Patent ID: 6268705
Filing Date: 2001-07-31
Classification: G09G,H01J,H04N

Abstract:
Apparatus for the demodulation of a raster for a display having a cathode ray tube, the cathode ray tube having a neck and a raster, the raster having scan lines, the apparatus comprising control means for varying the shape of at least one scanned electron beam (661-663) in a direction perpendicular to scan lines of the raster; characterised in that the control means comprises an electro-magnet (700) for mounting on the neck of the cathode ray tube, the magnet (700) having a eight pole pieces (720-790) spaced around the neck of the cathode ray tube (210) and a winding (710) passing around each of the pole pieces (720-790) in succession in opposite directions, the pole pieces being divided into a first group of four pole pieces comprising two pairs of pole pieces facing each other in a direction parallel to the lines of the raster and a second group of four pole pieces comprising two pairs of pole pieces facing each other in a direction perpendicular to the lines of the raster, the first group each having a greater number of turns of the winding than the second group so that, when an electrical current (I) is passed through the winding, adjacent pole pieces (720-790) around the neck of the cathode ray tube have different magnetic polarity to generate, within the neck of the cathode ray tube (210), a magnetic field in which magnetic lines of force (680) pass through the at least one electron beam (661-663) only in a direction parallel to the scan lines of the raster.