US2110284A - Repeater system - Google Patents

Description

March 8, 1938.` A. 'E BixzHlLl-:Ty ET AL 2,110,284 A REPEATER SYSTEM l v'r sheets-sheet 1 Filed Sept. 5, 1936 /NvE/vroks; e4f- BACHELET BRAND fron/VEP March s, 193s. A', E. BAHELT ET AL, 2,110,284,

RE'IPEATER SYSTEM .Filed Sept. 5, 1936 7.Sheets-Sheet 2 NE' W VOR/f BR/DG/NG POINT Af. BAC/Her INVENTOR$- -sI BRAND ATTORNEY March s, 1938.

AQ E. BACHELET ET AL REPEATER SYSTEM Filed Sept. 5, 1936 7y sheets-sheet 4 Il All vvvrrvv N. u rRANsM/TTER Af. BACHELET NVENTORS.

March 8, 1938. A. E. BACHELET ET AL. l 2,110,284

I REPEATER SYSTEM Filed Sept. 5,` 1936 7 sheets-Sheet 5 HG.v 5.

HARTFORD BR/DG//VG POINT i .AE BACHELET /A/L/E/vrofs.-4 5. BRAND A 7' TORA/EV March 8, 1938- A. E. BACHELET Er AL" l2,110,284

REPEAT-ER SYSTEM f Filed sept. V5, 195e 7 sheets-sheet e F IG. 7

A10/Vl 70k/N6 L 000 SPEAKER .21.5. HACHE/ ET WVM/70H5. s. BRAND ATTORNEY March 8, 1 938.

A. E. BAcHELET Er AL REPEATER SYSTEM Filed Sept. 5, 1936 7 sheets-sheet 7 Bos T oN f mANsM/TTER MIO HAR TFORD V TRANSM/ 7`7'ER V .74.5. BAG/ELU NVNTORS ATQORNEV Patented Mar. 8, 1938 fol-FICE aEPEA'rEa SYSTEM Albert E. Bachelet, Mount Vernon, and Smart Brand, Bronxville, N. Y., assignors to Bell Telephone Laboratories,

---N. Y., a. corporation o Incorporated, New York, i' New York Application September 5, 1936, Serial No. 99,564

` s claims.y

This invention relates to telephone transmission systems and more particularly to systems for transmitting radio broadcast programs over wires from one broadcasting station to another.

An object of the invention is to provide a simple and ilexible transmission network, employing one-way amplifiers, for interconnecting a plurality of broadcasting stations whereby any station may assume control of the network and condition all ampliers in the network to transmit from that stationto all other stations and to prevent interference, by any oi' the other stations, as long as said rst station retains control.'

Other objects and features of theinvention will appear from the following description, claims and appended drawings, Figs. 2 to 9 of which when placed adjacent to one another as shown in Fig. l, show a hypothetical network extending from Chicago in the middle west to Boston on the eastern seaboard with intermediate bridged connections to local broadcasting stations along the route and also to points to the south.

In the drawings, Fig. 2 shows a broadcasting station at Chicago connected by a program circuit to a bridging point (Cleveland) at which point a local broadcast station is connected.

From Cleveland the circuit continues east to the` next bridging point (New York, Figs. 3, 4, and 5), where a broadcasting station is connected and where two other circuits are branched, one to the south, Washington, for example, and another to the east. with a bridging point at Hartford (Figs. 6 and 7) where a local station is connected (Fig. 9) and from which point avcircuit is extended to Boston (Fig. 8).

A description of the circuit arrangement, in

accordance with the invention, will now be given.

