US2170941A - Transmission control in signaling systems - Google Patents

Description

Aug. 29, 1939. K. H. DAVIS- TRANSMISSION CONTROL IN SIGNALING SYSTEMS 2 Sheets-Sheet 1 Filed Sept. 15, 1938 NOGAD FIG. 2

MIXING C/RC lNl ENTOR K. H. DA V/S av ATTORNEY lllA v 2 Sheets-Sheet 2 VAR/ABLE AMP! LB/SABLER IVOJ 91.

VAR/451.4

K. H. DAVIS TRANSMISSION CONTROL IN SIGNALING SYSTEMS Filed Sept. 15, 1938 DISABLE? AMP Aug. 29, 1939.

P M 8 MW Y M M E R W m K A y B Patented Aug. 29, 1939 UNITED STATES PATENT OFFICE TRANSMISSION CONTROL IN SIGNALING SYSTEMS Kingsbury H. Davis, Closter, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

6 Claims.

This invention relates to control of transmission in signaling systems, and more particularly to arrangements for minimizing the effects of interfering waves in such systems.

Long distance telephone systems are often subject to noise waves of varying amplitudes, including ordinary line noise introduced into the system by the operation of line apparatus and the noise induced into the system from extraneous sources, particularly neighboring power lines. In these systems it is frequently the practice to employ voice-operated, switching devices such, for example, as voice-operated echo suppressors or antisinging devices. The noise waves in the system may materially interfere with the proper operation of the switching devices on speech energy in a manner such that noise waves of high amplitude may cause either false operation of the switching devices or necessitate reducing the sensitivity of the latter to such degree as to prevent proper operation on speech waves of low amplitude.

Certain circuits of the prior art attain proper discrimination against noise by utilizing the noise energy to automatically produce a continuous reduction in sensitivity in the voice-operated switching device proportional to the increase in amplitude of the line noise in the associated line so as to prevent operation of the device by the noise while maintaining the maximum effective sensitivity thereof to operation by speech for the amount of noise present. In one circuit of this type, disclosed in the patent of H. L. Barney No. 2,020,452 issued November 12, 1935, the sensitivity of the voice-operated switching device is controlled by utilizing the input energy to the device from the line or other circuit subject to the noise to control the gain of an amplifier in the input of the device in such manner as to cause very slow decrease in its gain when the input energy increases suddenly, i. e., when speech comes on the line, and a very rapid increase in gain when the input energy is reduced, i. e., when speech transmission and noise are removed from the line. To prevent excessive reduction in the gain of the amplifier by speech during strong parts of the received speech syllables, a marginal control device connected to the input of the vario-amplifier is arranged to respond to the peaks of the received speech impulses to disable the gain-controlling circuit.

A device of the type disclosed in the patent of H. L. Barney, supra, will be hereinafter referred to as a nogad, i. e., noise-operated-gain-adjusting-device. Nogads are designed for use on long distance telephone circuits in which it is imperative from the standpoint of most efficient operation that as much sensitivity of the echo suppressors be provided as noise in the circuit will allow. In addition, nogads are arranged to vary the sensitivity of the echo suppressors inversely relative to the amount of noise on the circuit, always maintaining the echo suppressors as sensitive as possible without permitting uncommercial amounts of operation thereof on noise.

Another circuit of the prior art improved the system disclosed in the above patent to H. L. Barney by associating with the latter a device which is unresponsive to noise but is responsive to weak as well as to strong speech impulses to disable the gain-controlling circuit. This improved nogad disclosed in the patent of B. G. Bjornson No. 2,081,422 issued May 25, 1937, includes, in addition to the marginal device connected across the input of the vario-amplifier and operated on strong parts of speech to disable the gain-controlling circuit as disclosed in the above patent of H, L. Barney, another device comprising a very sensitive syllabic amplifier-detector connected across the output of the vario-amplifier and arranged to respond to the lower range of speech voltages to disable the gain-controlling circuit.

