USRE22897E - Transmission system - Google Patents

Description

y 5, 1947. R. H. HERRICK Re. 22397" TRANSMISSION SYSTEM 7 2 Sheets-Shee 1 Original Filed Aug. 14, 1939 mmi mQ v9 INVENTOR. Roswell HHer/"l'ck July 15, 1947.

R. H. HERRICK TRANSMISSION SYSTEM 2 Sheets-Sheet 2 0 m m m w m MH H wa A N8 w MN .8 Md m. m, wfiq wmwm NEUKPU NJ m qwucfiam @MN Y E8 35mm .WQQN w mQN Reissued July 15, 1947 TRANSMISSION SYSTEM Roswell H. Herrick, Oak Park, Ill., assignor to Automatic Electric Laboratories, Inc., a corporation of Delaware Original No. 2,371,291, dated March 13, 1945, Serial No. 404,136, July 26, 1941, which is a division of Serial No. 290,090, August 14, 1939. Application for reissue July 26, 1945, Serial No.

11 Claims. 1

The present invention relates generally to im provements 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, more particularly, to improvements in telephone substation circuits havin incorporated therein coupled signal current channels for the transmission of incoming 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. 2,282,405, granted May 12, 1942.

In the usual telephone substation circuit a hybrid system or antiside tone impedance network is 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 efiiciency of the side tone 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, the hybrid system of each substation is usually balanced to provide maximum side tone suppression efliciency 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 subscribers lines are of widely different lengths. In order completely to obviate or to minimize the 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. In the usual arrangement of this 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 periods when signal currents are incoming to the 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 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 be exceedingl 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 effect 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 tubes each time the transmission of signal currents over either of the two channels is arrested.

It is a further object of the invention to provide an improved transmission system of the character described wherein the screen electrode of one of the amplifier tubes in each channel is utilized as the control element for blocking and unblocking the associated channel.

The novel features believedto be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as toitsorganization and method of operation, together 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 circuit having incorporated 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 illustrates a modified form of the substation circuit shown in Fig. 1.

Referring now more particularly to Fig. 1 of the drawings, the substation circuit there illustrated is adapted to be connected to a telephone line terminating at the terminals I which may, for example, form a part of a conventional automatic or manual telephone system, in which case, the distant end thereof will terminate in a line circuit the character of which is determined by the character of the exchange. Briefly described, the circuit comprises a transmitter or microphone HM and receiving means in the form of a loud speaker I02 which are adapted respectively tobe coupled t an antiside tone impedance network including a hybrid coil I03 and a balancing circuit I04 by means of signal transmission channels I05 and I06. The outgoing signal current channel I05 comprises a vacuum tube amplifier I08 including three stages of amplification respectively including the amplifier tubes I09, H0 and III, the first of which is adapted to receive signal currents from the microphone I0.I and the last of which is arranged to deliver its output to the hybrid system I03. More particularly, the microphone IOI is coupled to the input electrodes of the first amplifier tube I09 through a coupling network which includesa condenser II2 and an adjustable voltage dividing or signal level control resistor I I3. The output electrodes of the tube I09 are coupled to the input electrodes of the succeeding amplifier tube I I0 through a resistance'capacitance coupling network which includes a pair of resistors H4 and H5 and a coupling condenser IIB. Similarly the output electrodes of the second amplifier tube 0 are coupled to the input electrodes of the final amplifier tube I II through a resistance capacitance coupling network which includes a resistor I", ,a voltage dividing or signal level adjusting resistor II 8 and a coupling condenser H9. In order to bias the control electrode of the tube I09 to the proper negative potential with respect to the associated cathode, thereis provided in the cathode leg of the output circuit of this tube a pair of series connected cathode biasing resistors I20 and I2I which are shunted by a signal current by-pass condenser I22. The control grid of the tube H0 is similarly biased to the proper negative potential with respect to its associated cathode by means of a cathode biasingcircuit comprisinga resistor I23 shunted by a signal current by-pass condenser I24. A similar cathode biasing network comprising a resistor I25 shunted by a by-pass condenser .I26 is included in the cathode leg of the output circuit of the final amplifier tube III. Anode potentials are supplied to the anodes of the tubes I09, H0 and III from a source ofanode 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 I03. Screen potentials are impressed upon the screen electrodes of the indicated tubes from the anode current source over paths respectivel including-filter networks, that leading to the screen electrode of the tube I09 including a resistor I21 and a signal current by-pass condenser I28 and that extending to the screen electrode including a, resistor I29 anda bypass condenser I30.

