Patent GB-1015206-A (GB)
Title: Methods and arrangement for the operation of pick-up tubes, preferably of the type of image-orthicon
IPC: H04N5/18
Filed: 1963-09-27
Abstract: 1,015,206. Television camera tube systems. FERNSEH G.m.b.H. Sept. 27, 1963 [Sept. 29, 1962; Nov. 3, 1962], No. 38144/63. Heading H4F. In an arrangement for operating a television pick-up tube of the image-orthicon type, wherein the storage target is scanned by a beam of slow electrons and the picture signal is derived from the beam returning from the storage electrode, the tube is controlled during the blanking intervals to produce a first reference signal representative of "black," but which may be influenced by noise, and a second reference signal which is free from noise. The first reference signal is produced by biasing the target negatively so that the whole of the tube beam is returned to the collector electrode and electron multiplier. This signal is then used for establishing the " black " level of the video waveform with the aid of a clamp circuit which is given a sufficiently great time-constant that the noise fluctuations are integrated over many signals and a correct invariable black level is established. The second reference signal is produced by interrupting the scanning beam or keying the photo-multiplier so as to obtain an output corresponding to zero current and thus free from noise. Alternatively the beam current may be increased above normal so as to produce a signal in the blacker-than-black direction and this may be clipped to produce a noise-free pedestal. According to another alternative the beam may be established at its maximum " black " level by biasing the target and a pulse added in the blacker-than-black direction which is similarly clipped. The second reference signal produced as above falls outside the normal black-white signal range. If this is undesirable by reason of the design of signal transmission circuits, the reference signal may be brought within the black-white range by a counter pulse (see below). The second reference signal is employed for establishing the video waveform level with the aid of a clamp circuit which is given a smaller timeconstant so that it responds to eliminate low frequency interfering components, e.g. hum, which becomes superposed on the signal. In a first arrangement according to the invention, Fig. 2 (not shown), the first and second reference signals are produced in alternate line blanking intervals. Line synchronizing pulses which are sent from the camera control unit to control the camera scanning are also applied to bias the target and. additional pulses at half line frequency are also sent from the control unit to block the tube beam. The additional pulses are sent as part of the video signal which is returned from the control unit to the camera for the purpose of operating an electronic viewfinder. In a modification a line frequency train of pulses of alternating polarity is sent and is separated into two interlaced half line frequency pulse trains by polarity discriminating circuits. The video signal produced in this first arrangement is as shown in Fig. 5, 2 and 4 denoting the first reference (black) signals with noise and 1 and 3 denoting the second reference signals free from noise. In a second arrangement, Fig. 6, the two reference signals are produced one after the other in each line blanking interval (curve c) under the control of time-displaced line frequency pulses (curves a-b). In a third arrangement, Fig. 7 (not shown), the first (black) reference signal is produced in the field blanking interval and the second reference signal in the line blanking interval. The signal then takes the form shown in Fig. 8 (curve c). In connection with this arrangement there is described a circuit, Figs. 9 and 10, whereby the second reference signal may be brought back to "black" " level by a counter pulse. The camera, signal, curve a, Fig. 10, is applied to a valve 103 and has the " black " level 5 restored to earth potential by a clamp circuit 141, 142, which is rendered operative during field blanking intervals by pulses applied at 147, 148. The coupling capacitor 102 to the clamp circuit has a large valve, e.g. 0À1ÁF. Valve 103 has an anode load 105 in common with a valve 107 which is pulsed during the line blanking intervals so that the combined anode potential due to the video signal and pulse is as shown in curve b, Fig. 10, but with reversed polarity. The signal is now clipped by a diode 103 at the level a to produce the signal shown in curve c, Fig. 10, which is combined with the original video at the cathode of valve 120. By making the amplitude Z-Z<SP>1</SP> of the additional pulse equal to the clipping potential a-z the final signal takes the form shown in curve d, Fig. 10, with the second reference signal brought down to " black " level. Should the level Z change, e.g. due to change of the image-orthicon beam current, the clipped signal, curve c, Fig. 10, changes correspondingly and the level of the second reference signal remains unchanged at " black " By appropriate choice of the amplitude of the additional pulse, the reference signal may be brought to other levels, e.g. " white " level. The signal from valve 120 is applied to an output valve 124 and has its level restored during the second reference signal by a second clamp circuit 191, 192, which restores to earth potential and is rendered operative by pulses occurring during line blanking which are applied at terminals 197, 19. The coupling capacitor 123 has a small value, e.g. 1000 pF.