It will be assumed that the New York studio (Fig. 5) is to transmit a program and therefore control key I (Fig. 5) will be operated. When key I is operated an obvious circuit is closed to operate relay 2 and light guard lamp 3. Relay 2 operated connects 13D-volt battery, at its lower alternate contacts, to the middle of simplex coil 4 connected across the line between the studio and the nearest bridging point, in this case the New York toll ofllce. From this point the circuit can be traced to the middle of the corresponding simplex coil 5 (Fig. 4) at the toll oiiice or bridging point and from that point over contact 6 of relay 'I, winding of relay 8 and contacts of key 9 to ground. Relay 8 operates in this circuit and closes its contact I thereby connecting 13D-Volt battery I 53 to the mid-point of simplex coil II bridged across the multiple connection 49 between the amplifiers AI, A2, A3, and A4. Relay I2 connected to this simplex circuit operates and lights cue lamps I3 as an indication to the attendant at the bridging'point that one of the studios has control of the circuit. Operation of relay 8 also closes its contact I4 and short-circuits the normally operated relay I5 oi amplifier A3 which relay thereupon releases. When relay I5 releases it closes its contacts I6 therebyclosing a circuit to operate amplifier relay I'I. vRelay I5 in releasing also opens the circuit for normally lighted direction lamp I8 and the operation of relay I'I lights the opposite direction lamp I 9 which indicates to the attendant that amplifier A3 is now reversed and set to transmit from its associated studio into the network. Operation of relay I1 connects ground at its innermost upper contacts to place a second short circuit around relay I5 thereby holding relay I5 released in case the contact key` I at the New York studio should be released thereby removing battery irom the simplex circuit and releasing relay 8 which `would' remove ground, at its contacts I4, from relay I5.'

It should be here pointed out that the-amplifiers at the bridging point, i. e. Cleveland (Fig. 2), New York (Figs. 3 and 4) identified as AI, A2, A3 and A4 and Hartford (Figs. 6 and 7) identi- `iled as BI, B2 and B3, are arranged to normally transmit from left to right, i. e., the left-hand switching relays, for example relay 39 of amplifier A2 (Fig. 3) are normally operated and the right-hand switching relays, for example relay 4l) of ampliner A2 (Fig. 3), are normally released. Thiscondition will be indicated to the attendant at the bridging point or repeater station by the lighting of the direction lamps associated with the respective relays, for example lamp 4I associated with. relay 39 of amplifier A2. When an amplifier is reversed, i. e. arranged to transmit in the opposite direction, the left-hand switching relay is released and the right-hand relay operated as in the case just described in connection with amplifier A3, whereupon normally lighted direction lamp I8, for example, is extinguished and direction lamp IT is lighted. C

Continuing with the circuit description, release of relay I5 closes, at its contacts 42, a

-circuit to operate equalizer relay 43 which operates and connects the equalizing network 44 in shunt to the primary winding of input transformer 45 which feeds the amplier tube 46, this equalizing network being for the purpose of compensating for transmission distortion introduced in the program signalfrequencies by the circuit over which they arrive at the ampliiler.: in this case, the line 2l from the New York studio. Keys 41 and ll are for manually controlling the equalizer relay I3. Corresponding relays, equalizing networks and keys are associated with each of the other amplifiers.

Whenrelay 8 (Fig. 4) operated, and connected 13o-volt battery to the middle of simplex coil Il, current flowed over the multiple connection leads 49 in parallel whereupon relay 28v of am- -pliiler A2 and the corresponding relays of amf pliiiers Ai and A4 operate in a circuit including contacts OI .of relay B2, in the case of ampliiier A2. Relay 2lin operating vcloses its contacts and operates relay 21 in an obvious circuit. Relay 21 operated connects, at its contact 2|, 13D-volt batteryto the middle of simplex coil 29 and thence over the two conductors in parallel of cable line 30 to the next eastern bridging point (Hartford) where the circuit may be further traced from-the middleof simplex coil 3i of ampiifler B2 (Fig. 7) and contact 32 of relay 33, winding of relay Il and contact of the switch I5 to'ground. Relay 34 operates in this circuit and at its contacts 5I. short-circuits relay II which releases.` Operation of relay M also closes its contact I6- which connects 130-v6it battery to the middle of simplex coil l1 and thence to the multiple connection 52 between the ampliiiers BI, B2, and B3 which causes operation of relay 53. associated with the amplifier BI in a circuit including contact 6l of relay 64 (Fig: 6) and the corresponding relay associated with amplifier BI (Fig. 7). Also relay 38 operates to light cue lamp 29 as an indication tothe Hartford repeater attendant that the network is in use.