It has been found that the combination of the two disabling circuits embodied in the nogad according to the above patents of H. L. Barney and B. G. Bjornson prevent any change in the gain of the vario-amplifier during a very large part of any speech syllable and therefore materially reduce the amount of speech desensitization of the nogad. In addition to the aforementioned feature, this combination of two disabling circuits serves, when a second long distance telephone link equipped with echo suppressors is connected in tandem with a link having the speech desensitized nogad, to prevent false adjustments of the gain of the vario-amplifier due to noise being cut off by the operation of the echo suppressor in the other link of the system.

Without the two disabling circuits mentioned above, the gain of the vario-amplifier might increase to a maximum during the short interval after speech operated the desensitized echo suppressor associated with the nogad in the one link while the echo suppressor in the other link would cut off noise therefrom thereby causing a rapid gain in the sensitivity of the nogad. Then, when the latter released and noise could reach the nogad, the echo suppressor associated therewith would be falsely operated, thus causing mutilation of the speech which was continuing on the same branch of the system. The combination of the two disabling circuits illustrated in the patent of B. G. Bjornson, supra, prevents such false operations by operating a disabler master relay and applying a comparatively long hangover thereto so that no false adjustment in the gain of the vario-amplifiers is made while noise is cut oil by operation of the remote echo suppressor on speech in a tandem transmission system.

When the circuits disclosed in the patent of B. G. Bjornson, supra, are incorporated in a tandem system comprising at least tWo links each of which includes two oppositely directed one-way paths, an echo suppressor is usually connected to each of the one-way paths and functions in the well-known manner to disable the opposite of the two paths associated therewith. Also, in such system each echo suppressor is equipped with a nogad arranged so that the energy taken from the outputs of the two vario-amplifiers is combined and introduced into a common reducer circuit and the energy supplied to the inputs of the two vario-amplifiers is taken off the one-way paths ahead of the echo suppressors so that noise in each link can reach each nogad regardless of the operation of the suppressors associated therewith. The common reducer, as seen in the above patents of H. L. Barney and B. G. Bjornson, builds up a direct current voltage whose magnitude depends on the amount of noise present in the system. This direct current voltage is then applied simultaneously to both vario-amplifiers to adjust the gain thereof in such way that the noise is substantially constant in the output of the varioamplifiers.

Although there may be substantially no false adjustments of the nogad and operations of the echo suppressors in the tandem telephone system mentioned above, it does happen in such systems using a common reducer for each link thereon that the east and west sides of the nogad are both disabled for relatively long periods of time, necessitating, of course, the disabling of the nogad a large part of the time. This period of time would be the interval while speech waves are being transmitted in either direction. Because of the insensitivity of the disabling combination it would still be possible to cut off noise from a nogad without disabling it, if the speech waves were to have such amplitude as to operate the echo suppressors without operating the disablers. Cutting oif noise from and rendering ineffective the nogad for intervals of time may result in rapid increases in the gain of the vario-amplifiers thereby causing false operations of the echo suppressors. Such occurrences would obviously defeat the purpose for which the nogad was designed.

It is an object of the invention to discriminate between electrical waves of different characteristics, for example, between useful signals and interfering noise waves.

It is another object of the invention to improve the operation of a signal-controlled device for controlling transmission in a signaling system subject to interfering waves, such as line noises.

It is another and more specific object of the invention to provide that interfering noise waves can always reach each nogad embodied in a tandem telephone system.

It is a further and more specific object of the invention to control substantially at all times the effectiveness of each nogad in a tandem telephone system.

It is a still further and more specific object of the invention to control the gain of vario-repeaters in such manner that the possibility of false operations of the echo suppressors is sub stantially reduced.

