Patent GB-793988-A (GB)
Title: Video-frequency signal-translating apparatus
IPC: H04N11/14
Filed: 1956-01-30
Abstract: 793,988. Colour television. HAZELTINE CORPORATION. Jan. 30, 1956 [Feb. 11, 1955], No. 2920/56. Class 40 (3). In a compatible television system comprising a cathode-ray tube in which colour or monochrome images may be produced by periodic switching to different primary colours, the system being subject to undesirable red or blue colour effects when displaying in monochrome because of heterodyning between the switching signals and high-frequency video signals, means are provided for supplying the high-frequency video signals in proper phase to be switched to the different primary colours when displaying a colour image and for supplying the highfrequency signals only when the tube is switched to green when displaying a monochrome image. The invention is described as applied to a system of the type described in Proc. I.R.E., January, 1954, pages 299-308, where a signal according to the American N.T.S.C. standards is processed so as to be suitable for a one-gun cathode-ray tube display employing a striped phospher screen with post-deflection colourswitching grids and arranged for operation according to the reversing colour sequence R, G, B, G, R, &c. According to the invention, the undesirable red and blue patterns produced when the system is switched over to monochrome reception are removed by modifying the phase of the locally-generated subcarrier signal, either as applied to control the R-B axis selector or the colour-switching grids in the cathode-ray tube, so that the highfrequency video signals modulate the tube only whilst the beam is impinging the green phosphors. Provision is also made whereby this operation may be brought into effect if desired during colour reception so that a monochrome image may be produced in response to colour signals. In Fig. 1, signals received at 10 are applied to operate a one-gun cathode-ray tube 14, the system comprising four channels, the outputs from which are combined in adder circuit 13 and applied to the tube cathode. The first comprises a 0-3 Mc/s. luminance channel 15, 16, 17, the second a 3.0-4.2 Mc/s. channel 20-21 for translating colour information along the R-B axis of the 3.6 Mc/s. colour sub-carrier, the third a 6.6-7.8 Mc/s. channel 22, 23, 24 for translating colour information along the G-M axis of the sub-carrier and modulating it on to a second harmonic of the sub-carrier, and the fourth a 0-0.6 Mc/s. M-Y luminance correction channel 25, 26. Reference signals at 7.2 Mc/s. and 10.8 Mc/s. for operating stages 20, 22, 23 and 25 are derived by harmonic a Tl pliners 34, 31, from a 3.6 Mc/s. reference signal generator 28 synchronized with the standard frequency " burst " signal by an automatic-phase-control system 27 of the type described in Proc. I.R.E., January, 1954, pages 288-299. The reference signals are applied through phase-adjusting circuits 30, 32, 33 and 35, the last being arranged (see below) so that it may be switched between two different phase settings. A 3.6 Mc/s. signal from generator 28 is applied through amplifier 29 to control the colour switchng grids 14a, 14b in the cathode-ray tube. A colour-killer circuit 36 (see below) which is connected to line-scan generator 38 and automatic-phase-control circuit 27 and arranged to produce a negative bias in the absence of a synchronizing " burst " signal, e.g. during reception of a monochrome signal, controls stages 23, 25 and 35. The effect is to suppress the operation of the M-Y luminance correction channel and the G-M second harmonic channel during monochrome reception and signals to pass only through the luminance channel and R-B axis channel, the phase of axis selection being changed by 90 degrees however to correspond with beam impingement on the green phosphors instead of red and blue. In effect, the 0-3 Mc/s. portion of the monochrome signal passes to the cathode-ray tube directly and modulates the beam continuously so as to produce a desired black-and-white image, whilst high-frequency signals in the band 3-4.2 Mc/s., which may cause undesirable red and blue patterns, are passed to the tube only whilst the beam impinges the green phosphors. The arrangement in dotted line 12 is shown in greater detail in Fig. 2. Harmonic amplifier 34 is of conventional construction with the anode circuit 52 tuned to 7.2 Mc/s. The phase adjusting circuit 35 comprises triode 53 with a tuned cathode load 56 supplying signals to the grid of valve 65 forming the axis selector 20. The 7.2 Mc/s. signal from amplifier 34 is applied both to the grid of triode 53 via capacitor 57 and direct to the tuned circuit 56 via capacitor 59. With valve 53 conducting, the signal is translated through the stage without substantial phase change. The colour-killer circuit comprises a triode 61 with the anode connected to receive positive pulses during flyback from line scan generator 38 and the grid connected to the automatic-phase control-circuit 27. During colour reception, a negative potential is applied to the grid and the triode fails to conduct, but in the absence of a synchronizing " burst " the negative potential disappears and the triode conducts and produces a negative bias via filter 63. This bias is then applied to stages 23 and 25 and via resistor 60 to the grid of triode 53 to cut off the valve. The 7.2 Mc/s. is then transmitted to circuit 56 via capacitor 59 and a phase shift of approximately 90 degrees results. Switch 75 may be set to apply a permanent negative bias and permits the circuit to be set to produce a monochrome image in response to colour signals. In a modification of this arrangement, Fig. 3 (not shown), a single stage functions both for the R-B and G-M axis selection and includes a resistor network to matrix the R-B and G-M signals to derive an M-Y luminance correction signal for application to detector 25. In this embodiment the R-B component is allowed to pass through the luminance channel which is extended in bandwidth to 4.2 Mc/s. The combined axis selector is connected to receive its input at the cathode from the luminance channel and when rendered operative translates the G-M component to the anode whilst short-circuiting it from the luminance channel. In a further modified arrangement, Fig. 4 (not shown), the adjustable phase-shifting circuit is connected between reference-signal generator 28 and push-pull amplifier 29 and is effective to alter the phase of the 3.6 Mc/s. signal applied to the cathode-ray tube colour-switching grids whilst the R-B axis selector operation remains unchanged.