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Publication numberUS2137023 A
Publication typeGrant
Publication dateNov 15, 1938
Filing dateDec 20, 1935
Priority dateDec 20, 1935
Publication numberUS 2137023 A, US 2137023A, US-A-2137023, US2137023 A, US2137023A
InventorsMonk Newton
Original AssigneeBell Telephone Labor Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Carrier communicating system
US 2137023 A
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Description  (OCR text may contain errors)

Nov. 15, 1938. N, MONK 2,137,023

CARRIER COMMUNICATING SYSTEM Filed Dec. 20, 1935 2 Sheets-Sheet 1 .Shin SB Q mlbo.

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. 1V E@ Y. W W nel /m/EA/roe N. MOA/K BV A TTOR/VEV N. MONK Nov. 15, 1938.

CARRIER COMMUNICATING SYSTEM Filed Dec. 20, 1955 2 Sheets-Sheet 2 /A/ VEA/Tof? V. MUN/f er A Trom/EV Patented Nov. l5, 1938 PATENT OFFICE 2,137,023 CARRIER COMMUNICATING SYSTEM NewtonV Monk, New York, N. Y.,

Telephone Laboratories, York, N. Y., a corporation asaignor to Bell Incorporated, New

of New York Application December 20, 1935, Serial No. 55,313

9Claims.

This invention relates to an improved communicating system and more particularly to a communicating system of the type in which carrier waves or oscillations, either free or guided, are employed as the medium for conveying energy for effecting the operation and control of electrical signaling controlling and communicating apparatus located' at separate points in the system. 'I'he invention especially relates to a system in which carrier currents, in the form of high frequency waves guided upon wires, are used as the medium of electrical connection between the electrical devices and apparatus located at diil'erent points in the system. These carrier currents are assigned such frequencies that they are non-interfering with one another, so that each carrier serves as a separate and distinct connecting medium superposed upon the line or conductor over which thecarrier is transmitted. Each of the carriers is modulated in accordance with the voice currents or other signaling or controlling currents that are to be carried. The carriers thus modulated are transmitted over the toll or long distance line or any other connectingI medium such as, for instance, a coaxial cable, and are passed through individual receiving arrangements, each of which is selective of one carrier frequency to the exclusion of the others and each of which serves to reproduce from the modulated carrier the voice or signal currents with which the carrier was modulated at the distant station.

The apparatus which is used to transmit and receive by means of a wave of a different frequency is commonly known as a high frequency channel or a frequency selective channel.

It is the object of this invention to provide a highly efficient communicating system between a number of oilces connected by acommon medium by providing a number of single high frequency channels common to all the offices, by providing an individual high frequency channel to designate each office and by providing switching equipment at each oiiice so that, upon the initiation of a call from one oiiice to another by the transmission over the common medium of the individual frequency of the called omce, the switching equipment at the calling office will be operated to select an idle frequency channel over which the talking and sending connection shall take place, while the switching equipment at the called oiice will be operated to select the selected frequency channel through which the connection may be completed.

In the type of carrier communicating system to which the present invention particularly relates,

a common transmission medium having a number of stations associated with it is provided with a plurality of single frequencies for both directions of transmission, while at each station in the system there is included a plurality of similar trunks each provided with means to transmit and receive these frequencies. This apparatus include:t a modulator and its associated oscillator, a demodulator, a variable frequency filter and several rotary switches. oscillator are connected mechanically to one of these switches and are controlled by it in the The variable filter and sense that the switch operates to tune the cir-/ cuit to each one of the system frequencies in rotation. Additional apparatus is also included to start the operation of the rotating switch both when initiating a call and when completing the call.` By operating the rotating switches synchronously at any two stations, as will be hereinafter described, a connection is set up. There are also included in each set of apparatus a number of auxiliary relays and lamps for supervising the connection.

In the operation oi' the system, the operator chooses an idle trunk by plugging the calling cord into the jack of the trunk outlet and then manipulates the calling oscillator to transmit a spurt of calling frequency which is individual to the :sailed omce and at which office appropriate responsive apparatus picks it up. In the meanwhile, the mechanical switches are set in motion in cooperation with the variable filter and variable oscillator, the variable lter being automatically adjusted to respond to one of the communieating frequencies and the variable oscillator being variably adjusted to produce it. If this fre-l quency is in use at this time it will be picked up by the filter over the common medium and made to operate a demodulator which, in turn, operates a relay that grounds the hunting terminal upon which the frequency selective switch is standing whereupon the frequency selective switches at the originating and called oillces are sttpped to the next terminal, the filter and oscillator being tuned, in turn, to the next available frequency. The busy test is now repeated to detect the busy or idle condition of this frequency. If the frequency is found idle, the switches remain where they are; if the frequency is busy, the operations are repeated until an idle frequency is found.