Release of relay 5i of the amplifier B2 extinguishes the direction lamp 54, operates equalizer relay which connects the equalizing net- .work 51 across the primary windingY of input transformer 59 and operates relay 5I which in turn lightsdirectlon lampV 5I. Relay in operating connects, at its contact III, a second short circuit around relay 5I to maintain this relay released in case relay 34 is released due to release of the control keyI at the New York studio and the consequent sequential release of relays l, 21, and 24. V

n Relay B2 of amplifier BI in operating operates relay 85 ,which relay connects 13D-voit battery, atlts contact 08, to the mid-point of simplex coil 61 from which point the circuit can be traced over the conductors loi line 88 in parallel to the Boston repeater station and to the mid-point of the simplex coil 89 vconnected across the line at that-point (Fig. 8).

From the mid-point of coil 89 the circuit can be traced over contact 1li of relay 1I, and through relay 12 to ground. Relay `12 operates in this circuit and opens-its contact 1I thereby preventing the accidental operation of the control key 1l at the Boston studio from interfering with the connection established by the operationf of key I at the New York studio as long as key I remains operated.

When battery was connected to the multiple connection I2, (Figs. 6 and '1) by the operation of relay 3l, it also caused relays in amplifier BI, corresponding to relays 53 and 85 of amplifier BI to operate thereby connecting 13D-volt battery to the line leading to the Hartford studio (Fig. 9)r where it caused operation of relay 16 thereby opening contacts 11 of that relay and preventing accidental operation of control key 1l from intel'- 'and 21 of amplifier A2 to operate thereby con-` necting 13D-volt battery to simplex lines 1l to Cleveland and. ll to the south, respectively. The effect of connecting battery to the line 1| at New York is exactly the same as that when battery is` connected to line Il extending `to Hartford and Boston which is also true with respect to connecting battery to the line Il extending from the south.

In the case of line 1l 'to Cleveland the circuit operation at the Cleveland bridging point (Fig. 2) is exactly thesame as at Hartford (Figs. 6, '1, and 9). From Cleveland a circuit li extends to Chicago where the studio circuit functions exactly the `same as the eastern terminal of the circuit, i. e. Boston (Fig. 8). In other words battery connected to the line II at the Cleveland bridging point extends from the midpoint of the simplex coil U2 at Chicago through relay I8 to ground which relay operates to prevent accidental or other improper operation oi' control key Il from effecting the switching of the amplifiers as established by key i at New York, as long as that key is held operated.

The key I at New York will in practice, be held operated until the end of the New York broad- `'cast at which time it will be released thereby conditioning the network to be reswitched for a program from another studio, for example, .Boston which will be later-described.

It will now be seen that amplifier Ai at the New York repeater station or bridging point (Pig.

left but all other amplifiers remain set to transmit from left to right, which is their normal setting.

'I'he program transmission circuitv can now be tracedfrom, the studio microphone. at New York (Fig. 5') through the associatedamplifier, over contacts Il and of relay 2l now operated to the New York broadcasting transmitter 22, it

being noted that whenrelay 2 is operated due to operation of the control kkey an energizing circuit is closed at its upper contact to operate relay 2l from battery l1 through relay 2l to ground.

Relay 2l in operating closes its contact 24 thereby closing a holding circuit for itself to battery 01 independent of relay 2.

