion of the Fig. 14a wave and prevents circuit 201, 202 from being placed in the " block " state, whereas the positive potential during incorrect operation merely aids the triggering to the " block " state. A differentiation circuit 172, 173 and diode 174 provides a pulse on terminal 108 for the switch