At the called oillce, the spurt of calling frequency operates a circuit tuned thereto which in turn operates a relay that causes the frequency selective switch of an idle trunk to step to the frequency selected at the originating office and,

at the same time, to adjust the variable oscillator and filter of the trunk to produce and respond to this frequency. When the selected frequency is found, a calling lamp of the trunk is lighted and the call is completed thereover in the usual manner. A

The scope and purpose of the invention may be obtained from a consideration of the Vfollowing description taken in connection with the attached drawings in which:

Fig. 1 shows an outlet or terminal in one office connected to a common transmission medium such as, for example, a pair of coaxial conductors; and

Fig. 2 shows a similar outlet or terminal in another ofiice connected to said medium.

Both terminals are identical and each of them may be used either to initiate an outgoing call or to complete an incoming call.

In the system of my invention, the same frequency is employed for both directions of transmission, there being a plurality of such frequencies available for the entire system. At each station in this system there is included a plurality of similar terminals such as the one shown in either Fig. 1 or Fig. 2 and each includes similar equipment; that is, an oscillator and its assoclated modulator, a demodulator, a variable frequency filter and several rotary switches. The variable filter and oscillator are connected mechanically to one of these switches and are controlled by it. This switch operates to tune the circuit to each one of the system frequencies in rotation. Additional apparatus is also included to start the operation of the rotating switch, both when initiating a call and in completing the call. By operating the rotating switches synchronously at any two stations, as will hereinafter be described, a connection is set up. There are also included in each terminal a number of auxiliary relays and lamps for setting up and supervising the connection.

For clearness of description and for the purpose of more easily identifying corresponding elements in each of the terminals, said elements have been given identical designations in both figures except that the designations of the elements in the terminal shown in Fig. 2 have been primed to distinguish them from the corresponding elements in the terminal shown in Fig. 1.

Each terminal includes a jack JA through which a connection is established from a calling subscriber A, through the medium of a cord such as, for instance. cord B or similar instrumentalities, hybrid coils HYi and HY2, each terminating in appropriate balancing networks N1 and N2 respectively, a modulator MD1, a variable oscillator OS1 operatively connected with it, and a source of power PS1 operatively connected to both, a calling variable oscillator OS2 with its source of power PS2, key KY1, a fixed band filter BF2 capable of passing the various high frequencies produced by the oscillator OS2, and a variable band filter EF1 which is synchronously tuned to pass whatever frequency is being produced at any time by the variable oscillator OS1. The tuning of both the oscillator OS1 and the band lter BF1 is controlled through a stepping switch SM1 in a manner which is completely disclosed and described in the Patent 2,064,904 granted December 22, 1936 to E. I. Green. Two other rotary switches, namely, SM2 and SMa have their arms mechanically connected together so that the operation of one switch, besides causing the movement of its own arm over a cooperating contact arc, similarly influences the movement of the arm of the companion switch. The switches SM1, SM2 and SM2 are of the type which advance their arms on the energization of their stepping magnets. Each contact in each of the contact arcs of these switches is reserved to one Vof the available terminals and is usedto mark the idle or busy Vcondition of the terminal, as more completely described hereinafter.

The operation of the system will now be described by tracing a connection between subscriber A,whose line can be assumed to have been extended to the cord B in the usual manner and subscriber-A located in the distant office accessible through the terminal shown in Fig. 2, and that this connection is to be made over the common medium extending between the two terminals.

