US2101224A

US2101224A - Privacy system

Info

Publication number: US2101224A
Application number: US67722A
Authority: US
Inventors: Osborne Harold Smith; Wright Sumner Bisbee
Original assignee: American Telephone and Telegraph Co Inc; Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1936-03-07
Filing date: 1936-03-07
Publication date: 1937-12-07
Anticipated expiration: 1954-12-07

Description

Dec. 7, 1937.

H. S. OSBORNE ET AL PRIVACY SYSTEM Filed March 7, 1936 5 Sheets-Sheet l De@ 7, 1937 H. s. oslsoRNl-z ET AL 2,101,224

PRIVACY4 SYSTEM Filed March 7, 1956 5 Sheets-Sheet 2 Jet ` gli if ,l

ATTORNY H. S. OSBORNE ET AL PRIVACY SYSTEM 5 Sheets-Sheet [EHS 'IV 155 V@ y I; ll y; 'Il k IIl l Filed March '7, 1936 Dec. 7, 1 937.

s ME@ ATTORNEY 11726. Distributor Dec. 7, 1937. H. s. osBoRNE ET Al. 2,101,224

l ,PRIVACY SYSTEM i Filed March fr, 195e 5 sheets-sheet 4 Egg 5M E5. 05h07? mi ATTORNEY Dec. 7, 1937, H. s. osBoRNE ET Al.

PRIVACY SYSTEM Filed March 7, 1936 '5 .Shee'ts-Shee'l: 5

m95 Statons Tzpe Tramsmttwler INVENToRs h E i Osborne@ BY C ATTORNEY Reja. lh'strLb/.ctol' I Patented Dec. 7, 1937 PRIVACY SYSTEM narnia smith Osborne, Upper Montclair, and Sumner Bisbee` Wright, South Orange, N. J.;

said Osborne assignor to American Telephone Application March 7, 1936, Serial No. 67,722 18 Claims. (Cl. 17a-1.5)

UNMED s'rATEs PATENT oFFlcE This invention relates to a system for controlling circuit switching and other operations by wave controlled devices. One application of this principle is a privacy system, applicable to a group of telephone circuits operating between two points. Another application is to control the operation of an automatic transmission regu-- lator or gain changer. Many other applications will suggest themselves, such as a band splitting privacy system, in which the various speech bands are interchanged at intervals.

It is well known in the art that telephone waves in one direction of transmission that are produced by conversations are not continuous but occupy only about one-third of the holding time.

The other two-thirds is occupied by pauses be-' tween syllables and by answering from the distant end. In the privacy schemes herein described, conversations are switched from one circuit to another in the group as frequently as the condition is established of no voice current present on any circuit in the group. By having the proper number of circuits in the group, a person listening in on one circuit (or on one wave length if connections are made by radio) will receive only scattering syllables from the various conversations under way, the amountof each conversation overheard being only a small fraction of the total. In other applications, the usual functioning is held up until an interval occurs in which no current is passing, after which the mechanism is released by the Wave operated control system, to function in the usual way.

An advantage of this scheme over previous schemes suggested for switching circuit in use is that the switching is done when no current is in any of the circuits, thus avoiding extraneous clicking and quality effects. Another advantage over privacy schemes employing switching at a iixed rate is that when one or more of the channels is not being used the rate of switching is automatically increased. In the privacy while' Figs. 3(a) and 3(1)) similarly illustrate still a further modification.

Fig. 1 shows in schematic form circuits and apparatus connected to a number of transmission channels in such a manner as to accomplish these results over a group of one-way transmission channels. Thls drawing shows tour telephone connections being switched among four transmission channels but this numberis chosen for illustration only and the number might be reduced or increased as desired with- 4 a magnetic pattern from the line magnet A1, and

a delayed copy of the waves so produced is transmitted to a magnet B1, and thence through the Scrambler device tov a transmission channel. Magnets E1, etc., contain direct current for erasing the wave-pattern from the rotating tape. The A and B magnets are also provided with armatures which operate i'n the manner shown at A1 and B1 to interrupt current from a battery which is connected to a magnet P through the Scrambler device.

