Signaling system for radiotelephones
United States Patent 2064958

UNITED STATES PATENT OFFICE 2,064,958 SIGNALING SYSTEM FOR RADIOTELEPHONES Edmund R. Taylor, Mount Vernon, Charles C. Taylor, Flushing, and Paul W. Wadsworth, Elmhurst, N. Y.; said Wadsworth assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York,...

Taylor, Edmund R.
Taylor, Charles C.
Wadsworth, Paul W.
Application Number:
Publication Date:
Filing Date:
Edmund, Taylor R.
Charles, Taylor C.
Primary Class:
Other Classes:
24/265A, 370/343, 370/496, 455/504, 455/517
International Classes:
View Patent Images:



Taylor, Flushing, and Paul W. Wadsworth, Elmhurst, N. Y.; said Wadsworth assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York, and said Edmund R. Taylor and Charles C.

Taylor assignors to American Telephone and Telegraph Company, a corporation of New York Application May 26, 1934, Serial No. 727,800 4 Claims. (Cl. 250-6) This invention relates to radiotelephone or land wire transmission systems, to transmission of signaling pulses for alarms, controls or telegraph and to selective signaling means for use on such systems. More particularly it relates to arrangements for selectively calling any one of a number of stations in such systems from a central point, and in certain cases from any other station. Stations may also be called in groups. Likewise the selective apparatus may be used for other control arrangements and the means for transmitting the signal pulses may be utilized for other purposes.

In operating a radiotelephone circuit, such as a harbor radiotelephone system, it is necessary that a central station be able to call any one of the boats, and that the boatsi be able to call this central station. It may also be desirable for a boat to be able to call other boats. It also is essential that the calling of any boat by a central station be extended to a toll switchboard position permitting a toll operator to handle traffic efficiently over this system. In certain leased wire full or partial period talking circuits set up on land wires similar calling arrangements between the stations by using voice-frequency transmission of the signaling pulses may be required. It is also desirable to call the stations in large groups in cases of emergency or in smaller groups as the more efficient means for broadcasting information. The signaling system of this invention is designed to meet the requirements outlined above.

As is well known, a radio' transmitter and radio receiver may be connected by means of a combination of four-wire and two-wire circuits to a two-wire terminal at a switching point where connections may be established by a switching operator between the radio channel and various telephone circuits. It is also well known that terminal amplifiers, voice-operated devices, and other apparatus under the control of a technical operator may be associated with the four-wire circuit in such a manner that echoes and singing, which might be caused by radio or wire line unbalances, are effectively eliminated, and radiation by the radio transmitter of signals received by the radio receiver is prevented.

0g Likewise on shipboard a four-wire circuit may be made up of a radio transmitter on one branch, a radio receiver on the other, and various controls and necessary auxiliary apparatus.

In accordance with this invention circuit ar5 rangements are provided whereby pulse sending equipment is associated with the four-wire land terminal, control of which is located at the terminal and is also extended to a distant toll switchboard. This pulse sending equipment sends spurts of 1500-cycle tone under control of a dial or selector key, and also controls the terminal in such a manner that static crashes on the receiving branch may not break up the pulsing. At the ship end of the circuit a selective receiving ringer is connected to the output of the radio receiver causing an alarm when the code for that particular ringer is sent in spurts of 1500-cycle tone. Arrangements may also be set up to send pulses from the boat to signal other boats and the land station. Several combinations of these arrangements together with automatic calling-in of the land station under control of the ship's carrier are possible. It will be understood, of course, that the arrangements used at the ship and shore stations may be any of many well-known arrangements and that the invention is not to be limited to a ship-to-shore radiotelephone system.

The invention may now be more fully understood from the following description when read in connection with the accompanying drawings, Figure 1 of which shows the circuit arrangement for one terminal of the radio system embodying the signal sending arrangements; Fig. 2 of which shows a circuit arrangement embodying the signal receiving unit which may be used for one or more radio stations to be communicated with; Figs. 3 and 5 show modified forms of signal transmitters; Fig. 6 shows a modified form of signal receiver; and Figs. 4 and 7 show modified forms of terminal arrangements particularly adapted for use aboard ships.

