US2428520A - Transmission system - Google Patents

Description

Oct. 7, 1947. R. H. HERRICK 2,428,520

TRANSMISSION SYSF'EM Fiied May 1, 1943 g Sheets-Sheet 1' .OUT AMP. CHANNEL (I24 ENABLING a DISABLING UNIT 7 I "0 BALANCING CIRCUIT ENABLING 8 DISABLING UNIT,

FIG. I

Q65 l64l IN AMP INVENTOR. ROSWELL H. HERRICK M 1M7, M m

ATTORNEYS LINE I00 Oct. 7, 1947. HERRICK I 2,428,520

TRANSMIS S ION SYSTEM Filed May l, 1943 2 Sheets-Sheet 2 OUT AMP. CHANNEL ENABLING 8| DISABLING UNIT BALANCING cmcun ENABLING 8| DISABLING UNIT III FIG.- 2

LINE 200 AMP. CHANNEL INVENTOR. ROSWELL H. HERRICK ATTORNEYS Patented Oct. 7, 1 947 UNITED STATES PATENT omce Roswell H. Herrick;

Automatic Electric ration of Delaware Oak Park, Ill., assignor to Laboratories, Inc, 21/601110:

Application May 1 1943, Serial No. 485,269

'12-Claims. l

The present invention relates generally to improvements in signal current transmission system of the type in which signal controlledswitching circuits are provided for partially or completely blocking under certain conditions. certain of the signal channels included therein and, more particularly, to improvements in telephone substation circuits or the type disclosed in United States Letters Patent N0. 2,282,405, granted May 12, 1942, upon the application of Roswell H. Herrick, for Transmission'systems.

In the patent referred to there is disclosed an improved telephone substation apparatus of the character wherein the channels are preferably only partially blocked when not in use, and Where-, in the control of the channel blocking circuit is arranged to utilize a hot cathode controlled gas tube, whereby eifective channel blocking and unblocking operations are obtained Without any sub-. stantial speech clipping.

In such an arrangement there is provided a telephone substation connected to an associated line by a hybrid or anti-sidetone impedance .network and a'first channel'is provided for trans mitting signal currents ol the gas tubesto the as.- sociated channel-so that a predetermined potential is impressed on the control electrode of the tube :to render it conductive when signal current traverses its associated channel.

Each of the control circuits is provided with circuit meansoperative to lower the potential difrent arresting means are disclosed. In one of these embodiments a sloW-to-release relay is provided in the loador output circuit of the-rectifying means, which relay functions momentarily to interrupt the anode circuit of the associated gas control :tube when the input signal voltage derived from the associated channel, and impressed upon the input circuit of the rectifying means, is removed. In the other embodiment there is Q Y p o ded a source of alternating current which is utilized as the anode current source for each of two control tubes. With the latter arrangement, when the control electrode of one of the tubes is biased to a starting potential, the tube functions as a half-Wave rectifier so that direct voltages are developed in the output circuit thereof which are utilized to perform the desired char-1 nel control functions.

While such arrangements have proven to be entirely satisfactory it may be desirable to provide certain other characteristics and advantages not contained therein. For example, it may be desirableto utilize in place of a hot cathode controlled gas tube, a cold cathode controlled gas tube. The particular advantage to be gained is that the need for filament power for the control tube is eliminated, and furthermore, since many hot cathode controlled gas tubes have cathodes which require a longer heating time than the oathodes of the tubes used in the associated channel amplifiers, there of course is some delay between the times when the two types of tubes are conditioned for use after the heating circuit for the cathodes has been closed; Since controlled gas tubes must have the anode potential interrupted or reduced in order to restore the control of the control electrode, various methods and devices must be provided. In certain instances it is possible to provide improved arrangements winch, for certain situations, may have distinct advan tages over arrangements such as those already known in the art. For example, where arr-alternating current supply is used to bring about the periodic interruptionof the conductivity of the control tube, there is a possibility of some delay occurring in accordance with the probabilities relative to the positive half-cycles of alternating current. In other words',-in certain instances the initiation-0f speech'might fall within the negative loop of alternating current which would provide a maximum delay not less than 8.35 milliseconds or of a second, which, together with the two or three milliseconds required for a positive half-cycle of anode potential to reach a value at which the tube will become conductive, might produce at certaintimes some speech clipping. In order to reduce such possibilities of speech clipping or delay which might "be unde sirable for certain purposes, it is proposed to provide a direct current potential for thfi control tube which, together with some mean responsive to the conductivity of the tube Will periodically of proper polarity the particular arrangement of the render the tube non-conductive. Such periodic rendition of the tube to non-conductive state is I preferably produced at a frequency which is low compared to production voice frequencies, but which, however, is sufficiently high so that it will not be difficult to provide a filter arrangement adequate to remove the noise which is brought about as a form of an alternating current component due to interruptions of the conductivity of the tube. If such an arrangement has provided periodically interrupted conductivity of the tube by a variation of the anode voltage, the conductivity is re-established at the next half-Wave from the controlling voice or signal Wave. It therefore becomes apparent that it is desirable to avoid a relatively high frequency of interruption because this would result in a smaller proportion of time of conductivity of the control tube which, in turn, would produce a smaller average anode current flow and a resultant smaller control voltage for application to the associated channel amplifier.