The Scrambler device `at the transmitting end of the circuit group consists o! rotors I to 8 all operating in iixed relation to'cach other on a common shaft. Rotor l is a commutator for interchanging the various circuits lof the group. Rotors 2 to 5 are slip rings necessarily associated with the commutator. Rotor 6 is a second commutator for making the number of steps depend on the length of the common pause as will be described later, and rotor l is its associated .slip

ring. Rotor 8 includes a ratchet wheel G which is operated 'from the pawl K in response to magnet Q which is operated to cause l/n revolution of the shaft by each operation of magnet P, where n is the number of circuits in the group.

The four telephone connections from the delay arrangements are made to rotor I through the four stators S1, etc., and are led from the privacy device through the slip ring stators'Ri, etc., to four transmission channels M1, etc. In addition. magnet P can connect a synchronizing impulse from an oscillator to a synchronizing channel M5. This synchronizing channel might be obtained by any well known technique such as by using a. portion of the frequency band of one oi' the main transmission channels.

At the receiving end the four channels and the synchronizing circuit are brought to a common point at which is located a restoring device which consists of rotors I' to 6', inclusive, all operating on the same shaft and in synchronism with the Scrambler device at the transmitting end due to the impulses transmitted over the synchronizing channel M under the control of the magnet P. In this device, rotors I' to 5', with .their associated stators, switch the various 'circuits to the proper telephone receivers H1, etc. Operation of the shaft is caused through the ratchet G' and pawl K' in response to magnets P' and Q.

' In the drawings it is assumed that voice current is present in magnets A1 and B1 so that the armatures of these magnets arey drawn away from the contacts. This interrupts the current to magnet P which is therefore released. The path of voice currents can then be traced from line L1 through'the delay arrangement D1 to stator S1, commutator C1, ring R1, channel M1, ring R1', commutator C1', stator S1' to receiver H1. In a similar manner the path of currents that may be set up'in any of the other transmitters may be traced to its corresponding receiver th'rough the transmission channel having a corresponding subscript.

. The operation of the Scrambler system is as follows: Assume that voice currents in line L1 cease so that relay A1 is released. The energizing circuit for relay P is still broken by magnet B1 but as soon as the waves have died out in this magnet its armature falls back and if no voice currents are on the other lines P is operated. For the present it will be assumed that wie X is connected in which case the operation of P is continuous for the duration of the common pause. The operation of P causes Q to make the scrambler shaft rotate l/n revolution, at the same time sending a synchronizing'impulse from the oscillator over the synchronizing channel 'Ms and causing operation of P at the receiving end with consequent rotation of the receiving shaft in a direction opposite to the transmitting shaft by l/n revolution. This results in commutator C1 Abeing connected to stator S4, C4 being connected to Ss, Sa to Sz and Cz to S1. "At the receiving end C1 is connected to S4', C4' to Si', Ca to Sz' and Cz to S1. The next following conversation from telephone T1 will, consequently, appear on transmission channel M4 while if T2 has voice current it will get to receiver Hz over channel Ms. Currents from T: will reach VH1 over channel M4 and waves from T4 will travel over M1 to receiver H4. These voice currents will of course operate A and B magnets so as to. cause the release of magnets Pand P'. As soon as voice currents cease again in all the channels, magnets P and P' 4will again be operated causing the shafts to move another l/n revolution with a correspondingly diil'erent'configuration of the connections.

In the above description rit can be seen that the conversations will rotate from one channel to' another at unpredictable intervals but in a precise order which is never changed. By omitting the wire X and allowing the magnet P to be operated through the commutator Cs the magnets P-and P will be released immediately after a switching operation so that the motion oi. commutator C5 when completed .will cause P to be operated again, provided the common pause is not interrupted by someone speaking on one of' the channels. 'Ihis results in each of the commutators taking a number of steps which is roughly proportional to the length of the pause. Since the pauses occur at random the interchange of channels will also be at,random instead of in regular sequence, thus increasing the. diillculty of eavesdropping on a conversation by switching f from one channel to another. The magnetP' which. obtains its impulse from P will, of course, take exactlyA the same number of steps as the sending device so that no matter which channel the conversation is put on at the sending end it will be directed to the proper receiver at the distant end.

