GB635712A - Improvements in or relating to television and like pick-up or camera cathode ray tubes - Google Patents

Abstract

635,712. Television. RADIO CORPORATION OF AMERICA. March 7, 1947, No. 6495. Convention date, March 8, 1946. [Class 40 (iii)] In a television system employing a pick-up tube of the type in which the target electrode is scanned by a low velocity cathode-ray beam, the return video modulated beam being intensified by an electron multiplier arrangement, output video signals are derived from the target electrode and from the electron multiplier and the two groups of output signals are fed to a combining circuit through low-pass and highpass filters respectively to produce a corrected composite video signal. The invention has for its object to eliminate undesirable variations in the video signal series generated by picture signal generating tubes of the " orthicon " type which employ electron multiplier arrangements integral with the tube, these undesirable variations resulting from movement of the point of impact of the return beam on the first electrode of the multiplier so that the current into the multiplier varies as the electron beam impacts points of different emission response. These variations are stated to occur at a frequency of about 100-300 kc/s. video signals above this frequency range being free from this effect. In accordance with the invention, therefore, output video signals from the final stage 48 of an electron multiplier incorporated in an " orthicon " type picture signal generating tube 10 are fed through a H.P. filter 60, cutting off at about 200 kc/s., and combined with output video signals, derived from the signal plate 14 of the target electrode (in which signals the undesired variations do not occur), fed through a L.P. filter 62 also cutting off at about 200 kc/s., an amplifier 64 being provided in the latter circuit to simulate the gain of the multiplier in the former channel, so that the combined signals comprise a composite video signal series which is free from the undesired effects. During blanking intervals, a positive pulse 57 is applied to both the cathode and control grid of tube 10 to reduce the velocity of the scanning beam so that no electrons reach the target whilst at the same time maintaining constant the current density of the beam to prevent undesirable fluctuations in the video signal output.