US2079262A - Apparatus for communication systems - Google Patents

Description

May 4, 1937. w. P. PLACE ET AL 2,079,262

' APPARATUS FOR COMMUNICATION SYSTEMS Filed Jan. 8, 1935 2 Sheets-Sheet 1 Engnemanis Cab N" Q .gb as N a, v N Y INVENTORS 5 Q7 2N (Dillard P. Place and N k I s k 3 Paul MBossam k 1 #2 a t f THEIR ATTORNEY y 1937- w. P PLACE Er AL 2,079,262

APPARATUS FOR COMMUNICATION SYSTEMS Filed Jan. 8, 1955 v 2 Sheet-Sheet 2 Madalazor 08052162101" INVENTORS willavd P. Place and y Paul MBoss-am.

T l- W THEIR ATTORNEY 95 100 99 ,5 location.

Patented May 4, 1937 awaits APPARATUS FOR COMMUNICATION SYSTEMS Willard P. Place, Wilkinsburg, and Paul N. Bossart, Swissvale, Pa., assignors to The Union Switch & Signal Company, ,Swissvale, Pa; a

corporation of Pennsylvania 7 Application January 8, 1935, Serial No. 846

14 Claims.

15 munication the transmitting apparatus embodies a source of carrier frequency current which is modulated by the voice frequencies produced in a microphone, and means for supplying the resultant modulated current to acommunicating circuit, or for supplying a side band of such modulated current to the circuit. The receiving apparatus embodies a demodulator which requires a source of the carrier current for mixing with the incoming carrier telephone current. A feature 25 of our invention is the provision of novel and improved apparatus wherewith a single electron tube oscillator is operative to supply to the transmitting apparatus of a location the carrier current which is to be modulated, and also to supply to 30 the receiving apparatus at the same location the carrier current necessary for demodulation.

Two independent stations at one of such spaced locations are often desirable. A further feature of our invention is the provision of telephone ap- 35 paratus for systems of the type here involved Wherewith the operators at both of two such stations may hear the telephone message sent by the operator at the remote location, and wherewith the operator at either of the two stations n may reply to the remote operator with the operator at the other station hearingthe conversation. Furthermore, the operators at the two stations may converse with each other with or without their conversation being repeated at the remote Other features and advantages of our invention will appear as the specification progresses.

Apparatus embodying our invention may be useful in many communication systems and is 60 particularly adaptable to carrier telephone systems for railway trains. In telephone systems for railway trains it has been proposed to telephone between two vehicles of a train, such as the locomotive and the caboose of a freight train,

by transmitting, a carrier telephone current be- Electric train signaling systems, In many instances a cabin is built on the rear end of the top of the locomotive tender, and in this cabin a trainrnan rides as an outlook for derailed cars, hot boxes, etc. In such instances it is desirable that both the trainman in the cabin on the top of the tender and the engineman in the locomotive cab may hear the telephone message sent by the conductor in the caboose, and that either the trainman or the engineman may reply to the conductor with each party on the locomotive hearing what the other is saying. Again, it is desirable that the trainman and the engineman may conversewith each other with or without their conversation being repeated in the caboose. For a better understanding of our invention reference may be had to the accompanying drawings Wherein the invention is disclosed in connection with a railway train telephone system. It is to be understood, of course, that this specific application is by way of illustration only and apparatus embodying our invention is equally useful for other carrier telephone systems.

In the accompanying drawings, Fig. 1 is a diagrammatic view of one form of telephone apparatus embodying our invention as applied to the locomotive equipment of a telephone system for railway trains. Fig. 2 is a diagrammatic view of a second form of telephone apparatus embodying our invention as applied to the locomotive equipment when the transmitting apparatus and the receiving apparatus are each provided with an independent source of carrier current. Fig. 3 is a diagrammatic view of a modified form of the apparatus for intercommunication between the trainmans cabin and the enginemans cab, and which apparatus also embodies our invention.

In. each of the different views like reference characters designate similar parts.

Referring to Fig. 1, the reference character L designates a locomotive such as, for example, the locomotive of a freight train. The enginemans cab is indicated at EC and on the top of the tender a trainmans cabin TC is built. On the locomotive L two telephone stations are established, one in the trainrnans cabin TC and the other in the enginemans cab EC, and to this'end two telephone sets indicatedas a whole by the reference characters TS and ES are mounted in the trainmans cabin TC and the enginemans cab EC, re-

spectively. Each of these telephone sets comprises a loud speaker LS, a microphone M and manually operated circuit controlling contacts to be subsequently described. The microphone M of each telephone set is preferably mounted on a handle 53 to facilitate its being brought close to the person speaking. The handle 53 is adaptable of being manually operated to a normal or lefthand position, that is, the position illustrated in Fig. 1, and to a communication or right-hand position as indicated by dotted lines. As shown schematically, the circuit controlling contacts I03 and ID! are operatively connected with the handles of the sets ES and TS, respectively, the arrangement being such that each contact occupies the horizontal position when its handle is moved to the normal position and is swung downward when its handle is moved to the communication position. On each microphone handle 53 a push button PB of the spring return type is mounted and the contacts of which are operative to govern the associated loud speaker and microphone in a manner to be described hereinafter.

