Patent GB-1138911-A (GB)
Title: Improvements in or relating to colour television receivers
IPC: H04N9/44
Filed: 1966-05-04
Abstract: 1,138,911. Colour television receivers; transistor switching circuits. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. 4 May., 1966 [21 May, 1965; 12 June, 1965], No. 19600/66. Headings H3T and H4F. In a colour television receiver adapted to receive a signal which includes a chrominance signal having two different compositions in alternate line periods and which also includes in the intervals between lines colour synchronizing signals consisting of "bursts" of oscillation of colour carrier frequency which are subjected to phase change in alternate lines as indicated at 16 and 17 in Fig. 2, the synchronizing signals are employed to produce a first - control signal to establish the phase of a reference carrier oscillator and a second control signal which controls the operation of a switch which adapts the receiver to the two different signal compositions. Reference is made to receivers for both PAL and SECAM (Registered Trade Mark) systems but detailed description is given only of receivers for the PAL system. In such a system the chrominance carrier Q modulation axis remains fixed in phase whilst the I axis is reversed at line frequency. The phases of the synchronizing "bursts" are arranged symmetrically with reference to the - Q axis as shown in Fig. 2. In Fig. 1 the local reference carrier oscillator 8 is synchronized by the "bursts" with the aid of a conventional phase discriminator 5 and reactance stage 7, the arrangement including a circuit 6 having a time constant large compared with the phase change interval of the "bursts" whereby the oscillator is controlled to a phase (+I) related to the mean of the phases of "bursts" 16 and 17. The synchronizing "bursts" are applied to the discriminator through a gate stage 2 which is operated by line frequency pulses 3. To adapt the receiver for the I axis reversal, the local carrier oscillation is likewise reversed by applying the output of oscillator 8 to a changeover switch 12 by way of a first direct path 10 and second path including a phase inverter 11, the switch being actuated by a bi-stable multivibrator 15 which operates at half line frequency. The multivibrator is synchronized by line frequency pulses 3 applied via line 20 but, in accordance with the invention, is caused to operate in the correct phase by means of the synchronizing "bursts". To this end the output of discriminator 5 is also passed, without the intervention of a time constant circuit, via capacitor 14, to the multivibrator. The output consists alternately of positive and negative pulses 18 and 19 which indicate by their polarity the position which switch 18 should occupy. Since pulses 18 and 19 also contain the timing information to operate the multivibrator correctly the line pulse drive over line 20 may be dispensed with. However, if the use of the line drive pulses is retained the circuit will continue to operate satisfactorily in the event of interruption of pulses 18 and 19. Furthermore, one or the other of pulses 18 and 19 may be omitted. The pulse train 18, 19 may be converted to a sawtooth wave before being applied to control the multivibrator. A detailed transistor circuit for the arrangement shown in Fig. 1 is described with reference to Fig. 4 (not shown). In a second embodiment of the invention, Fig. 5 (not shown), the subcarrier oscillator is synchronized as described above but the pulse waveform 18, 19 which is applied to multivibrator 15 is first converted to a sine wave with the aid of a high Q circuit tuned to half line frequency. The circuit may comprise a resonant circuit, if necessary with the Q increased by feedback, or an oscillator. A detailed transistor circuit of this embodiment based on the use of an oscillator is described with reference to Fig. 7 (not shown). The oscillator furnishes a push-pull output for connection to the two sides of multivibrator 15. The sine wave may be rectified to furnish a control voltage which disappears if the synchronizing bursts disappear or assume a fixed phase. Such an arrangement may be used to control the receiver to adapt its operation automatically for colour and black-and-white signals or PAL and NTSC signals.