US2371291A - Transmission system - Google Patents

Description

March 13, 1945. R. H. HERRICK TRANSMISSION SYSTEM Original Filed Aug. 14, 1959 2 Sheets-Sheet 1 mmi .NBQ Qkmni 3 5 m+ m9 mw qz fimi g IN VENTOR. I B Roswell H Herrick March 13, 1945. R. H. HERRICK 2,371,291

' TRANSMISSION SYSTEM Original Filed Aug. 14, 1939 2 Sheets-Sheet 2 mow New mmw 39mm mmwtgdm QWN I T 3 1I\\!VEN'TOR. BY Roswell H Herrick v I fllfoknegs.

- 2,282,405, granted May 12, 1942.

. In the usual arrangement-of this Patented Mar. 13, 1945 i Roswell H. Herrick, Oak Park, Ill., assignor to Automatic Electric Laboratories, Inc., a corporation oi Delaware Original application An 1941, Serial No. 404,136

Claims. ('Cl.179-170) The present invention relates generally to improvements in signal current transmission systems of the type in which signal Controlled switching circuits are provided for partially or completely blocking, under certain conditions, certain of the .signal current channels included therein and, in telephone substation circuits having incorporated therein coupled signal current channels for the transmission of'incomin'g and outgoing signal currents. This application is a division of copending'application Serial No. 290,090, filed August 14, 1939, Roswell H. Herrick, now Patent No.

In the usual telephone substation circuit a hybrid system or antiside tone impedance networkis provided for preventing signal currents developed during operation of the transmitter from being transmitted to the receiver and for similarly preventing signal currents incoming over the line extending to the substation from being transmitted to the transmitter for reproduction. In this type of arrangement one of the factors which determines the efliciency of the sidetone suppression is the impedance of the talking circuit established by way of two connected subscribers lines. This impedance is not the same for any two different established connections, and, accordingly, each substation is usually balanced to provide maximum side tone suppression efiiciency for average line conditions. In installations wherein amplification of incoming, and outgoing signal currents is required, as, for example, in executive loud speaking sets, the conditions of unbalance introduced in the substation circuit by the impedances of the lines over which an established connection extends may become intolerable. This is particularly true in substation installations provided in an exchange area where the subscriberslines are of widely difierent lengths. In order completely to obviate or to singing which may result due to unbalance of the substation circuit occasioned by unfavorable line conditions, signal controlled switching means may be provided in the substation circuit for selectively blocking the channels when not in use.

character the signal channels are completely blocked when not in use. More particularly, during intervals when the transmitting means of the substation is being used to transmit outgoing signal currents, the incoming signal current channel is rendered completely inactive. Conversely, during those minimize the more particularly, to improvements the hybrid system of gust 14, 1939, Serial No. Divided and this application July 26,

substation, the transmitting or outgoing signal .current channel is rendered completely inactive.

An arrangement of this character, while satisfactory in operation, prevents any interruption of a speech train being transmitted in one direction in response to signal currents transmitted in the opposite direction. In other words, conversation break-ins are positively prevented.

It is an object of the present invention toprovlde improved telephone substation apparatus of the character described wherein the channels are preferablyonly partiall 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 be exceedingly fast in operation so that speech clipping is minimized.

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 gaseous discharge tubes of the three-electrode type, whereby the circuit is rendered exceedingly fast in operation to eiiect channel blocking and unblocking operations without any substantial speech clipping.

It is still another object of the invention to provide an improved and exceedingly simple arrangement for deionizing the gaseous discharge unblocking the associated channel.

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, to-- gether with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with-the accompanying drawings in which Figure 1 illustrates a substation therein certain of the features of the invention as briefly outlined above; 'Fig. 1a illustrates a slightly different arrangement of certain of the circuit elements shown in Fig. 1; and Fig. 2 lllus-. trates a modified form of the-substation circuit shown in Fig. 1.