The incoming signal current transmission channel I06 has included therein an amplifier I35 which includes two stages of amplification respectively comprising the amplifier tubes I35 and I31. More specifically, the incoming signal current terminals of the hybrid coil I03 are coupled to the input electrodes of the first amplifier tube I136 through a coupling'network which includes a transformer I38 and an adjustable Voltage i ding or signal level control resistor I39. The output electrodes of this tube are coupled to the input electrodes of the second amplifier tube I31 through ,a resistance capacitance coupling network which comprises a resistor- 140, a coupling condenser MI and an adjustable voltage dividing or signal level control resistor I43. The output electrodes of the second amplifier tube I31 are, in turn, coupled to the voice or signal current coil of theloud 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 I46. In order to bias the control electrode of the tube I31 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 I48. Control of the bias impressed upon the control electrode of the first amplifier tube I35. is effected through the provision of a cathode biasing network which includes a pair of resistors I40 and I50 which are shunted by a signal current 'by-pass condenser I5I. Anode potentials are impressed upon the anode electrodes'of the two tubes I36 and I3'I from the source of anode current over circuits 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 I36 from the source of anode current over a path which includes afilter network comprising a resistor I52 and a bypass condenser I53. Screen potential for the screen anode of the tube 31 is derived in a manner more fully explained subsequently.

In order to control the gains of the amplifiers I08 and I35, thereby to vary the signal current transmission efficiencies of the channels I05 and I-06 in accordance with'signal current fiow 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 I05 and the second of which operates in response to signal currents traversing the channel I06. More particularly, the first unit of the control circuit comprises a gaseous discharge tube I56 of the three-electrode type having an anode-I51 anda cathode I58 separated to define a space current path and a control or start electrode I59 interposed therebetween to control the flow of current over the space current path. The'startelectrode I59 is normally biased negatively with respect to the cathode I58 over a path which serially includes a resistor IEO, a source of bias voltage IBI, a resistor I62 and a resistor I63. The space current path of the discharge tube I56 is included in the output circuit of this tube in series with the source of anode potential mentioned above and the resistors I50 and I63, 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 channel I05 and the first amplifier tube I36 included in-the channel I06. More particularly, the voltage developed across the two series connected resistors I50 and I-53in response to space current traversing the tube I56 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 flow of current over the space current path of the tube I50. Since the resistor I50 is also included in the cathode biasing circuit of the amplifier tube I36, the voltage across this resistor also functions to control the signal current gain of the amplifier I35. The two serially connected resistors I50 and I63 are heavily by-passsed for signal currents through the provision of a low impedance condenser I64. In order to impress a start potential upon the start grid I59 of the discharge tube I56 when signal currents traverse the channel I05 a rectifying unit is provided which includes a three-electrode vacuum tube I65 having it input electrodes coupled to the channel I05 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 rectifier by means of a bias battery I61 connected between the input electrodes thereof. The output electrodes of the rectifier tube I65 are included in an output circuit which comprises 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 I is arrested, there is provided a slow-to-release relay I68 having its winding included in the output circuit of the tube I 65 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 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 I12 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 current thereover. The output electrodes HI and I12 of this tube are included in an output circuit which also includes the resistor I2I, a second resistor I 14 and the source of anode current connected to the terminal marked +B. The voltage developed across these two series connected resistors when space current flows through the tube I10 is utilized as the operatin potential for the screen electrode of the amplifier tube I31 included in the channel I06, these two resistors being heavily bypassed for signal currents by a low impedance condenser I15 connected in shunt therewith. Since the resistor I2I forms a part of the cathode biasing circuit for the first amplifier tube I 09 included in the outgoing signal current channel I05, the voltage across the resistor serves to determine the gain of the indicated amplifier tube and thus the signal current tranmission eificiency of the channel I05. 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 I11, a, second resistor I18 shunted by a signal current by-pass condenser I19 and the resistor I14. For the purpose of impressing a start potential upon the start electrode I13 when signal currents traverse the channel I06, a rectiher is provided which comprises a three-electrode vacuum tube I80 having its input electrodes coupled to the channel I06 at a point following 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 of a bias battery I82 connected between the input electrodes thereof. The output electrodes of the tube I are included in an output circuit which serially comprises the resistors I2I and I18. In order to control the deionization of the discharge tube I10 there is provided a sloW-to-release relay I83 having its operatin winding included in the output circuit of the tube I80 and provided with: 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 of the system illustrated in Fig. 1, it is pointed out that in order to condition the signal current channels for signal current transmission, the cathode heater circuit, not shown, for the various tubes i1lustrated,.is first closed. Witlr the apparatus in this condition the amplifier stages respectively comprising the tubes I09, H0 and I36 are all biased to operate at normal gain.. Since, however, the two discharge tubes 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, both channels are effectively blocked so that no circulation of energy around thet system can occur. If, with the apparatus in this condition, a user of the substation circuit speaks into the microphone IOI, the resulting signal voltage is impressed upon the input electrodes of the amplifier tube I09 and the amplified output voltage is, in turn, impressed upon the input electrodes of the tube I I0. As further amplified by the tube IIO the signal voltage is impressed across the input electrodes of the amplifier stage II I and also through the condenser I66 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 flows through this tube over a circuit which includes the winding of the relay I68, 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, a pulsating direct current 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 resistor does not fall to a zero value during alternate half cycles of signal current but is maintained at a value sufiicient 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 negative bias voltage afforded by the Cbattery I6I. This voltage is of suflicient magnitude to lower 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 I50 and also substantially greater than the resistance of the space current path through the tube I56. Accordingly, a substantial portion of the available voltage of the anode current supply source appears as a voltage drop across the re- 7 sister I 63. This voltage drop, as augmented by the vbltage drop across the resistor I50, 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 HI operative to transmit the generated signal currents therethrough and through the hybrid coill'03 to the line terminating at the terminals I 60. -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 II I, the first amplifier tube I36 of the channel I66 is either biased to have a very low gain therethrough or beyond cut-off, depending upon the constants of the control circuit. Thus, the current traversing the resistor I 50 and flowing through the space current paths of the tubes I56 and [65 causes a 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 effect of lowering the gain ofthe tube I36 and if of sufficiently large magnitude may serve to bias this tube to a point beyond anode current cut-oiT such that the channel I06 is completel blocked in the first amplifier stage thereof. Alternatively, if the resistance value of the resistor I50 is chosen so that the voltage developed thereacross by the space currents flowing therethrough does not exceed the anode current cut-01f value of the tube I36, this tube w-illpass incoming signal currents to the succeeding amplifier tube I31 at a substantially reduced gain. By virtue of the arrangement just described if the antiside tone network comprising the hybrid coil I03 and the balancing circuit -I'0'4 are greatly unbalanced due to the condition of the lines connected to the 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 currents from being fed through the channel to initiate operation of the second control unit comprising the rectifier tube 186 and the-discharge tube I10.