The present invention contemplates an improved operation of a tandem two-way telephone system comprising two or more four-wire links each of which embodies two oppositely directed one-way paths equipped with echo suppressors and a nogad. The latter includes means for producing two voltages corresponding to waves in the one-way paths and applied to the amplifiers in such manner that the gain thereof is automatically controlled in accordance with the larger of the two. Associated operably with each echo suppressor is an arrangement for disabling the voltage producing means whose input comes from the one-way path which is opposite to the oneway path Whose waves caused the operation of the echo suppressor. Thus, one side of the nogad is substantially always connected to the one-way path transmitting both noise and voice waves while the other side thereof is disconnected from the one-way path transmitting noise waves alone.

As a result high-sensitivity disabling of the voltage producing means is obtained Without cutting off the nogad from noise for long periods of conversation. Furthermore, this arrangement provides such control of the gain of the amplifiers that false operations of the echo suppressors associated therewith are substantially reduced, or eliminated entirely, after noise is returned to the voltage producing means that was disabled.

The invention will be more readily understood from the following description taken together with the accompanying drawings in which:

Fig. l is a schematic diagram showing an echo suppressor associated with a nogad in a singlelink telephone system;

Fig. 2 is a schematic diagram illustrating a tan dem telephone system comprising two links each u The arrows in parallel with the one-way paths indicate the direction of transmission. Embodied in the four-wire circuit is a nogad it! with which is preferably associated a central-type echo suppressor represented by the arrowheaded lines and designed to be applied to the circuit at a point intermediate and some distance from the ends of the four-wire circuit. As is well understood, the arrowheaded lines indicate control paths such that speech waves arriving at the points I l or [2 cause the operation of mechanisms that produce controlling efiects, preferably short circuits across the respective one-way paths. A more detailed description of echo suppressors may be had by referring to an article by A. B. Clark and R. C. Mathes, entitled Echo Suppressors for Long Telephone Circuits which was published in the Journal of the American Institute of Electrical Engineers, Vol. XLIV, pages 618-626, for June 1925. Control circuits l3 and I4 connect the nogad Ill to the paths WE and EW, respectively.

As shown in Fig. 1 the control circuit |3is connected to the path WE at a point l5 which lies between the west terminal and the echo-suppression point ll. Therefore, the operation of the echo suppressor in the pathWEl cannot prevent noise energy originating at the west terminal from reaching control circuit |3 of nogad H], assuming speech waves originating at the east terminal to cause the operation of the echo suppressor. Likewise, control circuit I4 is connected to the path EW at a point l6 which lies between the east terminal and the echo-suppression point I2. Consequently, the operation of the echo suppressor in the path EW due to speech waves originating in the west terminal cannot prevent noise energy produced in the east terminal from arriving at the nogad III. In both cases, then, noise can always reach the control circuits of the nogad ID to adjust sensitivity thereof to a sensitivity depending on the noise at the west or east terminals.

Fig. 2 is a schematic diagram of two fourwire links of the type shown in Fig. 1 connected in tandem and extending through a twowire circuit in station A between the west and east terminals. Assuming speech waves to originate at the West terminal, these would immediately cause the operation of the echo suppressor at point 20 in the path EW of link B, and eventually the operation of echo suppressor at point 2| in path EW of link C. In link C it is noted that control circuit 22 of nogad 23 is connected to the path EW at a point 24 which is intermediate the east terminal and echosuppression point 2|. Consequently, nogad 23 will always be exposed to noise originating in the east terminal, and because of this condition will adjust itself to a satisfactory sensitivity, regardless of the operation of echo-suppression point 2|.

However, in link B it is observed that control circuit 25 of nogad 26 is joined to the path EW at a point 21 which lies intermediate the echo- suppression points 23 and 2|. Because of the operation of echo-suppression point 2| as hereinbefore explained, noise from the east terminal will be cut off from control circuit 25 associated with nogad 26. This will cause a sudden increase in. the sensitivity of nogad 28 such that when noise is returned thereto due to a release of echo-suppression point 2|, the echosuppression point 28 in link C of the path WE will be falsely operated during the interval required for nogad 26 to adjust itself to the prevailing noise condition at the east terminal.