Operation of relay 20 also closed its contacts 8l, It thereby connecting the microphone 22 to the line 2l extending to the repeater station, Fig. 4, where the circuit can be further traced over contacts l! and III of operated relay I1 to the primary winding ot repeating coil Il, the secondary of which is connected to the primary of input transformer I6, the secondary of which supplies the amplifier tube The output circuit of tube It' includes output transformer 92 having a tapped secondary, the entire winding of which being connected to conlductors I2 and the socalled monitoring tap being connected to conductors 94. The output of the tube 46 therefore has two branches, one over conductors 83 carrying the full gain of the amplifier which can-be traced over contacts 95 and 98 of relay |1 and conductors 91 tothe monitoring loud-speaker located at the repeater station and the other branch, 94,- of reduced energy output, which can be traced over contacts 98 and 99 of relay |1 and conductors |00 to transformer |0|. Conductors are also connected to corresponding relay contacts of amplifiers AI, A2 and A4 but as the relays of these amplifiers, corresponding to relay |1, are unoperated the circuit to these amplifiers is open at this point.

Transformer |0| 'couples conductors |00 to multiple connection 49 and' consequently the output transmission from amplifier A3 is further traced to the input circuits of amplifiers AI, A2

and A4 over contacts such as contacts |02 and |03 of normally operated relay 39 of amplifier A2.

Following the circuit through amplifier A2, for example, the transmission path extends to the primary of repeating coil |04 and thence through input transformer |05 to amplifier tube |06. The full energy output vof tube |06 can be traced through output transformer |01 over contacts |08 and |09 of relay 39 and conductors 0 to the cable line 30-extending to Hartford where it enters associated amplifier B2, Fig. 7, over contacts and ||2 of relay 56, and through repeat.

ing coil ||3 and input transformer 59 to the amplifier tube H4. Fromthe. output of transformer ||5two branches extend, one branch including the full secondary winding of transformer 5, conductors ||6, contacts ||1 and ||0 vof relay 50 and conductors ||9 extend to the monitoring loud-speaker at the Hartford repeater station and the other branch, including only a portion of the secondary winding of trans` former H5, conductors |20, contacts 2| and |22 of relay 56 and conductors |23 extends to the 'repeating coil |24 which couples conductors |28 with the multiple connection 52 which is bridged to the left-hand terminals of all'three amplifiers. Due to the fact that relay 5| of amplifier B2 is released the connection is open between conducv tors 52 and this amplifier, but as relay |25 of amplifier BI and the corresponding relay (not.

shown) of amplifier B3 are normal, i. e., operated the transmission circuit now branches in two directions, one branch being over contacts |26A and |21 of relay |25 of amplifier B| to the primary winding of repeating coil |28 and thence through input transformer |29 to the amplifier tube |30.y

The loutput of tube |30 includes the transformer |3| which has two branch-es; one the low` level output, including the tappedportion of the secondary winding, connected to conductors |32 terminates in resistance |33 over contacts |34 and |35 of relay |25. The other, or high energy i level branch of the amplifier output, which includes the full secondary winding of transformer |3|. is connected to conductor |36 and extends over contacts |31 and |38, of relay |25 and conductors |39 to line |6 extending to the Boston station where it terminates in transmitter |40 from which point it goes on the air.

The other branch of the transmission circuit from repeating coil |24 is through amplifier B3 from left and right in exactly the same manner as that just described for amplifier BI and thence over line to the Hartford broadcast station where it terminates in the transmitter |4|.v

Returning to the New York repeater station (Figs. 3 and M the lprogram transmission circuit 'trol key 14 at that point.

can be traced tothe west from repeating'coil |0| through amplifier AI from left to right in exactly the same manner as that described in connection with amplifier A2 of Fig. 3 or BIv of Fig. 6, and thence over line 19 to yCleveland (Fig. 2) where it terminates in an amplifier (not shown) identical with B2, for example, at Hart-ford. The program signals received are amplified by this amplifier, passed to a second amplifier videntical with BI oi Fig. 6 Aand extend over line 8| to Chicago where it finally terminates in the mitter |42.