The purpose of the delay arrangements at the sending end is to allow time for the mechanism to operate without clipping the beginning of syllables. `Assuming no current in any line the commutators will be stepping around and if it were not for the delay arrangement theirinertia might cause switching as the initial currents were building up. The delay arrangement prevents this in the following manner: By having the magnet A1 interrupt the rotation, of the comvmutator at the same time that the voice waves are impressed on the delay arrangement, the latter stores up these waves for a sumcient interval for the commutator to come to rest before the waves are impressed on the device through magnet B1. However, it is also necessary to provide B1 with an armature since if this were not done the release of A1 would start the mechanism operating again before the ilnal part of the voice wave had been transmitted. It is also assumed that these voice-operated magnets will have slow release characteristics by the provision of circuits or other devices well known in the art so as to delay the operation' of the Scrambler for a suillcient period to permit all vof the trailing weak endings of words'to get through.

Whilethe commutation at the two ends is shown synchronous by a separate channel this channel could beavoided by providing at the receiving end voice-operated magnets and delay circuits similar to those at the transmitting end so that synchronization could be brought about by the use of the voice frequency currents themselves; in other words, in periods of no voice currents on any channel commutation would take place at both ends in the same manner.

In practical circuits using privacy systems employing the above principle, the two branches of the circuit which transmit in opposite directions might be combined in a control terminal. Such a terminal generally includes devices for controlling the gain `and the.l direction of transmission and for lsuppressing echoes and singing, and is K frequently referred to asavodas. It is fundamentally similar Ato the arrangement described in U. S. Patent No. 1,639,773, Two-way telephone traon," issued August 23, 1927, to H. S. Hamilton. The vodas contains delay circuits for insuring that the switching relays function satisfactorily and the same delay circuits 'and other devices-in the terminal might be used in conjunction with the privacy switching system. Also, the privacy system might be Aarranged to have the switching sequences controlled in a predetermined manner as disclosed in U. S. Patent No. 1,598,673, Secrecy communication' system, issued September 7, 1926 to O. B. Blackwell, D. K. Martin and G. S. Vernam.. Either a 1 the corresponding switching may be done at the commutator as shown n Fig. 1 or a relay switching system might be used.

Figs. 2(a) and 2(b) when suitably arranged show four vodas control terminals equipped with such a privacy system, switching at the sending end being done during intervals of no speech in any of the four circuits by means of a plurality of relays controlled by the vodas and a key tape. Whenever a switch takes place at the sending end, a corresponding switch is made at the receiving end. This is controlled from the sending end by means of synchronizing or control frequencies that are transmitted over two of the circuits while the speech is stopped, and from the receiving end by means of a key tape identical with that at the sending end. In each of the four circuits, the upper path is shown connected over a wire line to a radio station and the lower path is shown connected from a radio receiver to the control terminal over another wire line. It is assumed that similar arrangements, not shown, are used at the opposite end ofthe circuit.

Starting with circuit No. I, a connection is made from a subscribers set T1 to a four-wire terminating set including a balancing network Net1. Here the circuit -divides into transmitting and receiving branches. The upper or transmitting branch contains a potentiometer associated with an amplifier and a delay circuit TDL1. The

delay circuit stores up speech waves while the amplifier detector TD1 is functioning to operate relays E1 and S1, which suppress echoes and clear the transmitting path for speech. The receiving branch of the circuit at the terminal contains amplier detector RD1.which operates relay R1 on incoming speech thereby preventing false operation of TD1 on echoes. The incoming speech also passes through RDL1, and another potentiometer and amplifier to the terminating set. The above description of the vodas terminal is common to all four circuits simply by changing the subscripts of the various units. Fundamentally, it is the same as the arrangement described in U. S. Patent No. 1,639,773 to Hamilton, above referred to.

The application oi the privacy system to these four terminals requires the addition of amplier detectors PD1, etc., which operate to energize windings B1, etc., of relays whose cores are also wound with windings A1, etc., that are energized from amplifier detectors TD1, etc. The contacts of these four relays are wired in series with each other, and in circuit with a battery and with a magnet P which controls the operation of the transmitter privacyv switching circuit. The effect of windings A1 and B1 in combination with the delay network TDL1 is similar to that ascribed to magnets A1, B1 and delay arrangement D1 0f Fig. l.

