Patent GB-1109187-A (GB)
Title: Improvements in subcarrier circuits for colour television apparatus
IPC: H04N11/18
Filed: 1967-02-07
Abstract: 1,109,187. Colour television. COMPAGNIE FRANCAISE DE TELEVISION. 7 Feb., 1967 [16 Feb., 1966], No. 5884/67. Heading H4F. To reduce visibility of the sub-carrier in a colour television system in which the subcarrier is used for alternately at line frequency transmitting two colour signals D 1  and D 2  which are linear functions of the primary colour signals R, G and B, the sub-carrier is amplitude modulated by a signal A and frequency modulated by a signal M, signal A being a function of the primary colour signals which cannot become zero unless D 1  and D 2  are simultaneously equal to zero, e.g. A = D = #D 1 <SP>2</SP> + D 2 <SP>2</SP>, where D 1  = D cos # and D 2  = D sin #, and signal M being alternately at line frequency equal to D 1 /A and D 2 /A, e.g. cos # and sin #. The modulator circuit, Fig. 1, comprises a frequency modulator 36 to which the signal M is applied, the frequency modulator's output signal being applied as a carrier input to an amplitude modulator 37, preferably a balanced modulator, to which the signal A is applied. To derive the signals A and M (signal A being equal to signal D in the preferred embodiment) Fig. 2, signals D 1  and D 2  are applied to the inputs of balanced modulators 51 and 52, the carrier inputs of which receive signals sin wt and cos wt respectively from an oscillator 40 and a 90 degrees phase shifter 43. The resulting signals D 1  sin wt and D 2  cos wt are added in a network 47 to derive the signal D sin (wt + #) which is applied to an envelope detector 48, the output of which thus comprises signal D, i.e. A. The signal D (sin wt + #) is also applied through a device 39, which keeps the signal's amplitude constant, to the first input of a phase detector 49. The outputs of the oscillator 40 and phase shifter 43 are applied to the two inputs of a switch 45 which has a control input 44 and changes its state during every horizontal blanking interval so that two signals sin wt and cos wt appear alternately at its output. The output of switch 45 is connected to the second input of phase detector 49 which supplies alternately at line frequency the signals cos # and sin #, i.e. M. Two signals a 1  and a 2  of opposite polarities may be transmitted during checking periods in each vertical blanking interval for identification of the two colour signals which alternate at line frequency. Instead of switching the sinusoidal signals applied to the phase detector 49 the signals D 1  and D 2  may be applied alternately to each of the two modulators 51 and 52. In a further arrangement the signal A is applied to the modulation input of the amplitude modulator 37, Fig. 1, and the signal M, applied to the modulation input of the frequency modulator 36, is supplied by a feedback loop comprising a subtractor, one input of which is connected through an amplitude-responsive frequency discriminator to the output of the amplitude modulator 37, and the other input of which receives alternately the signals D 1  and D 2 , the output of the subtractor being connected to the modulation input of the frequency modulator 36. Instead of A being equal to D it may equal the sum of the absolute values of D 1  and D 2  or the absolute value of one of the two signals D 1  and D 2  which has the higher absolute value. At a receiver the signal is applied directly and via a one-line period delay line 81, Fig. 3, to the two inputs of a double switch 86 which supplies the sub-carrier at its output 91 when its frequency modulation is a linear function of cos # and at output 92 when its frequency modulation is a linear function of sin #, these two outputs being respectively connected to the inputs of two frequency discriminators 93 and 94. The discriminators respond linearly to the signal amplitude and instantaneous frequency of the sub-carrier to supply the signals D cos # = D 1  and D sin # = D 2  respectively. The sub-carrier may be shifted in phase by 180 degrees every third line in addition to 180 degrees every other field. The sub-carrier may be demodulated in its sequential form by means of a single frequency discriminator. If an NTSC-type signal is used the NTSC subcarrier may be used instead of the signal applied in Fig. 2, by adder 47, the burst being used to phase-lock an oscillator, the output of which is fed to the phase detector 49 via a phase-shifting device.