Referring now more particularly to Fig. 1 of the periods when signal currents are incoming to the drawings, the substation circuit there illustrated circuit having incorporated potentials are supplied that extending to the screen channel I has included therein -which includes two stages to the input distant end thereof will terminate in a line circuit the character of which is determined by the character or the exchange. Briefly described, the circuit comprises a transmitter or microphone IM and receiving means in the form of a loud speaker I02 which are adapted respectively to be coupled to an antiside tone impedance network including a hybrid coil I00 and'a balancing circuit I04 by means of signal transmission channels I05 and I00. The outgoing signal current channel I05 comprises a vacuum tube amplifier I00 including three stages of amplification respectively including the amplifier tubes I09, H0 and II I, the first of which is adapted to receive signal currents fromthe microphone IM and the last or which is arranged to deliver its output to the hybrid system I00. More particularly, the microphone IOI is coupled to the input electrodes of the first amplifier tube I00 through a coupling network which includes a condenser H2 and an adjustable voltage dividing or signal level control resistor I I0. The output electrodes of the tube I09 are coupled to the input electrodes of the succeed ing amplifier tube I I0 through a resistance capacitance coupling network which includes a pair of resistors II4 and H5 and a coupling condenser IIO. Similarly the output electrodes of the second amplifier tube IIO are coupled to the input electrodes of the final amplifier tube I II through a resistance capacitance coupling network which includes a resistor II1, a. voltage dividing or signal level adjusting resistor H0 and a coupling condenser I I9. In order to bias the control electrode of the tube I09 to the proper negative potential with respect to the associated cathode, there is provided in the cathode leg of the output circuit of this tube a pair of series connected cathode biasing resistors shunted by a signal current by-pass condenser I22. The control grid 01 the tube H0 is similarly biased to the 'proper negative potential with respect to its associated cathode biasing circuit comprising a resistor I20 shunted by a signal current by-pass condenser I24. A similar cathode biasing network comprising a resistor I25 shunted by a by-pass condenser I20 is included in the cathode leg of the output circuit 01 the final amplifier tube III. Anode to the anodes of the tubes I09, H0 and III from a source of anode current, not shown, but having its positive terminal connected to the terminal marked +B, over circuits which respectively include the resistors H4, H1 and certain of the windings of the hybrid coil I00. Screen potentials are impressed upon the screen electrodes of the indicated tubes from the anode current source over paths respectively including filter networks, that leading to the screen electrode of the tube I09 including a resistor I21 and a signalcurrent by-pass condenser I20 and electrode including a resistor I29 and a by-pass condenser I00.

The incoming signal current transmission an amplifier I05 or amplification 'respectively comprising the amplifier tubes I00 and I01. More specifically, the incoming signal current terminals of the hybrid coil I00 are coupled electrodes of the first amplifier tube I00 through a coupling network which includes a transformer I00 and an'adiustable voltage divid- I20 and I2I which are cathode by means of a ing or signal level control resistor I09. The output electrodes of this tube are coupled to the input electrodes of the second amplifier tube I01 through a resistance capacitance'coupling network which comprises a resistor I40, 9. coupling condenser I4I and an adjustable voltage dividing or signal level control resistor I40. The output electrodes of the second amplifier tube I01 are, in turn, coupled to the voice or signal current coil of the loud speaker I02 through a coupling transformer I44 and a T connected signal current level control resistance network comprising a pair of series resistors I45 and I45 and a shunt resistor I40. In order to bias the control electrode of the tube I01 to the proper negative potential with respect to the associated cathode, there is provided in the cathode leg of the output circuit'of this tube a biasing network which includes a resistor I41 shunted by a signal current by-pass condenser I40. Control of the bias impressed upon the control electrode of the first amplifier tube I00 is effected through the provision of a cathode biasing network which includes a pair of resistors I49 and I50 which'are shunted by a signal current by-pass condenser I:5I. ,Anode potentials are impressed upon the anode electrodes of the two tubes I00 and I01 from the source or anode current overcircuits respectively-including a resistor I40 and the primary winding of the coupling transformer I44. Screen potential is impressed upon the screen electrode of the tube I00 from the source of anode current over a path which includes a. filter network comprising a resistor I52 and a by-pass condenser I50. Screen potential for the screen anode of the tube I01 is derived in a manner more fully explained subsequently.

In order to control the gains of the amplifiers I00 and I05, thereby to vary the signal current transmission efliciencies of the channels I05 and I00 in accordance with signal current flow therethrough there is provided a control circuit I55 which includes two control units, the first of which operates in response to signal currents traversing the channel crates in response to signal currents traversing the channel I00. More particularly, the first unit of the control circuit comprises a gaseous discharge-tube I50 of the three-electrode type having an anode I51 and a cathode 450 separated to define a space current path and a control or start electrode I50 interposed therebetween to control the fiow of current over the space current path. The start electrode I50 is normally biased negatively with respect to the cathode I50 over a path which serially includes a resistor I00, a source of bias voltage IOI, a resistor I02 and a resistor I00.

The space current path of the discharge tube I50 is included in the output circuit of this tube in ch nnel I05-and the series with the source of anode potential mentioned above and the resistors I50 and I00, whereby control voltages are developed across these resistors when a discharge through the tube is established, which voltages are utilized to control the final amplified tube III included in the first amplifier tube i00 in cluded in the channel I00. More articularly, the voltage developed across the two series connected resistors I50 and I00 in response to space current traversing the tube I50 is utilized as the screen potential for the amplifier tube I I I, whereby this amplifier tube is only rendered operative to transmit signal currents therethrough in response to a fiow of current over the space current path 01 the tube I00. Since theresistor I00 is also in- I05 and the second of which opthereover. this tube are included in an output circuit whichalso includes the resistor I2-I, a second resistor I14 and the source of anode current connected to the at a ,point following the amplifier tube H and over a circuit which includes a coupling condenser I66. The tube I65 is normally biased to operate as a half wave rect' er by means of a bias battery I61 connected between the input electrodes thereof. The output electrodes of the rectifier tube I65 are included in prises the two resistors I50 and I62 and the source of anode current connected to the terminal marked +B. In order momentarily to interrupt the output circuit of the gaseous discharge tube I56 when signal current flow over the channel I05 is arrested, there is provided a slow-to-release relay I68 having its -winding included in the output circuit of the tube I65 to be energized by space current traversing this tube. This relay is provided with an armature I69 which is arranged to interrupt the output circuit of the discharge tube I56.

