Patent GB-643271-A (GB)
Title: Improvements in or relating to television systems
IPC: H04N3/32
Filed: 1946-05-23
Abstract: 643,271. Television. BELLECOMBE, F. M. L. G. DE. May 23, 1946, Nos. 15616 and 15617. Convention dates, March 26. 1945, and Nov. 12, 1945. [Class 40 (iii)] [Also in Group XL (a)] In a television system uniformly illuminated portions of the image are scanned at a higher speed than complexly illuminated portions. In one arrangement according to the invention the image to be transmitted is focused on to the mosaic screens of two iconoscopes 1, 2 (Fig. 2), the screen 4 of iconoscope 2 being provided (as shown in Fig. 3) with a signal plate in the form. of a conducting grating comprised of two groups of interdigitated bars 6 and 7 disposed at right angles to the directions of line scanning, the bars of each group being connected in parallel and the spacing between the bars being equal to the spacing between the scanning lines. The scanning spot of iconoscope 2 is of elongated shape and simultaneously covers mosaic elements lying in two successive lines so that a signal is generated in each group of bars for each position of the spot, these signals being equal if the image portion is uniformly illuminated and unequal if it is complexly illuminated. In the latter condition the difference of signal potential is arranged, by means of the circuit comprising valves 11 ... 14, to unblock oscillators 19-21, the output of which is rectified in valve 23 and fed to the transmitter 3 in which it operates to reduce the speed of operation of the scanning generators, which control deflections of iconoscopes 1 and 2, to a predetermined lower value. To compensate for the time lag between the generation of the speed change signals and the actual change of scanning speed the images on iconoscopes 1 and 2 are staggered by a suitable amount. The mosaic electrode of iconoscope 1 comprises two portions 26, 27 (Figs. 2 and 5), both backed by a conventional type signal plate, the former operating in known manner to supply vision signals to the transmitter 3 and the latter which is continuously illuminated by a lamp 28, serving to generate, at the end of each frame scan, a signal which unblocks oscillators 32, 33, the rectified output from which is employed to stop the operation of the frame deflection generator when motion picture film is transmitted, a further signal generated in synchronism with the apparatus projecting the film on to the iconoscopes serving to initiate the frame fly-back. Time base circuits suitable for controlling the variable-speed line and frame deflection generators are described with reference to Fig. 6 (not shown). In a modified arrangement (Figs. 8, 9, 10) a single iconoscope 101 is employed to generate the image signals and an electronic device comprising valves 103, 104 switched by a source 110 serves to supply signals (equivalent to the signals derived from the bars 6, 7 of Fig. 3) to a circuit arrangement comprising valves 111 ... 114 (equivalent to valves 11 ... 14 of Fig. 2) which supply speed change signals to a device C which operates to alter the scanning speeds of the line and frame deflection generators D and E respectively and also to produce synchronizing and speed-change signals for transmission by the transmitter L. With this arrangement compensation for the time-lag between the generation of the speed-change signals and the actual change of scanning speed is effected by means of a device comprising two electronic tubes 116, 117 of special construction, the former being provided with a curved end wall formed as a 45 degree prism 219, the inner face of which is coated with a fluorescent material and the latter containing a mosaic screen 121 inclined at 45 degrees and located under the prism 219 such that a luminous line 120 produced by the cathode 118 on the prism and modulated in intensity by signals from iconoscope 101 forms a luminous image 122 on the screen 121, the disposition of the two valves being arranged so that the line 123, formed on screen 121 by cathode 119, follows the line 122 at a predetermined distance when the electron beams of both tubes are deflected synchronously with the line deflection of iconoscope 101. In this manner image signals are generated by the mosaic electrode 121 which are delayed relatively to the generation of the speed-change signals and these image signals are fed to the transmitter I. To prevent distortion resulting from after-glow effects in the fluorescent material on the face of prism 219 the cathode of tube 116 is divided into two portions acted on by two grids 127, 128 which are switched alternately by means of a multivibrator circuit including valves 129, 130 and set to operate at the minimum line frequency so that the uppermost and lowermost portion of the face are alternately and successively scanned. A similar arrangement may be employed. with the tube 117 or the electron trace 123 may be of such a length as to extend over the limits corresponding to the two positions of the reflected luminous image 122 on the mosaic screen 121, a further cathode 135 being arranged to flood the screen between each scanning to equalize the potential of the mosaic cells. For transmitting motion picture film the frame fly-back is initiated by means operating in synchronism with the film driving apparatus. When transmitting other than film the frame fly-back initiation occurs at instants which are determined by the normal predetermined frame scanning period, when the actual time of scanning is equal to or exceeds the predetermined frame period, or by means of an auxiliary device operated by the scanning cathode-ray beam, when the time of scanning is shorter than the predetermined frame scanning period. An example of a suitable auxiliary device for this purpose is described with reference to Fig. 10 (not shown). To ensure that all portions of the image are scanned when the actual time of scanning tends to exceed the predetermined frame period, the latter period is made at least equal to the time necessary for scanning the most complex image. . Receiver.-A receiver suitable for reproducing images transmitted by either of the systems described above comprises a cathode-ray picture reproducing tube 151 (Fig. 11) which reproduces the image under the control of signals fed to grid 162 and an auxiliary cathoderay tube 154 provided with three conducting plates 158, 159, 161 (Fig. 12), the electron beam of the latter tube being deflected horizontally in synchronism with the horizontal deflection of the beam in tube 151 and vertically in accordance with the intensity of the image signals fed to grid 162. With this arrangement a signal is developed by means of plate 159 when a change-speed signal, which produces a drop in signal strength of short duration, is received, a further signal being developed by means of plate 161 when a similar signal is received at the end of a line scan or when a signal of longer duration is received at the end of a frame period. The signal developed at plate 159 is employed to control the scanning speed-change device 164 and the frame fly-back device 165 by means of a discriminator device 163 which distinguishes between the longer pause synchronizing signals and the shorter speed-change signals, whilst the signal developed at plate 161 controls through a device 166, the line fly-back in time base 160.