The privacy-device itself consists of a group of relays in the transmitting branch of the circuit, Nos. 9 to I6, inclusive, and other group of relays in the receiving branch of the circuit Nos. 9' to I6', inclusive. These relays are operated by other auxiliary relays energized from transmitting distributor 29, receiving distributor 29' and

tape transmitters

36 and 36 which are provided with` identical perforated tapes as de-V scribed in the above mentioned Patent No. 1,598,673. The trapsmitting distributor 29 is supplied with a synchronizing segment 3| .which controls two tone generators having frequencies f1 and ,f2 for sending impulses on the transmitting branches of circuits I .and 2 in order that distant communicating station. `This is accomplished by having amplifier detectors in the receiving paths of circuits I and 2 at the distant end, arranged like SDa and- SD4 which control the starting of a receiving distributor like 29 Aon incoming control signals. The effects of adding these auxiliary control signals are isolated from the main transmission circuit by ampliers, U1 and U2 in the transmitting branches of circuits I and 2 and by relays corresponding to K3 and K4 in the receiving branches of circuits I and 2 at the distant station. Delay circuits D1, etc., are inserted in the receiving branches to hold up the received speech Waves long enough for theprivacy switching to be completed. Filters Fsand F4 are used to select the control frequencies in the receiving branches, excluding extraneous frequencies such as speech and noise. Two control frequencies are used instead of one to reduce the possibility of speech or noise causing false operation of the receiving distributor.

The operation of the transmitting arrangethe relays, operated by the TD and PD amplifier' detectors, are deenergized. The battery is then closed to magnet P which releases the latch and allows the brush of distributor 29 to start rotating. It will also be assumed that the no signal time during which P is operated is sufficient to allow two., rotations of the brush but it will be obvious that a larger or smaller number of rotations may occur, depending on the length of time there are no speech waves in any of the four channels. 'I'he number two is chosen merely for illustration.

As the brush passes segment 3l, ground is supplied to and then removed from relays Q1 and Q2, which close the circuits to connect generators f1 and f2 to the transmitting branches of circuits I and 2. These generators may put out any frequency in the voice range since there can be no interference with speech because it has ceased. They are` prevented from interfering with the 'ID and PD amplifier detectors due to the unilateral devices U1 and U2.

Filters F1 and F2 smooth out any transients that may arise from the closing and opening of the Q relays.

magnet will be released and the contacts of I the ta'pe transmitter will be pressed against the key tape. If there are no holes in the perforated key tape, the

contacts

39 and 40 will remain open as shown. Under such conditions, none of 'the relays 9 to I6, inclusive, will be operated and the rst line will be connected'to radio transmitter I overthe back contacts of relays I2 and I6. Similarly, all the other subscribers will be con'- nected through to their respective radio channels. When the brush of distributor 29 passes segment 33, groundY will be applied to relay 38 which will operate but this will have no other effect since

contacts

39 and 40 will be held open by the tape.

Now, assume that forthe'second revolution of the brush, the perforations in the tape are so arranged that contact 39 will be closed and contact 40 will remain open. ,Before this closure takes place, however, a second pair yof impulses of frequencies f1- and fzwill he sent out due to the brush having again passed over segment 3|. When the brush again crosses segment 33, the relay 38 will be operated and ground will be connected through its lower contact and contact 39 so as to operate relays 4|, I3, |4, l5, and I3.

When the brush passes off segment 33, relay 33 will be released.v However, relays I3, |4, I5, I6,

and 4| will remain locked up as a locking circuitv will be closed over the contact and armature of relay 4| and the break contact of relay 33 to ground. v When this condition is established, subscriber '1 will be connected toradio transmitter 3 over the contacts of relay |4. Subscriber 2 will be connected to radio transmitter 4 over the contacts of relay I3. Subscriber 3 will be connected f to radio transmitter l over the contacts of relay I6 and subscriber 4 .will be connected to transmitter 2 over the contacts of relay I5'. In a similar manner, other positions of the relays will permit the subscribers to be switched to otherq different radio transmitters depending upon as follows:

It will be assumed that the operations of the y 'transmitting branch at the distant station were as just described, withfthe exception that thev two synchronizing impulse frequencies have been sent out on

channels

3 and 4 which are reserved for controlling the switching at vthe receiving end of the circuit shown in Figs. 2(a) to 2(d) Filters Fa and F4 will be designed to suppress frequencies other than the control frequencies, two impulses of which are received. The iirst impulse arriving in circuit 3, for example, will pass into delay circuit Da while a portion of the energy will pass through filter F3 and operate amplifier detector 'SDa so as to close the contacts of relays K3 and V3. K3 will place a short circuit at the output of delay circuit De to prevent the impulse from passing to the circuit terminal and thus being heard by thesubscriber. In a similar manner, a corresponding impulse will be at the same time received in circuit 4 and will operate relays K4 and V4.

