Patent GB-871996-A (GB)
Title: Improvements in or relating to receivers for colour television systems
IPC: H04N11/06
Filed: 1957-08-09
Abstract: 871,996. Television. PHILIPS ELECTRICAL INDUSTRIES Ltd. Aug. 9, 1957 [Aug. 13, 1956], No. 25206/57. Class 40(3) In a compatible colour television receiver for a system in which a broad-band luminance (or equivalent) signal is transmitted together with a narrow-band chrominance signal the H.F. components of the former signal are added to the latter signal by combining the broad and narrow band signals with opposite polarity, passing the combined signals through a low-pass filter, the pass band of which is not greater than the bandwidth of the narrow-band signals and then combining the broad-band signal with the output signal from the filter. With this arrangement colour errors in the reproduction of components (in colour or monochrome) having their frequencies located in the region in which the two frequency bands adjoin is prevented without the need to employ filters of high accuracy (and therefore of high expense) to produce the necessary sharp transition between the two signals, Fig. 3 shows one embodiment for use with a video signal (derived from video amplifier 6) of the kind shown in Fig. 2 which comprises a broad-band luminance signal, occupying the range o to fa, and two narrow-band chrominance signals, consisting of sub-carriers fh 1 , fh 2  modulated respectively by components related to the red and blue aspects of the scene transmitted. As shown, the two narrow-band signals are selected from the composite signal by means of respective band-pass filters 23, 24 the outputs from which, after detection at 25, 26 and amplification at 27, 28 are supplied to mixing circuits 33, 31 in which they are combined with the phase inverted (in circuit 29) output of video amplifier 6. The output signals of the circuits 33, 31 are then supplied via respective low-pass filters 34, 32, the pass ranges of which are, at most, equal to the bandwidths of the blue and red signals, to mixing circuits 37, 36 in which they are combined with the output of video amplifier 6. The resulting outputs of circuits 37, 36 which comprise low-frequency colour signals together with high-frequency luminance signals are then supplied to picture reproducing tubes 47, 46 producing, respectively, the blue and red components and additionally are fed to a matrixing circuit 35 together with the output of video amplifier 6 to produce signals for application to the "green" picture reproducing tube 45. In a modification employing a tri-gun type of tube the outputs of filters 32 and 34 are fed, via phase inserting stages, to the red and blue cathodes of the tube and additionally are combined in a matrixing circuit to produce signals for application to the green cathode, the luminance signal output of video amplifier 6 being supplied to the control grid of the tube (Fig. 6, not shown). In another embodiment for use with a video signal of the kind shown in Fig. 8, which comprises a broad-band signal o-fa, the low-frequencies of which relate to the green information and the high-frequencies to brightness and a narrow-band signal consisting of a sub-carrier fh modulated, alternately, during successive line periods, by red and blue information, the latter signal is separated out (from the output of video amplifier 6) by means of a band-pass filter 51 (Fig. 9) and after detection in 52 and amplification in 53 is combined with the phase-inverted (in unit 29) output of video amplifier 6 in mixing circuit 54. The output of the latter is then limited to a range not greater than the bandwidth of the red and blue signals in low-pass filter 55 and combined with the output of video amplifier 6 in mixing circuit 56 the output from which is supplied, alternately, at line frequency to the red and blue picture reproducing tubes 62, 61 respectively in a switch device 57 operated by line synchronizing signals derived from the line scanning generator 9, delay devices 63, 64 (introducing a delay of one line period) and adding circuits 65, 66 being included in the channels of tubes 61, 62 to produce a "continuous" signal input thereto. The green picture reproducing tube 60 is then fed directly from amplifier 6 since the output of the latter consists of green information together with high-frequency brightness information. In a modification of the embodiment (Fig. 11, not shown), the output of amplifier 53 is first split into two channels by means of a switch (similar to switch 57) operating at line frequency, each channel including delay and adding devices similar to 63 . . . 66, and after combination with the phase-inserted output of amplifier 6 and band-width limitation in low pass filters (similar to 55) the signals in each channel are combined with output of amplifier 6 and supplied to the red and blue picture reproducing tubes, the green tube, as in the last embodiment, being fed directly from amplifier 6.