Receiving apparatus and transmitting apparatus designated by the reference characters R and T, respectively, together with sources of current and other equipment to be later pointed out are mounted in suitable housing at any convenient place on the locomotive. The receiving apparatus R and the transmitting apparatus T may take different forms several of which are Well-known to the art, and they will be described in this description only insofar as is necessary for an understanding of our invention. As here shown, the receiving apparatus R includes a filter network F, a high frequency amplifier RF of one or more stages of amplification, a demodulator DM, a band-pass filter BPF and a single stage audio fre quency amplifier AF. The transmitting apparatus T includes an amplifier TF of one or more stages of amplification and an amplifying transmitter AT consisting of power tubes capable of amplifying the outgoing current to a relatively high ener gy level. The receiving apparatus R is operative to inductively receive current from the traffic rails through the medium of an inductor coil If! mounted on the locomotive in inductive relation with the rails, and the transmitting apparatus T is operative to supply current to the rails over wires I and |2 which have electrical contact with the rails by being connected to the journal boxes of the tender truck wheels.

Referring now to the receiving apparatus, the inductor coil I0 is connected by wires i3 and M with the input terminals of the filter F, the output terminals of which are connected by wires 5 and IS with the input terminals of the high frequency amplifier RF, and the output circuit of the amplifier RF in turn includes the primary winding I! of a coupling transformer Tl. It follows that a carrier telephone current inductively picked up by the coil l0 and passed by the filter F is amplified and reproduced in the primary winding I! of the transformer Tl where it induces an elec tromotive force of corresponding frequencies in the two secondary windings l8 and IQ of that transformer.

The demodulator DM is provided with two electron tubes 20 and 2| having balanced grid and plate circuits. The grids 22 and 23 of the respective tubes 29 and 2| are connected with the outside terminals of the two secondary windings l8 and 9 of the transformer TI, respectively, and the inside terminals of these two windings are joined and connected to the two cathodes 24 and 25 of the tubes 29 and 2| in parallel, a resistor 26 being interposed in the connection. The grid circuits thus formed for the tubes 29 and 2| are grounded at 21. The plates 28 and 29 of the respective tubes 29 and 2| are connected with the outside terminals of the two primary windings 39 and 3| of a transformer T2, respectively. The inside terminals of the windings 30 and 3| are joined and are connected over wire 33 and the back contact 34 of a relay DR. to be referred to later to the positive terminal of a source of plate voltage here shown as a generator 32, a reactor 46 and a resistor 41 being interposed in the wire 33.

The negative terminal of the generator 32 is connected with a ground electrode 35. The circuits for the filaments of the tubes 29 and 2| are not shown in order to simplify the drawings and it is to be understood they are heated in the usual manner. A carrier current for mixing with the incoming carrier side band telephone current applied to the grid circuits of the tubes 29 and 2| is supplied to the demodulator DM over a circuit connection designated as a whole by the reference character CF. As here shown this circuit connection CF includes a resistor 84 and a condenser 85 in series, the top terminal of the condenser 85 being connected by wire 48 to the common point of secondaries l8 and |9 and the lower terminal of the resistor 84 being connected with a source of carrier current to be explained hereinafter. A resistor I25 is connected between ground 21 and mid point of windings l3 and i9. The secondary winding 36 of the transformer T2 is connected with the input terminals of the bandpass filter BPF the output terminals of which are connected with the primary winding 37 of a transformer T3. The filter BPF is so proportioned and adjusted as to pass the desired audio frequencies and to substantially suppress the other products of demodulation. It is clear, therefore, that the carrier telephone current induced in the secondary windings l8 and I9 of the transformer TI is demodulated and the audio frequencies are passed to the primary winding 31 of the transformer T3 where they induce an electromotive force of corresponding frequencies in the secondary winding 38 of that transformer.

The audio frequency amplifier AF includes a pentode 39 of the heater element type. The control grid 40 of tube 39 is connected to a mid terminal of a resistor ||3 which is connected across the terminals of the secondary winding 38 of the transformer T3. The cathode 4| of the tube 39 is connected with the lower terminal of the secondary winding 38 over a biasing element 42 and the secondary winding H4 of a transformer T9 to be referred to later. It follows that the grid circuit thus formed for the tube 39 is adaptable of receiving energy through either the transformer T3 or the transformer T9. plate 43 of the tube 39 is provided with plate voltage from the generator 32 over the primary winding 45 of an output transformer T4 and wire 44, a resistor 83 being preferably interposed in the wire 44. duced in the secondary winding 38 of the transformer T3 are reproduced in the plate circuit of the tube 39 greatly increased in amplitude and are effective to induce an electromotive force of corresponding frequencies in the secondary Winding 49 of the output transformer T4. Again, an electromotive force induced in the secondary winding I M of the transformer T9 is amplified by the tube 39 and an electromotive force of corresponding frequency is induced in the secondary winding 49 of the output transformer T4.

