Patent GB-784055-A (GB)
Title: Image-reproducing system for a color-television receiver
IPC: H04N9/26
Filed: 1954-09-06
Abstract: 784,055. Television. HAZELTINE CORPORATION. Sept. 6, 1954 [Oct. 5, 1953], No. 25781/54. Class 40 (3). Colour television signals of the constant luminance type (e.g. N.T.S.C. signals) are modified for use with a single gun colour picture reproducing tube of the kind which produces a plurality of colours in a sequence in which one of the colours occurs twice. In general the invention employs the steps of( 1); modifying the luminance signed Y to produce a luminance signal M of the dot-sequential type; and (2) modifying the colour sub-carrier to produce a further modulated wave in which the difference colour signals occur in synchronism with the scanning of like colour elements of the tube screen. The latter comprises a repeating series of green, red and blue colour-producing strips (Fig. la) in the sequence GRGB the scanning beam being deflected from the G strips on to either the R or B strips (Fig. 1c) by means of a sinusoidal switching signal having a frequency related to the frequency of the modified colour sub-carrier wave applied to the tube. The considerations leading to the manner in which the signal modification is effected consist mainly of an analysis of the type of signal to be modified and of the type of signal which it is desired to produce. Fig. 1m shows a vectorial representation of the colour sub-carrier of the former signal (NTSC) in terms of the quadrature RY BY colour-difference components - and - 1.14 2.03 and the equivalent modulation components RGB and the analysis shows that correct colour reproduction (in so far as first order effects are concerned) is obtained when employing a picture tube of the type described by modulating the beam with a brightness signal M=0.33G+0.33R+0.33B together with the normal sub-carrier component and additionally modulating the beam with a (modulated) second harmonic of the sub-carrier the phase relationship between the colour switching wave (which has the same frequency as the subcarrier fundamental) and the fundamental and second harmonic modulated sub-carriers being such that the sine component of the switching wave is in phase with the (R-B) axis (Fig. 1a) of the NTSC signal and the cosine component of the switching wave in phase with the (G-0À5R-0À5B) axis of this signal. Additionally, the gains of the channels supplying these signals are arranged to preserve the relative intensities of the chrominance and luminance components in the original (NYSC) signal. In all the described embodiments the modifications of the brightness signal Y to produce the desired form M is effected by deriving a signal M-Y and combining this with the Y signal. Fig. 1 (and Fig. 1b which shows circuit details of some of the "blocks" of Fig. 1), show one embodiment in which the M-Y signal is derived by the synchronous detection at a phase angle of +19 degrees (in unit 50) of the colour sub-carrier wave supplied from video amplifier 28 and filter 43 the homodyning wave being supplied at the mean frequency of the subcarrier (i.e. 3.6 Mc/s) from amplifier 35 (which also supplies the colour switching signals to the beam colour switching grid 33 of the tube 30) via a phasing circuit 49 which introduces the 19 degrees phase variation. The low frequency components of the M-Y signal are then selected by filter 51 and added to the luminance (Y) channel in filter 41 so that the output of the D.C. restorer 42, which supplies the control electrode of the picture tube 30, comprises the desired M signal together with the (NTSC) colour sub-carrier wave. For correct operation the colour switching signals must be in phase with the R-B axis (Fig. 1m) of the latter subcarrier wave and this condition may be realized either by conventional phase control of amplifier 35 or of generator 21. As described above, the tube beam is also modulated by the second harmonic of the colour sub-carrier and this is produced in a modulator 44 to which the subcarrier from filter 43 together with the third harmonic of the output of amplifier 35 is supplied, the output of 44, comprising a 7.2 Mc/s colour modulated wave, being supplied to the cathode of tube 30 via filter 45. If desired the screen grid electrode of the picture tube may be supplied with a gating signal comprising the sixth-harmonic (derived via 46 and 47) of the sub-carrier fundamental frequency to produce a 60 degree gating of the beam but if gating is not employed no colour errors occur (considering first order effects) since the two successive red and successive blue areas (produced by 60 degrees gating) then merge into areas each effectively 120 degrees, whilst the green areas become two 60 degree areas. When gating is employed the relative intensities of the chrominance and luminance