In accordance with the present invention several embodiments are disclosed for periodically rendering the control tube non-conductive. In one of these embodiments circuit means are included in the anode circuit of the control tube which, due to the flow of anode current built up an IR drop which, in turn, suificiently reduces the anode potential so as to bring about themterruption of the conductivity of the tube. In the other embodiment other means are provided responsive to the conductivity of the tube for periodically rendering the tube non-conductive, and in this arrangement a predetermined control voltage is constantly applied to increase the gain of the associated channel amplifier and yet the anode supply to the controlled gas tube is periodically interrupted so as to periodically give the control electrode a renewed opportunity to control the conductivity of the tube. Upon cessation of voice or signal current flowing through the associated channel amplifier the failure of the control electrode to supply a potential sufiicient to renew the conductivity of the control tube brings about operation of a slow-to-release relay which thereby removes from the control electrodes of the tubes of the channel amplifier the fixed bias which was applied to increase the gain thereof.

It is an object of the present invention to provide improved telephone substation apparatus of the character described wherein the channels are preferably only partially blocked when not in use, the blocking of the signal current transmitting and receiving channels is accomplished in a simple and reliable manner, and wherein the control circuits are arranged to effect channel blocking and unblocking operations without any substantial speech clipping. 7

It is another object of the invention to provide an improved transmission system particularly suited for use in telephone substation circuits of the character described, wherein the control or channel blocking circuit is arranged in an improved manner to utilize a cold cathode controlled gas tube. a

It is still another object of the invention to provide an improved transmission system utilizing channel amplifiers wherein the control circuit employs a controlled cold cathode gas tube and wherein means are provided responsive to the conductivity of the tube for periodically rendering the tube non-conductive.

Further features of the invention pertain to circuit elements, whereby the above and additional operating features are obtained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages there- 'of-will bejst'be understood by reference to the following description taken in connection with theaccompanying drawings, in which Figure 1 illustrates a substation circuit having incorporated therein one embodiment providing certain features of the invention as briefly outlined above; and Fig. 2 illustrates a different embodiment of the improved transmission system as incorporated in the substation circuit.

Referring now more particularly to Fig. 1 of the drawings, the substation circuit therein illustrated is adapted to be connected to a telephone line I00 which may, for example, form a part of a conventional automatic or manual telephonesystem, in which case the distant end thereof willlterminate in a line circ t, the character of which is determined by the character of the exchange. Briefly described, the substation circuit comprises a transmitter or microphone ItII, receiving means in the form of a loudspeaker I02,

an anti-sidetone impedance network' or hybrid.

system I33, a. balancing circuit therefor I04, and outgoing and incoming amplifier channels I05 and I96. The outgoing amplifier channel may comprise a plurality of stages of amplification of any suitable type, such as resistive, capacitive or inductive coupled amplifiers. Since the particular type used is of no special significance to the present invention. only the input'and output tubes thereof have been disclosed to show how the control circuit associated therewith is to be connected to the amplifier channel. The outgoing amplifier channel is shown having an input vacuum tube Iil'l which has a grid energized from voice currents produced by the microphone "II. The grid to cathode circuit includes a self-biasing resistor IDBsuitably by-passed by a capacitor 109, both of which are connected to ground. A portion of the'output stage of the amplifier is shown comprising the output tube I2I having a grid connected to a coupling capacitor I22 and a coupling resistor I23. The anode of the vacuum tube I2I is connected to'a transformer I24,

the output of which is connected to the hybrid be amplified by a suitable amplifier I29 which is provided with an output transformer I30. The controlcircuit I25 includes a cold cathode gas tube I3I having a cathode, an anode, and a con trol or starting electrode. The control or starting electrode of the gas tube 'I3I is connected to a predetermined pointon a bleeder circuit comprising the grounded output winding of the transformer I3U'and' the resistors I32 and I33 which are connected'to the positive terminal of a source of voltage I34. The control electrode is connected to the common juncture of the resistors I32 andI33; The

