GB1070193A - Improvements relating to colour television receivers - Google Patents

Abstract

1,070,193. Colour television receivers; automatic phase-control systems. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. May 11, 1964 [May 10, 1963; Sept. 5, 1963; Oct. 5, 1963; Nov. 27, 1963]. No. 19501/64. Headings H3A and H4F. The sub-carrier oscillator in a colour television receiver is synchronized throughout the line period by deriving from the received subcarrier signal itself a carrier signal which is subjected only to amplitude modulation by a colour dependent signal and has the carrier suppressed, the modulation being such as to give rise to phase reversals of the signal, and phase ambiguity in the control exercized by the signal is removed by also synchronizing the oscillator by the conventional sub-carrier synchronizing " bursts " which occur at the beginning of the line. The invention avoids the need for a stable oscillator, e.g. a quartz oscillator, since it is synchronized continuously. The oscillator may operate on a start-stop basis in each line, which is of particular value where the signal is derived from a magnetic recording and is subject to line-by-line phase errors. The invention is stated to be of use with SECAM (R.T.M.) signals but is described in detail only as applied to NTSC/PAL signals. In one embodiment, Fig..9, the received subcarrier signal is separated into the P and Ql components by a conventional delay-line demodulator 8, 10, 11, the I<SP>1</SP> signal being subject to line-frequency phase reversals due to the PAL method of operation and both the I<SP>1</SP> and Q<SP>1</SP> signals being liable to further 180-degree reversals whenever the modulating video signal changes sign. The signals are amplitude demodulated in stages 30 and 31 to which signals at sub-carrier frequency are applied from an oscillator 14 to restore the suppressed carriers. The oscillator signal is applied to stage 30 through a phase inverter 32 which is operated at line rate whilst the signal to stage 31 is applied through a fixed phase shifter of 90 degrees. In accordance with the invention, oscillator 14 is controlled by (a) the synchronizing " bursts " 41 which are separated from the received signal by gate 23 and applied to the oscillator through a 57-degree phase shifter 49 which establishes the bursts in the I<SP>1</SP> phase, and (b) signals which are derived from the separated I<SP>1</SP> and Q<SP>1</SP> sub-carrier components. To this end the signals are applied to fullwave rectifier circuits 36, 37 which function as frequency doublers to produce signals of fixed phase despite the phase changes to which the original I<SP>1</SP> and Q<SP>1</SP> sub-carrier signals are subject. The diodes in one rectifier are reversed with respect to those in the other so as to introduce a 180-degree phase change whereby the double frequency signals are brought into phase as illustrated more fully in Fig. 11 (not shown). The " burst " and two sub-carrier signals control the oscillator by direct " pulling " as illustrated more fully in Fig. 10 (not shown), in a detailed oscillator circuit. The I and Q carrier signals which are brought to the same phase may be combined to form new signals. for example R-Y and B-Y signals. In a second embodiment, Fig. 7 (not shown), which is otherwise similar to that shown in Fig. 9, the phase shift in one of the double-frequency signals is introduced after the full-wave rectification and the two signals are combined in an adder stage before application to the oscillator. The addition may be weighted in favour of the signal derived from the I<SP>1</SP> sub-carrier component. In a third embodiment, Fig. 6 (not shown), a double-frequency signal is obtained from the Q<SP>1</SP> component only and the " burst " controls the oscillator by way of a conventional retroactive loop including a phase detector. The Q component signal may be made constant in amplitude either before or after doubling by limitation or rapid regulation. A filter may be used to eliminate noise. In a fourth embodiment (not disclosed in detail) the oscillator control signal is derived from the sub-carrier signal by passing the sub-carrier signal through a one-line-duration delay line so as to bring the conjugate signals in successive lines into time coincidence and then multiplying them together. The resulting signal, which is at double the sub-carrier frequency, is free from phase variations imparted by the colour signal modulation but contains other phase errors imparted to the sub-carrier. The doublefrequency signal may be passed to a frequency halver before being utilized to control the local oscillator. Alternatively it may control an oscillator the output from which is halved.