At cleveland a third manner (not shown) but branches in two directions, one through amplifier B| and over line 68 to Boston and the other through amplifier B3 and over line 15 tothe Hartford broadcast station.

At New York the second branch extends to amplifier AI and over line 19 to Cleveland where it is branched in two directions, one branch passing over line 8| to Chicago and the other to the Cleveland broadcast station.

The third branch includes amplifier A4 and line 80 extending to the south where it may terminate in another bridging amplifier arrangement andbe branched in two or more directions or it may -extend to and terminate directly in a terminal 'broadcast station such as Washington, for example.

It will be understood that the line 30 fromNew York to Hartford, line 68 from Hartford to Boston, line 19 from New York to Cleveland, line 82 from Cleveland` to Chicago and line 80 from New York to the south may, and in practice will, include further reversible amplifying means adapted to be switched or set to transmit in` the direction of the program signals under control 'of the key at the studio originating the program. These amplii'ying means will be identical with the amplifier-s shown, as before described, and further description appears tobe unnecessary. f

It will now be assumed that when the program being transmitted by the New York studio is finished and some well known cue signal, 'such as a musical note, is transmitted over thecircuit which will be heard by an attendant of each of the connected broadcast stations, the attendant at New York will release key and, asBoston is scheduled to transmit for the next period, an attendant at the Boston studio will operate the conat` New York will release relay 2 thereby disconnectingbattery from the mid-point of simplex coil 4 connected across line 2|. Relay 2 in releasing also opens at its upper contact the original energizing circuit for relay buty this relay is now held over its own contact 24 and consequently dces not release at this time.

Removal of battery from the simplex'circuit comprising line 2| releases relay 8 of amplifier A3 which disconnects battery from the mid-point of simplex coil and also removes, at its conta'ct |4, the original short circuit from relay |5. lay, however, is now short-circuited at the inner` Chicago trans- The release of key This reuppervc'ont'acts of operated relay I1 and therefore circuit around this relay closed atcontacts 50 4of falsely operated to operated relay 55. y

The removal of battery from simplex coil 21 relases relay I of amplifier Bi and the corresponding relay of amplifier B3. Release of relay 5I releases relay 55 which removes battery from simplex coil 51 which, in turn, releases relay 12 at the Boston studio which closes its contacts 13 thereby preparing a circuit to operate relay 1|, under control" of key 14, which we have assumed will be next operated.

The removal of battery from simplex 'coil 31 aflso causes the sequential release of relays (not shown) in amplifier B8 corresponding to relays 5l and 55 of amplifier BI which causes battery to be removed from the simplex'path over line 15 to the Hartford studio which causes the release of relay 15.

It will be noted that at all studios other than the one originating a program the operation of relays, such as 12 of Fig. 8v and 15 of Fig. .9, in addition to opening their upper contacts.' also close their lower contactsthereby short-circuiting the corresponding switching relays |44 and |45 thereby insuring that these relays will not be interrupt the program being received and broadcast. Similar relays release in the same manner at Cleveland and Chicago (Fig. 2) due to disconnection of battery from the midpoint of simplex coil of theANew York amplifier A3.

` As before stated. it will now be assumed that Boston is to originate a program for the next broadcast period and therefore when the cue signal is heard by the Boston attendant he will 'operate key 14. With key 14 operated, as soon as relay 12 releases. as Just described. a circuit is closed to operate relay 1| whichconnects battery at its lower contacts, Vto simplex coil 5s and at its upper contacts closes an energizing circuit for relay |44 which operates and connects the studio microphone |44 through the amplifier to line 8l in multiple with the transmitter |45.

