US2264395A - Power line carrier frequency telephone system - Google Patents

Description

Dec. 2, 1941. D. MlTCHE LL POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM Filed Oct. 22, 1940 b Oh WE w mi 2 w Z @W 05 E /H &N mm M V/ WM. w

Patented Dec. 2, 1941 POWER LINE CARRIER-FREQUENCY TELEPHONE SYSTEM Doren Mitchell, Bound Brook, N. J., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 22, 1940, Serial No. 362,249

4 Claims.

This invention relates to electric wave transmission systems, and, more particularly, to a power line carrier frequency telephone system.

Carrier frequency telephony on transmission lines primarily intended for the transmission and distribution of low frequency high voltage electric power has, of course, been known for some time although never very extensively applied. It has, however, become of increasing interest in View of the somewhat extensive electrification of rural areas and the possibility provided by the rural power lines of providing telephone service thereover to the scattered inhabitants of the sparsely settled regions through which the power line extends.

An object of this invention is to provide an improved arrangement for transmitting and receiving telephonic communication on high frequency waves over a transmission path, such as a high voltage power line.

In accordance with the invention, a plurality of telephone stations and a central office for the stations are coupled to a power line. Transmitting from each station is on a common high frequency carrier wave and receiving at each station is on a second common high frequency carrier wave. The central oifice acts not only as a means for connecting parties on the power line telephone system with an outside or general telephone system, but constitutes a relay or frequency change-over station for the telephone stations of the power line system, that is, the outgoing transmission from a telephone station on the one high frequency carrier wave is received at the central office, and, after suitable demodulation, the modulation products are remodulated onto the second high frequency carrier wave for transmission to the called or listening telephone station. No voice-operated devices are necessary at the central ofiice although voice-operated devices are employed at the telephone station to enable the transmitting circuit thereat. When voice-operated devices for enabling the transmitting circuit are provided at the telephone stations and the latter are arranged for transmitting on the same carrier wave, intentional or inadvertent operation of the voice-operated devices at a called station while the party at the calling station is talking may produce an unpleasant or undesirable interruption or interference with the transmission from the calling station.

A feature of this invention is the provision at the telephone station of means in the receiving circuit to disable the transmitting circuit enabling means while transmission is incoming to the receiving circuit.

A more complete understanding of the invention and the various features thereof will be obtained from the detailed description which follows taken in conjunction with the appended drawing, the single figure of which illustrates an embodiment of the invention.

The Single figure of the drawing shows a power line carrier frequency telephone system comprising a power line I00 and a plurality of telephone stations A, one of which is shown in detail, and a central office C0 for the stations coupled to the power line. The system is arranged so that the stations communicate with one. another, or with a general telephone system through the central ofiice. Transmitting, or transmission outgoing from each station, is on a common high frequency or carrier wave F1, and receiving, or transmission incoming to each station, is on a second common high frequency or carrier wave F2, the central ofiice constituting a relay or frequency change-over station for receiving the transmission of the calling or talking party on the one frequency and for converting it to the frequency on which the called or listening part is able to receive it. e

The power line has one wire grounded and is adapted to transmit low frequency, high voltage electric power simultaneously with its use as a transmission line for carrier frequency telephonic communication. The power line, of course, could'be an insulated single phase line, or comprise a pair of phase wires of a multiphase line, or other type of power line.

Each telephone station A comprises a transmitting circuit In, a receiving circuit H and a coupling line l2. The transmitting circuit comprises a transmitter I'3, a volume control amplifier M, a modulator IS, a high frequency or carrier wave generator or oscillator l6, an output amplifier I! and a band-pass filter IS. The transmitter I3, for example, of the carbon granule type, is connected to the volume control amplifier through a transformer l9, and is connected in series with one transformer winding and a source 20 of talking current. The volume control amplifier may be of the voice-operated gain adjusting or volume limiter type, or may be provided with a manual control to provide" for talkers of different transmission levels so that the voice current level into the modulator will be the same at all times, or to insure that there will be a high ratio between the communication current and the noises in the transmission line, The