The second control unit embodied in the control circuit I55,'namely, that associated with and responsive to signal currents traversing the channel I06 is essentially identical to that just described and provided in association with the channel I05. More particularly, the second control unit comprises a three-electrode gaseous discharge tube I10 having an anode HI and a cathode l12,separated to define a space current path and a start electrode I13 interposed in this path for the purpose of controlling the initial ,fiow of 1 current terminal marked +B. The voltage developed across these two series connected resistors when an outputcircuit which com-- The output'electrodes. Ill and I12 of of a bias battery I82 connected between'the input electrodes thereof. The output electrodes of the tube I80 are included in an output circuit which speaks into the plified by the tube IIO the signal space current flows through the tube I10 is. uti'---- lized as the operating potential for the screen electrode of the amplifier tube I31 included in the channel I06, these. two resistors passed for signal currents by condenser I15 connected in shunt therewith.

- Since the resistor I2I forms a part ofthe cathode biasing circuit for the first amplifier tube I09 in cluded in the outgoing signal current channel I05, the voltage across this resistor serves to determine the gain of the indicated amplifier tube and thus the signal current transmission em ciency of the channel I 05. The start electrode I13 of the discharge tube I10 is normally biased negatively with respect to, its associated cathode over a path which includes a resistor I16, a source of bias voltage 111, a'second resistor I18 shunted by'a signal current by-passcondenser I19 and the resistor I14; For the purpose of impressing a start potential upon-the start electrode I13 when signal currentsltraverse the channel I06, a rectifier is provided which comprises a three-electrode vacuum tube I80 having its input electrodes coupled to the channel I06 at a point followin the amplifier tube I36 over a circuit including a coupling condenser I8I. This tube is normally biased to operate as a half wave rectifier by means being heavily bya low impedance lower maintained at a negative bias voltage afforded by the an armature I84 which is operative to make and break the output circuit of the discharge tube I10. Referring now more particularly to the operation 01- the system illustrated in at normal gain. Since, however, the two dischargetubes I56 and I10 are at this time in a deionized condition, no screen potentials are available for the screen electrodes of the final amplifier tubes III and I31 respectively included in the amplifiers I08 and I35. Thus, blocked so that no circulation of energy around the system can occur. If, with the apparatus in this condition, a user of the substation circuit microphone IOI, the resulting signal voltage is impressedupon the input electrodes of the amplifier tube I09 and the amplified output voltage is, in turn, impressed upon the input electrodes of the tube IIO. As further amvoltage is impressed across the input electrodes fier stage III and also through the across the input electrodes of the rectifier tube I65. During alternate half cycles of this voltage the bias potential impressed upon the control electrode of the tube I65 by the battery I61 is overcome so that space current tube over a circuit which includes the winding of the relay I 68, the resistor I62 and the resistor I50. As indicated above, during signal current transmission through the channel I05, due to the magnitude of the bias impressed across the control electrodes thereof by the battery I61, the vacuum tube I65 functions as a, half wave rectifier-.- Accordingly, traverses the space current path of this tube and the resistors I62 and I50. Due to the smoothing action of the condenser I62? shunting the resistor I62, however, the, voltage across this resistordoes not fall to. a zero value during alternate half cycles" of signal current but is value sufficient to permit ionization of the discharge tube I56. The resulting voltage developed across the resistor I62 is impressed across the input electrodes I58 and I59 of the discharge tube I56 in opposition to the C battery This voltage is of suflicient magnitude to the potential of the start grid I59 to a point where a discharge through this tube occurs. When the tube I56 is ionized in this manner, current is caused to traverse the space current path thereof; this current also flowing through the series connected resistors I50 and I63. The last-mentioned resistor has a resistance value substantially greater than that of the resistor I 50 and also substantially greater than the resistance of the space current path through the tube Accordingly, a substantial portion of the available voltage of the anode .current supply source appears as a voltage drop across the recondenser I66 IGI.