7 source of potentiall34 has its negative terminal connected through 'a 5 resistor I35 to "ground. The cathode of the control"tube*'I3I is connected through a resistor I36 to ground. The anode of the control tube I3I is arranged'to be connected through a suitable circ'uit to the positive terminal of the source of voltage "F34. As has been stated, the control electrode "of the tube I3] is connected to a bleeder circuit-which is arranged so that the voltage drop appearing across the resistor I32 supplies the greater portion of the voltage required by the control electrode to render the control tube conductive. The remainder of the necessary required voltage is obtained from the voice components 'f 'ppearingacross the output winding of the transformer I30. While the control electrode of the vacuum tube I31 might be energized solely from voice components appearing across the transformer 1-30 by supplying sufficient stages of amplification in the amplifier I29, it is believed to be an uneconomical arrangement. When anode current flows it will be noted that the anode to cathode circuit includes both the resistors I35 and I36, the common juncture of which is connected to ground. The opposite terminals of the resistors I'35and I36 are arranged to supply suitable-potentials to the incoming channel amplifier thereby to disable that channel amplifier, and tofthe outgoing channel amplifier thereby to enable that amplifier. Accordingly, the remaining terminal of the resistor I36 is connected to a suitable circuit including a resistor I37 and a grounded capacitor I38 and the grid coupling resistor I 23 of the outgoing channel amplifier outputtube 'I2I. The other terminal of the resistor I35 'isconnected to a circuit including a series resistor I39 and a grounded capacitor I40 and the grid circuit of the inputtube of the incoming amplifier channel I06.

As has previously been stated, it is desired to provide "a control circuit wherein, in response to the conductivity of the tube, means are operative for periodically rendering the tube non-conductive One of the simplest circuit arrangements which would thus provide such operation would be to "provide a resistor I44 connected in series between the anode of the control tube I 3| and thepositive potential of the anodesupply I 34-by connecting the strap I45 and disconnecting the straps 146 and 141. By proper selection of the valueof the resistor I44 in series with the anode to cathode circuit,'the resulting IR drop produced inresp'onse to the conductivity of the tube I3I will be such as to reduce sufiiciently the efiective anode voltage that the tube will no longer remain conductive. .Thereupon the voltage appearing across the. series resistor I44 also disappears and again the'full anode potential is applied to the control tube I3I. If it is assumed that signal currents arestill being transmitted through the'outgoin channel amplifier the next succeeding positive half-wave of voice voltage appearing at the controlelectrode of the control tube I 3I will again render the control tube conductive. This cycle of operation therefore will continue to recur as long as voice currents are being transmitted through-the outgoing amplifier channel. Such an arrangement, however, would require that considerable filtering action be provided for the resistors I35 and I 36 in order to suppress the rather low interruption frequency resulting from such operation, which of course would reduce seriously the-rapidity with which the enabling and disabling unit I25 could enable and disable the outgoing. and incoming channel amplifiers I35 and I06. 'In 'order to obviate this disadvantage another arrangement has been provided, whereby "the initial enabling and disabling potentials supplied to thechannel amplifiers have -a relatively small "amount of filtering action provided and subsequently additional filter circuit elements are introduced in order to filter out the eife'cts of starting and stopping the control tube I3'I.

This improvement which provides for more rapid enabling and disabling action is accom'- plished by disconnecting the strap I45 and connectingthe stra'ps I46 and I4! of a circuit which may be'traced from the anode of the control tube I3I,strap I46, a'relay I it connected in series with a 'resi'stor I48 which, together with the relay H0, is byepassed by a capacitor I49 through the strap 1'4! to the positive side-of the anode potential I34. A voltage-divider circuit comprising the resistors and I5:I is connected across the source of potential I 34. The common juncture between the resistors I5ll-and I-5I'is connected to ground. The relay-III) is provided with break contacts I I I connected between a resistor I52 which is connected to-anad-justable contact on the resistor I50, and a'filter capacitor I53 which is connected to ground. Make contacts IIZ are provided for connecting the capacitor I53 in parallel with a small filter capacitor Ml] which is connected to one terminal of a resistor I39. Another pair of contacts are also provided for the relay IN] which include the break contacts II3 connected between a resistor I54 which is connected to the adjustable contact of resistor I 5I, and a filter capacitor I55. By means of make contacts II4 the filter capacitor I55is arranged to be connected in parallelto the,

smallercapacitor I353 which is connected to one terminal of resistor I31.

In thecircuit just described, initial energizati-on of the control tube -I3I produces a fiow oi anode current which rapidly produces potentials across theresistors I35 and I36 so that these potentials-are impressed through the resistors I 39 and I31- respectively upon the incoming and outgoingchannel amplifiers. Thereupon the-relay I Illattractsits armature thereby to break con tacts-J-II and I-I3'and to make contacts H2 and II4. Thus shortly after the initial enabling and disabling-voltages are supplied to the channel amplifiers, additional filtering action is supplied by connecting in circuit the capacitors I53 and I555. The relay I I0 is provided with the capacitor I 49 so asto providea holdover action, so that the relay does not respond to-the periodic and momentary interruptions of the control tube I3I. The anode circuit of the control tube I3I is therefore provided with a relay I I 0 and with a bridgingcapacitor I49 which offers the required impedance, together with the resistor I48 which is suflicient to periodically render non-conductive the control tube I3 I due to the IZ drop. In this new arrangement therefore the interruption frequency is dependent upon the supply voltage I34 and the total'ei fect of resistors I35, I36-and I48. In order that the connection of the'capacitors I53 and I55 should-not disturb the value of the enabling and disabling voltage already applied to the channel amplifiers, the capacitors I53 and I 55 are initially charged through the break contacts II I and 'I I3 ofthe-relay H0 and the current limiting resistors I 52 and I54from the voltage divider resistors 15!) and I5I which are connected across the sourcerof potential I34.