modulator may be of any suitable type, as may the oscillator I 6. The oscillator generates a high frequency F1 which, during transmitting, is supplied to the modulator and is modulated by the voice frequency currents impressed on the modulator from the amplifier M. The modulation products are amplified in the amplifier l! and either the upper or the lower side-band, or both side-bands of the modulated high frequency wave are passed by the filter Hi to the line. The filter is adapted to transmit a band of frequencies comprising the upper or the lower side-band or both side-bands of the modulated high frequency wave for outgoing transmission from the station, i. e. it transmits the band F1 to Fri-F3, F1 to F1F3 or F1 to FliF3, where F3. is the upper limit of the audio frequency band beingtransmitted in the system. Normally, however, no high frequency wave is delivered to the modulator since the oscillator is short-circuited by the normally closed contact 2| of relay 22-. The nor mal condition of the station would be that in which the subscriber is not talking. An amplifier-detector 23 is connected across the output of the amplifier i4, and is adapted, when sound waves are impressed on the transmitter l3, to amplify and detect a portion of the voice frequency currents being delivered to the modulator and to deliver such detected currents to the relay 24 to cause it to close its normally open contact 25.

One end of the winding of relay 22 is connected through the normally closed contact 26 of the signaling key 21 to the junction of a resistance 53 associated with the contact 25 and a condenser 28, one terminal of wihch is connected to ground. The other end of the winding of relay 22 is connected to ground through a relay 29, a resistance 30 and a source 3| of current. Another resistance 32 is connected between the positive terminal of source 3| and the normally open contact 33 of key 21. An interrupter 34, to vibrate, for example, at 20 cycles per second, is connected for operation on closure of normally open contacts 33 and 54 of the key 21.

The receiving circuit comprises a band-pass filter 35, an amplifier 36, a second amplifier 31 for additional amplification if desired, a demodulator 3'8, station signaling or ringing means 39', an attenuator network 40 and a. telephone receiver 4|. The filter is adapted to transmit a band of frequencies comprising the upper or lower side-band, or both side-bands, of the modulated high frequency wave employed in the system for transmission incoming to the station, that is, the filter passes the band F2 to F2+F3, F2 to Fz-Fs or F2 to Fzi-Fs, where F2 is the carrier wave frequency for receiving and F3 is the upper limit of the audio frequency band being transmitted in the system. The amplifier 36 may comprise an electron discharge device whose input control grid biasing potential is determined by the potential drop in the cathode resistor 42, and in the resistor 43, which is supplied with the direct current component of the output of the demodulator 38. Variation in the output level of the demodulator from a preassigned level varies the value of the current fed back to the resistor 43 through the connections 41, to vary the bias on the control grid of the discharge device to change the gain of the control amplifier and to restore the demodulator output to its preassigned level. The variation in the modulator output may result from change in the level of the transmission incoming to the station because of different distances between any one station and the other stations coupled to the power line. The resistance 44 and condenser 45 act with retard coil 48 and the winding of relay to suppress alternating current feedback in the controlled amplifier. The demodulator is shown as comprising a plurality of variable resistance devices such as copper-copper oxide rectifier units in bridge configuration. As already indicated, the gain control resistor 43 is connected across the terminals 46 of the demodulator through connections 41, a retard coil 48 being provided in one connection. Condenser 49 is a blocking condenser. The station signaling means 39 may comprise a ringer adapted to be operated, for example, on a 20-cycle ringing current. The network 48 compensates for the lower volume level required for listening as compared with that required for operation of the signaling means 39. A relay 50 is included in the feedback connection from the demodulator, and has a normally open contact 5! associated therewith closure of which, while normally closed contact 52 of relay 29 remains closed, short-circuits the input to the amplifier-detector 23. Relay 50, however, will be energized only when transmission is incoming to the station. The object of this arrangement will be pointed out in more detail hereinafter.

The station is connected to the ower line by the coupling line [2, one conductor of the latter being connected directly to the grounded wire and the other to the non-grounded wire through a coupling condenser 56. With an insulated single phase line, or in case of connection to a pair of phase wires of a multiphase system, a coupling condenser would be provided in each conductor of the coupling line. Coupling might also be accomplished by use of a power distribution transformer, for example in the manner described and claimed in L. K. Swart application, Serial No. 359,879 filed' October 5, 1940. In any case a suitable protector 55 is provided to safeguard the subscriber and the station equipment against the hazard of the high voltage on the power line.

