Patent GB-1144075-A (GB)
Title: Improvements in or relating to colour television systems and apparatus
IPC: H04N11/18
Filed: 1966-04-12
Abstract: 1,144,075. Colour television. CSF-COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. 12 April, 1966 [13 April, 1965], No. 16089/66. Heading H4F. The invention relates to a compatible SECAM (Trade Mark) system referred to as the COSECAM system which may be transcoded to agree with other colour television systems such as the PAL and NTSC systems provided that they employ the same two chrominance signals A1 and A2, e.g. I and Q or the colour difference signals (with the possible exception of band width) and the same subcarrier frequency, i.e. the systems are harmonized. According to the invention the signal of the COSECAM system comprises a wide band luminance signal Y and a sub-carrier, of angular frequency w amplitude modulated with suppressed carrier alternately at line frequency by two chrominance signals, of the form: P1 = A1 sin wt. P2 = A2 cos wt. A phase reference signal, i.e. burst signal, is preferably transmitted during alternate intervals of line suppression and may be made the same as the half line frequency identification signal used in versions of the SECAM system. COSECAM transmitter, Fig. 1.-Block 50, which receives the three primary colour signals, forms the luminance signal Y at output 54, and the two chrominance signals A1 and A2 which are applied to balanced amplitude modulators 57 and 58. An oscillator 60 delivers a stable oscillation at the subcarrier frequency directly to modulator 57 and via a 90 degree phase shifter 61 to modulator 58. The outputs from modulators 57 and 58 are fed to a switch 73 controlled to connect its output alternately to the two inputs 71 and 72 in the intervals of line suppression by means of switching control signals on input 75 at half line frequency from a monostable flip-flop 88 controlled by a " divide by two " divider 90 which receives line frequency pulses at input 91. The half line frequency pulses from 90 are also fed to a control pulse generator 92 to enable it to supply during alternate line suppression intervals a pulse of duration equal to that of the burst to a gate 93 which receives via a phase shifter 99 the wave from oscillator 60. A burst is thus added in adder 74 during alternate line suppression intervals to the signal from switch 73. The transmitter may alternatively comprise a single balanced modulator whose modulation input receives alternately, by means of a switch, the two signals Al and A2, whereas its carrier input receives alternately, by means of a further switch, actuated in synchronism with the first, the oscillations sin wt and cos wt. COSECAM/NTSC/PAL receiver, Fig. 2.- The modulated subcarrier at input 2 derived in known manner from the received videocomplex signal is fed in parallel via a direct path 3 and a one line period delay path 4 to a double electronic switch 24 the outputs of which are fed to two synchronous demodulators 11 and 12. Gate 19 feeds the burst signals to a monostable flip-flop 20 which processes the control signals applied to switch 24, and to an oscillator and phase synchronization circuit 21 which delivers an oscillation (sin wt) at constant amplitude B, i.e. B sin wt, to synchronous demodulator 11 and via a 90 degrees phase shifter 22 to demodulator 12, i.e. B cos wt. With received signals according to the COSECAM system switch 24 directs the subcarrier signal P1 to output 7 and the signal P2 to output 8 so that demodulator 11 delivers the signal A1 and demodulator 12 the signal A2. If the video signal complex is of the NTSC type, but the burst is transmitted only each second line so that the phase of the burst is related to sin wt in the same way that it is in the COSECAM transmitter, the switch 24 functions as before and the subcarrier PO = A1 sin wt + A2 cos wt will appear at the two outputs of the switch, and demodulator 11 will supply Al, and demodulator 12 will supply A2. Monostable flipflap 20 may be replaced by a bi-stable flip-flop in which case the burst may be transmitted at line or half line frequency. If the burst is at line frequency and circuit 20 is a monostable flip-flop the switch will be blocked in a predetermined position so the demodulator 12 continuously receives the " direct " subcarrier wave and demodulator 11 continuously receives the delayed sub-carrier wave. This is particularly desirable when the signals A1 and A2 are respectively the signals Q and I of the NTSC system since I has a band-width greater than Q. With received signals according to the PAL system, i.e. P<SP>1</SP>1 = A1 sin wt + A2 cos wt and P<SP>1</SP>2 = A1 sin wt-A2 cos wt, the switch 24 will direct P<SP>1</SP>2 to demodulator 11 which will then supply the A1 signal and P<SP>1</SP>1 to demodulator 12, which will then supply the A2 signal, provided that the burst occurs at half line frequency preceding P<SP>1</SP>2 or P<SP>1</SP>1 according to whether the COSECAM burst precedes P1 or P2 respectively. COSECAM receiver, Fig. 3 (not shown), capable of receiving only from a COSECAM transmitter, includes a less precise delay device for delay path (4). The differences from the receiver of Fig. 2 are as follows: Synchronous demodulators 11 and 12 are replaced by ordinary amplitude detectors (111) and (112), and the carrier frequency is reinjected into the subcarrier wave by means of a combiner (30). The combiner is controlled by a switch (34) which has two signal inputs B sin wt and B cos wt, and a switching control input comprising one of the output signals of the monostable flip-flop (20). Transcoding device for transcoding from a COSECAM to NTSC harmonized therewith, Fig. 4 (not shown), comprises a combiner (200) which is fed in parallel via a direct path 203 and a one line period delay path 204 with the COSECAM video signal complex. If the burst of the COSECAM system has the same phase relative to the oscillation sin wt as has the NTSC burst, the COSECAM burst is split by the delay device and will appear at line frequency at the output of the combiner (200). If A1 = Q and A2 = I, then in the NTSC system as the signal Q has a band width less than I whereas in the COSECAM they are equal, a filter effective for alternate lines may be provided at the input of the circuit. Transcoding device for transcoding from COSECAM to PAL harmonized therewith and vice versa, Fig. 5, comprises a combiner 345 to which the input signal is fed directly over path 303 and via a one line period delay circuit 304 which feeds one input 343 of a switch 340 directly and the other input 344 via a phase inverter 342. The switch has a single output fed to the input of combiner 345 and a switching control input 341 receiving switching signals from the monostable flip-flop 320. Assuming A1 = Q and A2 = I, when COSECAM is transcoded to PAL the phase of the switching signals applied to switch 340 are so chosen that the switch connects its output to delay line 304 when the signal Q sin wt passes over this path, and to the phase inverter 342 when the signal I cos wt passes over the dalayed path so that the combiner supplies alternately the signals P<SP>1</SP>1 and P<SP>1</SP>2. If the combiner 345 supplies half sum rather than the sum of the input signals thereto, the circuit may be used for transcoding from PAL into harmonized COSECAM since ¢ (P<SP>1</SP>2 + P<SP>1</SP>1) = Q sin wt and ¢ (P<SP>1</SP>1 - P<SP>1</SP>2) = I cos wt.