When speech waves originate at the east terminal, an entirely analogous situation exists in the case of nogads 23 and 26 but in this case it is evident that noise from the west terminal is cut off from control circuit 33 of nogad 23. This will cause a sudden increase in the sensitivity of nogad 23 so that when noise is returned to the latter due to the release of echo-suppression point 28, the echo-suppression point 2| in link C will be falsely operated during the period required for nogad 23 to adjust itself to the prevailing noise condition at the west terminal. It is obvious that frequent false operations of the respective echo-suppressing points in the manner aforementioned would interrupt a talker, thereby impairing the efiiciency of speech transmission.

While Fig. 2 shows each of the two links equipped with a nogad, it is understood that this showing is only for the purpose of this description and the important circuit requirement to be rememberedis that the explanation applies to a signal transmission system comprising at least two four-wire links each of which has an echo suppressor and at least one of the four-wire links also includes a nogad associated with its echo suppressor. Therefore, in either link B or C the nogad could be eliminated without interfering with the operation of the invention which will now be explained in detail.

This invention therefore contemplates a tandem telephone system of the type described in Fig. 2 in which the sensitivity of the nogads is so maintained that false operations of the echo suppressors associated therewith are substantially reduced, and will now be described.

Fig. 4 shows a four-wire link comprising a one-way transmission path WE including a oneway amplifying device WA and a one-way transmission path EW embodying a one-way amplifying device EA, the respective amplifying devices serving to transmit waves in the directions indicated by the arrows. The one-way paths WE and EW may be coupled at their terminals in conjugate relation with each other and in energy-transmitting relation with the two-way lines or circuits. between which signal transmission is desired by the usual transformers and associated balancing networks (not shown) or by other suitable means.

The incoming portion of the path WE and the input of transmission control circuit 40 are connected by a network 4| in substantially conjugate relation with each other and in energy-transmitting relation with output of the amplifying device WA. This is to prevent the short-circuiting of the outgoing portion of path WE'by echo suppressor ESz associated therewith from affecting the input of control circuit 48. Likewise, the incoming portion of the path EW and the input of transmission control circuit 42 are connected by a network .3 in substantially conjugate relation with each other and in energytransmitting relation. with the output of the amplifying device EA so as to preclude the shortcircuiting of the outgoing portion of path EW by echo suppressor ESi from affecting the input of control circuit 42. The networks 4| and 43 are preferably of the type disclosed in the patent of G. Crisson No. 1,755,243, issued April 22, 1930.

The control circuit 40 comprises vario-amplifier VA1 and an echo suppressor ES].. The input of vario-amplifier VAI is coupled to the network M by a transformer 44. The vario-amplifier VA1 may include any suitable variable gain thermionic amplifying device which, for the purpose of this description, may be of the type disclosed in the patent of B. G. Bjornson, supra. Output transformer 45 couples the output of vario-amplifier VA1 to the input of the echo suppressor ES1 which may be of any suitable type to disable the path EW in response to voice energy impressed on its input and, for the purpose of this description, comprises an amplifierdetector 46 in whose output is a mechanical relay41 responsive to detected current to close short-circuiting connection 50 across the one way path EW.

Across the output of the vario-amplifier VA1 is also connected a transformer 48 coupling the former to reducer No. 1 whose function it is together with reducer No. 2, as will be hereinafter explained, to automatically regulate the gain of the vario-amplifier VA1 so that the noise input to the echo suppressor ES1 is always below that level of amplitude at which the operation of the echo suppressor ESi is effected. Reducer No. 1 includes rectifying and direct-current amplifying means, and may be preferably of the type disclosed in the patent of B. G. Bjornson, supra. In' the output of reducer No. 1 is connected a resistance 49 through which flows direct current whose magnitude depends on the amplitude of the waves in the output of the varioamplifier VA1. The potentials developed across the resistance 49 by this flow of current serve to control the gain of the vario-amplifier VA1 in a manner that will be subsequently explained.

