Phantom Telephone Circuits, and Combined Telegraph and Telephone Circuits, Worked at Audio Frequencies

HILL: PHANTOM TELEPHONE CIRCUITS. 675 PHANTOM TELEPHONE CIRCUITS, AND COMBINED TELEGRAPH AND TELEPHONE CIRCUITS, WORKED AT AUDIO FREQUENCIES. By J. G. HILL, Associate Member. (Paper first received l&ih October, 1921, and in final form 28th April, 1922; read at THE INSTITUTION lGth March, 1922.) SUMMARY. (2) The Equipotential method of providing simul- The superposing of additional circuits on telegraph and taneous channels of telegraphic and telephonic telephone conductors, so as to obtain two or more channels communication over the same wires, and the of independent communication from the same physical application of this method to the balancing circuit, now occupies a very important place in the design of telephonic relayed circuits. of such circuits. The loading of telephone circuits, combined with the (1) IMPEDANCE METHOD OF SIMULTANEOUS TELEGRAPH use of thermionic amplifiers in them, now renders it AND TELEPHONE WORKING APPLIED TO SINGLE- possible to provide efficient long-distance telephonic com- WIRE WORKING. munication—including the provision of phantom circuits— on small-gauge conductors in underground cables carrying This method was first introduced by F. Van Ryssel- a large number of circuits. The provision of these cables berghe,* a Belgian telegraph engineer, in 1882. Com- involves the gradual replacement of overhead open telephone bined working is rendered possible by the different circuits in a large measure, and constitutes a revolution in impedance of inductance coils and condensers respec- modern circuit provision. The object of this paper is to tively to high- and low-frequency currents. The review the present position of the art as applied to super- action depends upon the fundamental difference posed circuits worked at audio frequencies. of telegraph Morse signals and telephone speech The theory of simultaneous telegraph and telephone currents. working on the same circuit is first dealt with, and the Definition of telephone and telegraph signals, limitations of this method of working are stated. (a) Telephone signals.—The voltages and currents The comparative efficiency of side and phantom circuits produced in a telephone circuit by the voice are assumed is then dealt with, and the principal problems arising from to vary in a simple harmonic manner. The voice the loading and relaying by amplifiers of phantom circuits covers a wide range of frequencies varying, say, between are outlined. The solution of the problems is indicated. 100 and 2 500 periods per second ; the mean frequency Finally an appendix is given, dealing with the derivation for purposes of calculation is taken at 800 periods per of formulae for the calculation of electric wave filters, for second, f application to the circuits dealt with. (b) Telegraph signals.—In the Morse code these con- sist of " dots " and " dashes." In hand signalling at the normal rate of 30 words per minute a dot has a INTRODUCTION. duration of 1/12 second and this will be used as a basis The problem of. utilizing telegraph and telephone of comparison. circuits to transmit two or more simultaneous electrical Explanation of the electrical action in an impeded communications over one circuit without interference combined telegraph and telephone circuit.—If an electric between them, has from an early date occupied the condenser of sufficient capacity (say 5/xF) is placed attention of those interested in telegraphy and telephony. in series with a telephone receiver in an ordinary During the past few years a considerable development telephone circuit, the attenuation of speech currents of some of the methods employed has taken place due to the condenser is so small as to be only just with far-reaching results ; and the object of this paper perceptible to the ear. The impedance of the condenser is to review the present position of the subject so far to very low-frequency currents is, however, very high. as the simultaneous transmission of telegraph signals On the other hand if a suitable inductive impedance and speech, at normal frequencies, and of phantom be placed in the line lead of a telegraph set the speed telephone working at such frequencies are concerned. of telegraph signalling is slowed down, but it is still In view of the very wide range of the problems possible to signal at hand speed on the telegraph circuit. involved it will be possible to deal only very briefly In addition to these direct actions in the telegraph with them. The methods most generally employed to and telephone circuit, and other direct actions which provide extra channels of telegraph or telephone com- will be shortly described, the action of each circuit on munication may be classified under the following the other requires consideration. The reduction in the heads : rate of rise of the telegraph signals reduces the normal * British Patents Nos. 1 303 and 2 466 of 1882, and 3 621 and (1) The Impedance or Retardation method of com- 5 503 of 1883. bined telegraphy and telephony over the same t " Conference Internationale des Techniciens des Administra- tions des Tel6graphes et Telephones de l'Europe." Paris, 1910. wire or wires. Comptes Rendus, p. 99. 