The operator at the originating office first takes into use an idle one of the available terminals whose idle condition is indicated by the fact that no plug PL of a cord such as cord B is in a terminal jack JA or, if multiple jacks are employed, busy lamps may be provided which are lighted by means of a signaling circuitl controlled through the ground on the sleeve of the cord plug PL inserted into the jack. The operator next inserts the plug PL into the jack JA. This connection places ground through the calling supervisory cord lamp SU1 on the sleeve of the jack and an obvious circuit is thereby completed for relay RL1 which operates to perform a number of functions. First, it closes a partial path through its left inner contacts for supervisory lamp SL1 thereby permitting this lamp to be controlled through the left contacts of relay R111; second, at its left middle contacts, it breaks the connection through the call lamp CL1, thus preventing this lamp from lighting when relay RLG operates as described hereinafter; third, 1t completes a circuit which extends from grounded battery through its right inner contacts, right winding of relay Rin to ground; fourth, through its right outer contacts, it breaks the path from the arcs of the strapped terminals of rotary switch SM1 to the winding of the rotary magnet SM1 of said switch; and fifth, it connects ground through its left outer contacts to the contact of the arc of switch SM: which is reserved for the terminal shown in Fig. 1 and marks said terminal as busy. Inasmuch as the arm of switch SM; is mechanically connected to that of switch SM2. then, if the arm of switch SMa is on the terminal to which the contact of relay RL1` is connected,

the above ground completes a circuit to battery through the winding of switch magnet SM: causing the arm of said switch as well as that of switch SM2, to advance to the next contact on the respective arcs which are reserved to another terminal. If this terminal is busy, ground will be connected to the contact on the arc accessible to the arm of switch SM3, said ground being extended thereto by relay RL1 of said terminal, or on the arc accessible to the arm of switch SM2 from the contacts of relay RL: as described hereinafter. In either event, the operation of magnet SMa or SM2 will continue for each succeeding set of contacts on either one of which ground is present until a set of contacts is reached having no ground thereon; that is, until a free outlet or terminal is reached, at which time the switch arms will rest on said contacts.

The operator now manually or by other appropriate means, adjusts the frequency of the calling oscillator OS2 to a particular frequency Gil which, for calling purposes, is assigned to the desired olce. 'I'he operator next momentarily depresses key KY1 which permits a spurt of that particular frequency to pass through the filter BF: which likewise passes all the calling frequencies for the entire system, over the common medium to said desired oilice. Key KY1 also closes an obvious circuit for relay Ris which is momentarily energized, first to connect ground to the contact on thearc of switch SM2 for the purpose of driving said switch ofi' the contact if the arm thereof is standing on said contact, which would be the case if the key KY1 were operated prior to plugging into the jack JA of the idle terminal taken into use; secondly, to close a circuit which extends from ground through its contacts, through the normal contact on the arc S of switch SM1, winding of the switch magnet SM1 to battery. This circuit causes magnet SM1 to operate and adjust, me chanically, the filter BF1 to, say, the frequency f1 and adJusts the oscillator CS1 to this same frequency since the rotor of condenser C by which said oscillator is adjusted, is carried on the same shaft of switch SM1 as the moving elements for tuning the filter BF1. If, now, this frequency f1 is in use over the common medium and hence is being transmitted back and forth thereover, it will be applied from an oscillator such as OS1 tuned to generate the frequency f1 to the hybrid coil HY: and thence to the demodulator DM1 and relay RL4 which is connected to the plate circuit (not shown) of the demodulator DM1, will be operated which, in turn, closes a circuit for relay RLs extending from negative battery through the right contacts of relay RL4, right inner contacts of relay RLs, winding of relay RLs to ground. Relay RLs operates and connects ground to one side of relay RLs and to the strapped contacts of arc S of switch SM1 whereupon the operating circuit of magnet SM1 is completed which, on operating, causes the arms of said switch to be advanced to the next terminal, thereby tuning the oscillator OS1 and the filter BF1 to the next frequency, say fz. If this frequency is also in use at the time, it will be present on the common medium and will cause the response of demodulator DM1 which, in turn, causes the operation of relay RL4 followed by that of relay RLs, followed by that of magnet SM1 and by the stepping of the switch arms to the next set of terminals to which are connected the electrical elements for tuning the filter BF1 and the oscillator'OS1 to the next frequency, say f3 of the total number of communicating frequencies available to the system. The above operations will continue until an available frequency is found. relay RLs remaining short-circuited so long as relay RLs remains loperated in order not to disturb the operating path of the switch SM1. Furthermore, relay RLs (and likewise relay RL's) has a slow-to-operate characteristic which will prevent its false operation in advance of the operation of relays RL4 and RLs at the time the arm of switch SM1 steps on the strapped terminals of arc S, at which time, should the switch then be connected to a busy channel, relay RL4 operates and closes the circuit of relay RLs, in which event, relay RLS being slow to operate, the operation of relay R115 is assured and relay RLG is prevented from operating by the connection of ground to both sides of its winding. Assuming, however, that frequency f2 is not in use, then, when the arms of switch SM1 reach the second set of terminals, relays RL4 and Ris will not operate, no current 3 will flow through the winding of the stepping -magnet SM1v and the switch will remain set upon the set of contacts designating the frequency fz. Relay RL; will now operate in series with the winding of magnet SM1 in a circuit to be later described.