The secondary winding 49 of the transformer T4 is connected by an output circuit to the operating windings of the loud speakers LS of the two telephone sets TS and ES in parallel. This output circuit can be traced from the top terminal of winding 49 over wire 50, operating winding of the loud speaker LS of the telephone set ES, a normally closed contact 5| of the push button PB of that set and thence over wire 54 to the The Hence, the audio frequencies inlower terminal of the winding 49. A parallel path branches'from the wire 59 over resistor 55, operating winding of the loud speaker LS of the telephone set TS, a normally closed contact 56 of the push button PBof that set and wire 59 to the wire 54 leading to the lower terminal of the winding t9. Consequently, with the push button PB of each telephone set ES and TS released and the respective contacts BI and 55 closed, the audio frequencies induced in the secondary winding 49 are simultaneously reproduced by the loud speaker at each telephone station TC and EC on the locomotive.

Referring now to the transmitting apparatus T of Fig. 1, the input terminals of the amplifier TF are connected with the secondary winding Ell of an input transformer T5, and its output terminals are connected to a circuit which includes the primary winding 6! of a transformer T6, the generator 32 and a front contact 62 of the relay DR, as will be readily understood by an inspection of Fig. 1. The amplifier TF is here shown conventionally only for the sake of simplicity since its specific type forms no part of our present invention and it is to be understood that it preferably consists of one or more stages of electron tube amplifiers, the plate voltage for the tubes being supplied by the generator 32 over the front contact 62 of the relay DR. It follows that with the relay DR deenergized and its front contact 62 open, the amplifier TF of the transmitting apparatus is inactive, but that with relay DR energized and its front contact 62 closed, plate voltage is applied to the amplified TF and that amplifier is effective to amplify a carrier telephone current induced in the secondary winding 60 of the transformer T5 in a manner to shortly appear. The secondary winding 63 of the transformer T6 is connected with the input terminals of the amplifing transmitter AT which is also shown conventionally only in i order to simplify the drawings as much as possi ble and which transmitter preferably consists of power tubes having the conventional push-pull arrangement. The output terminals of the amplifying transmitter AT are connected with the primary winding 64 of a transformer T1, the secondary winding 65 of which isconnected by the wires H and 12 with the traffic rails through the wheels of the locomotive tender as explained hereinbefore. Hence, the telephone current passed by the amplifier TF to the transmitter AT is supplied to the rails at a relatively high energy level, the transmitting apparatus T being active, however, only at such time the relay DR is energized and its front contact 62 is closed.

The locomotive equipment is provided with a carrier current generator G which includes an electron tube 66 and an oscillating circuit 9. As here shown, the tube E6 is of the screen grid heater type having three control grids 61, 68 and 69, and two screen grids H5 and H6. The oscillating circuit 9 consists of an inductor l3 and a condenser l4, and has its lower terminal connected with the cathode element'lll of the tube 66 over wire 15 and a biasing element 12, the wire 15 being also connected with a ground electrode H. The top terminal of the oscillating circuit 3 is connected with the control grid 69 over wire I01, a condenser 16 and a resistor l! in multiple being interposed in the wire NH. The anode grid 68 is permanently connected with the positive terminal of the generator 32 over" a resistor 19 and an inductor 18, the latter being coupled in the correct phase with the inductor 13 of the oscillating circuit 3. A condenser I02 may be connected between the lower terminals of the inductors l3 and 18. The screen grids H5 and H5 are connected in parallel with the positive terminal of the generator 32 over a resistor H1 and are by-passed to ground over a condenser H8. The plate of the tube 66 is supplied with voltage by the generator 32 and to that end is connected with the generator 32 over a plate circuit including a resistor 8i, primary winding 82 of the transformer T5, wire 44, resistorfili, ground electrodes 35 and H, biasing element 12 and the plate impedance of tube (56. The parts are so proportioned and adjusted that as long as the generator 32 is operated and the filament of the tube 65 is heated in the usual manner, oscillations of, say, 7500 cycles per second are established between the control grids B8 and 69, and the plate circuit current has an alternating current component of the carrier frequency of 7500 cycles per second. a It will be understood, of course, that our invention is not limited to a carrier frequency of 7500 cycles per second, and other frequencies may be selected if found desirable to do so.

The circuit connection CF which includes the resistor 84 and the condenser 85 has its lower terminal connected with the plate 8! of the tube 56, while its top terminal is connected by wire 48 to the demodulator DM in the manner previously pointed out. Consequently, a carrier current of the frequency of 7500 cycles is supplied from the oscillating tube 66 to the demodulator DM over the circuit connection CF as well as being supplied to the transmitting apparatus T through the transformer T5.

