Patent GB-558298-A (GB)
Title: Improvements in or relating to electric selective signalling and control systems
IPC: H04Q1/446
Filed: 1942-06-26
Abstract: 558,298. Electric. selective signalling. JONES, C. E. P., and BIRSS, R. R. June 26, 1942, Nos. 8788 and 14191. [Class 40 (i)] [Also in Group XXXIX] Information e.g. digit values in automatic telephony, is transmitted by means of two currents differing in amplitude, frequency or phase by an amount corresponding to a number of steps, the two currents being separated on reception and used to set a selective device in accordance with the number of steps. If carrier current is used, the information may be transmitted by carriers modulated to different depths or by varying the amplitude and depth of modulation respectively of a single carrier. As described, one of the currents is of constant amplitude, frequency or phase and in the case of amplitude, this current may set the gain of the receiving amplifier of the other current in accordance with variations of the constant current as received, in which case the amplitude of the other current itself furnishes the information. Figs. 1 and 2 show the transmitter and receiver of a system in which digit values are transmitted as relative amplitudes of two alternating currents of frequencies, X, Y, frequency X being transmitted at constant level and Y at levels dependent on the digit, 2¢ decibels being used. as a unit for example. At the transmitter, contacts d, k of the dialling key transmit frequency X and connect up a 10-step attenuator VA for frequency Y, the attenuator being set by switch DS in response to an impulse train. At the receiver, the frequencies are separated by filter amplifiers SAX, SAY, rectified and applied to "logarithmic units" LUX, LUY the outputs of which are applied in opposition to resistance YA. The logarithmic units may be such as are referred to in Specification 438,839, [Group XXXIX], and convert the decibel units. of the input into linear units of D.C. corresponding to the digit transmitted. The voltage across. YA may be used to determine a train of impulses over circuit 20, 21 Fig. 6 prepared by relays RX, RY, Fig. 2 on receipt of a digit. The impulses. step a switch FS, Fig. 6 until the bias on valve V or polarizing voltage of a relay due to the voltage across YA is balanced by that across YB. As shown, relay DC in the anode circuit of V is normally operated, is released by bias voltage across YA on receipt of a digit to close the impulse sending circuit, and is re-operated to open the circuit when the normal bias condition is restored by switch FS. As a refinement, the output of the X-frequency amplifier may with the help of a control circuit COC be used to maintain constant the output of both amplifiers. If the frequencies XY, are close together they may be treated on reception as a carrier and single side band whether or not they are transmitted as such. At the transmitter, the variable attenuator, instead of being set in steps, may be set by one operation subsequent to storage of the impulses. The two amplitudes may also be the constant amplitude of a carrier frequency and the variable depth of modulation (upper and lower side bands) of the carrier controlled by a variable attentuator similar to VA Fig. 1, At the receiver, Fig. 5 the wave is amplified at AS, the carrier and modulation outputs after rectification at MR, MR1 respectively are smoothed at SC, SC1 and applied in opposition to a D.C. amplifier DCA to provide a significant output across resistance YA. When the digits are transmitted as variable frequency differences, a transmitter similar to Fig. 1 is used but the variable attenuator VA is replaced by a. variable frequency oscillator, the receiver being similar to Fig. 2 with the Y frequency amplifier responsive to the entire Y band and the logarithmic units replaced by known units which have D.C. outputs proportional to the frequency of their inputs. In utilizing phase variations, an A.C. generator which supplies constant reference current to the line also supplies a. frequency doubler the output of which is connected to line over a stepped phase-changing network as set described above. At the receiver, a fixed relative phase shift is applied to compensate for incidental phase shifts and preferably to bring the currents into phase opposition if there has been no phase shaft at the transmitter. The reference frequency is then separated, doubled and combined again after both currents have been limited in amplitude and filtered to remove harmonics. The resultant, after rectification, represents the number transmitted and may be converted into numerical form as previously described. Frequency multiplication, division or modulation may be used to provide the phase-shifted current, or separate transmission channels for the two currents may be provided. In a modified receiving arrangement Fig. 8 (not shown) for deriving information from the opposed voltages, the resultant voltage may be applied to say 10 relays connected in parallel and biased to graded extents. The highest biased relay which is operated opens the circuits of all relays biased to a lesser extent.