The incoming channel amplifier I06 may comprise any desired number of stages ofamplifica tiondependent upon the requirements to'be met, and -for purposes of simplicity only two amplifier tubesian input 7 tube I56 and an output tube, I51, The control grid electrode of tube I56 is energized from a have been shown.

the vacuum tube I56 may be biased negatively so as to disable the incoming channel amplifier whenever the outgoing channel amplifier is used,

and this is accomplished by connecting one terminal of the resistor I58 to one terminal of a re- .sis'tor I39 of the enabling and disabling um't I associated with the outgoing amplifier. The output tube I51 is connected to an output transformer I59 which, in turn, is connected to the loudspeaker I62. The grid circuit of th output tube I61 may include a resistor I60, one terminal of which is connected to an enabling and disabling unit I6I associated with the incoming channel amplifier.

A portion of the incoming energy supplied across the input resistor I58 of the incoming channel amplifier is coupled through a capacitor I62 and a voltage divider resistor I63 to a suitable amplifier I66 which is connected to a coupling transformer I55. The secondary winding of the transformer rounded, is connected through a resistor I66 to the control electrode of a controlled gas tube I61. This gas tube IE1 is of the same type as the gas tube I3I of the other enabling and disabling unit I25. While in each of these cases the gas tubes represented have been shown as comprising an envelope containing a gas or other suitable ionizable medium with an anode, a cold cathode, and a control electrode shown in the form of a starting electrode, it of course is understood that the control electrode may comprise any one of the other forms of control electrodes commonly used with this type of cold cathode gas tube including a control grid. The control electrode of the gas tube I61 is connected to the common juncture of the resistor I66 and a resistor I68 I65, which has one terminal which, in turn, is connected to thepositive terminal of a suitable source of potential I69 so that th circuit comprising the transformer I65 and the series resistors I66 and 966 constitute a bleeder circuit similar to the bleeder circuit provided for the enabling and disabling unit I26. The cathode of the gas tube IE1 is connected to the negative side of the source of voltage 566 through a pair of resistors I10 and HI, the common juncture of Which is connected to ground.

the secondary winding of posted to ground to and I83 which'are connected to grou'nded'ca 'pacitors I18 and I16 and to current limiting'resistors I85 and I86. The resistors I85 and I86, in turn, are connected to movable contacts on a voltage divider comprising a pair of resistors I81 and I86 connected across a source of voltage I69. The common juncture of the resistors 'I81and I88 is connected to ground. The cathode of the gas tube W1 is connected eluding a resistor I89 to one terminal of the grid coupling resistor I60 of the output vacuum'tube 61; The juncture between the resistors I89 and I6!) is provided with a capacitor I96 which is con-'- filter'action. The negative terminal of the battery I69 and one terminal of the resistor 'I1I are connected by a circuit having a series resistor I9I which is provided with a grounded filter capacitor I92, to the grid circuit of the input tube I61 of the outgoing channel amplifier.

,In response to signal energy appearing across the input of the incomingchannel amplifier the control electrode of the gas tube I61 has its potential increased sufficiently so as to'ionize the tube, thereby to render it conductive. Thus anode current will flow through the ,relayIBll and therresistor I16 so that shortly thereaiter the IZ (honor those two elements will be sufiicient to reduce the anode voltage below that re-. quired to maintain conductivity of the gas tube I61. When this occurs control is again restored to the control electrode of the gas tube. If signal energy is still being impressed on the in-,