The carrier current generated by tube 66 is modulated by the voice frequencies produced in the microphones M of the telephone sets TS and ES in the following manner. winding 86 of a transformer T8 is connected between the control grid 61 and the cathode it! of the tube 66, and the primary winding 88 of that transformer is included in circuits with the microphones. convenient source of current, such as a battery not shown, a circuit is traced over wire 89, a normallyopen contact 90 of the push button PB of the set TS, microphone M of that set, primary winding 88 of the transformer T8, resistor ill and to the opposite terminal C of the same source of current which should be grounded at a convenient point. A similar circuit may also be traced from the B battery terminal over wire 92, a normally open contact 93 of the push button P3 of the set ES, microphone M of the set ES and thence through the primary winding 88 of the trans former T8 and resistor 91 to the opposite battery terminal 0. It follows that, when the push button PB of the telephone set TS is depressed and the contact 90 is closed, the voice frequencies produced in the microphone M of the set TS are effective to induce an electromotive force of like frequencies in the secondary winding 85 cf the transformer T8 and these frequencies are applied to the control grid 6'! of'the tube 66 with the result that the carrier current is modulated by the yoice frequencies, Again, when the push button PB of the set E5 is depressed and the con tact 93 is closed, the voice frequencies produced in the microphone M of the telephone set ES are applied to the control grid 61 of tube 66 through the medium of the transformer T8 and the carrier current is modulated thereby.

The secondary Starting from the B terminal of any For intercommunication between the two telephone stations on the locomotive, that is, between the trainmans cabin TC and the enginemans cab EC, a shunt circuit is provided for connecting the microphone of each telephone set with the grid or input circuit of the tube 39 of the audio frequency amplifier AP. The shunt circuit for the microphone for the telephone set TS extends from the lower terminal of the microphone over condenser 94, resistor 95, wire 96, the primary winding 97 of the transformer T9 and wire 98 to the upper terminal of the microphone of the same set. For the microphone of the telephone set ES, the shunt circuit extends from the lower terminal of the microphone over condenser 99, resistor Hi9, wire 96, winding 9'! and Wire 98 to the top terminal of the same microphone. It is clear that, when the push button PB of the telephone set TS is depressed and voice frequencies are produced in the microphone of that set, a current having the voice frequencies flows in the shunt circuit including the winding 91, resistor 95 and condenser 94, the condenser 94 being effective to block the direct current of the microphone circuit out of the winding 9'! but permitting the voice frequencies to now. In like manner, when the push button PB of the telephone set ES is depressed and voice frequencies are produced in the microphone of that set, a current having the corresponding voice frequencies flows in the shunt circuit including the winding 91, resistor I and condenser 99. In either case the voice frequencies in the winding 91 of the transformer T9 are effective to induce an electromotive force of corresponding frequencies in the secondary winding 1 i 4 of that transformer with the result that these voice frequencies are amplified by the tube 39 and are supplied to the loud speaker circuit. Assuming that the trainman depresses his push button and speaks into his microphone, a portion of the voice frequency current thus produced is supplied to the audio frequency amplifier AF through the transformer T9 and the voice frequencies are sounded by the loud speaker of the enginemans set, the loud speaker of the trainmans set being now on open circuit since the contact 56 is open with the associated push button depressed. It is necessary to open the loud speaker circuit at 55 to prevent singing caused by acoustic feedback. Again, if the engineman depresses his push button and speaks into his microphone, a portion of the voice frequency current thus produced is supplied through the transformer T9 to the audio frequency amplifier AF and the voice frequencies are sounded by the loud speaker of the trainmans set, the loud speaker in the enginemans cab being now on open circuit since the contact Si is open with the push button of the set ES depressed. It is to be noted that the winding 91 of the transformer T9 is coupled to the two microphones in parallel but is not so coupled to the winding 88 of the transformer T8. The condensers 94 and 99 keep the direct current from being by-passed around the microphone in use. The resistors 95 and H19 are for tone correction of the intercommunication circuit since otherwise the high frequencies would be weaker due to the relatively low impedance to high voice frequencies of the shunt path through the unused microphone. The two condensers 94 and 99 and the two resistors 95 and H10 are used since this puts all four impedances in series for this undesired shunt path but only two of them in series in the desired path through the winding 91, the resistors 95 and I00 being so proportioned as to improve the fidelity of reproduction.

In describing the operation of the locomotive equipment of Fig. 1, we shall assume the remote station with'which telephone communication is desired is located in the caboose of a freight train attached to the locomotive L. We shall further assume that the equipment occupies its normal position, that is, the position shown in Fig. 1, the generator 32 being in operation to supply plate voltage to the electron tubes. Although the filament circuits for the several tubes have been omitted in Fig. 1, since it is thought they would add nothing to the understanding of our invention, it is to be understood the filaments of the electron tubes are heated according to standard practice. Under this normal condition of the locomotive equipment a carrier telephone current supplied to the traffic rails by the equipment in the caboose is inductively received by the coil H) on the locomotive, filtered, amplified, demodulated, the audio frequencies further amplified and the telephone message sounded simultaneously by the loud speakers LS at the two telephone stations TC-and EC on the locomotive. In case the engineman wishes to reply to the conductor in the caboose, he moves his microphone handle 53 to the communication position and depresses his push button PB. The circuit controlling contact i03 attached to the handle 53 is operated downward into engagement with a stationary contact I04 as the handle is moved to the communication position, and the normally open contact 595 of the push button is closed when the push button is depressed. With the contacts 73-104 and IE5 closed, current is supplied to the winding of the directional relay DR and that relay is energized. This circuit extends from the B terminal of the current source over winding of the relay DR, wire I56, contact I 05, contact i93ltl4 and to the opposite terminal C of the current source. Relay DR on picking up closes the front contact 62 and plate voltage is applied to the amplifier TF of the transmitting apparatus T. At the same time the receiving apparatus is rendered inactive since plate voltage is removed from the demodulator DM when the back contact 34 of the relay DR is opened. The voice frequencies produced in the microphone of the telephone set ES by the engineman answering the conductor are applied to the carrier current through the transformer T8. The resulting carrier telephone current is then amplified by the transmitting apparatus and supplied to the traffic rails over the wires II and i2. Voice frequencies resulting from the engineman speaking into his microphone are also supplied to the winding 91 of the transformer T9 and are amplified at the audio frequency amplifier AF and are sounded by the loud speaker in the trainmans cabin since the amplifying tube 39 of the receiving apparatus remains active even with the directional relay DR energized. By depressing and releasing his push button to operate the directional relay DR and to switch between his loud speaker and his microphone, the engineman may converse with the conductor in the caboose, the full conversation being repeated in the trainmans cabin. Inthe event the trainman is to reply to the conductor, the trainman moves the handle 53 of his set to its communication position and depresses his push button PB. The contact i9! is operated downward into engagement with a contact I08 as the handle 53 is in oved to the communication position, and hence the contact 35.