In the meanwhile, and at the time that the operator depresses key KY1 to transmit a spurt of calling frequency to the required office which, it may be assumed, is that represented by the terminal shcwn in'Fig. 2, the carrier current at.

this frequency is transmitted over the common medium and effects the tuned circuit or filter TC'. at the wanted oi'ilce, which filter or circuit is tuned only to the frequency individual to the wanted oflice and transmitted from the originating office. A rectifier RF: is associated with the tuned circuit TCs and the received spurt of current operates through the rectifier to close momentarily the contacts of relay RL1. This relay timed to operate simultaneously with relay RLs at the originating oiiice.

The operation of relay RL'1 momentarily con-A nects ground to the winding of the magnet of switch SMz and through the switch arm controlled thereby to the normal contact of the arc S' of switch SM'1 of the terminal taken into use whereupon a circuit is completed through the winding of magnet SM'1 and another circuit is completed through the winding of magnet SMz. Switch SM'1 now operates to flnd an idle frequency in the same manner as switch SM1 at the originating olce and since relay RLv operates simultaneously with relay RL: at the calling ofce, switches SM1 and SM'1 will step in synchronism and bring their switch arms to rest on corresponding terminals allocated to the same Y available frequency.

The ground connected to the winding of switch magnet SM'a by the contacts of relay RL'1 causes said magnet to step the arms of both switches SM: and SMa to the next set of contacts. Now if this next set of contacts is associated with a busy terminal, which fact is determined by the connection of a cord plug PL' into jack JA and the consequent operation of relay RL1, a circuit will be completed extending from ground on right outer contacts of relay RL1, corresponding terminal on the arc of switch SM'a and its arm, winding of magnet SMx to battery. The arms of both switches will now be advanced to the next set of contacts and continue to step over successive contacts until the arm of switch SMs engages an ungrounded terminal of its arc corresponding to an idle terminal. The switches SMz and SM; thus always maintain an idle terminal preselected. It is obvious that, lf desired, a single switch with two sets stationary contacts and two mechanically associated arms could be employed instead of the separate switches SM'1: and SM's.

Returning, now, to the operations taking place in the calling ofice, after the switch SM1 has been stepped to the terminal representing an idle frequency, a circuit is completed for relay RLS extending from ground through the winding of relay RLS, resistance R1, terminal in the arc S of switch SM1 upon which its associated arm is brought to rest, winding of magnet SM1 to battery and ground. The quantity of current flowing in this circuit is such that relay RLe will operate but magnet SM1 will not. Relay RLS, upon operating, connects ground to its left group of contacts. 'I'he ground connected to its left inner contacts extends to the left middle contacts of relay RL1. Since, however, said relay is operated at this time, lamp CLr will not light. The ground through the left outer contacts of relay RL@ extends through right contacts of relay Rin, which is operated, to an open circuit on the right outer contacts of relay RL1, while the ground through left middle contacts ofrelay Rlls serves to lock up relay RL: through its left Wintiing and contacts to battery. 4 y

Relay RL. also performs two otherfunctions. Over its right outer contacts it connects power supply PS1 to the oscillator OS1 which is now mechanically adjusted to produce a current of frequency fn and, through its right inner contacts it opens the path of relay Rh; thereby preventing the operation of this relay until the completion of the call.

