Lichtpunktabtaster mit einer Kathodenstrahlroehre

1,053,900. Television; automatic gain control; valve supply arrangements. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 7, 1964 [Dec. 31, 1963], No. 49619/64. Headings H3Q, H3W and H4F. Undesired variations imparted to the peak amplitude of the video signal produced by a photo-multiplier tube 25 viewing a document 22 scanned by a cathode-ray tube flying-spot scanner 10 are reduced by negative feedback to the photo-multiplier power supply 32 utilizing a first channel comprising a peak detector 28 and amplifier 29 responsive to D.C. and lowfrequency variations and a second channel comprising a peak detector 30 and amplifier 31 responsive to mid-range frequency variations. The output from amplifier 29 provides the input for a D.C./A.C. inverter supplying rectifier circuits for the high current dynodes of the tube, whilst the output from amplifier 31 is applied in series with a potentiometer network supplying the low current dynodes, Fig. 2 (not shown). Additional negative feedback may be applied to the grid of cathode-ray tube 10 by closing switch 33. A further photo-multiplier tube 38 with a negative feedback system 39, 41 controlling its power supply 42 responds to light obtained directly from the cathode-ray tube screen to produce a signal which is subtracted in amplifier 46 from the signal produced by photo-multiplier tube 25 for the purpose of removing high-frequency variations imparted by fine irregularities in the tube phosphor. In the example of the invention described, amplifiers 26, 29 and 31 have pass-bands extending from D.C. to 2 Mc/s., whilst amplifiers 31 and 41 have pass-bands extending from 10 to 5000 c/s. Reference is made to the application of the invention to television and character recognition systems.