speaking into his'microphone are effective to modulate the carrier current and the resultant carrier telephone current is supplied to the traffic rails. The voice frequencies are also supplied to the winding ill of the transformer T9, are amplified at the amplifier AF and are sounded at the loud speaker of the enginemans set ES. The

trainman by depressing and releasing his push button to operate the directional relay DR and to switch between his loud speaker and his microphone may carry on a conversation with the conductor, the engineman hearing the entire conversation.

Operation of the push buttons of the respective telephone sets at the twostations on the locomotive at a time when the associated microphone handles remain in the normal position permits the trainman and the engineman to converse with each other without their conversation being repeated in the caboose since the directional relay DR remains deenergized under the normal position of the microphone handles. The trainman by depressing his push button with the handle 53 in its normal position renders his loud speaker inactive and his microphone active with the resuit that the voice frequencies produced by his speaking into the microphone are applied to the amplifying tube 39 "throughthe transformer T9 and are sounded by the loud speaker in the enginemans cab. In a similar manner the engineman by depressing his push button when his microphone handle 53 is in the normal po sition is able to speak to the trainman. During such conversation between the trainman and the engineman, it is to be noted that the voice frequencies are effective to modulate the car rier current and the resultant modulated carrier frequencies are applied to the demodulator DM at a time it is in an active condition. The balanced circuit arrangement for the demodulator DM is efiec'tive, however, to balance out such frequencies without demodulation and the only frequencies applied to the audio amplifying stage AF are the frequencies applied through the transformer T9.

Referring to Fig. 2, the receiving apparatus R includes a filter F and a high frequency amplifier RF the same as Fig. 1. The demodulator DM is here shown conventionally only and may be electron tubes with balanced grid and plate circuits the same as in Fig. 1, except in this form of the invention the demodulator is provided with its own source of carrier current, whereas in Fig. l

the demodulator received carrier current from the same source as the transmitting apparatus. The plate voltage for the demodulator DM is obtained from the generator 32 over the back contact 34 of the directional relay DR as in Fig. 1. A transformer TIE] which is interposed between the filter BPF and the amplifier AF, is provided with two primary windings H and H9 and a secondary winding IZEI. The primary winding l is is connected with the output'terminals of the band-pass filter BPF and the secondary winding I20 is connected between the control grid ii! and the cathode 4'] of the electron tube. 3.9, a voltage divider in the form of a'resistor I24 being preferably interposed in the grid circuit thus formed for the tube 39. The primary winding N0 of the transformer Tm has one terminal grounded at H! through a condenser H2, and its other terminal is connected to the microphone circuits over wire Hi, and this winding I It is operative to effect intercommunication between the trainmans' cabin and the enginemans cab, as will shortly be explained. The output transformer T4 of the receiving apparatus of Fig. 2 has its secondary winding 39 connected with the loud speakers of the telephone sets TS and ES in multiple, the'norvmally closed contacts 5| and 56 of the push buttons being included in the respective circuits as will be readily understood by an inspection of Fig. 2. Consequently, a carrier telephone current picked up by the inductor coil It on the locomotive of Fig. 2 is demodulated, amplified and reproduced in both the. trainmans cabin TC and in the enginemans cab EC at such time as the directional relay DR is deenergized. The trans" mitting apparatus T of Fig. 2 consists of an amplifying transmitter AT and an amplifier TF the same as Fig. 1. In this form of our invention the transmitting apparatus is provided with a modulator-oscillator shown conventionally at MO. The modulator-oscillator MO may take different forms among them being the vacuum tube type and it will be understood that it includes a source of carrier frequency current. The transmitting apparatus -T is normally inactive and plate voltage is applied to the amplifier TF and the apparatus rendered active in response to the directional relay DR being energized and its front contact 62 closed. The directional relay DR of Fig. 2 is provided with a control circuit which is governed by contacts of the telephone sets TS and ES in exactly the same mannerdescribed in connection with Fig. 1, and it is thought unnecessary to repeat the description.