coming amplifier the gas tube I61 will again be' rendered conductive upon the next positive halfcycle of voice or signal energy. The relay I86, when energized, will disconnect the capacitors I18 and I19 from the resistors I85, I81, I86 and I88 whereby they were being charged to a predetermined potential, and thesecapacitors will then be connected by the make contacts I82 and I84 7 thereby to filter adequately, the disabling voltage supplied to the outgoing channel amplifier and the enabling voltage supplied to the output tube of the incoming channel amplifier The capacitor I11 connected in parallel to the resistor I16 and the relay I8!) provides sufiicient holdover or slow release characteristics so that the additional filter capacitors I18 and I16 are not disconnected at the time. that the anode voltage of the gas tube I61 is reduced sufficiently If an desired to provide a simplified former tube, a series resistor I12 may be connected by completing the anode circuit through the strap J13 and disconnecting the straps I14 and I15. Theoperation of this circuit would'be similar, therefore, to the circuit described for the outgoing amplifier enabling and disabling unit I25 whenthe resistor I44 was connected in the anode circuit of the gas tube I3I. Q 1 e l t is preferable, however, to use a circuit which has certain, advantages over this simple circuit and, hence, the strap I13 is disconnected and the straps I14, and I15 are connected in order to disconnect the resistor I12 and to connect in the anode'circuit of the gas tube I61 a relay I86 and a resistorsI 56 connected in' series with each other to restore control to the control electrode of the tube. If, however, no further incoming signal energy is present thegas tube I61 will not be rendered conductive upon the next positive half-cycle of signal voltage which of course is no longer there, and hence the relay I66 will release. It is believedthat no further explanation of the operation of. this enabling and dis' abling unit ISI is necessary in View of the simie larity of circuit elements and operation to the circuit elements and operation of the enabling and disabling unit'I25 associated with the outgoing channel amplifier which previously was described in considerable detail;

From the above description it therefore becomes apparent that each channel amplifier has associated therewith an enabling anddisabling unit which includes a cold cathode controlled 'gas, tube whichjin response to signaljcurren'ts traversing its associated channel amplifier, sup:

'pli'e's control potentials; one fof' which enables its channel amplifier and the other of which disables theother channel amplifier; Each of the control tubes is'connectd in a circuit which through a circuit in-' provide a small amount of is responsiveto the. conductivity of the tube to: periodically interrupt the. conductivity thereof so as to restore the; control of the. tube to the control electrode. In the preferred circuit arrangement the initial enabling and disabling voltages are provided with a relatively small amount of filter action which has a relatively short time constant so that rapid enabling and disabling may be accomplished, thereby to minimize or preventspeechclipping. Subseq-uentto the initial application of the rapid" enabling and disabling control voltages additional filter means are connected so as to provide controlvoltages which have no appreciable alternating current components, and the additional filter means also serve to prevent feedback between the two channel amplifiers through the controlcircuits.

Referring to Fig. 2, another embodiment is shown wherein means are provided for disabling and enabling incoming and outgoing channel amplifiers. In order to simplify the disclosure and the description, those elements which are similar to th elements: previously shown in Fig. 1 have been given reference numerals one hundred greater than the numerals in Fig. 1. The substation circuit there illustrated is connected to a telephone line 200 which, in turn, is connected to a hybrid unit 203 which interconnects a balancing circuit 204, an outgoing channel amplifier 205 and an incoming channel amplifier 295. The microphone is connected to the outgoing channel amplifier and a loudspeaker 282 is connected to the incoming channel amplifier. The outgoing channel amplifier is provided with In order that the enabling and disabling unit 225-may be responsive to signal: currents passing through the outgoing channel amplifier 205, a portion of the signal energy is obtained by a voltage divider 226 and a coupling capacitor 221 from the input circuit of the vacuum tube 201. This energy maybe amplifiedby a suitable amplifier 229 which is" connected to a transformer 230,, A cold cathode controlled gas tube 23| has a control electrode connected to av bleeder circuit comprising the grounded secondary Winding of the transformer 230 and resistors 2.32 and 233 which are connected to the positive side of a source of voltage 234. The cathode of the gas tube 2M is connected to the negative side of the source of voltage 234 through a pair of resistors 235 and 236, the common juncture of which is connected to ground. Before the gas tube 23| is rendered conductive there is. normally provided an anode circuit which includes a relay 2 l0, break contacts 2| a current limiting, resistor 248, and a choke coil 300' which is connected to the positive side of a'source of voltage 234. The juncture between'the choke coil 300- and the current limiting resistor 248' is connected to a grounded filter capacitor 3M. The choke coil 300 and the capacitor 3M are provided to filter out the transient impulses occurring from repeated opening and closing of the contacts 2| I. The value of the resistance and impedance of the choke coil 300', the relay- 2I||l and the resistor 248 is such that-the: anode current is limited to a predetermined maximum amount, The juncture of the resistor 248 and the choke 300 is connected to a resistor'392 which, in turn, is connected to a resistor 333, one terminal of which is grounded. The juncture. oftheresistors 302' and 303.'i connected to the break'contact 2|'2 of the relay 2H); The break contact 2|2 is connected to a capacitor 394 which, in turn, is connected'to the negative terminal of the voltage source 234. The capacitor 304 is of such size as to provide sufii cient h-old'over characteristics to the voltages. developed across the. circuit comprising the resistors-235 and 236: inparallel with the capacitor 3G4, so that the control voltages supplied to the incoming and outgoing channel amplifiers hold over word and syllable pauses. The capacitor 334, by virtue of its connection to the juncture between. the resistors 332 and 303, is ini ially con nected to a bleeder circuit which charges the capacitor so that when it is connected by the make. contacts 2|3 to one terminal of the resister 236 there Wilt not be any anp eciabe cha e inthe voltages appearing across the resistors 235 and 23.6 which are abpled to the cha nel amplifiers. One terminal of the resistor 236 is connected throu h a resis or 231 to the grid coup ing resistor 223. of the output vacuum tube 22! of the outgoing channel amplifier. nal' of the resistor 231' is provided with a capacitor 238 conne ted to ground. One terminal of theres stor 235 is connected through a resistor 239 which is provided With a grounded capacitor Miland thiscircuit is connect d to the input resistor 258 of the i ut tube 256 of the incoming channel amplifier 286.