Momentary operation of both relays V3 and V4 will cause operation of magnet P' which will release the brush of the receiving distributor 29 so that it will make o`ne revolution. As the brush passes segment 32', it will operate the magnet 35' of the receiving tape transmitter and as it leaves this brush, contacts 39.' and 40' will move upward and will complete circuits depending upon the position of the holes in the key tape exactly as in the case of the transmitting tape transmitter. It is assumed that the perforations in the tapes associated with the distant transmitting station and the local receiving station are in exact agreement so that synchronism will be maintained.

Under this assumption the first impulse will cause no change in relays 9' to I6', inclusive, and the four circuits will be 'connected straight through'as at .the transmitting end." However, the second synchronizing impulse will cause a revolution of the receiving brush and contact 33' will be closed and will cause the operation of relays 33', 4|', I3', I4', I5', and i6' to connect the fourv receiving circuit branches to the radio channels vto which the corresponding transmitting relays have switched them. Each of the receiving circuits has a delay circuit, D1, etc., to so retard the control impulses and the succeeding voiceA waves that before the voice waves arrive thev receiving paths will have been cleared by the release of relays Ka and K4 after the completion of the operation of any of the switching relays 9' and I6' that may have been operated by the particular configuration of the holes in the key tape.

It will of course be clear in connection with thearrangement just described, that While each switching combination is determined by the setting of the key tape,'the final switching combination that is used after a no signal interval depends onthe length of the interval, i. e., upon whether the dstributor has had time to operate onlyv once or more than once. 'I'hus the switching combination is theresult of two unpredictable factors, the random perforations of the vkey vtape and the length of the no signal interval.

f It will be understood that while the arrangements shown have been specifically described as associated with a radio telephone transmission circuit, the channels connecting the two terminals might equaliy well be wire lines'throughout, or concentric conductors, or repeatered toll circuits of any description, Withoutchanging the terminal arrangements. yIt shouldy be noted alsothat the vodas terminal itself may be of some other form without essentially changing the purpose of this invention. For example, jthe vodas might be so arranged .that the circuit will be normally disabled in the receiving direction instead of the transmitting direction, or even in both directions together, in which case it would be possible yto utilize receiving amplifier detectors and receiving delay circuits to control the switching at the receiving end, thus dispensing with the synchronizing frequencies. This would, of course, require the addition of amplifier detectors at the outputs of the receiving 'delay circuits, these detectors being connected to relays similar to those with A and B windings to start the receiving distributor.

Still another variation within the scope of this invention would be to transmit synchronizing impulses outside the speech range on all the chan-v nels along With the speech. This arrangement is shown in Figs. 3(a) and 3(b). The vodas operates relays S1, etc., which send out frequencies f1, etc., (which are outside the voice range) while the speech is on. These frequencies hold relays V1, etc., at the receiving end in operated condition so as to prevent the receiving distributor from rotating. When speech ceases in all channels the relays with the A and B windings will beV released Aat the sending end, and rela-ys V will be released at the receiving end, so that switching will take place in the proper sequencen and synchronizaton. The relays' K1, K2, etc., are released during no signal intervals and hence short circuit the 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 signaling system, a plurality of terminal circuits, a plurality of transmission channels, one connected to each terminal circuit, switching means for simultaneously interchanging the connections between the terminal circuits and the transmission channels, said means operating to produce such interchange only when no signals are being transmitted in any terminal circuit or transmission channel of the system and before the transmission of signals begins.

2. In a signaling system, the methbd of obtaining secrecy which consists in switching the signal terminal from one transmission channel to another at times when no signal is present in said i signal terminal or in any transmission channel of any one of a number of transmission channels,

the method of obtaining secrecy which consists in simultaneously switching a plurality ofthe signal terminals of the group to different transmission channels only when no signals are present in any terminal of the group or in any transmission Vchannel and before the transmission of signals begins.

5. In a signaling system in which any one of a group of signal terminals may be switched to any one of a number of transmission channels, the method of obtaining secrecy which consists in simultaneously switching a plurality of the signal terminals of the group to different transmission channels each time a no signal condition occurs simultaneously upon all of the signal terminals of the group and before the transmission of signals begins.

6'. In a signaling system in which a group of signal terminals may be connected with a group of transmission channels in a plurality of combinations, the method of obtaining secrecy which consistsinshifting the connections from one combination to'another only when no signals are present in either the signal terminals or the transmission channels and before the transmission of signals begins.