Fig. 1, it is p0inted out that in order to condition the signal curboth channels are effectively -of the ampliflows through this.

a pulsating direct current the succeeding amplifier tube.

sister I63. This voltage the voltage drop across the resistor I60, is impressed as an operating potential upon the screen electrode of the final amplifier tube III included in the channel I05, thereby to render the tube III operative to transmit the generated signal currents therethrough and through the hybrid coil I03 to the line terminating at 'the terminals I00. As indicated above, the operations just described all occur during the first cycle of the signal current developed in response to the sound waves impinging upon the .diaphragm of the microphone IOI. Accordingly, no substantial clipping of the first syllable of the speech represented by the sound waves occurs. Simultaneously with the activation of the amplifier tube III, the first amplifier tube [36 of the channel I06 is either biased to have a very low gain therethrough or beyond cut-oil, depending upon the constants of the control circuit. Thus, the current traversing the resistor I50 and flowing through the space current paths of the tubes voltage to be developed across this resistor which is impressed through the resistor I49 upon the cathode of the tube I36 as'a positive bias. This bias voltage has the of the tube I36 and if of sufiiciently large magnitude may serve to bias this tube to a point beyond anode current cut-ofi such that the channel I06 is completely blocked in the first amplifier stage thereof. ance value of the resistor I50 is chosen so that the voltage developed thereaci'oss by the space' currents flowing therethrough does not exceed the anode current cut-off value of the tube I36,

signal currents to I31 at a substan tially reduced gain. By virtue of the arrangement just described it the antiside tone network comprising the hybrid coil I03 and the balancing circuit I04 are greatly unbalanced due to the condition of the lines connected tothe terminals I00, such that signal currents are transmitted from the channel I05 through this network to the input side of the channel I06, the partial or complete blocking of the latter channel in the first amplifier stage comprising the tube I36 prevents the signal through the channel to initiate operation of the second control unit comprising the rectifier tube I80 and the discharge tube I10.

From the foregoing explanation itwill be apthis tube will pass incoming I56 and I65 causes a eiiect of lowering the gain Alternatively, if the resistcurrents from being fed drop, as augmented by -mal value. During .value permitting throughthe tube I56 tion of the rectifier nel- I06 momentarily to be returned to the northis operation the potential of the start electrode I59 is maintained at a the flow of space current due to the continued operatube I65. Accordingly, imimmediately the output circuit of the discharge tube-I66" is recompleted at the armature I69 and its associated front contact, space current again fiows through the tube I56, thereby to render the II I operative and partially or completely to block the signal transmission channel I06. During the interval when the armature I69 is operating between its two positions no signal current transmission through the channel I05 can occur. Accordingly, a very slight clipping of the intermediate syllables o! the speech train may occur during this switching interval. Such clipping is of no consequence, however, since the listener, upon hearing the first syllables of the speech train and the syllables following the interval during which clipping may occur, can readily ascertain, by interpolation,

\ the syllable or partial syllable which is cut off ing the interval when the armature parent that the current traversing the space current path of the required for operation of the armature I68 fromits retracted position to its attracted positionthe output circuit of the tube I56 is held open. Accordingly; the now of space current through this tube and through the resistors I and I6! is momentarily interrupted, causing the final amplifier 'tube III of the channel I05 momentarily to be rendered inoperative and the bias voltage impressed across the input electrodes of the first amplifier tube I36 included in the chanthe relay 7 during the switching operation. V

When the flow of signal currents through the channel I05 is arrested, the flow of current through the space current path of the rectifier, tube I65 'is reduced to zero or to a negligible value. Accordingly, the potential impressed upon the start electrode I59 of the discharge tube I56 is increased in a negative sense to a value which will prevent further current flow through this tube after the tube is deionized. As explained above, the character of the discharge tube I56 is such that when ionization therein occurs the start electrode I59 has no further control over the current fiow therethrough. Since, however, the operating winding of the relay I68 is 'deenergized when the fiow of signal currents through the channel I05 is arrested, this relay is caused to restore after an interval. Dur- I69 of the relay I68 is movingbetween its attracted and retracted positions the output circuit of the discharge "tube I56 is interrupted in an obvious manner so that no voltage is available for sustaining the discharge through the tube. Accordingly, the tube I56 is deionized, at which time the control of the space current path in the tube is restored to the negatively biased start electrode I59. Due to the negative potential impressed upon this electrode by the battery I6 I ionization of the tube is prevented when the armature I6! is moved into engagement with its associated back contact to again impress the voltage of the .anode current source across the anode I51 and the cathode I58. When the space current now through the two tubes I65 and I56 is interrupted in the manner just explained, the voltage drops across the resistors I50 and I63 are reduced substantially to zero. Accordingly, the operating potential is removed from the screen electrode of the final amplifier tube III and this tube is rendered inoperative to transmit signal currents to the hybrid coil I03. Also, the value oi! the negative bias impressed across the input electrodes of the amplifier tube I36 is decreased to normal so that this amplifier *stage is conditioned to operate with normal gain.