The reay 2 ll! is so arran ed that upon the con ductiv ty of the gas tube 23| the first operations that will occur w ll be the interruption of the break contacts 212' and the closing of the make contacts 2| 3; During the initial flow of current from the anode to the cathode the v ltage develooed' across the resistors 235 and 236 is suppl ed to the control circuits of the vacuum tubes 255 and 22f of' the incoming and outgoing channel amp ifiers, respectively, thereby to disable rapidly the incoming channel amplifier and to enable rapidly the outgoing channel amplifier. Subsequentto the initial application of the rapid enabling and disabling voltages the capacitor 334 is connected across the resistors'235 and 236 to prov de a certain amount of smoothin and filtering action which also serves to stabilize the control voltages so that they will be effective to hold over word andsyllable pauses. As the relay 21m continues to attract its armature, eventually the contacts 2| I will be interrupted. The relay 2|0, having interrupted the anode voltage, restores control of the tube 23| to the control electrode. If signal currents are still passing through the outgoing channel amplifier the tube 23! Will again become conductive, thereby a ain attracting thev armature of the relay am. The contacts 2H are so arrangedthat making and breaking of these contacts may occur as long as the speech continues' o be transmitted by the outgoing channel amplifier, make contacts 2 2 and 2| 3'. Therefore While signal' currents are being t ansmitted through the outgoing channel amplifier, the relay 210 is in-' termittently energized and deenergized so as to make and break the contacts 2 at a relatively rapid rate, thereby periodically restoring the control of the tube 23| to the control electrode. This operation continues until there is a cessation of the-voice or signal currents through the outgoing channel amplifier, whereupon the relay 2|ll is rel-easedso that the contacts 2 are made and the contacts 2|2 and 2|3 are restored to the positions shown in the figure. Therefore, in the arrangement-above described means are provided One termiwithout actuating the break or While a' single relay Zillhas responsive tothe conductivity of the cold cathode formthe operations of theanode voltage and 304 across the resistors 235 and 236, it of course will become apparent that these operations could be performed by several relays, or a relay and a vibrator.

The incoming channel amplifier 20B is provided with an enabling and disabling unit 26! which is similar in circuitarrangement and operation to the control unit 225 associated with the outgoing channel amplifier. nal energy appearing across the input resistor 258 of the incoming channel amplifier is obtained by a' circuit comprisin a coupling capacitor 262 and a resistor 263 which is provided with an adjustable contact connected to a suitable ampli- The amplifier 264 is connected to a transformer 255, the secondary winding of which is grounded and connected in a bleeder circuit including a pair of resistors 266 and 268 connected to the positive terminal of the source of voltage 269. The control electrode of a controlled cold cathode gas tube 261 is connected to the juncture between resistors 265 and 268. The anode circuit of the gas tube 261 includes a relay 280, break contacts ZBI of the relay 280, a current limiting resistor 216, and a choke coil 40!! which is connected to the positive terminal of the source'of voltage 269. A suitable grounded filter capacitor 40! is connected to the juncture of the resistor 216 and the choke coil 400. A bleeder circuit comprising resistors 402 and403 connected between ground and one terminal of the choke coil 40!] is arranged for charging a capacitor 484 through the break contacts 282 of the relay 280 to'a potential which approximates the average potential appearing across the resistors 210 and 2H. -One terminal of the resistor Z'IlLis connected through a resistor 289 to the grid coupling resistor 26!] of the output tube 251 of the incoming channel amplifier 206. The terminal of the resistor 289 which is connected to the resistor 260 is also connected to a capacitor 290 which is grounded so as to provide a small amount of filter action. When the charged capacitor 404 is connected by the make contacts of relay-280 to the juncture of the resistor 289, the resistor 21!) and the cathode of the gas tube 261, additional filtering action is obtained, One terminal of the resistor 2' is connected through a resistor 29! to the grid circuit input tube 201 of the out oing channel amplifier. A suitable capacitor 292 is connected between ground and one terminal of resistor 29I to provide a certain amount of filter action.

