| 摘要 |
<p>1,155,621. Television distribution systems; colour television. COMMUNICATIONS PATENTS Ltd. 16 March, 1967 [22 March, 1966], No. 12593/66. Headings H4F and H4R. In a wired broadcasting system for colour television the signal fed to the circuit comprises a modulated carrier brightness information signal, a suppressed carrier colour information signal and a pilot which has a frequency equal to a sub-multiple of the suppressed carrier of the colour information signal and which is used at the receiver to ensure that the reinserted carrier of the colour information signal is equal to the suppressed carrier frequency. The phase of the pilot signal is modulated in synchronism with the variations in phase of the colour component to provide synchronization for a PAL system signal. The Figure shows an embodiment for the reception and rebroadcast over a wire system of a " PAL " transmission. In this arrangement the brightness signal together with the colour signal on a suppressed carrier is applied to a modulator 19 where it modulates a carrier of about 5.4 Mc/s from oscillator 20, from which the signal is applied, via a vestigial sideband filter 22 and a gate 23 to the mixing circuit 24 for application to the lines 2 and 3. Also applied to gate 23 are signals derived from the incoming signal and applied over line 17 to enable colour burst signals at the frequency of the suppressed carrier of the colour components, i.e. 9.83 Mc/s., to pass to comparator 26 where they are compared with the output signal of a frequency tripler 27, which is fed with the locally generated pilot from oscillator 28, to provide an a.f.c. signal on line 29 which ensures that the pilot signal is maintained at one third the frequency of the suppressed colour signal carrier. The a.f.c. signal is applied as one input to a phase modulator 30, to the other input of which is applied colour recognition signals on line 18. The phase modulation by the colour recognition signals is such that the phase of the pilot is modulated by 60 degrees in synchronism with the phase modulation of 180 degrees of one of the colour components in the transmitted signal. At the receiver 6 the brightness and colour information signals are selected by a filter 34 and amplified by amplifier 35. The brightness component is demodulated by an envelope detector 36 and the colour components are applied, via filter 38, to synchronous detectors 40 and 41. The carrier for detector 40 is derived from the pilot signal by a wide bandpass filter 42, which passes all the sidebands due to the phase modulation applied at the transmitter, and a frequency tripler 43 so that the carrier is synchronously demodulated by a carrier whose phase reversals are synchronized with those of the colour component. The carrier for detector 41 is derived from the pilot by a narrow band pass filter 45 which passes phase stable pilot signal to tripler 46 from which a phase stable carrier is supplied to demodulate the phase stable colour component.</p> |