The central ofiice will now be described. It comprises an incoming or receiving circuit and an outgoing or transmitting circuit 10 between the coupling line l2 and the operators line jack 6|. The incoming circuit 60 comprises a filter 62 similar to filter H!- of the telephone station, an amplifier 63, a demodulator 64 with a suitable automatic volume control feedback connection 65 from, the demodulator to the amplifier, a relay 66 adapted to respond to the low frequency signaling current, for example, 20 cycles per second originating at a telephone station, and an attenuator network 61 to equalize for the different levels required for the signaling current and for the voice frequency currents. The. armature associated with the relay 66 is connected to ground through a current source 68, and its normally open contact 69 is connected to ground through the winding of relay H and through the operators signal lamp 12. The outgoing circuit 1.0 comprises an amplifier 13 similar to the arnplifier f4, a modulator M, an output amplifier I5 and a band-pass filter 16 similar to filter 35. An oscillator Tl generating the high frequency carrier wave F2 is coupled to the modulator, the connections including the normally closed contact 18 of an interrupter or buzzer 19 adapted to respond to a low frequency current, for example, 20 cycles per second, to be supplied over the connection 8!) through the normally open contacts 81 of relay H when the latter is energized ,receiver 4| would be supported on aswitchhook which, as will be evident to those skilled in the art, could have associated therewith normally open contacts controlling the talking current circuit of the transmitter I3 and the power supply circuit for the oscillator and the transmitting circuit amplifiers. As shown in the drawing, the receiver has been removed from its hook and the talking current circuit for the transmitter I3 is closed and the oscillator and the amplifiers of the transmitting circuit are understood to be connected to their power supply circuits. The receiving circuit amplifiers are continuously energized o that the receiving circuit is at all times in condition to receive incoming transmission. By listening for a moment, the calling party ascertains if the power line is already in use. If it is idle, he depresses the key 21, the number of times and for a period specified for the called station by the signaling code of the power line telephone system. Each time the key is depressed, energizing current is supplied from source 3| to the interrupter through contact 33, and energizing currents for relays 22 and 29 are supplied from the same source, the current circuit for the relays being completed by closure of contact 54. Operation of relay 34 causes the contact 2| of relay 22 to break-and-make at the interrupter frequency, for example, 20 cycles per second. This modulates, at a corresponding frequency, the carrier wave F1 supplied to the modulator from the oscillator l6, and the modulated carrier wave is transmitted through amplifier I1 and filter l8 to the line and over the latter to the central office and through the central office to all of the stations. The modulated carrier wave is transmitted by the filter 62 to the amplifier 63 and is demodulated by the demodulator 64, the demodulated signal energizing the relay 66. Closure of the normally open contact 69 energizes relay H and the signal lamp 12. Operation of relay 1| closes the normally open contacts 8| and low frequency signaling current is supplied over connections 80 to the transmitting circuit. The interrupter I9 is operated thereby and modulates the carrier wave F2 by the corresponding break-and-make of contact 18. The modulated carrier wave is amplified in amplifier l5, and transmitted through the filter 16 to the power line. The modulated carrier wave F2 is received at all of the stations, being passed by filter 35 to the amplifiers 36 and 31 and demodulator 38. The demodulated signal is delivered to and operates the signaling means 39. When the called party answers, the calling party talks into the transmitter I3, and the voice frequency currents developed in the transformer are amplified in amplifier l4 and impressed on the modulator. Simultaneously the voice currents are impressed on the amplifier-detector 23. The rectified voice current output of the amplifier detector energizes the relay 24, and normally open contact 25 closes. The condenser 28, normally under charge from source 3 I, discharges through resistance 53, and relays 22 and 29 energize to open the normally closed contacts 2| and 52 for as lon as the calling party continues to talk. The carrier wave F1 supplied to the modulator is modulated by the voice frequency currents, and the peech modulated'carrier wave is amplified in amplifier I! and transmitted to the line through the filter I 8. The modulated carrier wave enters the central ofiice and is transmitted through the filter 62 through the amplifier 63 and demodulator 64. A preassigned output level from the demodulator is maintained through the automatic volume control connection 65. The demodulation voice frequency products pass through the network 61 and the normally closed contacts 82 of relay H to the outgoing circuit 10. The voice frequency currents are amplified in amplifier l3, modulated onto the carrier wave F2. and are transmitted through the output amplifier 15 and filter 16 to the power line. The modulated carrier wave travels over the power line to the called station and enters the receiving circuit H thereat. The modulated wave is transmitted by the filter 35 to the amplifiers 36, 31, and demodulated by the demodulator 38. The demodulated voice frequency currents are delivered to the receiver 4| through network 40. The direct current component of the demodulation products is fed back through the connections 41 to the gain control resistor 43 to maintain the demodulator output level at a preassigned value. At the same time, the feedback current energizes the relay 59 and causes closure of the normally open contact 5|. Since at the called station, relay 29 would not be energized, contact 52 is closed and the amplifier detector circuit at the receiving tation is short-circuited or disabled. The called or the listening party cannot intentionally 'orv accidentally take control of his transmitting circuit by talking, or perhaps coughing, into his transmitter while transmission is incoming through the station. If the called or listening party were free to use his transmitting circuit, it is apparent that interference with transmission from the other station would result because of the presence on the power line of outgoing transmission from two stations on the same carrier wave. Since the receiving circuits of all of the stations in the system are normally energized when any two stations, or any one station and the central office have established a connection, the transmitting circuits in each of the other stations will be disabled during the talking period of such connections.