The control circuit or enabling and disabling unit ZEI' operates in response to signal currents appearing across the input of the incoming channel amplifier in a manner identical to the response of the other control unit 225 so as to enable its channel amplifier and to channel amplifier.

While two'different arrangementshave been shown for controlling the gain of a pair of chafinel amplifiers operating in different directions, it will be appreciated that in each instance the control circuit utilizes a cold cathode controlled gas'tube and that means are provided in the anode circuit thereof which are responsive to the A certain amount of the sigdisable the other 1'2 conductivity of the control tube to periodically render the tube non-conductive so as to periodically return the control of the tube to the control electrode. As long as signal currents continue to traverse the channel particular control tube, the control tube will again be rendered conductive. The anode current which flows produces control potentials across'a pairof resistors, and

a valuesufiicient to hold over the enabling and disabling action through the intervals during which the control tube is periodically rendered non-conductive.

While two embodiments been disclosed, it will be understood that various modifications may be made therein which are within the true spirit and scope of-the invention.

What is claimed is: V

1. The combination of and a control circuit therefor, said control circuit including a controlled gas tube having a'control'electrode, a source of anode potential con nected to said tube, means responsive to signal currents passing through said amplifier for impressing a control potential on trode thereby to render said means connected in the anode-cathode circuit of said tube and having a relatively short time constant for generating a voltage to be applied to said amplifier to control the gain thereof, de-

lay means, means operated by said tube for connecting to said anode-cathode circuit said delay means to delay the time constant of said voltage generating means; and means connected in said anode-cathode circuit so as to be responsive to the conductivity of said tube for periodically rendering said tube non-conductive.

2. The combination of a channel amplifier and a control circuit therefor, said control circuit including a controlled gas tube having a control electrode, a source of anode potential connected to said tube, means responsive to signal currents passing through said amplifier for impressing a control potential on said control electrode thereby to initiate. conductivity of said tube, means anode circuit of said tube and responsive to theconductivity thereof a for periodically rendering the said tube non-conductive.

3. The combination of a channel amplifier and a control circuit therefor, said control circuit including a controlled gas tube having a control electrode, a source of anode potential connected to said tube, means responsive to signal currents passing through said amplifier for causing said control electrode to initiate conductivity of said tube, means connected in the anode circuit of said tube including an interrupter responsive to the conductivity of said tube for periodically disconnecting said source of anode potential during the period of time the signal currents pass through said amplifier, and means responsive to the average anode current flow for applying abias to said amplifier to control the gain thereof.

4. The combination of a channel amplifier and amplifier associated with a:

the potential thus obtained is maintained by suitable capacitors at of the invention have a channel amplifierthe control elec-' tube conductive,

13; az-control: circuit thereto-r, said control: circuit in-- cluding accntrolled gas'tub-e, a source of potena tial for said tube, means responsive to=signa1 currents passing through said amplifier for causing said tube tobecome conductive, meanshaving a relatively short time constant associated with said tube for deriving control voltages therefrom to con-trol the gain of said amplifier, means i'n-l cludin a circuit interrupter connected in series Withsaid tube and said source, of potential, said interrupter being arranged in response to theconductivity of said tubetodisconnectsaid sourceof potential from said tube, and means to be connected subsequently by said circuitw inter-- rupter in circuit with said. means for deriving control voltages therefrom to increase the time constant thereof sufiiciently to supply to. said amplifier a relatively stable potential to hold over Word and syllable pauses in the signal currents passing through said amplifier,

5. The combination of a channel amplifier and a control circuit therefor, said control circuit including a controlled gas tube, a source of potential for said tube, means responsive to signal currents passing through said amplifier for causing said tube to become conductive, means including an interrupter connected between said source of potential and said tube for periodically disconnecting said source of potential from said tube in response to the conductivity" of said tube, means associated with said tube forderiving'control Voltages therefrom for controlling the gain of said amplifier, switching means controlled by said tube, and means arranged to be connected by said switching means in circuit with said means for derivin control voltages from said tube to increase the time constant thereof sulficiently to supply a control potential proportional to the average current transmitted by said gas tube. A

6. A transmission system comprising a pair of channels for transmitting signal currents in different directions, each channel being provided with a vacuum tube amplifier, a control circuit for each amplifier including a controlled gas tube arranged to be rendered conductive in response to signal currents passing through said amplifier, means associated with each gas tube for deriving control voltages therefrom, means for applying said control voltages to increase the gain of the associated amplifier and to decrease the gain of the other amplifier, means actuated by said gas tube for subsequently connecting to said means for deriving said control voltages means to increase the time constant thereof, and means responsive to the conductivity of said tube for periodically rendering said tube non-conductive,

7. A transmission system comprising a pair of channels for transmitting signal currents in different directions, each channel being provided With a vacuum tube amplifier, a control circuit for each amplifier including a controlled gas tube arranged to be rendered conductive in response to signal currents passing through its associated amplifier, relatively lightly filtered means having a relatively short time constant associated with each gas tube for deriving control voltages therefrom, means for applying said control voltages to increase rapidly the-gain of the associated amplifier and to decrease rapidly the gain of the other amplifier, means for supplying additional filter action, means actuated by said gas tube for subsequently connecting to said means for deriving control voltages said means for supplying additional filter action, and means responsive to 14 the conductivity ofrsaid' tube for periodically rendering said:tubenon-conductive.