components is maintained by proportioning the amplifier 28 (or filter 41) to produce a relative gain of 1.35 for the sub-carrier and its side bands whilst providing a gain of unity for the luminance signal and by proportioning the channel including the elements 44, 45 to provide a relative gain of 1.2. When gating is not employed these gains (i.e. 1.35 and 1.2) are changed to 1.41 and 1.44 respectively for correct colour production. In a second embodiment (Fig. 2), second order distortions caused by the derivation of colour from components in quadrature with the (R-B) and (G-0.5R-0.5B) axis (due to the effect of the gamma of the picture tube and other non-linear characteristics in the circuitry) are prevented by supplying the subcarrier output from filter 243, equivalent to filter 43 (Fig. 1), to two channels the first of which translates the 3.6 Mc/s (i.e. the fundamental) modulated sub-carrier to the picture tube is a transformer 116 and the second of which supplies modulator 44 and each of which is arranged to provide a short path to earth (specifically by means of switched diodes 115 115 and 109 respectively) during occurrence of sub-carrier components which are in quadrature with the desired components, i.e. the first path shunts (via diode 115) all components in quadrature with the (R-B) axis and the second path shunts (via diode 109) all components in quadrature with the (G-0.5R-0.5B) axis. In this embodiment the filter 241, equivalent to filter 41 (Fig. 1), cuts off at 3 Mc/s to prevent direct application of the sub-carrier to the picture tube and the output of this filter, after connection by the addition of the M-Y signal (as described in connection with Fig. 1) is supplied to the picture tube via an amplifier 102 to which is also fed a 3.6 Mc/s signal, this latter signal serving to balance the light quantity emitted by the different phosphors by increasing the energy of the beam when it impinges on the less efficient phosphor, e.g. the red phosphor. In a further embodiment (Fig. 3, not shown) the two switched diode shunt paths for selecting the (R-B) and (G-0.5R-0.5B) axes as described in connection with Fig. 2 are replaced by a single triode valve and the colour balancing 3.6 Mc/s signal fed to the picture tube directly from amplifier 35. Fig. 4 shows an embodiment in which the I and Q components of the N.T.S.C. sub-carrier wave signal are derived by means of samplers 154 and 158 respectively, the axes of both components being arranged to be in phase with the 3.6 Mc/s signal supplied from amplifier 35 to the samplers via transformer 155. The I signals are supplied to a circuit 162 which develops a signal +1.1I and the Q signals are supplied to the series arrangement of a circuit 163, which develops a third harmonic (10.8 mcls) signal effectively modulated by a - 1.2Q signal, a circuit 160, which developes a second harmonic (7.2 mcls) signal effectively modulated by a +0.77Q signal and a circuit 161, which develops a low frequency 0.4Q signal. The latter signal (i.e. 0.4Q) is shown to represent the M-Y correction signal and this signal combines with the signals developed in circuits 160, 162 and 163 to provide the colour modulation signals which are supplied to the picture tube via amplifier 164. In this embodiment compensation for the relative inefficiency of the red phosphor in the picture tube screen is effected by applying a bias potential, by means of potentiometer 167, to the colour switching electrodes 33 to cause the undeflected position of the beam to be shifted from the green phosphors towards the adjacent red phosphors. Since this results in the times during which the beam infringes on the green phosphors being no longer 180 degrees apart the modified colour signals in the embodiments of Figs. 1, 2 and 3 are not suitable for use with this form of colour balance compensation. In a further embodiment (Fig. 5, not shown) the colour switching frequency is 1.8 Mc/s and the picture tube is fed with a modulated 1.8 mc/s colour subcarrier and with the corrected luminance (M) signal together with the normal 3.6 mc/s colour sub-carrier in the luminance channel. In this case the 1.8 Mc/s wave is equivalent to the normal colour sub-carrier of the other embodiments and the normal 3.6 Me/s wave equivalent to the second harmonic colour sub-carrier wave of these embodiment. Colour balance compensation is effected by supplying the picture tube with a seventh harmonic (12.6 Mc/s) signal which causes the beam to impinge three times on the red phosphors and only twice on the blue and green phosphors for each 1.8 Mc/s cycle. With this form of control colour balance compensation the modified N.T.S.C. sub-carrier wave described in connection with the embodiment of Fig. 4 will produce improved colour fidelity. Specifications 772,791, 784,056 and 784,057 are referred to.