Patent GB-779579-A (GB)
Title: Improvements in and relating to cathode ray tube systems
IPC: H04N9/24
Filed: 1953-08-12
Abstract: 779,579. Automatic control systems for cathode-ray tubes. MORLE, C. W. (Philco Corporation). Aug. 12, 1953, No. 22234/53. Class 38 (4). [Also in Groups XL (a), XL (b) and XL (c)] Dot-sequential colour television signals are displayed by a cathoderay tube the screen of which is made up by a regularly repeating sequence of red, green and blue phosphor stripes, such as 26, 27 and 28 in Fig. 3, disposed in the direction of line scan. The lines of the raster are normally positioned on the green stripes 26 and an auxiliary deflection 37 at the dot cycle frequency is added in the vertical direction to cause the beam to scan the adjacent red and blue stripes, whereby the beam impinges the three colour phosphors in synchronism with the modulation of the beam by the colour signals. Indexing means associated with red stripes produce pulses upon the passage of the beam across the stripes and any displacement upwards or downwards of the line scan as indicated at 38 or 39 due to non-linearity in the vertical scanning is detected as a change in the phase of such pulses and corrected by a supplementary deflection. The circuit for carrying out the invention is illustrated in Fig. 1, in which the colour component of the signals from a source 10 are applied through amplifier 11 to modulate the grid 12 of the cathode-ray tube whilst the synchronizing component controls the horizontal and vertical deflection generator 20, 21. The colour synchronizing bursts pedastalled on the back porch are selected by circuit 22 and applied to synchronize a sine-wave oscillator 23 from which the auxiliary deflection wave is derived by a circuit 24 and applied to coils 24a. The auxiliary wave may be in the form of a saw-tooth as shown in Fig. 3 or a sine-wave. Blanking signals derived from this wave may be employed to suppress the cathode-ray beam and eliminate the output of light from the red phosphor and the production of an indexing pulse during the downward sweep of the wave. As illustrated in Fig. 1, the indexing means takes the form of stripes 30a over the red phosphor stripes and formed of a material, such as gold, having a secondary emission coefficient detectably different from that of an aluminium backing layer disposed over the phosphors. Other indexing means are referred to, such as phosphor stripes with a secondary emission coefficient different from the others and a photocell responsive to light of one colour only. The pulses due to the passage of the beam across the indexing stripes are derived from across resistor 31 and applied to synchronize an oscillator 34. The output from this oscillator is then compared with the output from oscillator 23 in a phase comparator 35 to derive a D.C. potential for application to correction coil 36. When the line scan is correctly positioned on the green phosphor, such that the auxiliary deflection takes place as indicated at 37 in Fig. 3, the output of the phase comparator is arranged to be zero. Any departure upwards or downwards then results in an output of appropriate polarity to restore the beam to its correct position. Electrostatic deflection may replace the electromagnetic deflection shown in Fig. 1, D.C. amplification may be provided between phase-comparator 35 and coil 36, and oscillator 23 need not be synchronized from the colour bursts where the colour repetition is substantially uniform.