If the calling party had desired to call the operator at the central ofi'ice instead of any telephone station on the line, the procedure would have been the same as described above; the operator responds to the central office signal by plugging her telephone set (not shown) into the jack 6|. The operator, therefore, receives on carrier wave F1 and transmits and signals on carrier wave F2. The operator may connect a power line telephone station and a party on an outside or general telephone system through conventional switchboard facilities.

The system described with reference to a power line transmission path is applicable also to a transmission path provided by a pair of telephone wires. If the line I00 comprised two conductors balanced with respect to ground instead of having one grounded as shown in the drawing, and a condenser 99 were inserted as indicated, such an arrangement would be realized. This would permit private or semiprivate service to be provided over an ordinary rural telephone line.

What is claimed is:

1. A carrier frequency telephone system comprising a telephone station coupled to a line, said station comprising a. transmitting circuit for transmittinglon a high frequency- :electric wave,

an=amplifier for an incoming modulated highfrequency wavey' a demodulator for demodulating the amplified modulated high frequency wave, i a feedback connection from said demodulator -tosaid-amplifier, and means insaid feedback con nectibn fordisabling the transmittingcircuit en abling -means during receiving by said receiving 2. "A' carrier frequency telephone systemcomprising a telephone-station coupled to a 1ine,--saidstation comprising a transmitting circuit for transmitting -on a-high frequency electric Wave; and areceiving circuit for receiving on another high-frequency electric wave, said transmitting circuit being-normally disabled andincluding voiceenergy operated means for enabling it, said receivingcircuit being normally enabled and including-anamplifier for an incoming modulated-:- high frequency waveya demodulator for demodu-- latingthe amplifieclmodulated high frequency wave means for disabling the transmitting circuit enabling means during receiving by said re-- ceiving' circuit, and means for feeding' backanoutput-component of said demodulator-to regu-- late the gain of said amplifier and-to operate-said disabling meanse 3; IA carrier 2 frequency: telephone :system com-' prising a telephone station coupled to aazline, said station: comprising: a. transmitting circuit Y for transmittinggvon; a high; frequency electric wave,

2 saidrtransmittinga circuit being normally disabled. andincludingmeansito enable it for transmission v to th'eilirien and: a :receiving circuit for vreceiving onitanother :high frequency :electric wave, said re- 1 ceivingcircuitbeingcontinuously enabled and includingaanxamplifier for modulated high frequencyeleotric wavesz incoming to lthe station fromathe: line; a ifeedbackwconnection fromthe.

outputricircuit :ofJsaidamplifier to its input circult, and means in;said;;feedbackl connection for ;disablingdaheztransmitting; circuit. enabling means duringireceivingby said receiving circuit.

4. A carrier;frequencyatelephone system comprising .a;.telephone station coupled to a line, said station; comprising a r transmitting circuit for transmitting aongaehighflfrequency electric wave,

saidctransmittingmircuit being normally disabled and: includingimeans to enable it. for transmission toi'th'e line, :andra, receiving circuit .for receiving on; another high vfrequency electric .Wave, said receivingicircuit :being :continuously enabled and. inoluding :an; amplifierwforqmodulated high. frequencyelectric wavesincoming from the line, an

amplifier gain: control. feedback connection from the output circuit of said: amplifier to its input -circuitr, and means said feedbaclc connection i for disabling the transmitting circuit enabling meansduri-ng receiving by-said receiving circuit.

DOREN MITCHELL.

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