Referring to Fig. 1, SL designates the twowire line connecting the control station to the switchboard. The two wire line SL is connected through a hybrid coil, HBi, to a transmitting path TLi, and a receiving path RL1, being balanced by a suitable network N1. Path TLi and the following parts of the transmitting circuit include the transmitting volume control variable attenuator TVC, amplifier TA, hybrid coil HB2, transmitting path TL2, transmitting path TL3, transformer Ti, and radio transmitter RTi. The receiving path includes receiving amplifier RA2, radio receiver RRi, receiving path RL2, transformers T3 and T4, resistance termination RT2, receiving volume control attenuator RVC, amplifier RAi and receiving path RLi.

In order that when transmission is taking place over either the transmitting or receiving path the other path will be disabled, echo suppressor devices are associated with each path.

The echo suppressor associated with the transmitting path TL1 comprises an amplifier-detector unit TAD, of a well-known type, with its input connected through variable attenuator TSC and band-pass filter Fi to hybrid coil HB2 in conjugate relationship with transmitting path TL2, the output of amplifier TA being balanced by a suitable network, N2. The output of the amplifier-detector unit TAD controls through master relay TM the relays TSS and TEHO for performing the blocking or disabling operations whereby transmission is permitted in only one of the paths TL2 and RL1 at a time. Normally, transmission path TL2 is disabled by the back contacts of relay TSS, which short-circuit the conductors, and transmission from transmission path TL1 to, the radio transmitter can not take place. Blocking means comprising transformer arrangements T3 and T4 are associated between receiving lines RL1 and RL2. These disabling means comprise transformer arrangements which are so connected as to be balanced when the contacts of relay TEHO are opened, thereby preventing transmission through the transformer arrangements. When the contacts of relay TEHO are closed, however, the transformer circuits instead of being balanced are, in effect, connected so as to be parallel-aiding, thereby permitting transmission to take place readily. Normally, relay TEHO (which is controlled by the amplifier-detector TAD) is deenergized so that the receiving path is unblocked to permit transmission to take place from receiving path RL2 to the two-wire line SL.

The input of an amplifier-detector arrangement RAD is connected through band-pass filter F3, receiving sensitivity control RSC and resistance termination RTi to the secondary windings of transformer T4, so as to be effectively in series with the resistance termination RT2 in the receiving path RL1, and has in its output a relay RM, which when operated opens the control circuit from the amplifier-detector TAD to switching relays TSS and TEHO, thereby maintaining the transmitting path TL2 blocked and receiving path RL unblocked when signal currents are being received from the radio receiver. Static or other interference is prevented from causing operation of relay RM duing idle periods by the action of relay COD which maintains a short circuit on receiving path RL2 whenever receiving carrier f2 is not present. As will be explained later in connection with Fig. 2, the distant or boat transmitters are so arranged that carrier frequency f2 is radiated only when communication from a boat to the shore is desired. The control circuit of relay COD includes the following. The input band-pass filter F4 which passes substantially only carrier frequency f2 is connected in multiple with the input of radio, receiver RRi, and the output of filter F4 is connected to the input of detector CD, the D. C. output of which controls the operation of relay COD. The operation of the apparatus so far described for signal transmission is as follows: Signal currents incoming over the two-wire circuit SL pass through the hybrid coil HBI and through the amplifier TA. Some of the energy enters the amplifier-detector unit TAD and causes relays TM, TSS and TEHO to be energized.

Relay TSS removes the short circuits from transmitting line TL2 while relay TEHO by opening its contact causes the transformer arrangements Ta and T4 to block the receiving path.

In the meantime, a part of the energy passes from hybrid coil HB2 through the transmitting paths TL2 and TLs to radio transmitter RTi and is transmitted on carrier frequency fi. The receiving amplifier-detector RAD associated with the receiving path cannot be actuated to disturb the circuit condition now existing because the receiving path is disabled in the manner already described.

If, in the normal condition of the circuit, carrier frequency f2 modulated with a signal is received through amplifier RA2, part of the received energy passes through band-pass filter F4 to detector CD, causing the operation of relay COD which removes the short circuit from the receiver RR1 and is demodulated, the resultant mainder of the received energy passes into radio receiver RRI and is demodulated, the resultant signals passing through receiving path RL2 and transformer arrangements T3 and T4. Part of the energy passes through receiving amplifier RAi and hybrid coil HBi to the two-wire line SL. The remaining received energy passes through receiving sensitivity control RSC to the amplifier-detector RAD to cause the operation of relay RM, which opens the circuit controlled by the amplifier-detector unit TAD, thereby preventing energization of relays TSS and TEHO so that the transmitting path is disable and the receiving path remains unblocked while the signal currents are being received.

