Patent GB-787050-A (GB)
Title: Improvements in or relating to scanning circuit
Inventor: LYNCH WILLIAM MINOS
IPC: H04N3/233
Filed: 1956-03-03
Abstract: 787,050. Cathode-ray tube circuits. TECHNICOLOR MOTION PICTURE CORPORATION. March 3, 1956, No. 6766/56. Class 40 (7). Relates to a circuit adapted to correct for keystone distortion in a cathode-ray tube scanned by an off-axis electron gun, the invention being described as applied to an arrangement as shown in Fig. 1, where the screen of a colour television cathode-ray tube is scanned by off-axis green, red and blue guns disposed at 120-degree intervals around the tube axis. The scanning coils or plates for each gun are orientated to scan in transverse and longitudinal directions T and L as indicated, the resulting rasters appearing as shown in Figs. 2, 3 and 4. The line deflection is applied in the transverse direction T for the green gun, but in the longitudinal direction L for the red and blue guns. To produce correct registration of the three rasters, it is necessary first to correct for the keystone distortion, then to reorient electrically the red and blue rasters through 30 degrees, and finally to correct for non-linearity (see, for example, the line spacing in the green raster and the frame linearity in the red and blue raster) due to off-axis scanning. Keystone distortion correction. In the case of the green raster, the keystone distortion is corrected in a conventional manner by modulating the line-scan wave by the frame-scan wave to produce a resultant wave as shown in Fig. 5. In the case of the red and blue rasters, however, a converse operation is employed, the frame-scan wave being modulated by the linescan wave as shown in Figs. 6a and 6b for the red and blue guns respectively. The waves shown in Figs. 5, 6a and 6b each comprise a wave as shown in Fig. 7, corresponding to the side-band components produced by modulating the line-scan by the frame-scan, together with the appropriate scan wave. In accordance with the invention, a keystone distortion correction wave is generated as shown in Fig. 7 and then employed for each of the guns by appropriate combination with the line- or frame-scan wave. The wave is generated by a circuit as shown in Fig. 14, the line- and frame-scan waves being combined linearly at point A by cathode followers V1, V2 and applied to a non-linear stage V3 acting as a modulator to produce a composite wave comprising the original line- and frame-scan waves together with desired wave (Fig. 7) in the form of the modulation side-band products. These products are then extracted by reversing the phase of the composite wave in stage V4 and combining it at point B with the original line- and frame-scan waves so that they are cancelled out. To achieve accurate cancellation, it is necessary first to distort the line- and frame-scan waves in stages V7 and V6 similar to stage V3. Reorientation of rasters. This is effected for the red and blue guns by applying an appropriate proportion of the scanning wave for each direction to the scanning wave for the other direction. Thus the transverse deflection comprises the frame-scan wave and keystone distortion correction wave together with a sufficient proportion of the line-scan wave to achieve the desired 30-degree shift in frame-scan direction. Similarly the longitudinal deflection comprises the line-scan wave together with a proportion of the frame-scan plus keystone correction waves. Fig. 10 (not shown) illustrates in schematic form a suitable circuit for effecting this operation. Since both high- and low-frequency (line and frame) signals are employed for each deflection, it is necessary, where electromagnetic deflection is employed, to form the scanning yokes for each direction with two coils constructed appropriately for the frequencies involved. Scanning linearity. To produce equal line spacing in the green raster, the frame scan is distorted from a linear to an exponential curve by passage of the wave through a resistorcapacitor network. No change is made to the line scan. In the case of the red and blue guns, distortion is necessary in both line and frame scans, the line distortion for the blue gun being of opposite form to that for the red gun. Figs. 11 and 12 (not shown) illustrate suitable resistorcapacitor networks.