Patent GB-547218-A (GB)
Title: Improvements relating to telephone and like systems
IPC: H04Q1/446
Filed: 1940-11-13
Abstract: 547,218. Voice-immunity circuits. TELEPHONE MANUFACTURING CO., Ltd., PADDLE, L. H., and DRAKE, B. Nov. 13, 1940, No. 16435. [Class 40 (iv)] [Also in Group XL] In an electric signalling system using a basic frequency modulated by a lower frequency and having means for discriminating between signal currents and fortuitously occurring currents of the same frequency, means are provided to increase the depth of modulation of received signals. The invention is described in connection with a telephone system for discrimination between a signal of 500 c.p.s. interrupted at 20 c.p.s. and speech which is subject to smaller amplitude variation than the signal. In the case of a signal originally interrupted, the gaps become partially filled with mush during transmission. The relative depth of modulation may be increased simply by passing the signals, desired and otherwise, through a non-linear impedance or by reducing the level of a carrier or basic frequency relatively to the side bands, or by the arrangement shown in Fig. 2, which is almost identical with a corresponding Figure in Specification 541,731. In this Figure, in which only signal currents are required to operate relay RL1, the modulated 500 c.p.s. signal is applied to the control grid of valve V, an output being taken from the screen grid by transformer T2 tuned to 500 c.p.s. and rectified by bridge D1, the 20 c.p.s. output across load resistance R2 being applied to the control grid in phase with the original signal as in the Specification referred to. The D.C. component across R2 may be removed. The valve in the present case is arranged by biasing over resistance R1 as a class A/B, B or C amplifier to operate non-linearly so that the modulation peaks are amplified more than the troughs. In the arrangement shown in Fig. 3 which responds to a larger range of input levels, increase of modulation of low level signals is effected by a non-linear impedance D3. In the case of signals at a higher level, the impedance D3 which is a metal rectifier would operate to cut off say the positive peaks but it is coupled to valve V by filter C3, R7 so that modulation on the positive. side is not entirely removed. However, since rectifier D3 increases modulation independently of frequency in the case of low levels and reduces modulation on high levels, additional protection against false operation is necessary. For this purpose, the feed back circuit includes rectifier . D4 fed from an additional output transformer T3 tuned to a frequency other than the basic frequency, the resulting voltage in the feed-back circuit being opposed to that provided by transformer T2 and rectifier R2 in response to the basic frequency. Furthermore, to prevent overloading on loud signals, the voltage across the primary of T2 is rectified and smoothed at D6, R8, R9, C4 and applied to the feed-back circuit in a sense which tends to make the bias on valve V more negative. The transformer T3 may be tuned to a harmonic of the basic frequency so that it also tends on high levels to compensate for harmonics produced in the circuits. At low levels, harmonics due to rectifier D3 are neutralized by the action of the valve V. Preferably, T3 is slightly off-tuned to the 2nd harmonic. Amplification and modulation control may be effected in separate valves or a dry rectifier type modulator may be used. In another modification, Fig. 4 (not shown) rectifier D3 may be replaced by a diode in the same envelope as valve V, and minor alterations occur in the protective circuits. Specification 539,595 also is referred to.