The apparatus and circuits employed at the ship station may be of several well-known types. as, for example, those shown in Fig. 2. This shows an antenna normally connected to radio receiver RR2, which is arranged to receive carrier frequency fi, and the radio transmitter RT2 arranged to transmit carrier frequency f2.

The antenna may be switched from the radio receiver RR2 to radio transmitter RT2 by relay AR, which is controlled by key AK. The output of radio receiver RR2 is connected to operator's receiver OR2 and the input of radio transmitter RT2 is connected to operator's transmitter OT2.

Accordingly the operator at the ship station may normally receive signals transmitted on carrier frequency fi and may transmit signals on carrier frequency f2 by operating key AK, which enables radio transmitter RT2 and disables radio receiver RR2. Thus carrier frequency f2 is radiated only when signals are to be transmitted from the boat.

The signal receiving unit RUi is bridged across the output of the radio receiver along with the operator's receiver OR2.

The apparatus so far described relates to telephonic or other communication between the twowire line SL and a distant radio station. The additional equipment and circuits which are associated with the shore terminal in order to afford dial signal transmission and alarm features will now be described.

Referring to Fig. 1 and the operation of the echo and singing suppressing means described above, it will be seen that relay TM must be operated whenever signals are transmitted from the two-wire line SL or from the oscillator O0 to the radio transmitter RTi, and that relay COD must be operated whenever signals are transmitted from the radio receiver RRi to the two-wire line SL and, hence, one or the other of these two relays TM or COD must be operated whenever communication is attempted in either direction of transmiission. When relay TM is operated, relay RM cannot be actuated to disturb the circuit condition. Accordingly, in sending signal spurts from the central station, the signal tone is within the voice range and operates the relay TM. The operate contacts of relay TM are also closed to hold the circuit in the transmitting position. The operation of the circuit is as follows: The 1500-cycle oscillator 01 is bridged across the input of amplifier TA through the high resistances HRi and HR2. The oscillator output is normally shorted through the pulsing contacts of dial D2, and the contacts of relay R4. When dial D2 is moved from its normal position, its lower contacts close, shorting the contacts of relay TM and operating relays TSS and TEHO.

When dial D2 is released, its upper contacts are opened once for each unit of the digit dialed. This sends spurts of 1500-cycle tone through amplifier TA and the remainder of the transmitting path in the manner already described.

The lower contacts of dial D2 open when the dial returns to normal.

The sending of signals from the central station may be accomplished from a distant toll switchboard as follows: Dial Di and dial key Ki are connected through a telegraph repeater and line DCLi to relay Ri. When key K1 is operated, relay Ri operates, in turn actuating the slow-release relays R2 and R3. Relay R3 shorts the contacts of relay TM, operating relays TSS and TEHO. Relay R2 connects the upper contact of Ri to the winding of R4. When dial Di is operated, relay Ri releases once for each unit of the digit dialed.

Relays R2 and R3, being slow-acting, do not release, however. Therefore, relay R4 is operated by each release of Ri and 1500-cycle spurts are sent out as previously explained. When key Ki is released at the conclusion of the dialing period, a final spurt is sent by the release of Ri before R2 has released. This is made use of as will be explained in connection with Fig. 2. For giving alarms or signals when relay COD is operated, suitable circuits are set up to indicate that a call is desired by some distant station or that a distant station is answering after being dialed. The circuit includes the following: The lower contacts of relay COD, key K2, relays R5, Re and R7, key K3 and bell or buzzer B2 for giving an alarm at the central station and, in addition, line DCL2, relays Rs, R9 and Rio, lamps Li and L2 and bell or buzzer Bi for giving an alarm or signal at the switchboard. The operation of the circuit is as follows: The operation of relay COD by carrier frequency f2 connects ground to key K2, thereby short-circuiting the winding of relay R5, which is normally operated as shown if key K2 is in its normal position, that is, with the contacts closed.

Key K2 is provided so that the alarm arrangements may be disabled when alarm operation is not desired. Relays R5 and R6 are made slowacting in a well-known manner such as shortcircuited windings or the addition of a copper slug to the core in order that intermittent short duration operations of relay COD, such as might be caused by static, may not give an alarm. To continue the circuit operation, relay R5 releases, after a predetermined interval of time has elapsed, and its contacts close short-circuiting in turn, the windings of relay Re. After a second predetermined interval of time, relay R6 76 releases, causing the operation of relay R7.

This, through its lower contacts, completes the circuit to the bell or buzzer B2, giving an alarm.