At the incoming oilice, when the switch SMi comes to rest the contact on the arc S which corresponds to the selected frequency fz, relay RL'a operates in the manner described for an identical operation at the outgoing oillce and completes a circuit for lamp CL1 extending from grounded battery through the lamp CL'1, right middle contacts of relay RL'1, right inner contacts of relay RL'@ to ground. Lamp CL1 lights to inform the operator that a call is awaiting on the terminal designated by the lighted lamp. The operator then inserts the plug PL of an available cord B' into the jack JA of the desig nated terminal and, by operating KYz connects her telephone set to the talking circuit. After ascertaining the number of the wanted subscriber she inserts the plug PL'z of a cord circuit into the jack of the called subscriber A. In addition to completing the talking connection, plug PL' causes a circuit to be completed extending from ground on the sleeve of the plug through the called supervisory lamp SU1 of the cord,'

winding of relay RL'1 to battery. The operation of relay RL1 causes lamp CL1 to be extinguished and further connects ground through its right outer contacts to the contact on the arc of switch SM: reserved for the terminal taken into use to mark said terminal as busy.

A path is now completed for lamp SL1 extending from grounded battery through said lamp, right inner contacts of relay R111 right contacts of relay RL'4 to ground. The demodulator DM1 responds to the incoming carrier so that, so long as the connection is maintained, relay RL'4 is operated and lamp SL'1 will be lighted to signify that the two subscribers are conversing.

When the call is completed and the calling operator is apprised of the fact through the usual signaling equipment associated with the cord circuit, she removes the plug PL from the jack JA, thus deenergizing relay RL1. A circuit is now completed extending from ground through the left outer contacts of relay RLe, right contacts of relay RLn, right outer contacts of relay RL1, the arc S of switch SM1, winding of magnet SM1 to battery. The arms of switch SM1 will now be stepped around to their normal position at which point the stepping circuit is broken and the switch will come to rest. This also serves to bring the filter BF1 and the oscillator OS1 to their normal position. The switch SM1, upon returning to itsnormal position, also breaks the circuit through th'e winding of relay RLS which, in turn, unlocks the relay RLa. The terminal at the outgoing oillce is now in readiness for use in setting up a new call.

When the relay RL releases at the calling oflice and the power supply PS1 of the oscillator CS1 is opened at the right outer contacts of this relay, frequency la is removed vfrom the line whereupon the demodulator DM1 at the called ofilce causes the release of relay RL4 and the opening of the circuitof 'lamp SL'1. 'I'his lamp,

*when extinguished, signals the operator at the incoming ofiice that the call'is terminated, whereupon she removes the plug PL' from the' jack and the apparatus at the incomingl oilice restores to normal in the manner already described for the calling omce.

What is claimed is:

1. In a high frequency communicating system having a plurality of stations associated with a common transmission medium, the combination with a plurality of trunks at each station of means for transmitting and receiving a number of high frequencies, each of said high frequencies designating a particular station, means in each trunk 'for transmitting and receiving a. number of other high frequencies used for communicating purposes, means responsive to the transmission of a high frequency designating a called station from one of said stations when calling for selecting an idle communicating frequency and associating it with a trunk taken into use thereat for establishing an outgoing connection to said called station, means at said called station responsive to the reception of said high frequency for selecting an idle trunk and operatively associating it with the same communicating frequency which was selected at the calling station.

2. In a high frequency communicating system, the combination with a first station having a plurality of trunks and associated high frequency apparatus in each trunk adapted to transmit and receive a number of high frequencies, of means at said station for originating an outgoing call, a distant station, a plurality of trunks and associated high frequency apparatus in each of said trimks at said distant station adapted to transmit and receive the same number of high frequencies as the trunks at said first station, and apparatus individual to each trunk in said first station for selecting the same idle frequency over which the call is to be established, and apparatus common to said trunks in said second station responsive to the origination of anoutgoing call at said rst station for selecting an idle trunk and operatively associatingv it with said selected frequency.

3. In a high frequency communicatingv system having a plurality of stations, means. at each station for transmitting and receiving a number of high frequencies each designating a particular station, and a plurality of trunks at each station, the combination in each trunk of means for transmitting and receiving a number of -high frequencies used for establishing communication between the different stations, a selecting mech.- anism adapted to select an idle one of said communicating frequencies when said trunk is used for originating an outgoing call, and means under the control of the high frequency designating a station in which the trunk is located for operating said selecting mechanism to select the same communicating frequency as at an outgoing station when said trunk is used for completing an incoming call.

