Patent GB-318160-A (GB)
Title: Improvements in television and like systems
IPC: H04N5/16
Filed: 1928-04-26
Abstract: 318,160. Beatty, W. E., (Bell Telephone Laboratories, Inc.). April 26, 1928. Television.-Relates to a television system in which the direct and very low-frequency components of the picture current are first suppressed by condensers and then reintroduced by manual adjustment, so that accumulation of blocking charges on the amplifier grids may be prevented. Means are provided for selecting suitable operating ranges on the characteristics of the valves and neon tubes, the selection being such as (1) to compensate when necessary for the differential colour-sensitivity of the photo-electric cell at the transmitter, by the use of the curved part of the characteristic of one of the valves, and (2) to ensure the full use where necessary of the straight part of the valve characteristics by increasing or decreasing the d.c. component of the required picture current when the average tone of the picture brightens or darkens. The d.c. component of the picture current may be reinserted either at the receiver (Fig. 1) or at the modulation stage of the transmitter (Fig. 4). Means may be provided for indicating whether the right adjustments have been made. D.C. components reinserted at receiver. In the case where the d.c. and low-frequency components of the picture current are suppressed at the transmitter and where any further such components introduced at A-17 and A-25, Fig. 1, are eliminated by the condenser 85, the desired component is reintroduced by means of a tapping from the battery Vs which can be varied by hand. 'The gain and grid voltage of amplifier 26 having been adjusted by means of tapping 91, 90 so that full use is made of the straight part of the valve characteristic, the tapping 180 is adjusted until detail is equally clear in the high lights and shadows of the reproduced picture, so that the straight part of the neon lamp characteristic is being employed. D.C. component reinserted at transmitter. In the form of transmitter shown in Fig. 4 a modulator M-98 receives carrier oscillations from an oscillator CO-97 and picture signals from a circuit 102 which is tapped off the plate battery 95 and resistance 96 of an amplifier A-93, the grid of which takes the signals from a photo-cell 12, through an amplifier A-13 and an intervalve coupling wherein the low-frequency components are suppressed by a condenser 40. The gain is adjusted by means of the tapping 90, and the grid bias is adjusted by means of the tapping 91. These adjustments are normally so made as to ensure the use of the straight part of the characteristic 80, Fig. 4b, of the valve 26; and the tapping 91 is further varied in such a way as to restore the d.c. or low frequency components of the picture currents, corresponding to the average illumination. Alternatively this latter result may be achieved by adjustment of the plate-current tapping 98 of the modulator. Similarly the tapping 95 is adjusted to ensure the use of the straight part of the characteristic of the modulator M-98. Indicating operating-range of valve. A further amplifier 112 connected in shunt with the amplifier A-93 feeds picture current to neon lamps A - - D, which indicate whether the gain and grid bias settings 90, 91 are correct. The batteries 115, 116 feed the lamps C, D in opposition to the potential drop in the plate-circuit resistance 113, while the batteries 117, 118 feed the lamps B, A in support of that drop. It is shown in the Specification that if the resistances and voltages be suitably chosen in relation to the critical voltages of the lamps, then so long as the voltage applied to the valve 26 is varying over the desired range the lamps A, D remain alight: should one of these go out, or should one of the lamps B, C light up, the parts 90, 91 are adjusted by hand accordingly. If high lights and deep shades are naturally absent from the object they may be introduced to ensure the occurrence of peak currents to operate the indicator. Compensation for differential colour-sensitivitv of photo-electric cell. The photo-electric cell is less sensitive to red than to green light. If, therefore, the relation between the current in the photo-electric cell and the illumination at the receiver be linear, a red face is reproduced with insufficient relative brightness as compared with an olive-coloured face. Means are, however, provided whereby the above relation can be made non-linear in such a way as to brighten the duller parts of the picture, the relation being changed at the discretion of the operator. To this end (1) the operating point of the valve 26 is moved to the lower bend of the characteristic by adjustment of the grid bias contact 91: (2) the variations of input picture-current applied to the valve 26 are reversed in sign so as to represent a negative of the picture, either by reversal of the connections of one of the transformers in the line, or by an odd-number alteration in the number of stages of amplification: (3) the variations of outgoing picture-current from the valve 26 are reversed in sign, so as to yield once more a positive picture, the voltage Vs applied to the neon lamp being simultaneously raised, so that the picture voltage is subtracted from the voltage Vs instead of being added to it. The switches 88, 89 are raised accordingly. It is explained in the Specification with reference to Fig. 5 (not, shown) that this arrangement produces a non-linear relation between the incoming picture current and the reproduced image, such that the weaker tones are brightened in reproduction. An alternative method of reversing the sign of the picture-voltage variations consist in substituting for the apparatus to the right of the line XX, Fig. 1, a resistance coupled neon lamp in series with a battery (Figs. 6, 7, not shown) or tapped off the anode battery (Fig. 8, not shown). Neon lamp-circuits. The picture voltage, less the d.c, and very low-frequency components, may be passed to the neon lamp 30, Fig. 1, through a condenser 85, while chokes 86, 87 prevent it from being short-circuited by the anode battery VB of the valve and priming battery Vs of the neon lamp. As is explained above, the picture voltage may aid or oppose the priming voltage, and in alternative arrangements (Figs. 6, 7, 8, not shown) the lamp may be resistance-coupled. In a further modification, Fig. 9, the chokes 86, 87 are replaced by current-limiting devices 86<1>, 87<1>, comprising valves connected as diodes and operating at saturation current. The circuit shown in Fig. 9 is substituted for the circuit to the right of the line XX, Fig. 1, and serves to prevent undesired oscillation. It corresponds to the arrangement shown in Fig. 1 with switches 88, 89 down: the equivalent of the same arrangement with switches up (so that picture-voltage variations are reversed in sign) is obtained (Fig. 10, not shown) by reversing the parts Vs, 86<1>, 30 in Fig. 9 individually.