If key K3 is in its normal position with its contacts closed, relay R7 will lock itself operated through its upper contacts and the alarm will continue until Ks is operated to break the battery circuit.

The remainder of the circuit operation required to operate the alarm and signals at the switchboard is as follows: Relay Rs, in releasing, places ground on the line DCL2, causing the operation of relay Rs.

Relay Rs through its contact operates relay R9, lights lamp Li and causes bell or buzzer Bi to operate. Relay R9 locks up through the contact of relay Rio and the alarm and lamp will remain actuated until the operator at the toll switchboard places a cord in the line jack J1 when battery flowing through the sleeve circuit will operate relay Rio, permitting R9 to release and removing battery from lamp Li and bell Bi.

If the operator has placed a cord in the line jack and has dialed a ship she will receive an indication that the ship has answered by the lighting of lamp L2. The cord in the line jack has operated relay Rio, and when relay COD is operated by the boat answering, relay Rs will be operated4 as previously explained, which will send battery through its contact and through the upper lower contact of relay Rio to lamp L2. During the period of a call, it is expected that key K3 at the central terminal will be operated so that further operation of relay R7 and bell or buzzer B2 will not occur as COD is operated. At the conclusion of each call, key K3 will be restored to normal. This will permit relay Ra to operate and light lamp L2 whenever relay COD operates for a length of time sufficient to release relay R6. Intermittent flashing of L2 will therefore occur during conversation, but a slow steady flash of L2 for switchhook supervision may be obtained by the boat station operating and releasing at a slow interval the switch AK in Fig. 2.

Referring to Fig. 2, which represents the ship apparatus for ship-to-shore communication, the carrier frequency fi modulated by the 1500-cycle spurts is received by radio receiver RR2, and when keys SH and AK are not operated the 1500-cycle output of RR2 enters the signal receiving unit RUi. This is essentially a twostage amplifier with a volume limiter action having two tuned output circuits working into rectifying arrangements whose D. C. outputs are made equal and control a pulse dividing relay circuit. This circuit controls a railway selector in such a manner that one step of the selector occurs for a marking and spacing pulse together.

The selector consists essentially of a polar relay with a ratchet attachment so arranged that successive operations of the relay at proper speed cause the stepping around of a contact wheel.

Stop pins at certain points prevent the contact wheel from returning to its normal position when the regular sequence of stepping is interrupted at these points. Any interruption of the regular sequence of stepping when the contact wheel is at any other point causes it to release.

The use of these selectors on railway telephone dispatch systems is well known. The speed of pulsing used in this system is about seven cycles per second.

The signal spurts of 1500 cycles enter the twostage amplifier through the transformer IN.

The permanent grid potential on the first tube is small ard the input potential if of sufficient magnitude drives this positive, charging the condenser Ci and thereby lowering the gain. When the input is removed, the resistance LR1 discharges condenser Ci and the gain increases.

3 These units are so chosen that the gain of the amplifier increases somewhat during a spacing pulse while dialing. Also, the gain of the amplifier is lowered in a period of time which is longer than the operate time of relay R1. Dialing spurts tend to lower the gain and thereby decrease the static currents in the relay but the initial spurt of rectified current at the beginning of each marking pulse before the gain is lowered is large and by its action in operating relay Rn enables signaling through a higher noise level than would be possible without this action.

When no dial spurts are being received the static and voice currents lower the amplifier gain, thereby reducing in magnitude the peaks of rectified current in the balanced branches, helping appreciably to eliminate false operation from any unbalance which may occur in the static or voice currents. This is particularly valuable in the case of voice currents where the characteristics of speech are such that there may be considerable frequency inequality.

The output of the first stage tube VTi is impedance-coupled to tube VT2 by resistance PRi, condenser C2 and resistance GRi. Tube VT2 is transformer-coupled to two Rectox circuits RRn and RR12 by means of two output transformers OTi and OT2. Transformer OTi is tuned to 1500 cycles by means of retard coil TRi and condenser TCi while output transformer OT2 is tuned to 500 cycles by retard coil TR2 and condenser TC2.

The rectified output currents are opposing when applied to relay Rn, the current from OTi being in the direction to cause relay Rn to operate while current from OT2 is in the direction to aid the permanent biasing current supplied through resistance BRi and hold relay Ru in the nonoperated position. Noise and voice currents, therefore, tend to balance out and produce no operation of relay Ru, although actual voice currents have such unbalanced frequency characteristics that occasional operation of relay R1 may occur particularly at the start of an exclamation before any gain regulation of the amplifier occurs. The sensitivity of the amplifier is such that relay Ru may be successfully operated for an input range from about 25 db. above 1 milliwatt to about 20 db. below 1 milliwatt.