The time constants of reducer'No. 1 are such that an impulse of high amplitude applied suddenly to the input thereof requires a comparatively long time to decrease the gain of varioamplifier VAI and when this impulse is suddenly removed, requires a relatively short time to increase the gain of vario-amplifier VA1.

Across the input of vario-amplifier VA1 is a transformer 55 coupling the former to disabler No. 1 whose function is to prevent strong parts of applied speech syllables in one-way path WE from causing reducer No. 1 to make false gain adjustments of vario-amplifier VA1. Disabler No. 1 also serves an additional purpose that will be hereinafter explained, and comprises a thermionic amplifier-detector AD1 in the output of which is a relay 56 whose operations are controlled by the amplifier-detector ADi.

The control circuit 42 comprises. vario-amplifier VA2 and echo suppressor ES'z. The input of vario-amplifier VAZ is coupled by a transformer 51 to the network 43. Output transformer 58 couples the output of vario-amplifier VA2 to the input of echo suppressor ESz which comprises amplifier-detector 53 in whose output is a relay 54 responsive to detected current to close shortcircuiting connection 6! across the one-way path WE. The vario-amplifier VA2 is identical with vario-amplifier VA1.

Across the output of vario-amplifier VA2 is bridged a transformer 59 coupling the former to reducer No. 2 whose function it is together with reducer No. 1, as will be hereinafter explained, to automatically regulate the gain of the varioamplifier VA2 so that the noise input to the echo suppressor E82 is always below that level of amplitude at which the operation of the echo suppressor ESz is accomplished. Reducer No, 2 is of a type the same as previously mentioned reducer No. 1. In the output of reducer No. 2 is connected a resistance 60 through which flows direct current whose magnitude depends on the amplitude of the waves in the output of the waves in the vario-amplifier VA2. The potentials developed across the resistance 69 by this flow of current serve to control the gain of the varioamplifier VA2 in a manner that will be hereinafter described. The time constants of reducer No. 2 are preselected such that the operation thereof is similar to that of reducer No. 1 with respect to the abrupt application of an impulse of high amplitude to associated vario-amplifier VA2 and the sudden removal of this impulse therefrom for controlling the gain thereof.

In bridge of the input of vario-amplifier VA2 is a transformer 62 coupling the former to disabler No. 2 whose function is to prevent strong parts of applied speech syllables in one-way path EW from causing reducer No.2 to make false gain adjustments of vario-amplifier VAz. Disabler No. 2 also serves an additional purpose that will be subsequently explained, and comprises a thermionic amplifier-detector AD2 in the output of which is a relay 63, the operation of which is controlled by the amplifier-detector ADz.

Associated with the relay 4'! of echo suppressor ESi is a relay 66 which is connected with relay 63 included in the disabler No. 2. Output circuit 61 of the amplifier-detector 46 in echo suppressor ESi is connected to relay 54 of echo suppressor 1332. Associated with the relay 54 of echo suppressor ESz is a relay 68 which is connected with relay 56 embodied in the disabler No. 1. Output circuit 69 of the amplifier-detector 53 in echo suppressor ESz including the operating circuit of relay 54 is connected to relay 4! in echo suppressor ESi.

Bridging the resistance 49 and 60 of the respective reducers Nos. 1 and 2 are condensers and H charged by the voltages built up across the respective resistances 49 and 60 and whose charges are applied through mixing circuit 12, Fig. 3, to the vario-amplifiers VA1 and VAZ to control the gain thereof in a manner that will be subsequently explained.

The mixing circuit 12 shown in Fig. 3 comprises, looking from reducer No. 1, a thermionic tube 73 whose input is connected across condenser 70 and whose output includes anode battery 14 and resistance 15. The condenser 10 bridges resistance 49 with disabling points 11 or 18 therebetween. The mixing circuit 12 also comprises, looking from reducer No. 2, a thermionic tube 19 whose input is bridged by condenser H and whose output embodies anode battery 89 and resistance 8!. The condenser H is also applied across resistance 60 with disabling points 84 or 85 interposed therebetween.