:From the foregoing explanation it will be apparentthat the current traversing the space current path of the tube I65 as a result of signal voltage impressed upon the input electrodes of this tube also traverses the operating winding of 'theslow-to-release relay I68. Shortl following the first few cycles of this voltage, the relay I68 operates to break, at its armature I69 and the associated back contact, the previously traced output circuit including the space current path of the discharge tube 56. This circuit is recompleted at the armature I69 and its associated workin contact when the armature I69 is moved to its attracted position, During the interval required for operation of the armature I69 from it retracted position to its attracted position the output circuit of the tube I56 is held open. Accordingly, the "flow of space current through this tube and through the resistors I50 and I63 is momentarily interrupted, causing the final amplifier tube III of the channel I05 momenitarily to be rendered inoperative and the bias voltage impressed across the input electrodes of thetfirst amplifier tube I36 included in the channel I66 momentarily to'be returned to the normal value. During this operation the potential of the start electrode I59 is maintained at a value permitting the flow of space current through the tube I56 due to the continued operation of the rectifier tube I 65. Accordingly, immediately the output circuit of the discharge tube I56 is recompleted at the armature I69 and its associated front contact, space current again fiows through the tube I56, thereby to render the final amplifier tube III 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 syllable of the speech train may occur during this switching interval. Such clipping is ofno 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 during the switching operation.