Battery on the simplex circuit of line il can be traced to the mid-point of coil 51 of the Hartford amplifier Biwhere it operates relay I4 which connects battery to simplex coil |41 and short-- circuits relay` |25 which releases. The release of relay |25, closes its of amplifier BI which is identical with the reversal operation of 4amplier AI as more fullyv battery to simplex coil 3| and. at its contacts |54, short-circuits relay 55 which now releases.

The release of relay 50 opens its contacts 50 thereby removing the remaining short circuit around relayll which relay now reoperates. Amplifier 'false operation of key inner lower contacts, thereby' operating relay |45, thus completing the reversal l B2 is now reversed and arranged to transmit from right to left, i. e., from Hartford to New York.

Battery connected to simplex coil |41 operates relays (not shown) in amplifier Bl corresponding to relays |49 and 33 of B2 which connects battery over the simplex circuit line at the Hartford 'studio which operates relay 15 which again opens contact 11 to prevent 18 from interfering with the switching arrangement established by key 14 at Boston. Relay 16 in operating closes its lower contacts thereby short-circuiting relay-'|45 and insuring that it will not be falsely operated;

As amplifier B3 was not reversed for the preceding broadcast itsswitching relay corresponding to relay 5| of B2 was operated and the relay :corresponding to 58 was released and consequently operation of the relay corresponding torelay Il has no effect on the relay corresponding to relay 56 and the amplifier remains in its normal condition, i. e., arranged to transmit from left to right into line 13 to the Hartford studio. Up to this point it will be noted that amplifier Bi'is reversed, i. e., is adapted to transmit from right to'left from line 6I coming from Boston into the other two amplifiers B2 and BI.- Amplifier B2 which was in a reverse condition. i. e., arranged to transmit from the NewYork line 3l' to the other amplifiers BI and B3 during the previous broadcast is now turned around into its normal condition, i. e.. adapted to transmit from the output of amplier BI into the New York line Il.

Amplifier BI remains ,in its normal condition, i. e..

Varranged to transmit from the output of amplifier Bi into the Hartford studio line 15.

Battery connected to coil Ily of amplifier B2 by the operation of relay passes over line Il to the New York repeater station and operates relay t2v of amplifier A2 which connects battery toslmplex coil 25 andshort-circuits relay 35 which now releases and' in turn operates relay 4l thereby completing the reversal of this amplifier and conditioning it'to transmit from right to left, i. e., from line Il into the other ampliers of the station, i. e., AI, AI, and A4.

Connection of battery at the mid-point of coil 25 operates relay |5| of amplifier AI which in turn operates relay 1. Relay 1 in operating connects battery to the simplex coil 5 and short-circuits relay |1 which releases and removes .the short circuit from relay i5 which thereupon reoperates thereby restoring this amplifier toits normal oondition, i. e., for transmission from left to right and into line 2l to the New York studio. Connection of battery to the simplex coil 5 connected across line 2| Icauses relay. |62 at-the New York superimposed on in ampliers AI and A4 corresponding to relays |5| and 1, for example, of amplifier AI which connects battery over the simplex circuit of linev 1s to Cleveland and line 55 to thev south, respectively. l

Battery on the simplex of line 1! causes relays. not shown, at Cleveland to operate which connects battery to the -simplex circuit of line Il at Chicago and tothe Cleveland studio line for the purpose of locking out the control keys at these points, key 84 at Chicago, for example. As the amplifier at- Cleveland waspointed in the normal direction, i. e., from left -to right to transmit from line 1l to Chicago, and to the Cleveland studio, this amplifier remains in this condition for` the present broadcast which also arrives over line 19.

'Ihe program transmission circuits now estab- -arrangements, employing the same method and instrumentalities for securing transmission over a network in either direction with one-way amplifiers or repeaters may be used if desired with# out departing from the spirit of the present invention as covered in the appended claims.