It is to be seen that with either the trainman or the engineman operating the microphone handle of his telephone set and depressing the push button mounted on the handle thereof, the loud speaker of that set is disconnected from the receiving apparatus, the microphone circuit is closed and the directional relay DR is picked up. Voice frequencies produced in the microphone of the telephone set thus operated are applied to the modulator-oscillator through the transformer T8 and the resulting carrier telephone current is amplified and supplied to the trailic rails. Such voice frequencies are also supplied to the winding lid of the transformer TH! and are effective to induce an electromotive force of corresponding frequencies in the winding lZii of the grid circuit of the tube 39, with the result that these frequencies are sounded at the loud speaker at the other station on the locomotive. Since the demodulator DM of Fig. 2 is provided with its own source of carrier current, no harmful feed-back due to the coupling between the primary windings HE] and N9 of transformer Till occurs, and the impedances consisting of condensers B l and 99, and of resistors 95 and Hill as shown in the intercommunication circuits of Fig. 1 may be omitted. With a common carrier oscillator for receiving and transmitting as in Fig. 1, a slight disturbance in the demodulator circuit whichis not perfectly balanced out might produce a disturbance in the primary of the transformer T8 which disturbance might be fed back over the CF connection to the demodulator with the result that the circuits might sing. It is clear, there-.

fore, that the operation of the apparatus of Fig. 2 is the same as described for the apparatus in connection with Fig. 1 and a telephone message received from the conductor in the caboose is sounded in both the trainmans cabin and in the enginemans cab. Either the trainman or the engineman may reply to the conductor with the other party on the locomotive hearing the full conversation, and intercommunication between the trainman and the engineman with or without their conversation being repeated in the caboose is permitted.

In Fig. 3, an autotransformer having an auxiliary winding is provided for supplying the final amplifying stage of the receiving apparatus with the audio frequencies resulting from demodulation and with the local voice frequencies for intercommunication between the two stations on the locomotive. An autotransformcr TH has its main winding I22 connected between the control grid 40 and the cathode 4B of the electron tube 39. The winding I22 is also connected with the output terminals of the band-pass filter BPF, with the result that the audio frequencies resulting from demodulation of a carrier telephone current picked up by the receiving apparatus are amplified as the amplifier AF and are supplied to the loud speaker circuits through the output transformer T4. The transformer Tll is provided with an auxiliary Winding I23 which is connected with the microphone of the telephone sets TS and ES, the resistor and condenser Si l being interposed in the shunt circuit for the microphone of the trainmans set TS, and the .resistor ll!!! and condenser 99 being interposed in the circuit for the microphone of the enginemans set ES. It follows that the voice frequencies produced in either microphone of Fig. 3 are applied to the winding l 23 of the transformer Tll as well as being supplied to the input transformer T8 of the transmitting apparatus. Such voice frequencies suppiled to the winding I23 of the transformer TH are amplified by the final amplifying stage AF of the receiving apparatus and are sounded by the loud speaker at the other station on the locomotive. It is clear that the transformer TH of Fig. 3 may replace the two transformers T3 and T9 of Fig. 1 or it may be used in place of the transformer TH] of Fig. 2 without changing the operation of the apparatus.

Although we have herein shown and described only three forms of apparatus embodying our invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim is:

1. In a telephone system comprising a communication circuit, transmitting apparatus operative to supply a telephone current to said circuit, receiving apparatus responsive to telephone current flowing in said circuit and including an electron tube amplifier, and switching means for rendering either the transmitting apparatus or the receiving apparatus active; the combination with the above instrumentalities which consists of a first and a second station each provided with a loud speaker and a microphone, a first circuit means to connect said loud speakers in multiple to the output of the amplifying tube for reproducing simultaneously at said stations the message of a telephone current picked up from the communication circuit, 9.

second circuit means to connect either micro= phone at will to the transmitting apparatus for supplying to the communication circuit a telephone current produced at either station, and a third circuit means to connect said microphones with the input of the amplifying tube for reproducing at either station the voice frequencies produced in the microphone of the other station.

2. In a telephone system comprising a communication circuit, transmitting apparatus operative to supply a telephone current to said circuit, receiving apparatus responsive to telephone current flowing in said circuit and including an electron tube amplifier, and switching means for rendering either the transmitting apparatus or the receiving apparatus active; the combination with the above instrumentalities which consists of a first and a second station each provided with a loud speaker and a microphone, a first circuit means to connect said loud speakers in multiple to the output of the amplifying tube for reproducing simultaneously at said stations the message of a telephone current picked up from the communication circuit, a second circuit means to connect either microphone at will to the transmitting apparatus for supplying, to the communication circuit a telephone current produced at either station, a third circuit means to connect said microphones with the input of the amplifying tube for reproducing .at either station the voice frequencies produced in the microphone of the other station; and a manually controlled circuit controller at each station for governing said first, secondand third circuit means.