Relay Rn is operated by a marking signal (spurt of 1500-cycle current) for each dial pulse. The biasing current returns it to the non-operated position for spacing pulses. With relays R12 and Ri3 in the position shown in Fig. 2 when relay Ru operates, relay R13 operates reversing the polarity across the condenser C3 and the selector winding and operating the polar relay in the selector RS while the condenser is charging.

When Ru releases, Ri2 operates and R13 remains operated. When Rn again operates, Ri3 releases and reverses polarity on the selector RS while R12 remains operated. When Ru next releases, R12 releases and Ri3 remains non-operated. Thus relay R13 reverses polarity on and operates the polar relay in selector RS for each operation of relay Rn or for each unit of the digit dialed. The polar relay operates while the condenser C3 charges but releases when the charge completes.

The condenser Ca and resistance unit CRI are such that the polar relay does not release during the dialing of a digit (at the speed of pulsing 15 used in the system) but does release between digits permitting all contact wheels in the selectors not resting on stop pins to return to normal. When any contact wheel has been stepped up and rests on the final stop pin, battery flows from terminal 2 through the contact wheel to ., terminal 3, 4, 5 or 6 and the bell or buzzer Bi.

Terminal 2 is the arm which holds the stop pin and prevents the wheel from returning to normal. The bell rings then only when the contact wheel comes to rest on the final stop pin. The bell will ring until a final dialing spurt is sent or until the switch SH is operated. As mentioned above when dialing from the toll switchboard, this final spurt is sent when the dial key is returned to normal by the operator. The key SH I5 is normally open but is closed when the handset is lifted from the switchhook. This key operates relay Ru which discharges the condenser C3 through the selector windings, causing it to operate once and return to its normal position. No operations of selector RS may occur while relay R14 is operated.

Returning to Fig. 1, the dial Di being an ordinary telephone dial, signals by sending a marking and spacing impulse for each digit unit and is not capable of sending signals corresponding to digits of more than ten units. The railway selector of Fig. 2 is so arranged that where any particular station is to be called the selector responds to impulse in groups corresponding to a plurality of digits, as for example 3-7-7 or 4-6-7. As each digit does not exceed ten pulses these digits may be sent by successive operations of the dial Di. Where, however, it is desired to select a group of stations, the dial Di cannot ordinarily be used due to the limitations of the selector RS which is so arranged that for group selection a continuous train of pulses must be received which is equal in number to the sum of the pulses of the code digits of any individual 4 station. In a system having a considerable number of stations this will involve sending a continuous group of pulses greater than ten and hence the dial Di is not adapted for group calling. A common arrangement of the selector is to make it responsive to, group calls involving: continuous trains of say 17, 19 or 21 pulses, Thus one group of stations might be set to respond to a train of 17 pulses, another group to a train of 19 pulses, etc. The groups of stations might include in part the same stations, or one group might be a sub-group of the other.

The transmission of these large trains of impulses makes it necessary to supplement the dial Di by one or more railway selector key sending units of the type shown at SK in Fig. 3, one 55 such unit being provided for each group to be called. Each selector key unit involves a code disc having a number of teeth suitably arranged to operate pulsing contacts. The dial may then be used for calling any one of a small number 60 of stations and the selector key unit may be used to call a group. For a large system it will be preferable to omit the dial and use a master railway selector key of known type both for calling individual stations and for calling, 65 groups. The master railway selector key is so arranged that by setting certain combinations of keys the proper series of pulses for calling any individual station or any group may be sent.

If either the selector key unit or master rail- 70 way selector key is provided, since the dial D1 sends a marking and a spacing pulse for each unit of the digit dialed, whereas the selector key unit and the master railway selector key each send only one pulse (either marking or spacing), 75 for each unit of the digit, it will be necessary to provide the selector keys with pulse multiplying relays such as Rai and R= of Fig. 3. These put out a marking pulse followed by a spacing pulse for each pulse sent by the selector key whether the selector key pulse be marking or spacing.