I The manner in which the second control unit comprising the rectifier tube I and the dischargetube I10 functions partially or completely to block the transmission channel I05 and tonetivate the final amplifier stage of the channel I06 terminating at the terminals I through the of the loud speaker I02 the anode I'II.

two resistors I'ZI of the channel 105 when signal currents are transmitted from the line brid coil I03 to the input side of the channel I06,

is substantially identical to the mode of operation of thefirst control unit as described above. .More particularly, signal currents transmitted to the input electrode of the tube I36 through the coupling transformer I38 and the voltage divider I39 are amplified by signal voltage is impressed across the input electrodes of the two tubes I31 and 30., The resuiting space current flow through the rectifier tube I80 causes the ionization of the discharge tube-I10, whereby a relatively heavy current is caused to traverse the control resistors I2I and'I 14.

this tube and the amplified The voltage drop across these two serially connected resistors is utilized as an operating voltagefor the screen electrode of the tube I 31, where by this tube is rendered operative to amplify the signal currents and transmit the same through the transformer I and the T connected volume control resistor network to the signal current coil for reproduction. The voltage developed across the resistor I2I is impressed as a negative bias between the'input electrodes of the first amplifier tube I08 included in the channel I05, whereby this tube is either rendered completely inoperative to transmit signal currents therethrough or the gain of the tube is reduced to a very low value. Shortly following the initial flow of signal currents through the channel I06, the slow-to-release relay I83, which is energized bythe space current of the tube I80, operates momentarily to extinguish the flow of space current throughthe discharge tube I10 with the result that the channel I06 is momentarily blocked and the amplifier I09 is momentarily reconditioned to operate with normal gain in the manner described above with reference to signal transmission through the channel I05.- During the transmission of signal currents through the channel I06 to the loud speaker I02, the 1' connected resistance network included in this channel between the coupling transfermer I and the signal current coil of the loud speaker operates in a well-known manner to maintain the sound output level of the loudspeaker substantially constant regardless of changes in the level of the signal currents transmittedto the input side of .Q

the channel. When signal current transmission through thechannel I06 is arrested the, flow 01' space current through the rectifier tube I80 is arrested. Thus, the operating winding of the relay I 83 is deenergized and the potential impressed upon the start electrode I73 of the discharge tube period of the-relay I 83, the armature I84 tracted position. During such movement. the output circuit of' the tube I10 is interrupted, thereby to disconnect the source of anode voltage from As aresult, the tube is deionized and control of the space current path thereof is restored to the negatively biased start electrode I13. When the flow of space current through the and I'll is arrested the potential impressed upon the screen electrode of the amplifier tube I31 is reduced substantially to zero, whereby this tube is rendered inoperative. Also, the voltage drop across the resistor. IN is suificiently reduced to permit the amplifier tube I09 to operate with substantially normal'gain.

v 5 The purpose of providing the relays I68 and I 83, having slow-to-release characteristics, for controlling the output circuits of the tubes I56 and I10 respectively is to prevent clipping. More particularly, the period or interval required for the release of either of these two relays prevents the pauses may occur.

A modified arrangement for obtaining the delay intervals just described is illustrated in Fig. 1a. The relay arrangement illustrated in this figure may brewed in lieu of the relays I68 and I 83 as shown in Fig. 1 without changing the armature and contact spring assemblies. In the Fig. 1a arrangement the relay coil oomprisesa pair of differentially related windings, the lower winding. of which is connected in series with acondenser I and a resistor II across theupper winding. IAISO, the lower winding is constructed to provide a substantially greater number of ampere turns than the upper winding. With this arrangement included in the circuit of Fig. 1- the initial rush of current through the upper winding occasioned by a flow of space current through the tube I65, for example, causes the relay armature rapidly to be attracted. During such energization, the condenser I90 charges relatively slowly so that the flow ci posing action occurs. When the flow of current through the upper winding is decreased or completely arrested, the condenser I90 discharges through the two windings of the relay in series so that the ampere turns represented by the two windings are in aiding phase relation and the armature of the relay is maintained in its attracted position for a short time interval after the current flow through the path ceases.

If the circuit constants of the circuit illustrated in Fig. l are so proportioned that each of the first amplifier stages respectively comprising the tubes I09 and I36 is not completely blocked when associated space current .the complementary transmission channel isactive, the arrangement permits what is known as conversation break-ins to occur. In this regard it will be apparent that signal currents trans-' mitted to the substation circuit over the line connected to the terminals I00 the hybrid coil I 03 to the input side ofthe channel I 06 at a reasonably high intensity as compared with the currents transmitted from the,