3. In a carrier telephone system comprising a communication circuit, transmitting apparatus including a source of carrier current effective to supply such current to said circuit, receiving apparatus effective to pick up such current from said circuit and including a demodulator and an audio frequency amplifier, and switch means for rendering either the transmitting apparatus or the receiving apparatus active; the combination with the above instrumentalities which consists of a first and a second station each provided with a loud speaker and a microphone, a first circuit means to connect said loud speakers in multiple with the output circuit of the audio frequency amplifier for simultaneously reproducing at the stations the message of a carrier telephone current received from the communication circuit, a second circuit means to connect either microphone at will with said. source of carrier current to modulate the carrier current supplied to the communication circuit with the voice frequencies produced in the microphone at either station, a third circuit means to connect the microphones to the input of the audio frequency amplifier to sound at the loud speaker of either station the voice frequencies produced in the microphone at the other station; and a manually operated circuit controlling device at each station for governing said first, second and third circuit means.

4. In a carrier telephone system comprising a communication circuit, transmitting apparatus including a source of carrier current effective to supply such current to the circuit, receiving apparatus effective to pick up such current from the circuit and including a demodulator and an audio frequency electron tube amplifier, and switching means for rendering either the transmitting apparatus or the receiving apparatus active; the combination with the above instrumentalities which consists of a first and a second station ach provided with a loud speaker and a microphone, a first circuit means including a normally closed contact at each station for connecting the loud speakers in multiple with the plate circuit of the electron tube amplifier to reproduce at both stations the message of a carrier telephone current picked up from the communication circuit, a second circuit means including a normally open contact at each station for connecting either microphone at will with said source of carrier current to modulate the carrier current supplied to the communication circuit with the voice frequencies produced in the microphone at either station, and manually controlled means at each station for operating the contacts at the same station. 7

5. In a carrier telephone system comprising a communication circuit, transmitting apparatus including a source of carrier current effective to supply such current to the circuit, receiving apparatus effective to pick up such current from the circuit and including a demodulator and an audio frequency electron tube amplifier, and switching means for rendering either the transmitting ap paratus or the receiving apparatus active; the combination with the above instrumentalities which consists of a first and a second station each provided with a loud speaker and a microphone, a first circuit means including a normally closed contact at each station for connecting the loud speakers in multiple with the plate circuit of the electron tube amplifier to reproduce at both stations the message of a carrier telephone current picked up from the communication circuit, a second circuit means including a normally open contact at each station for connecting either microphone at will with said source of carrier current to modulate the carrier current supplied to the communication circuit with the voice frequencies produced in the microphone ateither station, a third circuit means including a winding coupled with the input'circuit of said electron tube amplifier for soundingat the loud speaker of either station the voice frequencies produced in the microphone at the other station, and manually controlled means at each station for operating the contacts at the same station.

6. In a carrier telephone system comprising a communication circuit, transmitting apparatus including a source of carrier current efiective to supply such current to the circuit, receiving apparatus effective to pick up such current from the circuit and including a demodulator and an audio frequency electron tube amplifier, and switching means for rendering either the transmitting apparatus or the receiving apparatus active; the combination with the above instrumentalities which consists of a first and a second station each its microphone with said source of the transmitting current, 'a third circuit for each station for connecting its microphone with the input of the electron tube amplifier, and manually controlled means at each station for operating the contacts at the same station whereby the message of a carrier telephone current picked up from the communication circuit is simultaneously reproduced at both stations and the message sent out from either station is reproduced at the other station.

7. In combination; an electron tube provided with a first, a second and a third control grid as well as a plate and a cathode; means including a source of plate voltage and an output winding connected between the plate and cathode to form a plate circuit, means including said source and a first inductor connected between the first control grid and the cathode, an oscillating circuit including a condenser and an inductor connected between said second grid and the cathode and arranged with its inductor coup-led with said first inductor for causing a carrier frequency component in the plate circuit current, means including a microphone connected between said third grid and the cathode for at times modulating saidv and a cathode, means including a source of platevoltage'and an output winding connected between the plate and the cathode to form a plate circuit, oscillating circuit means including an inductor and a condenser and efiectively connected with one of said grids and the cathode-for causing a carrier frequency component in the plate circuit current, means including a source of signaling current connected between the other grid and cathode for at times modulating said carrier frequency component, transmitting apparatus including a winding coupled with said. output winding for receiving such modulated carrier energy, a circuit connection including a condenser connected effectively between the plate.

and cathode and operative to pass such carrier frequency energy, and demodulating apparatus coupled with said circuit connection to receive carrier frequency energy for mixing with-an incoming signaling current. i

9. In cornbinationwith receiving apparatus operative to pickup and demodulate a carrier telephone current and transmitting apparatus operative to send out such telephone current, an electron tube audio frequency amplifier, a first and a second station each provided with a loud speaker and a microphone, a circuit for each station to connect its loud speaker with the plate cir cult of said amplifier tube, means for coupling the grid circuit of said tube withthe demodulator for reproducing at each station the message of a carrier telephone current picked up by the receiving apparatus, atransformer having a secondary winding coupled with the grid circuit of said tube; a first circuit means at each of said stations including the microphone, a source of direct current and a winding coupled with said transmitting apparatus; a second circuit means for each station including the microphone, a condenser and a resistor; means for connecting said two second circuit means in multiple with a primary winding of said transformer for reproducing at either station the voice frequencies produced in the microphone at the other station when a telephone message is being sent out over the transmitting apparatus from said other station, and manually controlled means at each station for rendering either its loud speaker or microphone efiective.