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 I 56 is such that when ionization therein occurs th start electrode I59 has no further control over the current flow therethrough. Since, however, the operating winding of the relay I68 is deenergized when the flow of signal currents through the channel I05 is arrested, this relay is caused to restore after an interval. During the interval when the armature I69 of the relay I66 is moving between its attracted and retracted positions theoutput circuit of the discharge tube I 56 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 I6I ionization of the tube is prevented when the armature I69 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 I56. When the space current flow through the two tubes I65 and I56 is interrupted in the manner just explained, the voltage drops across the resistors I50 and I63 are reduce "substantially to zero. Accordingly, the operating potential is removed from the screen electrode of the final amplifier tube II I and this tube is rendered inoperative to transmit signal currents to the hybrid coil I03. Also, the value of 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.

The manner in which the second control unit comprising the rectifier tube F and the-discharge tube II'0 functions partially or completely to block the transmission channel I05 and to activate the final amplifier stage of the channel I06 9 when signal currents are transmitted from the line terminating at the terminals I through the hybrid coil I03 to the input side of the channel I06, is substantially identical to the mode of operation of the first control unit as described above. More particularly, signal currents transmitted to the input electrodes of the tube I36 through the coupling transformer I38 and the voltage divider I39 are amplified by this tube and the amplified signal voltage is impressed across the input electrodes of the two tubes I31 and I80. The resulting 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 I14. The voltage drop across these two serially connected resistors is utilized as an operating voltage for the screen electrode of the tube I31, whereby this tube is rendered operative to amplify the signal currents and transmit the same through the transformer I44 and the T-connected volume control resistor network to the signal current coil of the loud speaker I02 for reproduction. The voltage developed across the resistor I2I is impressed as a negative bias between the input electrodes of the first amplifier tube I09 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 by the space current of the tube I80, operates momentarily to extinguish the flow of space current through the 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 T-connected resistance network included in this channel between the coupling transformer I44 and the signal current coil of the loud speaker operates in a well-known manner to maintain the sound output level of the loud speaker substantially constant regardless of changes in the level of the signal currents transmitted to the input side of the channel. When signal current transmission through the channel I06 is arrested the flow of space current through the rectifier tube I80 is arrested. Thus, the operating winding of the relay I83 is deenergized and the potential impressed upon the start electrode I13 of the discharge tube I10 is restored to a negative value which will not permit ionization of the tube after the tube has been deionized. After an interval equal to the release period of the relay I83, the armature I84 is'moved from its attracted position to its retracted position. During such movement, the output circuit of the tube I10 is interrupted, thereby to disconnect the source of anode voltage from the anode I1 I. As a result, 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 two resistors I2I and I14 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 I2l is sufficiently reduced to permit the amplifier tube I09 of the channel I to operate with substantially normal gain.

The purpose of providing the relays I68 and I83, 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 blocking of the channels during momentary pauses in signal current transmission through the respective associated channels. To this end, the relay I68, for example, provides a hang-over period, during which the discharge through the tube I56 is sustained, which exceeds the short pauses occurring in normal speech. Similarly, the slowto-release period of the relay I83 serve to sustain the flow of space current through the discharge tube I10 for intervals during which short speech 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 be used in lieu of of the relays I68 and I83 as shown in Fig. 1 without changing the armature and contact spring assemblies. In the Fig. 1a, arrangement the relay coil comprises a pair of difierentialy related windings, the lower winding of which is connected in series with a condenser l and a resistor I9I across the upper winding. Also, 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. l 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 of current through the lower winding is relatively low and no substantial opposing 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 associated space current path ceases.

If the circuit constants of the circuit illustrated in Fig. 1 are so proportioned that each of the first amplifier stages respectively comprising the tubes I09 and I36 is not completely blocked when the complementary transmission channel is active, the arrangement permits what is known as conversation break-ins to occur. In this regard it will be apparent that signal currents transmitted to the substation circuit over the line connected to the terminals I00 are passed through the hybrid coil I03 to the input side of the channel I06 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 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 fiow of space current through the discharge tube I10 the required operating voltage is impressed upon the screen nal voltages appearing across the output circuit I of this second tube are impressed across the input electrodes of the third amplifier tube 2 II and also across the input electrodes of the rectifier tube 265. The resulting space current fiow through the tube 265 causes a voltage to be developed across the resistor 281 which reduces the negative potential upon the start electrod 259 of the discharge tube 256 to a point where this tube becomes ionized and current fiows through the two load resistors 250 and 263. The voltage developed across the resistor 263 opposes the voltage of the 0 battery 290 and is of such magnitude that the bias between the input electrodes of the tube 2| l is sufficiently reduced to permit this tube to operate with normal gain. Accordingly, the amplified signal currents delivered to the input electrodes of the amplifier tube 2 I l are amplified therein and transmitted through the coupling transformer 285 and the hybrid coil 203 to the line extending to the terminals 200. The voltage developed across th 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 beyond cut-off such that the channel 206 is completely blocked, or to bias this tube so that it operates 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 203 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 268 included in the output circuit of the rectifier 265 functions, in the exact manner previously explained with reference to the corresponding relay of Fig. 1, momentarily to break the space current path of the discharge tube 256 shortly following the initial signal current fiow 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 26B is fully operated, 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. Upon operating, the relay 268 completes, at its armature 269', an obvious path for shortcircuiting its upper winding, thereby to render itself exceedingly slow to release in a manner well understood in the art. By virtue of this arrangement, 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.

