1,193,547. Colour television. COMPAGNIE FRANCAISE DE TELEVISION. 15 Oct., 1968 [17 Oct., 1967], No. 48800/68. Heading H4F. Specification describes a compatible colour television system of the PAL or N.T.S.C. type wherein the required high degree of accuracy and stability in the relationship between the line frequency and the sub-carrier frequency is attained not by highly accurate and expensive oscillators but by passing the synchronizing pulses through a variable delay means 5, Fig. 1, before their addition to the video signal, the delay introduced by means 5 being dependent on the initial phase of the synchronizing pulses with reference to the output of the sub-carrier oscillator 3. The transmitter, Fig. 1, generally comprises a camera 11 supplying the colour signals and a matrix 12 producing the chrominance signals on lines 21 and 22 and the luminance signal on line 61. The chrominance signals modulate the sub-carrier in means 2. The synchronizing pulses, produced by generator 4 and after phase shifting at 5, are used to control the sweep circuits of the camera and are also added to the luminance signal in adder 6, a second adder 7 summing the modulated subcarrier and the luminance/synchronizing signal. For the PAL 625 line system the relationship between the sub-carrier line frequencies and frame frequency F s , F h and F v respectively is The term F v /2 necessitates a highly accurate generator, and this necessity can be overcome using a simpler (nŒ“)F h generator by causing the phase shifter 5 to introduce shifts of 0 and # with respect to the sub-carrier oscillation on alternate frames. A suitable arrangement of shifter 5 is shown in Fig. 2, comprising, input 51, output 52, a two-way switch 520, a # delay line 510 and a bi-stable circuit controlling switch 520 and controlled by frame sync. pulses on line 531. For the N.T.S.C. system the frequency relationship is This results in the phase difference between the line pulses and the subcarrier being 0 and # on alternate lines for each frame, interlacing of successive frames producing relative phases for successive lines of a complete picture of 0, #, #, 0, 0, #, #. . . . By use of the invention separate unlocked line and subcarrier oscillators 4 and 3 can be used having stabilities of 10<SP>-6.</SP> If for frame 1, the relative phases of the line pulses from oscillator 4 are α<SP>1</SP> and α<SP>11</SP> for two consecutive lines and the phase of the pulses when the leave phase shifter 5 are #<SP>1</SP> and #<SP>11</SP> respectively, then the phase shift # introduced by 5 is (#<SP>1</SP> - α<SP>1</SP>) = (#<SP>11</SP> - α<SP>11</SP>). The values of α<SP>1</SP> and α<SP>11</SP> will vary slowly due to the use of unlocked oscillators, but substantially α<SP>11</SP>=α<SP>1</SP>Œ#. Thus phase shifter 5 comprises, Fig. 3, a variable delay 500 producing a delay # = # 0 + aV c controlled by a voltage V c applied at 501. A pulse generator 510 receives the line pulses and generates on line 91 those line pulse signals occurring during the frame blanking interval and on line 92 a sampling pulse occurring during said blanking interval. Voltage V c is produced by a correction voltage generator 9 fed by lines 91 and 92 and with the sub-carrier on line 53. Generator 9, comprises, Fig. 4, a comparison pulse generator 900 receiving line 91, an AND gate receiving line 92 and the output of 900, a bi-stable circuit 920 set by the output of the gate 910 and reset by line frequency pulses on line 922 synchronous with the sub-carrier (i.e; produced by dividing down said carrier in 930) and an integrator 940 receiving the output of circuit 920 and reset at the beginning of each sampling period by line 92. In the N.T.S.C. system, every other line pulse has the same relative phase, thus generator 900 can be a simple divide by two circuit passing on every other pulse. In the PAL systems every fourth line has the same relative phase, thus generator can be a divide by four circuit. The pulse exiting gate 910 represents those lines occurring during the sampling period and the widths of output pulses of circuit 920 represent α. Integrator 940 then produces voltage V c for controlling the delay 500 for the ensuring frame For sensitizing the Fv/2 frequency shift in the PAL system, the arrangement of Fig. 2 may be added. Generator 900 may alternately comprise, for N.T.S.C. a switchable delay circuit as shown in Fig. 2 operable at line frequency whereby all the line pulses are used and are brought into phase, or for PAL, a delay circuit operating on all the lines and introducing successive delays of 0, #/4, #/2, 3#/4. Generator 510, Fig. 3 may be alternatively fed with the output of the delay 500 and the generator 9 used to provide an error voltage, in servo-loop manner. Fig. 6 (not shown) shows the use of the invention in a SECAM to N.T.S.C. converter wherein no sync. pulse generator is needed, the sync. pulses being extracted from the SECAM signal.