4. In a high frequency communicating system having a plurality of stations, the combination of a plurality of trunks at each station with means in each trunk for transmitting and receiving a number of high frequencies available at each station for communication purposes, means in each trunk when taken into use for originating a cali for selecting one of said available frequencies, means at each station when called for selecting for use one oranother of the associated trunks and means responsive to the selection of a trunk at said called station for selecting the high frequency utilized by the trunk at the originating station and associating it with the selected trunk.

5. In a`high frequency communicating system, the combination with a station having a plurality of trunks and communicating apparatus adapted for connection with any of said trunks, of apparatus in each trunk for transmitting and receiv-` ing a plurality of high frequencies to be used for communication purposes, means responsive to the connection of said communicating apparatus with a trunk for testing for and selecting an idle communicating frequency over which a connection is to be extended to and from said trunk, means responsive to the selection of said high frequency for adjusting said apparatus to transmit and receive the selected communicating frequency and means responsive to the use of said frequency for supervising a connection.

6. In a high frequency communicating system, the combination with a station having a plurality of trunks and associated apparatus in each trunk for transmitting and receiving a plurality of communicating frequencies, of selecting means common to said trunks adapted to select a particular one of said trunks. selecting means in each trunk adapted to select a free communication frequency and means for rendering said selected trunk responsive for the transmission and reception of said selected high frequency.

7. In a high frequency communicating system, the combination with a plurality of stations each having a plurality of trunks each equipped with a variable oscillator and a variable lter respectively capable oi' producing and responding to any one of a number of high frequencies available for use at each of said stations, of a switching mechanism in each of said trunks mechanically connected to said oscillator and to said ilter, and means responsive to the taking into use of any one of said trunks for a operating said switching mechanism to select a frequency not atthe time being produced and received at any of the stations, and to adjust said oscillator to produce said frequency and said filter to respond to said same frequency.

8. In a high frequency communicating system having a plurality of stations connected to a common medium and a available at each station, the combination with a plurality of trunks in which each is provided with apparatus capable of adjustment to transmit and receive each of said high frequencies, of a mechanism in each trunk to select an idle frequency when said trunk is used at its associated station to originate a connection and to adjust said apparatus to transmit and receive the selected frequency, and of another mechanism responsive Ato a calling signal from another station to select an idle trunk for operative association with said selected frequency for completing the connection.

9. In a high frequency communication system having a plurality of stations associated with a common transmission medium, the combination with a plurality of trunks at each of said stations of means in each trunk for transmitting and receiving a plurality of high frequencies for communication purposes, means for selecting an idle trunk at a calling and at a called one of said stations, and means at each of said calling and called stations responsive to the selection of a trunk thereat for selecting the same idle communication frequency and associating it with the selected trunk of that station.

NEWTON MONK.

plurality of high frequencies

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2458558 *Jul 26, 1946Jan 11, 1949Rca CorpRadiotelephone equipment
US2479701 *Feb 7, 1944Aug 23, 1949Edward B De VryTransmitting system
US2512613 *Jul 25, 1947Jun 27, 1950Int Standard Electric CorpSelective call system
US2520007 *Nov 21, 1947Aug 22, 1950Bell Telephone Labor IncTesting circuit for carrier communication system with automatic reset timer
US2536825 *Mar 26, 1948Jan 2, 1951Stromberg Carlson CoMultiplex telephony
US2566106 *Dec 9, 1947Aug 28, 1951Farmers Engineering & Mfg CoEmergency communication system, particularly for mines
US2607853 *Jan 24, 1948Aug 19, 1952British Telecomm Res LtdTelecommunication system
US2629092 *Dec 31, 1948Feb 17, 1953Automatic Elect LabMultichannel mobile telephone system
US2655652 *Jun 28, 1948Oct 13, 1953Homrighous John HMultiplex time division telephone systems
US2665375 *Sep 9, 1949Jan 5, 1954Gen Electric Co LtdRadio telephone system
US2694140 *Nov 24, 1951Nov 9, 1954Bell Telephone Labor IncSelection of low interference radio channels
US2733337 *Oct 18, 1954Jan 31, 1956 Multichannel radiant energy signaling system
US4996709 *May 12, 1988Feb 26, 1991Tandy CorporationIntercom telephone
Classifications
U.S. Classification370/295, 220/2.30R, 455/59, 370/480, 455/516, 340/6.13
International ClassificationH04M7/16
Cooperative ClassificationH04M7/16
European ClassificationH04M7/16