7. In a signaling system in which a group` of signal terminals may be connected with a group of transmission channels in a plurality of combinations, the method of obtaining Isecrecy which consists in shifting the connections from one combination to another each time a no signal condition occurs simultaneously upon all of the signal terminals of the group. l

8. In a signaling system in which a group of signal terminals may be connected with a group of transmission channels in a plurality of combinations, the method of obtaining secrecy which ,onsists in shifting the connections from one combination to another each time a no signal condition occurs simultaneously upon all of the signa1 terminals of the group, and determining the combination to which the shift is made in accordance with the duration of the no signal condition.

9. In a signaling system, a group of signal terminals, a plurality of transmission channels, switching means to variably connect said group of signal terminals to said transmission channels, said switching means operating to simultaneously vary the connections of a plurality of signal terminals only when no signals are present in any signal terminal of the group or in any transmission channel and before the transmission of signals'begins.

10. In a signaling system, a group of signal terminals, a plurality of transmission channels, switching means to variably connect said group of signal terminals to said transmission channels, said switching means operating to simultaneously vary the connections of a plurality of signal terminals whenever no signals are present in any signal terminal of the group or in any transmission channel and before the transmission of signals begins.

l1. In a signaling system, a group of signal terminals, a plurality of transmission channels, switching'means to interconnect said terminals and said channels in a plurality of combinations, said switching means operating to shift the connections to a new combination Whenever no signals are present in any signal terminal of the group or in any transmission channel and before the transmission of signals begins.

12. In a signaling system, a group of signal terminals, a plurality of 'transmission channels, switching means to interconnect said terminals and said channels in a plurality of combinations, said switching means operating to shift the connections to a new combination whenever no signals are present in any signal terminal, and means to determine the new combination in accordance with the duration of the no signal condition.

' 13. In a signaling system, a group of signal terminals, a plurality of transmission channels, switching means tointerconnect said terminals' and said channels in aplurality of combinations, said switching means operating to shift the connections to a new combination whenever no signals are present in any signal terminal, means whereby the new combination is determined in part by the duration of the no signal condition, and an arbitrary combination determining device for in part determining the new combination.

14. In a signaling system, a group of signal terminals, a plurality of transmission channels, switching means to interconnect said terminals and said channels in a plurality of combinations, said switching means operating to shift the connections to a new combination whenever no signals are present in any signal terminal, means whereby the new combination is determined in part by the duration of the no signal condition, and a combination recording tape for in part determin- A ing the new combination.

15. In a signaling system, a group of transmitting signal terminals at one station, a corresponding group of receiving signal terminals at another station, a plurality of transmission channels beto each other whenever no signals are present in any signal terminal. Y'

16. In a signaling system, a group of transmitting signal terminals at one station, a corresponding group of receiving signal terminals at another station, a plurality of transmission channels between the stations, switching means at each station to interconnect corresponding signal terminals with said transmission channels in a plurality of combinations, means controlling the switching means at both stations to shift the connections to new combinations which correspond to each other whenever no signals are present in any signal terminal, and means to determine the new combinations in ac cordance with the duration of the no signal condition.

17. In a signaling system, a group of transmitting signal terminals at one station, a corresponding group of receiving signal terminals at another station, a plurality of transmission channels between the stations, switching means at each station to interconnect corresponding signal terminals with said transmission channels in a plurality of combinations, means controlling the switching means at both stations to shift Y the connections to new combinations which correspond to ech other Whenever no signals are present in any signal terminal, means whereby the new combinations are determined in part by the duration of the no signal condition, and an arbitrary combinationdetermining device at each station for in part determining the new combinations.

18. In a signaling system, a group of transmitting signal terminals at one station, a corresponding group of receiving signal terminals at another station, a plurality of transmission channels between the stations, switching means at each station to interconnect corresponding signal terminals with said transmission channels in a plurality of combinations, means controlling the switching means at both stations to shift the connections to new combinations which correspond to each other whenever no signals are present in any signal terminal, means whereby the new combinations are determined in part by the duration of the no signal condition, an arbitrary combination determining device at each station for in part determining the new combinations, and means to operate said devices` synchronously at both' stations whenever no signals are present.

- `HAROLD S. OSBORNE.

SUMNER BISBEE WRIGHT.