Also included in the mixing circuit 12 is a rectifier 99 comprising thermionic rectifiers 9| and 92. The input of rectifier 9| is connected to resistance 75 while the input of rectifier 92 is connected to the resistance 8 I. The anodes of the rectifiers 9| and 92 are joined by a strap 93 so that the outputs are applied across resistance 94. The voltage developed across 94 is applied through leads 95 and 96 to vario-amplifiers VAI and VAz to control the gain thereof in a manner that will be hereinafter explained.

The operation of the system will now be described.

When neither noise nor speech is present in the path WE, reducers Nos. 1 and 2 produce no output voltages, the biases applied to the grids of the vario-amplifiers VAI and VA2 are such that both of the latter are at maximum gain, and the echo suppressors E81 and E82 are effectively at maximum sensitivity.

Now, let it be assumed that there are no speech waves being transmitted over the system, but that noise of a fairly steady character is present therein. Referring to Fig. 4, a portion of the noise in path WE is diverted through network 4| into control circuit 49. The portion of the noise transmitted through control circuit 39 and transformer 55 into disabler No. 1 will be dissipated therein due to the fact that the detector thereof is set so as not to be operated on the maximum noise anticipated in the path WE. The remaining portion of the noise directed into control circuit 40 will be impressed by transformer 44 on vario-amplifier VA1 to be amplified thereby. The amplified noise will be divided between the input of control circuit 38 for reducer No. 1 and the input of echo suppressor ESi.

The first portion of the noise impressed on echo suppressor ESi, due to the initial high gain of vario-amplifier VA1. before the reducers have commenced to function as will be hereinafter explained, may be of an amplitude that will cause momentary operation of echo suppressor ESi to disable the path EW. However, as soon as the reducer No. 1 has built up an output voltage as will be subsequently described, the gain of variorepeater VA1 will be reduced to the proper point to cut down the level of the noise applied to the echo suppressor E51 so that such level of noise will be inadequate to effect the operation thereof. The remaining portion of the amplified noise in the output of VA1 will be impressed on control circuit 38 and applied through transformer 48 on reducer No. 1 in whose output is a resistance 49 across which is built up a voltage that serves to charge condenser 10.

Similar operation occurs in the opposite side of the circuit with the result that amplified noise in the output of vario-amplifier VAz is applied to control circuit 36 and transformer 59 to reducer No. 2 in whose output is a resistance 69 across which is developed a voltage that serves to charge condenser ll.

Referring to Fig. 3, the charges on the condensers ill and ii are applied through tubes 13 and. 19 to the respective resistances l5 and 8!. The voltages developed across the latter resist-- ances are app-lied through the rectifiers 9| and 92 to the resistance 94 in their combined output. Whichever one of resistances l5 and Bi produces the larger voltage at a given instant that one will cause current to flow in resistance 94. The voltage built up across resistance 94 will be substantially equal to the voltage across either resistance 15 or 8| that caused current to flow in resistance 94, as the latter is relatively large compared to either of the resistances "l5 and 8!.

The voltage produced across resistance 94 will be applied over leads 95 and96 through the secondary windings of the transformers 44 and 5'! to the grids of the respective vario-amplifiers VA1 and VA; to control the gain thereof as described in the patent to B. G. Bjornson, supra. The smaller of the voltages developed across either resistance 75 or 8| will in no wise affect the voltage across the resistance 94, as the smaller voltage will be in the non-conducting direction of the respective rectifiers 9i and 92 associated therewith. Therefore, the voltage across the resistance 94 serves to automatically control the bias on both vario-amplifiers VA1 and VA2 such that the sensitivity thereof is constantly adjusted to the higher level of noise.

