Patent GB-1068872-A (GB)
Title: Improvements in or relating to colour television systems
IPC: H04N9/44
Filed: 1964-05-05
Abstract: 1,068,872. Colour television systems. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. May 5, 1964 [May 18, 1963], No. 18682/64. Heading H4F. In a colour television system of the kind in which there is transmitted a luminance signal and a chrominance signal, of which the latter is produced by modulation of a sub-carrier wave by means of two colour dependent signals, in such a way that the chrominance signal has one or the other of two different compositions in alternate line periods, a signal is produced which is capable of controlling switching operations at a receiver to take account of the changes of said composition by transmitting a reference signal during intervals between field scan which comprises an oscillation of the frequency of said sub-carrier wave and which is modulated by a characteristic A.C. voltage signal to identify one or other of said compositions. The invention is stated to be of application with the SECAM (Registered Trade Mark) system where the reference signal may be produced by applying four 200 kc/s. pulses to the F.M. modulator, but is described in detail only as applied to the NTSC/PAL system. In this system the reference signal may comprise pulses occurring every other line, each pulse having a duration of twenty microseconds and occupying a position in the first half of the line interval as illustrated at 18 in Fig. 2, where 16 denotes the line synchronizing pulse and 17 the normal colour synchronizing " burst ". The A.C. signal 2 may have a frequency in the range 100- 500 kc/s., and preferably the frequency of an I.F. commonly used in radio receivers (so as to permit the use of a readily available selective filter). The signal amplitude modulates the colour carrier with carrier suppression. Fig. 1 shows the arrangement at the transmitter where the I' and Ql signals modulate the colour carrier in modulators 7 and 3, the carrier being applied directly to modulator 3 and via a 90-degree phase shifter 9 to modulator 7. In accordance with the PAL method of operation the phase of the I<SP>1</SP> signal is inverted in alternate lines by a circuit 10 under the control of line-frequency waveform 11. Normal colour synchronizing " bursts " are provided by oscillator 12. In accordance with the invention the phase of the operation of circuit 10 is signalled to the receiver by A.C. impulses 2 which are added to the Q<SP>1</SP> signal in adder 1 so that they modulate the Q<SP>1</SP> phase of the colour carrier during the vertical blanking period. At a receiver, Fig. 3, the colour carrier signal received at terminal 20 is separated into the I<SP>1</SP> and Q<SP>1</SP> components by delay line demodulator 21 and is then demodulated by demodulators 22 and 23, a local signal at carrier frequency from terminal 24 being applied to demodulator 23 directly and to demodulator 22 via a 90-degree phase shifter 26 and a further phase shifter 25 which is changed from 0 to 180 degrees on alternate lines under the control of a line-frequency signal. The additional synchronizing pulses modulated on the Q<SP>1</SP> phase cancel in the P channel due to the delay operation of the demodulator except for the first and last pulses. The pulses are taken from the channel through a switch, e.g. a transistor switch, which is rendered transmissive only during vertical blanking by pulses 31 and are applied to a synchronous demodulator 33 supplied with local colour carrier signal at the Q<SP>1</SP> phase. The demodulated A.C. signals are passed through a selective amplifier 84 to output terminal 35 and are then applied (by means, not shown) to control the switching phase of phase shifter 25. Reference is made to the use of automatic-gain-control for amplifier 34 and the use of amplitude limitation. An amplitude detector may be used in place of synchronous detector 33. The reference signal may alternatively comprise pulses transmitted in every line of the interval between field scans. In this case the impulses in successive lines are distinguished by the use of A.C. voltages of different frequency.