When the flow of signal currents through the channel 205 is arrested, space current flow through the rectifier tube 265 ceases. As a result. the relay 268 is deenergized and the potential upon the start electrode 259 is restored to a value which will prevent space current fiow through the tube 256 after this tube is deionized. After an interval determined by the release period of th relay 268, this relay restores to interrupt, at its armature 269', the path short-circuiting its upper winding, thereby to render itself reasonably fast to operate. At its armature 269, therelay 268 momentarily interrupts the output circult of the discharge tube 256, whereby this tube is deionized and control of the space current fiow therethrough is restored to the start grid 255. 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 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 th channel 205. From the foregoing explanation it will be ap-par out that if the resistors 250 and 2'2l ar properly proportioned so that the associated amplifier tubes 236 and 2! are not biased beyond cut-off whence space current traverses these resistors, each of the two channels 205 and 206 is partially active when the other channel is in use. Accordingly, conversation break-ins may occur in a manner which will be apparent from the above description. In this regard it will be apparent 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 the substantial side tone suppression achieved through operation of the hybrid system 203.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may 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 amplifying means, said amplifying 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 amplifier 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 flow of current over said path, means included in said input circuit for impressing 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 difference 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 fiow of current over said path and ing said source to normally maintain the said device in said receiving channel in blocked condition and the said device in said transmitting channel in operative condition, means responsive to the signal strength impressed on said receiving channel to efiect a condition, means responsive to the signal strength existent in the output line of said transmitting channel to eifect a condition in opposition to said first mentioned condition, and means responsive to the balance between said two cOnditions in control of the potential of said source whereby said channels are alternately conditioned for signal passage in accordance with directional signal conditions.

9. Telephone substation apparatus comprising in combination a receiving amplifying channel and a transmitting amplifying channel each having a space discharge device operative to alternately block and condition its associated chan nel, a control network for said devices, a signal strength responsive device operative to effect a condition in proportion to the incoming signal strength impressed on said receiving channel, a second signal strength responsive device operative to effect a condition in proportion to the strength of output of said transmitting channel and in opposition to said first mentioned condition, means to control said network in accordance with the degree of balance between said two conditions, and means to diminish the efiicacy of said first mentioned responsive device upon increase of signal strength impressed on said sec ond mentioned responsive device.

10. Telephone substation apparatus comprising in combination a receiving amplifying channel and a transmitting amplifying channel each having a control device therein operative to alternately block and condition the channel, a control circuit for said devices Op r ive to effect the blocking of one channel while the other is conditioned and vice versa, a signal strength responsive device operative to eifect a condition in proportion to the incoming signal strength impressed on said receiving channel, a second signal strength responsive device operative to effeet a condition in proportion to the strength of output of said transmitting channel and in opposition to said first mentioned condition, means to control said circuit in accordance with the degree of balance between said two conditions, and means to diminish the efficacy of said first mentioned responsive device upon increase of signal strength impressed on said second mentioned responsive device.

11. Telephone substation apparatus comprising in combination a pair of conductors adapted to be connected to a line, a receiving amplifying channel coupled to said conductors, a transmitting amplifying channel coupled to said conductors through a voltage transforming network, a device in each of said channels operative to alternately block and condition the channels, a control circuit for said devices operative to effeet the blocking of one channel while the other channel is conditioned and vice versa, means to effect a condition in response to the signal strength impressed on said receiving channel, means to effect a condition responsive to the signal strength existent in the interconnection between said transmitting channel and said voltage transforming network and in opposition to said first mentioned condition, and means to control said circuit in accordance with the degree of balance between said two conditions whereby said channels are alternately conditioned for signal passage in accordance with directional signal conditions.

ROSWELL H. HERRICK,

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