Patent GB-955267-A (GB)
Title: Improvements in or relating to colour television receivers
IPC: H04N11/12
Filed: 1960-09-15
Abstract: 955,267. Colour television; multiplex radio signalling. PHILIPS ELECTRICAL INDUSTRIES Ltd. Sept. 15, 1960 [Sept. 18, 1959], No. 31813/60. Headings H4F and H4L. The invention relates to a receiver circuit for use in a colour television transmission system in which the transmitted signal, modulated on a carrier, comprises a component which relates mainly to the brightness of a scene and a second component consisting of a sub-carrier modulated by one or more signals relating to the colour content of the scene, whilst the frequency band of the second component coincides with that part of the frequency band of the first component which comprises the higher frequency components, said receiver circuit comprising two detectors for the modulated carrier, one, 5, for providing the component relating to the brightness and having a bandwidth characteristic embracing the complete frequency band of the brightness component, the other, 15, for providing the component consisting of the modulated carrier. In accordance with the invention a suppression filter 60, 61, 62 for suppressing at least partly the second component in the signal derived from the first-mentioned detector is connected between the point of the receiver where the channel common to the two modulation components of the carrier divides into a channel leading to the detector from which the brightness component is derived and a channel leading to the detector from which the modulated sub-carrier component is derived and the non-linear element of the first mentioned detector. By locating the filter in an intermediate-frequency portion of the receiver before the detector, as distinct from known arrangements where the filter is located in the videofrequency portion of the receiver, the arrangement according to the invention prevents the non-linear element of the detector having a rectifying effect on the sub-carrier contained in the brightness signal, which effect may give rise to a brightness increase in the reproduced image. Elements 44 and 45, together with the transformer 42, constitute the band-pass filter for the brightness signal modulated on an I.F. carrier. A series resonant circuit consisting of capacitors 46, and 47 and coil 48 constitutes a sound suppression filter. 49 designates the detector diode and elements 50, 51 and 52 constitute a filter to suppress harmonics of the I.F. signal in the output of the detector. In a first modification, Fig. 2 (not shown), the signal modulated on the I.F. carrier is fed via separate amplifiers to the two detectors. The penultimate section of the I.F. stage includes an amplifying valve from which the output signal is fed via a filter network to the control grids of two parallel connected amplifying valves. One amplifying valve is coupled via a transformer with the detector for the brightness component; the other amplifier is coupled via another transformer with the detector for the modulated sub-carrier. In this arrangement the suppression filter consists of a series resonant circuit included in the output of the amplifier which feeds the detector for the brightness component, connected in parallel with the primary winding of the transformer. In a second modification, Fig. 3 (not shown), the suppression filter consists of a parallel resonant circuit connected in series in the control grid circuit of the amplifier valve which feed the detector for the brightness component. In yet a further arrangement, Fig. 4 (not shown), the suppression filter is included in the feed-back circuit of the amplifying valve. The filter is formed in this case by a parallel resonant circuit. Where the carrier modulated by the sound signal is derived from detector 15, as in the case illustrated in Fig. 1, the suppression filter for suppressing the second component in the signal may be constructed so that it is capable of suppressing also the sound carrier. If the transmitted signal contains more than one component consisting of a modulated subcarrier, suppression filters for the further component or components is/are provided in the same manner as described above.