channel I05 to the channel I06 through the hybrid coil during operation of the microphone IOI. With the circuit so arranged that the gain of i the amplifier stage comprising the tube I36 is only reduced to a low value during signal transmission through the channel I05, the incoming signal currents are transmitted at reduced gain through this first amplifier stage of the channel I06 and are impressed across the input electrodes of the rectifier tube' I80, whereby the discharge tube I10 is ionized in the manner previously explained; In response to the flow of space current through the discharge tube I10 the required operating voltage is impressed upon the screen current through the lower winding is relatively low and no substantial opare passed through user of electrode of the amplifier tube I31, thereby to render this tube operative to transmit the incoming signal currents to the loud speaker I02 for reproduction. The activation of the amplifier stage comprising the tube III is accompanied by a decrease in the gain of the amplifier stage comprising the tube I36 for reasons which will be ap parent irpm the preceding explanation. After the initial conversation break-in, the user of the illustrated substation circuit is expected to cease speaking, whereby the control unit comprising the rectifier tube I65 and the discharge tube I56 operates to block the channel I05 and to increase the gain through the amplifier stage comprising the tube I36, in the manner pointed out above. In a manner similar to that just described, the the substation circuit illustrated may break in upon a speech train being transmitted to the circuit by speaking into the microphone IM to cause the activation of the channel I05 and a decrease in the gain of the amplifier tube I56, providing the circuit constants of the control circuit I55 are chosen so that the first amplifier stage of the amplifier I08 is not completely blocked when signal currents are being transmitted through the channel I06.

While it will be understood that tubes of any desired type having the proper operating characteristics may be used in the circuit illustrated in Fig. 1, preferably, the amplifier tubes I09, I I0 and 136 are of the type commercially known as the GJ'IG tube; the amplifier tubes III and I31 are of the type known as the A6G tube; the rectifier tubes I65 and I80 are of the type known as the 6C5G tube; and the gaseous discharge tubes I56 and III] are of the type commercially known as the 884 tube.

The arrangement illustrated in Fig. 2 of the drawings is substantially similar to'that shown in Fig. 1, but differs therefrom in that a different method is employed for controlling the gainiof the amplifier stages included in the resgactive channels and a difierent'type of relay arrangement is utilized for controlling the deionlzation of the two gaseous discharge tubes embodied therein. Due to the similarity between the two circuits, corresponding element thereof have been identified by reference characters having the same tens and units digits but different hundreds digits respectively corresponding to the two figures. In the arrangement oi. Fig. 2. the first stage'of amplification 209, included in the channel 205 is diagrammatically illustrated and the output electrodes of the last amplifier tube included in this channel are coupled to the outgoing signal current terminals of the hybrid coil 203 through a coupling transformer 285. The control circuit 255 illustrated in Fig. 2 differs fromthe corresponding circuit I55 illustrated in Fig. l in that each of the two relays 268 and 283 provided for controlling the deionization of the two discharge tubes 256 and 210, respectively, are double wound relays, the extra winding being arranged to be short-circuited in response to energization of the relay, whereby an additional slowto-release period is imparted thereto. The two control sections of the circuit are each arranged to control the gain of one amplifier stage in each of the two signal transmission channels. Thus, the control section comprising the rectifier tube 265 and the discharge tube 256, which is coupled to respond to signal currents traversing the channel 205, is arranged to control the bias voltage impressed between the input electrodes of the first amplifier tube 236 included in the channel 206 and the bias voltage impressed between the input electrodes of the amplifier tube 2 I I included in the channel 205, More particularly, signal voltages developed across the channel 205 at a point following the amplifier tube 2I0 are impressed upon the input electrodes of the rectifier 265 through a coupling condenser 266. This tube is normally biased to operate as a half wave rectifier by the voltage of the C battery 261 which is connected between the input electrodes of the tube 265 in series with a resistor 286. In order to overcome the bias normally impressed upon the start electrode 259 of the discharge tube 256 by the bias battery 26I when signal currents traverse the channel 205, there is provided in the cathode leg of the output circuit of the rectifier 265 a biasing network which comprises a resistor 261 shunted by a signal current by-pass condenser 288. The load resistors 263 and 250 included in the output circuit of the discharge tube 256 are arranged respectively to control the gains through the amplifier tubes 2 and 236. More particularly, the junction point between these two resistors is grounded and the negative terminal of the anode current source for the tube 256 is connected to the right terminal of the resistor 250.

. the amplifier tube fier stage 209 and the 22I being impressed During space current flow through the discharge tube 256 the voltage drop developed across the resistor 263 is impressed through a resistor 289 between the input electrodes of the tube 2 in a direction to overcome the normal negative bias afforded by the C battery 290, thereby to increase the gain of the tube 2I I. The voltage developed across the resistor 250, on the other hand, is impressed through a filter network which comprises a resistor 29I and a condenser 292, and the voltage divider 239 between the input electrodes of 236 in a direction to decrease the gain of the last-mentioned tube. In a similar manner, the control voltages developed'across the resistors 22I and 214 when space current traverses the discharge tube 210 are utilized reversely to vary the gains through the amplifier tubes 2H] and 231, the voltage developed across'the resistor in a negative sense between the input electrodes of the tube 2I0 through a filter network which comprises a resistor 293 and a condenser 284, and the voltage developed across the resistor 214 being impressed in a positive sense between the input electrodes of the tube 231 through a resistor 295. As illustrated in the drawings, the rectifier tube 280 is coupled through a condenser'28l to be controlled by signal currents traversing the channel 206, the input electrodes of this tube being biased substantially to the anode current cut-off point through the provision of a source of bias voltage 282 which is connected across the input electrodes of the tube 280 in series with a resistor 296. In this unit the control voltage for overcoming the bias voltage of the battery 211, thereby to condition'the discharge tube 210 for ionization, is developed across a resistor 291'which is included in the cathode leg of the output circuit of the rectifier tube 260 and is shunted by a smoothing condenser 298.

