Patent GB-538006-A (GB)
Title: Improvements in or relating to electrical communication systems
IPC: H04M11/06
Filed: 1940-05-07
Abstract: 538,006. Code telegraphy ; impedance networks. ELECTRICAL RESEARCH PRODUCTS, Inc. May 7, 1940, No. 8213. Convention date, July 8, 1939. [Classes 40 (iii) and 40 (v)] [Also in Group XXXIX] In a telegraph gain-regulating amplifier-detector (" telegrad ") for use with signals subject to fading or the like, the gain is at first increased very rapidly on the receipt of a signal impulse, and the gain-increaser is then disabled. The pulses of alternating current which constitute the signal impulses are caused to be of a standard amplitude by the joint action of a gaindecreaser (vario-losser) and a variable-mu amplifier. The gain-increase-disabler is operated by the pulses after they have been rectified, and the resulting D.C. pulses are cut to a uniform length before being passed to a telegraph distributer. The device resembles a vogad. The telegrad forms part of a two-way repeater system (Fig. 1, not shown) which bears a broad resemblance to that described in Specification 531,394. Two-way speech signals pass between a telephone line and a pair of aerials, one for sending and one for receiving, through circuits which provide for voicecontrolled disablement, whenever outgoing speech occurs, of telegraphic circuits associated with the same two aerials. Incoming voice-frequency telegraph impulses a, Fig. 3, pass from the radio receiver to the receiving distributer through a band-pass filter 31 and the telegrad, Fig. 2. In the latter, the signals have their amplitude corrected by the vario-repeater 55, 52 and acquire a form b, Fig. 3, with an enlarged head d; they are converted at 68 into D.C. pulses whose length is corrected by relays 70, 71, 74, and pass to the receiving distributer through lead 75. As the plate-current from the variable-mu amplifier 52 increases, voltage is applied through lead 89 to a vario-losser comprising a rectifier bridge 55, so as to decrease the signal strength. The amplification effected by the pentodes 52 is controlled by the charge on the condenser 61, which is normally at mid gain, and the vario-repeater 55, 52 makes all signals to have the same amplitude. The signal impulses a pass through pentodes 77, 78 and crossed-coil device 79 to the cold-cathode gas-filled tubes 81, 83, which break down and convey positive potential to the condenser 61, suddenly increasing the gain, so that each impulse acquires an enlarged head d, Fig. 3. Immediately afterwards, however, the gain is decreased in two stages. The relays 70, 71, 74, which are normally held in the positions shown in response to plate current from the pentode 69, reverse their contacts when an impulse (mark) is received, in response to the action of rectifier 68 through pentode 69. The gain-increaser 81, 83 is thus disabled in two steps, the voltage concerned being too great for disablement by the breaking of a single contact. First the contact 70a by breaking circuit shuts the signal path through the crossed-coil device 79 and thus cuts off the activating (signal) voltage from tube 81 ; then on reaching its back contact it removes the enabling voitage from the tube. In order that the initial rapid increase of gain may be stopped quickly, a gain-decreasing device comes into operation shortly after the gain-increase. It comprises a coldcathode gas-filled tube 86 which, on the arrival of a signal, admits negative voltage to the condenser 61, but ceases to function as soon as mid gain is reached. The head d is cut off the final impulse c by delay in the action of relay 74 which, when relay 71 reverses its contact, is held on for a short time while a current is charging condenser 73. The impulse c, Fig. 3, applied through lead 75 to the telegraph receiving distributer, is therefore cut to the right length, though slightly delayed. Impedance networks. A vario-losser for variably attenuating voice-frequency signals comprises a bridge 55, Fig. 2, of copper-oxide rectifiers with a diagonal ohmic resistance 59. The shunting effect of the bridge is controlled by a D.C. voltage applied to the other diagonal from the resultant voltage drop across the two resistances 60, 87. Specifications 324,639, [Class 40 (iv)], and 367,616 also are referred to.