The operation of this circuit is as follows: When key SK turns so that its contacts close relay R21 operates. Battery flows through resistance CRi, and the lower contacts of R21 to charge condenser Ci. Relay R22 operates on this charge current, condenser Ci and resistance CRI being so chosen that the time during which R22 remains operated is 1/2 that during which the contacts of SK remain closed. When the contacts of SK open R21 releases and Ci discharges through resistance CR2 and the windings of R22 causing R22 to operate for the same period of time that it operated when Ci was charging through CRi. Therefore, relay R22 operates and releases putting out a marking and a spacing pulse of equal length each time the contacts of SK close or open. The contacts of R22 are shown in series with a dial Di shorting the output of an oscillator when Ai connects to A2 and Bi to B2. Spurts of tone will be sent from the oscillator as R22 operates or the dial opens its contacts. If remote control of the oscillator is desired as' shown on Fig. 1 the contacts of the dial Di and relay R22 may be placed in series between key Ki and telegraph repeater TR as shown in Fig. 3 by connecting Ai to A3 and Bi to B3 in which case the telegraph circuit will be controlled directly instead of the oscillator output. This control may then be applied to a distant oscillator in the manner shown on Fig. 1 and explained previously. It should be noted that the combination of dialing equipment and oscillator may be used with central control equipment as shown on Fig. 1, or may be multipled with operator's transmitter OT2 as in Fig. 4 and used to modulate radio transmitter RT2 or may be associated with any signal transmission system which will transmit the frequency output of oscillator O1.

The circuits shown in Figs. 1 and 2 are arranged for ship-to-shore communication and hence the shore station sends on frequency fi and receives on frequency f2 while each ship circuit as shown in Fig. 2 sends on frequency /2 and receives on frequency fl. For communication between ships it is desirable that a different frequency be used as otherwise the supervisory signals BI and B2 of Fig. 1 would be operated at the shore station every time a call between ships took place. Hence the ship stations should be arranged to transmit and receive on some third frequency f3 for intership communication as shown in Fig. 4 in which a separate set of transmitting and receiving equipment is provided for communication between ships.

If we now consider Fig. 4 we see a ship station arranged for two-way selective signaling and using the receiving unit amplifier as an oscillator when the dialing key DK is thrown to permit dialing from this station. The operation of the unit as a signal receiving device is as previously outlined for Fig. 2. When it is desired to send pulses from this station key DK is operated. This puts the contacts of dial D across the input of radio transmitter RT'2 the input being shorted except when these cons75 tacts open once for each unit of the digit dialed.

Key DK also operates relays Ri5 and Ri6, R16 in turn operating R14, R17 and AR. Rin connects the tuned output transformer OTi primary winding through its lower contacts and condenser C5, resistance FBR and inductance FBI to the input transformer providing a feedback circuit which may be adjusted by well-known means to give proper phase relations and amount of energy fed back so that a two-stage oscillater is set up. The frequency of oscillation will be that of the tuning of OTi, 1500 cycles in this case. The amount of energy fed back is such that condenser Ci is charged and volume limiting action is set up. The oscillator will, therefore, maintain a constant output level because of the volume limiting action of its amplifier.

Relay R15 connects the output of the oscillator through output resistances ORi and OR2 to the input of the radio transmitter RT'2. As dial D is operated spurts of 1500-cycle tone enter the radio transmitter. Relay Ri6 when operated puts ground on the leads from switches A.K. and S.H. This operates relay AR switching the antenna to the transmitter RT2, relay Ri7 removing the plate supply from the receiver RR2 and Ri4 disabling the selector RS operating circuit.

The circuit is thus set up to transmit the dial pulses. When dialing is completed key DK is returned to normal and the circuit is under control of switches A.K. and S.H. for normal service. It is obvious that the combined transmitting and receiving unit SRUi may be used with any transmission system either two-wire or four-wire by proper application and, therefore, that its use is not to be confined to the example here given. We have described Figs. 1, 2, 3 and 4 showing a system of signaling using spurts of 1500-cycle tone. The successful operation of this system through heavy static is somewhat dependent upon having tone on only for the dial spurts the amplifier constants being adjusted for maximum efficiency when using spurts. However, the system may be set up to have steady tone on when dialing is in progress and to interrupt that tone by the dial. Such a system under most static conditions will work as effectively as the spurt system. This system will be referred to as the interrupted tone system and this invention should not be confined to the spurt system, but should be understood to include the interrupted tone system.