The mode of operation of the arrangement illustrated in Fig. 2 is, in general, substantially similar to that described above with reference to the arrangement shown in Fig.1. More particularly, with the cathode heater circuits of the various tubes illustrated in Fig. 2 energized the ampliamplifier stages respectively comprising the tubes 2I0 and 236 are conditioned to operate with normal gain. The two amplifier tubes 2H and 231, on the other hand.

, circuiting its upper completely blocked so that the system is positively prevented from oscillating. With the apparatus in this condition, it. sound waves impinge upon the diaphragm of the microphone l, the resulting signal currents are amplified in the amplifier stage 209 and are delivered to the input circuit of the second amplifier tube H0, The amplified signal voltages appearing across the output circuit of this second tube are impressed across the input electrodes of the third amplifier tube 2 and also across the input electrodes of the rectifier tube 265. The resulting space current flow through the tube 265 causes a voltage to be developed across the resistor 28'! which reduces the negative potential upon the start electrode 259 of the discharge tube 256 to a point where this tube becomes ionized and current flows through the two load resistors 250 and 263. The voltage developed across the resistor 263 opposes the voltage of the C battery 290and is of such magnitude that the bias between the input electrodes of the tube 2| I is to operate with normal gain. amplified signal currents delivered to the input electrodes of the amplifier tube 2 are amplified sufficiently reduced to permit this tube;

Accordingly, the

' which will prevent space channel 205 is arrested, through the rectifier tube 265 ceases. As a result, the rela 268.is deenergized and the potential upon the start electrode 259 is restored to a .value current now through the tube 256 after this tube is deionized. After an interval determined by the release period of the relay 268, this relay restores to interrupt, at its armature .269, the path short-circuiting its upper winding, thereby to render itself reasontherein and transmitted through the coupling transformer 285 and the hybrid coil 203 to the line extending to the terminals developed across the resistor 250, on the other hand, is impressed as a negative bias voltage between the input electrodes of the amplifier tube 236, thus reducing the gain of this tube. By suitably proportioning the resistance value of the resistor 250, the voltage developed thereacross when space current traverses the discharge tube 256 may be proportioned to bias the tube 236 be-v yond cut-oif such'that the channel 206 is completely' blocked, or to bias this tube so that it opcrates with only a small gain, whereby only signal currents of substantial magnitudes may be transmitted therethrough. By thus reducing the gain of the amplifier tube 236, signal currents which may be transmitted through the hybrid system 263 to the input side of the channel 206, due to an unbalanced condition of the hybrid system, are prevented from initiating the operation of the control unit comprising the rectifier tube 280 and the discharge tube 210. The relay 266 included in the output circuit of the rectifier 265 functions, in the exact manner previously explained with reference to of Fig. 1, momentarily to break the space current path of the discharge tube 256 shortly following the initial signal current flow over the channel 205, During its operating period, therefore, the relay 268 momentarily blocks the transmission of signal currents through the channel 205 and increases the gain of the amplifier 236 to its normal value. When, however, the relay 268 is fully opcrated, the ionized condition of the tube 256 is reestablished, whereby the channel 205 is again rendered operative and the gain of the amplifier tube 236 is again reduced to an exceedingly low value. Up'on operating, the relay 266 completes, at its armature 269', an obvious path for shortwinding, thereby to render itself exceedingly slow to release in a manner well ment, delay or hang-over periods are provided to prevent the blocking of the channel 205 and the unblocking'of the channel 206 during the short intervals which occur between speech syllables.

200. The voltage the corresponding relay understood in the art. By virtue of this arrangeis deionized and control of ably fast to operate. At its armature 269, the relay 266 momentarily interrupts the output circuit of the discharge'tube 256, whereby this tube the space current flow .therethrough is restored When the flow of space current through the tube 256 is arrested, the voltage drops across the resistors 263 and 250 are reduced to values such that the amplifier tube 2 I I is biased beyond anode current cut-off and the amplifier tube 236 is biased to operate with normal gain.

The mode of operation of the control unit comprising. the rectifier tube 280 and the discharge tube 210 and associated with the channel 206 is identical with that just described with reference to the unit associated with the channel 205. From the foregoing explanation it will be apparcut that if the resistors 250 and 2H are properly that in the usual installation complete blocking of each channel when the other channel is in use is not required in order to prevent singing due to thesubstantial side tone suppression ach eved through operation of the hybrid system 203.