10. In combination with receiving apparatus operative to pick up and demodulate a carrier telephone current and transmitting apparatus operative to send out such telephone current, an electron tube audio frequency amplifier, a first and a second station each provided with a loud speaker and a microphone, a circuit for each station to connect its loud speaker with the plate circuit of said amplifier tube, means for coupling the grid circuit of said tube with the demodulator for reproducing at each station the message of a carrier telephone current picked up by the receiving apparatus, a transformer having a secondary winding coupled with the grid circuit of said tube; a first circuit means at each of said stations including the microphone, a source of direct current and a winding coupled with said transmitting ap- 0 paratus; a second circuit means for each station including the microphone and a condenser, means for connecting said two second circuit means in multiple with a primary winding of said transformer for reproducing at either station the voice frequencies produced in the microphone at the other station when a telephone message is being sent out over the transmitting apparatus from said other station, and manually controlled means at each station for rendering either its loud 210 speaker or microphone effective.

11. In combination with receiving apparatus operative to pick up and demodulate a carrier telephone current and transmitting apparatus operative to send out such telephone current, an electron tube audio frequency amplifier, a first and a second station each provided with a loud speaker and a microphone, a circuit 'for each station to connect its loud speaker with the plate circuit of said amplifier tube, means for coupling the grid 40 circuit of said tube with the demodulator for reproducing at each station the message of a carrier telephone current picked up by the receiving apparatus; a first circuit means at each of said stations including the microphone, a source of 5 direct current and a winding coupled with said transmitting apparatus; a second circuit means for each station including the microphone, a condenser and a resistor; means for coupling each of said second circuit means with the grid circuit of said amplifier tube for reproducing at either station the voice frequencies produced in the microphone at the other station when a telephone message is being sent out over the transmitting apparatus from said other station, and

manually controlled means at each station for rendering either its loud speaker or microphone eifective.

12. In combination, an electron tube provided with at least two control grids as well as a plate and a cathode, means including a source of plate voltage and an output winding connected between the plate and the cathode to form a plate circuit, oscillating circuit means including an inductor and a condenser and eifectively connected with one of said grids and the cathode for causing a carrier frequency component in the plate circuit current, means including a source of signaling current connected between the other grid and cathode for at times modulating said carrier frequency component, transmitting apparatus including an electron tube amplifier and a Winding coupled with said output winding for sendi'ng out such modulated carrier energy, a circuit senses:

connection including a condenser effectively con nected between the plate and cathode of the first mentioned electron tube and operative to pass such carrier frequency energy, receiving apparatus including an electron tube demodulator having its grid circuit coupled with said circuit connection to receive carrier frequency energy for mixing with an incoming signal current impressed upon the receiving apparatus, a source of plate voltage for the several electron tubes, and means to selectively connect the plate voltage source either to the tube of the transmitting amplifier or to the demodulating tube.

13, In combination, an electron tube oscillator operative to generate a carrier frequency curient, a first circuit means including a microphone coupled to a grid circuit of said tube to modulate the carrier with voice frequencies, transmitting apparatus including an electron, tube amplifier and a winding coupled with the plate circuit of the oscillator tube for sending out carrier telephone current, receiving apparatus operative to pick up such carrier telephone current and including a balanced electron tube demodulator, a second circuit means including a loud speaker and a Winding coupled with the output of the receiving apparatus, means to couple the plate circuit of the oscillator tube with the grid circuits of the demodulator tubes to mix the carrier current with incoming carrier telephone current, a relay, a source of plate voltage, means including a back contact of the relay to connect the plate voltage source with the plate circuits of the demodulator tubes, means including a front contact of the relay to connect the plate voltage source with the plate circuit of the tiansmitting amplifier tube, and an energizing circuit for the relay including a circuit controller associated with the microphone.

14. In combination with receiving apparatus operative to pick up and demodulate a carrier telephone current and transmitting apparatus operative to send out such current, an output transformer, means to couple the primary winding of said transformer with the output of the receiving apparatus, a first and a second station each provided with a loud speaker and a microphone as well as a push button and another circuit controller, a first circuit means for each station to connect its loud speaker with the secondary winding of said transformer and including a normally closed contact of the push button, a second circuit means for each station including the microphone and a normally open contact of the push button as well as a winding coupled with the input of transmitting apparatus, a third circuit means for each station including the microphone as well as a condenser and a winding coupled with the primary winding of said transformer, a directional relay, means including a back contact of the relay to render the receiving apparatus active, means including a front contact of the relay to render the transmitting apparatus active, and an energizing circuit for the relay consisting of a branch path at each station and each of which paths includes a normally open contact of the push button and a normally open contact of said other controller in series.

WILLARD P. PLACE. PAUL N. BOSSART.

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