Likewise we may set up this signaling system so that each unit of the digit or code is sent by either a marking or a spacing pulse. Such a system is partly spurt and partly interrupted tone since the pause between digits in the code may be either a marking or spacing pulse depending on the preceding digit being odd or even. Fig. 5 shows the dialing arrangements for such a system. The selector key SK operates relay R2i opening the lead Ai Bi. Ai Bi will connect to an oscillator or D. C. control circuit as explained for Fig. 3. When key SK is in its normal position R21 is released. It is controlled by SK to send either a marking or spacing pulse for each unit of the code. Dial Di normally sends a marking and spacing pulse for each unit of the digit so that pulse dividing relays R22 and R23 are associated with it here.

These put out a marking pulse or a spacing pulse for each code unit comprising a marking and a spacing pulse from Di as explained in connection with the pulse dividing relays Ri2 and Ri3 in Fig. 2. Key K1 when closed as shown always assures that relay R23 is non-operated and lead Ai Bi closed. Opening Ki permits the dialing from Di.

With this system of pulsing the pulse dividing relays Ri2 and R13 are no longer required in the signal receiving unit. The selector RS is then controlled directly by relay Rn as shown in Fig.

6. When Rn operates condenser C3 is charged from +200 through CRi, the windings of RS and the lower contacts of Rn operating RS. When Rn releases Ca discharges through CRi, the upper contacts of Rn and the windings of RS operating RS in the opposite direction.

One additional example of the use of a selective signaling unit is illustrated in Fig. 7, in which the features of Figs. 2 and 4 are combined. In this figure a radio transmitter RT is provided on board ship, and by switching to either of two oscillators 02 and 03, generating frequencies f2 and f3, the transmitter may be used to transmit either to the shore station or to another ship. Two radio receivers are provided on board the ship, the receiver RR2 being used to receive from the shore station and the receiver RR'2 being used to receive from another ship. For signaling between ship and shore the frequency fi is used from shore to ship and the frequency f2 is used from ship to shore, but in communicating between two, ships the same frequency fa may be used in both directions.

Two antennae Ai and A2 are provided. The antenna Ai is used for both sending and receiving under the control of the key AK, the antenna being connected normally for receiving but being connected to the radio' transmitter RT when the key AK is actuated. By means of the key CK the antenna At may be connected to! either the radio receiver RR2 when communicating with the shore station or to the receiver RR'2 when communicating with another vessel. In the stand-by position, antenna At is connected to, the receiver RR2 to receive signals from the shore station and the antenna A2 is connected to the receiver RR'2 to receive signals from another vessel. When, however, the key CK is operated in order to, establish connection with another vessel, the antenna Ai is connected to the receiver RR'2 and the antenna A2 is shifted to the receiver RR2 in order to receive a code signal from the shore station if one should be received while the vessel is communicating with another vessel. The two receivers RR2 and RR'2 are connected to two input transformers in series, the transformer IN2 being bridged to the output of the receiver RR2 and the transformer IN'2 being bridged to the output of the receiver RR'2. These two transformers are so poled with respect to the input of the amplifying arrangements VTi and VT2 that a static crash reaching the outputs of the two receivers will balance out if the crash is of equal volume in the two input transformers.

As the outputs become unequal the balancingout effect becomes less but the effect of the static crash will nevertheless be of smaller magnitude than if only one receiver were connected to the amplifier.

The operation, in so far as it may not be understood from the previous description of Figs. 2 and 4, is as follows: With the circuit in the condition shown, the apparatus is arranged to receive from the shore station, the antenna Ai being connected over the back contact of relay AR and the back contact of relay RAR to radio receiver RR2 to the output of which is connected the operator's receiver OR2. If a code signal corresponding to the ship station is received from the shore station the interrupted tone will be transmitted from the output of the receiver RR2 through the transformer IN2 to the input of the amplifier of the receiving unit SRUi. The amplifier rectifying arrangements of the combined transmitting and receiving unit SRUI operate as described in con- nection with Fig. 4 to set the receiving selector RS which will connect the point 2 of the selector to point 5 of the selector, completing a circuit for the bell B2 over the upper contact of the relay R14. Upon hearing the bell the operatormay actuate the key K2, thereby energizing relay R14 which opens the bell circuit and reverses the connection to the polar relay of the receiving selector RS to restore the selector to normal, as previously described. The ship operator may then by throwing the key AK talk to the shore station since the radio transmitter is normally supplied with carrier frequency from the oscillator Oz. Whenever the key AK is restored the antenna Ai will be disconnected from the output of the transmitter RT and will be connected to the input of the receiver RR2 so that the ship operator may listen to the shore station.