What is claimed is:

l. In a two-Way program transmission system, a plurality of program stations adapted to transmit or receive a program, a repeater station, a line for each lprogram station connected in multiple to all other'station lines at said repeater station, a one-Way voice frequency amplifier in each line at the repeater station and normally pointed toward its respective program station, a simplex circuit, comprising the conductors of all said lines, extending from each station to all other stations, sources of direct current, manually controlled switching means at each station .for connecting one of said current sources to said simplex circuit. means associated with each am-I plifier adapted to repeat said direct current around the amplifier in either direction, other means associated with each amplifier responsive to direct current arriving over the simplex circuit from. its respective program station to reverse said amplifier and point it toward theother lines, and other means at each station responsive to direct current arriving over the simplex circuit for disabling the switching means thereat as long as said incoming current persists.

2. In a program transmission system, a plurality of program stations, a line for each station connected' to all other station lines, one-way amplifiers in each line, means associated with each amplifier adapted, when operated, to reverse the connection of the amplifier with respect Y to its normal transmission direction in the line, manually controlled switching means at each station for transmitting a control current over its line to all other lines to actuate all reversing means associatedwith amplifiers whichareI normally connected to transmit toward the station at which the switching means is operated, and means at .each station responsive to the contro-l current received over the respective lines to disable the switching means thereat as long as said control current is received.

3. In a two-way program transmission system, a plurality of program stations, lines for eachstation connected in multiple to all other station lines, a one-way amplifier in each line adapted to be reversed with respect to its line, relay means associated with each amplifier for reversing the amplifier, manually controlled switching means at each station for operating all reversing means associated with ampliers in the system which are not normally connected to transmit away from the station at which the switching means is operated, and other means at each station responsive to operation of av switching means at one station4 to disable the switching means at each of the other stations as long as the`switching means at the one station is operated.

4. In a two-way program transmission system, a plurality of program stations, lines i'or each station connected in multiple to all other station lines, a one-way ampliflervin each line,'means associated with eachamplier to reverse the pointing of said amplifiers in their respective.

lines, manually operated switching means at each station adapted to transmit direct current over all of the lines to operate the reversing means associated with amplifiers pointed in the direction of the station at which thelswltching means is operated,- and other means at each station responsive to direct current, transmitted from anotherstation, to prevent transmission of direct current therefrom.

. 5. In a transmission system, a plurality of lines connected together at one end and extending to stations at the other ends thereof, a. one-way amplifier in each line, eachamplifler being normally connected, in its respective line, to transmit in the same direction with respect to the convnecting point, means associated with each amplifier to reverse its transmitting direction With're- 'spect to its vassociated line, manually controlled switching means at each station for transmitting control current over all of said lines toactuate all amplifier reversing means associated with amplifiers which 4are normally pointed in the directionof the station at which the switching means is actuated, said reversing means being arranged 'to operate only when said control current is received from the direction ltoward which the associated amplifier is normally pointed, and means at each station responsive to control current transmitted from another station to disable. f

the switching means thereat while said control current is connected to the line. n

6. In a program transmission system, a plurality of program stations,'a line for each station connected in multiple to all other station lines, one-way'ampliers .connected in each line, sources of control current, manually controlled switching means at each station for connecting one of said Vcurrent sources to the respective line,

means associated with each amplifier adapted to normally connected to transmit, and other means' at each station responsive to control current received over the respective line to disable the switching means thereat and prevent the connection of other control currents to the line as long e as said first control current is received.

'LA system in accordance with claim 6, in which each of said amplifiers has two output circuits, each of different energy level, and in which the higher energy output is normally connected to the line, characterized in this that means are provided responsive when said amplifier is reversed to disconnect the higher energy output from' the line 'and substitute the lower energy output therefonand to connect the higher energy output to a monitoring receiver'.

8. A system in accordance with claim 6, in which each amplifier has an input. circuit and two output circuits, each output circuit being of amplifier are iirst disconnected from the line and after a momentary delay the input circuit and the low level output circuits are connected to the line in the opposite transmission direction.

ALBERT E. SMART BRAND.

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