While there has been described what is at present considered to be the preferred embodiment of 6176 invention, it will be understood that various modifications ma be made therein, and it is contemplated to cover in the appended claims all such modifications as fall within. the true spirit and scope of the invention.

. I claim:

1.. In a transmission system, a signal current transmission channel including means, said ampl fying means including a stage comprising an electron discharge tube having a screen electrode, a control circuit having an input circuit coupled to said channel at a point preceding said amplifierstage and comprising a gaseous discharge tubeincluding an anode and a cathode separated to define a space current path and a start electrode for controlling the flow of current over said path, means included in said input circuit forimpressing a unidirectional start potential upon said start electrode when signal currents traverse said channel,

means controlled by the resulting flow of' space current over said path for impressing an operating potential upon said screen electrode, thereby to render said amplifier stage operative, and relay means controlled by said input circuit operative to lower momentarily the potential difierence between said cathode and anode of said gaseous discharge tube when the flow of signal currents through said channel is arrested, thereby to arrest the flow of current over said path and When the flow of signal currents through the space current flow to' the start grid 259.

amplifying cuit serially to restore the control of said space current path to said start electrode.

2. In a transmission system,a pair of channels [or transmitting signal currents in different directions, an amplifier included in one of said channels and including one amplifying stage comprising an electron discharge tube having a grid electrode, a second amplifier included in the other of said channels, a control circuit coupled to said one channel at a point preceding said one stage and comprising a gaseous discharge tube includingan anode and a cathode separated to define a space current path and a start electrode for controlling the flow of current over said path, means including a rectifier for impressing a start potential upon said start electrode when signal currents traverse said one channel, means controlled by the resulting flow of space current over said path for decreasing the gain of said second amplifier and for impressing an operating potential upon said grid electrode, thereby to render said one stage operative, and means including a relay serially connected in the rectifier output circuit and operative to lower the potential difierence between said anode and cathode of said gaseous discharge tube when the fiow of signal currents through said one channel is arrested, thereby to arrest the flow of current over said path and to restore the control of said space current path to said start electrode.

3. In a transmission system, a pair of channels for transmitting signal currents in different directions, an amplifier included in one of said channels and including one amplifying stage comprising an electron discharge tube having a screen electrode, a second amplifier included in the other of said channels, said second amplifier comprising one stage including an electron discharge tube having a pair of input electrodes, an input circuit coupled to said input electrodes and including an impedance element for impressing a biasing potential therebetween, thereby to control the gain of said last-mentioned stage, a control circuit coupled to said one channel at a point preceding said one stage and comprising a gaseous discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the fiow of current over said path, means for impressing a start potential upon said start electrode when signal currents traverse said one channel, thereby to initiate current fiow over said path, a cirincluding said impedance element, an additional impedance element and said space current path,-said last named circuit being so connected and arranged that'the gain of said last-named amplifier stage is substantially decreased when current traverses said path, means for impressing the voltage developed across, said additional impedance element upon said screen electrode, thereby to render said first-named amplifier stage operative, and means operative to lower the potential difference between said anode and cathode when the flow of signal currents through said one channel is arrested, thereby to arrest the fiow of current over said path and to restore the control of said space current path to said start electrode.

4. In a transmission-system, a pair of channels for transmitting signal currents in different directions, an amplifier included in each of said channels, each of said amplifiers including one stage comprising an input circuit having an input electrode biasing element included therein, a control circuit coupled to one of said channels and comprising a gaseous discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the flow of current over said path, a circuit serially including said biasing elements and said space current path, means for impressing a start potential upon said start electrode when signal currents traverse said one channel, whereby current is caused to traverse said space current path and said biasing elements, said biasing elements being so connected and arranged that the gain of the amplifier in said one channel is increased and the gain of the amplifier in the other of said channels is decreased when current traverses said space current path, and means operative to lower the potential difference between said anode and cathode when the flow of signal currents through said one channel is arrested, thereby to arrest the fiow of current over said path and to restore the control of said space current path to said start electrode.

5. In a transmission system, a pair of chan-- nels for transmitting signal currents in different directions, a control circuit comprising a gaseous discharge tube including an anode and a cathode separated to define a space current path and a start electrode for controlling the flow of current over said path, a first output circuit coupled to said anode and cathode and serially including a source of current and an impedance element, rectifying means coupled to one of said channels and including a second output circuit coupled to impress'a start potential upon said start electrode when signal currents traverse said one channel, thereby to initiate the flow of current through said first; output circuit, means controlled by the voltage developed across said impedance element forcontrolling the signal current transmission efiiciency of at least one of said channels, a relay comprising ,a winding serially included in said second output circuit, and means controlled by said relay for momentarily opening said first output circuit when signal fiow through said channel is started and stopped, thereby to arrest the flow of current over said first output circuit and to restore the control of said space current path to said start electrode.

ROSWELL H. HERRICK.

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