Should the operator of the ship station desire to call another vessel, the operator will throw the key CK, thereby energizing relay RCR, RAR, and OR. Relay OR connects the oscillator 03 to the radio transmitter RT and enables transmission at the frequency fs. The relay RAR at its left-hand front contact connects the antenna At to the radio receiver RR'2, which receives on the frequency ft used in communicating between ships. At its right-hand front contact the relay RAR connects antenna A2 to the receiver RR2, thereby acting as a stand-by connection to enable the shore station to send a code signal to the ship during intership communication.

The operator then throws the key DK, actuating the relay Ri1, to set up an oscillating circuit through the amplifier VTI-VT2 to produce the signaling tone. The relay Ris is actuated at the same time to connect the output transformer OTI to the radio transmitter RT. By actuating the dial D the tone may be interrupted to send the code combination to call another ship through the radio transmitter RT. At the end of the dialing operation the key DK is restored, thereby returning the amplifying arrangement to its normal amplifying condition and again connecting the transformer OTi to the rectifying arrangement RRu. Having called the other ship, the operator may now, by throwing the key AK, connect the antenna At to the radio transmitter RT and transmit to the other ship at the frequency f3. Upon restoring the key AK to normal, the antenna Ai will be connected over the upper contact of the relay AR and the left-hand front contact of the relay RAR to the radio receiver RR'2 in order to enable the operator to receive from the distant station. In this connection it should be noted that the relay RCR, which was actuated when the key CK was thrown, connects the operator's receiver OR2 to the output of the radio receiver RR'2.

When the key CK is restored to normal the relays OR, RAR and RCR are de-energized and the antennae Ai and A2 are connected to the radio receivers RR2 and RR'2, respectively, so that code signals may be received from either the shore or another ship. If another ship should send in a code signal, such code signal would be received at the frequency f3 over the antenna A2 and be transmitted to the radio receiver RR'2 whose output is connected through the transformer IN'2 to the combined transmitting and receiving unit SRU'. Preferably the code combination transmitted from another ship will be different from that transmitted from the store station in calling the same ship station. The combined transmitting and receiving unit operates as previously described to actuate the selector RS which may connect the points 2 and 3, thereby completing a circuit for the bell B'2 over the upper contact of relay Ris. Upon hearing the bell B'2 the ship operator will actuate the key K'22 thereby energizing the relay Rs1 and reversing the connection through the polar relay of the selector RS, which will be restored to normal. The ship operator will now throw the key CK to enable communication with the calling ship by sending over the radio transmitter at the frequency f3 and by receiving over the antenna Ai and the radio receiver RR'2 at the same frequency. While this communication is going on, the code signal received from the shore station would come in over the antenna A2 and be received by the receiver RR2. In such case the combined transmitting and receiving unit which is not used during voice transmission would receive the code signal through the transformer IN2 and the selector RS would be actuated as before to ring the bell B2.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is: 1. In a signal transmission system, a master station, a plurality of associated stations, means at said master station to transmit calling code signals individual to each associated station, means at individual associated stations to transmit calling code signals to other associated stations, said calling codes transmitted by said master station and said associated stations being converted into audible tone wave groups, separate receivers at each associated station for picking up signals from the master station and signals from other associated stations, a wave converter at each associated station to convert received tone waves to code signals, said receivers being connected to said wave converter in opposing relation, and signal responsive means associated with said converter.

2. In a signal transmission system, two signal receiving circuits adapted respectively to receive independent signals on corresponding different frequencies, an amplifier common to both circuits, said amplifier having a different gain when signaling current is present than it has in the absence of signaling current, and a common indicator associated with said circuits, said circuits being connected with opposite effect to said indicator whereby they will give a null response for static.

3. In a signal transmission system, a master station comprising a radio transmitter and a radio receiver, a plurality of associated stations each comprising two antennas, two radio receivers and a radio transmitter, one receiver being normally connected and adapted to receive signals through one antenna from the master station, the other receiver being normally connected and adapted to receive signals through the other antenna from other associated stations, and means under the control of an operator at an associated station to change these connections for signaling between such associated station and the master station or another associated station and for talking with either such station.

4. In a signal transmission system, two receiving circuits adapted respectively to receive waves of different frequencies from different parts of the spectrum of voice and static frequencies, an amplifier common to both circuits, said amplifier having a different gain when signaling current is present than it has in the absence of signaling current, a common indicator associated with said circuits, said circuits being connected with opposite effect to said indicator, and means for sending signals on one of said frequencies to be received by the appropriate receiving circuit of the said two receiving circuits.