676 HILL: PHANTOM TELEPHONE CIRCUITS, AND COMBINED TELEGRAPH tendency of those signals to cause disturbance in the matical point of view in the relatively simple circuit telephone apparatus via the condenser, in view of the shown in Fig. 2. simple fact that the disturbing signals vary in intensity in proportion to the rapidity of the rate of change Let E = a constant applied voltage at A. of voltage at the terminals of the condenser. The v = the voltage at the terminals of the condenser retarding device in the telegraph circuit, however, has at the time t. a very high impedance to telephone speaking currents R = the resistance of the retardation coil. at the mean frequency of speech, and this high impe- L = the inductance in henrys of the retardation dance acts beneficially as regards the transmission of coil. speech, by reducing the transmission loss due to the Ri = the unvarying resistance of a long line telegraph apparatus acting as a shunt to speech currents. supposed to be non-inductive. The disturbing effect of speech currents on ordinary C = the capacity of the condenser in farads. Morse telegraphic apparatus is absolutely negligible. t = the time in seconds after the application of the voltage E to the circuit. VAN RYSSELBERGHE SYSTEM. Then the equation connecting the voltage v with the time during the transient period when the E.M.F. is The working of this system depends fundamentally rising from zero to its steady value E, is on the actions described. A high inductive impedance RL is placed in the path of the telegraph apparatus dv R E and a condenser C in the path of the telephone apparatus, the two sets of apparatus being joined in parallel and connected to the line as indicated in Fig. 1 in which both ends of the circuit are shown. A single-current The corresponding equation for the value of v at any circuit is shown for simplicity, but double-current instant t after the application of E, is working is generally used. ERX _ l] (2) Line where 2 2/ - 2\L^CR] E RL -oww° f 'III—IHH nnnnr< Telephone Telegraph, Telegraph Telephone set set set set FIG. 1. FIG. 2. The foregoing description shows in a general way If R\L = 1/CRi and R = the equation for v may that combined telegraph and telephone working on the be written same wire is possible, but it is of interest to study more closely the effect of the inductive impedance and the v = \E\\ — x/2e-tlcRsin {t/CR + {TT)} . (4) condenser in order to find to what extent (if any) the retarded telegraph signals cause disturbance in the In any case if jS is imaginary the equation assumes a telephone set, and, further, how far the retardation form similar to (4). affects the telegraph signals themselves as regards Examination of these equations shows that the rate both shortening them and slowing down the rate of of rise of the voltage is logarithmic, and that by signalling. These matters will now be investigated. arranging suitable values of JB, L, C and Rx the rate In commercial combined telegraph and telephone of rise of the volts may be either advanced or retarded. circuits the apparatus arrangements are complex, as If RL represents a relay at the receiving end of a line may be seen by inspection of the diagrams here given, and C a condenser shunted by Rx at the same point and in addition the electrical action is materially we have the well-known arrangement of the shunted complicated by the fact that the permeability of the condenser as may be seen from Fig. 2. Fig. 3 gives iron cores and the effective resistances of the telegraph an oscillogram of the normal rise of current in an apparatus in circuit change materially with different unshunted Wheatstone telegraph receiver, and Fig. 4 current strengths. For these reasons experiment is shows the more rapid rate of rise obtained by adding usually preferred to calculation in determining the the shunted condenser to the same apparatus, the best values of the inductance and capacity required. adjustment being made to produce a signal as nearly The fundamental theoretical effect of the devices intro- square (flat-topped) as possible. duced to control the rate of rise of the current may, On the other hand, in order to produce the retardation however, be studied with advantage from the mathe- which is desired in a combined telegraph and telephone AND TELEPHONE CIRCUITS. 677 circuit, the following values which have been actually retardation which has just been examined has slowed used may be taken as an example : down the signal considerably.

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