8. A transmission system comprising azpairi of,"

channels for transmitting, signal currents in dif ierent. directions, each channel beingprovided with. a Vacuum tube amplifier, a control circuit, for: each amplifier including a controlled gas tube arranged to berendered conductive in response to; signal-currents passing through its. associated: amplifier, a source of potential for said tube, meansassociated with each gas tube for deriving cmtrol voltages therefrom, means for applying; said control voltages, to, increase the gain of. the: associated amplifier and to decrease the gain, of; the other amplifier, means connected in circuit with said gas tube potentialapplied thereto sufficiently to interrupt; the. conductivity of said tube, means for supplying; filter-action, and means operated by said tube for connecting, said filter means in circuit with said: means for derivin control voltages to increasethe time constant thereof sufficiently to,

hold, over the derived voltages for the time; of the periodic interruptions of the conductivity of said tube;

9, A transmission system comprising a pair; of channels for transmitting signal currents in dlf ferent directions, each channel being provided with a vacuum tube amplifier a control circuit for each amplifier including a controlled cold cathode. gas tube, means for rendering said gas tube conductive in response to signal currents passing throughits, associated amplifier, a source of potential for said gas tube, means having a relatively short time constant associated with each gas tube for deriving control voltages therefrom, means for applying said control voltages to increase rapidly the gain of the associated amplifier and to decrease rapidly the gain of the other amplifier, means for increasing the time constant of said means for deriving control voltages, means for connecting said last-mentioned means to said means for deriving control voltages in response to the conductivity of said tube, and means connected in circuit with said tube for producing in response to the conductivity of said tube a potential drop sufiicient to render said tube non-conductive.

10. A transmission system comprising a pair of channels for transmitting signal currents in different directions, each channel being provided with a vacuum tube amplifier, a control circuit for each amplifier including a controlled cold cathode gas tube arranged to be rendered conductive in response to signal currents passing through said amplifier, a source of anode potential for said gas tube, means connected in the anode circuit of said tube and responsive to the conductivity of said tube for producing a potential drop sufiicient to interrupt the conductivity of said tube, means having a relatively short time constant associated with each gas tube for deriving control voltages therefrom, means for applying said control voltages to increase rapidly the gain of the associated amplifier and to decrease rapidly the gain of the other amplifier, a relay connected in the anode circuit of said gas tube, and an energy storage device arranged to be connected by said relay in circuit with said means for deriving control voltages from said tube thereby to increase the time constant thereof sulficiently to hold over the average derived voltage for a time interval equal to or greater than the time of interruption of the conductivity of said tube.

for periodically reducing the cathode gas tube arranged to be rendered conduc tive in response to signal currents passing through said amplifier, a source of anode potential for said gas tube, means having a relatively short time constant associated with each gas tube for'deri ving control voltages therefrom, means for applying said control voltages to increase rapidly the gain of the associated amplifier and to decrease rapidly the gain of the other amplifier, a relay connected in the anode-circuit of said gas tubeand responsive to the conductivity of said tube for interrupting the conductivity thereof each time it is rendered conductive, an energy storage device arranged to be connected by said relay in circuit with said means for deriving control voltages from said tube thereby to increase the time constant thereof sufi'iciently to hold over the average derived voltage for a time inter; val equal to or greater than the time ofinterruption of the conductivity of said tube, and means for initially chargingsaid energy storage device to a potential substantially equal to the average control voltage derived by the means therefor. V

12. A transmission system comprising a pair of channels for transmitting signal currents in different directions, each channel being provided with a vacuum tube amplifier, a control circuit 16, for each amplifier including a controlled cold cathode gas tube arranged to be rendered conductive in response to signal currents passing through its associated amplifier, a source of potential for each tube, means having a relatively short time constant associated with each gasv tube for deriving control voltages therefrom, means for applying said control voltages to increase rapidly the gain of the associated amplifier and to decrease rapidly the gain of the ther amplifier, means including a circuit interrupter con nected in the circuit supplying potential to said tube for periodically disconnecting from said tube said source of potential, and means arranged to' be connected by said interrupter in circuit with said means for deriving control voltages from said tube to increase the time constant thereof suffi-. ciently to supply an average control voltage to said amplifiers as long as signal currents are passing through the channel associated 'With said control tube. I

ROSWELL H. HERRICK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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