Now, let it be assumed, that While noise is still present in the path WE, a subscriber at the west terminal commences speaking thereby producing speech waves that are impressed on the WE path of link B in Fig. 2. As explained above, this will cause first, the operation of echo-suppression point 20 in link B and in due course the operation of echo-suppression pointZI in link C. Also, as previously described, the operation of echosuppression point 2! serves to cut off from the control circuit of nogad 26 such noise as originated at the east terminal. Also, as hereinbefore mentioned, the effect of this is such that in the absence of the invention the sensitivity of nogad 26 increases rapidly to cause false operation of echo-suppression point 28 during the interval required for nogad 26 to adjust its sensitivity after echo-suppression point 2| had been released to allow noise from the east terminal to reach control circuit 25.

It will now .be explained how, in accordance with this invention, abrupt changes in the sensitivity of nogad 26 and the false operations of echo-suppression point 28 are substantially reduced. It is to be kept in mind that for the purpose of this description the circuit shown in Fig. 2 embodies in each of links B and C the circuit illustrated in Fig. 4, although the circuit in Fig. 2 may be further modified as herelnbclore explained. In Fig. 2 the nogads 22 and 26 represent circuits in accordance with Fig. 4. Therefore in Fig. 4 a portion of the speech waves together with noise in the path WE will be diverted by network 4i into control circuit 49. The diverted energy will be divided between circuit 39 for disabler No. l and the input of vario-amplifier VA1.

The portion of speech energy diverted into vario-amplifier VA1, Whose gain has been adjusted by the reducers in accordance with the amount of noise in the system as above described, will be amplified therein. The main portion of the amplified speech in the output of vario-amplifier VA1 will be applied to the amplifier-detector 46 of echo suppressor ESl to cause the energization of winding of relay ll. The latter operates to close across the path EW the short-circuiting contact 56 which, since the circuit according to Fig. 4 is embodied in both links B and C, is the equivalent of operating echo-suppression points 20 and 2i in Fig. 2.

The operation of relay 4? also shifts its armature I00 from its back contact to its front contact thereby causing the energization of the winding of relay 66 through a circuit comprising negative terminal of battery Ifll, winding of relay 66, series resistances Hi2 and IE3, closed front contact of armature we of relay 4! and ground I04. Relay 66 operates to open contact 85 there- .by disabling reducer No. 2. Now, condenser 15 charged normally by battery lill through winding of relay 66 and resistance W2 is discharged to ground I04 through resistance H13 and closed front contact of armature I99 of relay 4?. When relay 4! releases to return armature I99 to its back contact, the operating winding of relay B9 is not immediately broken, although ground [04 is removed therefrom. Relay 66 will not immediately release but will be held operated to keep contact 85 open for a hangover period of time while condenser I05 is being charged up to the potential of battery l0! by a flow of current therefrom through winding of relay 66 and resistance I02.

Since reducer No. 2 is connected through control circuit vario-amplifier VAz, circuit 42 and network 43 to the path EW, the disabling of reducer No. 2 by the operation of relay 66 has the effect that whatever noise prevails in the path EW such noise will be precluded from affecting the charge on the condenser II in each circuit according to Fig. 4 embodied in both links B and C inFig. 2. Consequently, at least the amount of charge on condenser ll will be rendered effective through the mixing circuit 12 in Fig. 3 to bias the grids of both vario-amplifiers to control the gain thereof in the manner hereinbefore explained.

Thus, when relay 66 operated to open its contact 85 in each of the circuits according to Fig. 4 embodied in both links B and C, it was thereafter unimportant whether the control circuits 22 and 25 of the respective nogads 23 and 26 in Fig. 2

were either exposed to or cut off from noise originating at the east terminal. As a matter of fact the control circuit 22 remained exposed to such noise while the control circuit 25 was cut off therefrom. The reason for this unimportance is that the disabling of the reducer No. 2 in Fig. 4 renders it impossible for further noise energy in the path EW to aflect the charge on the condenser 1! in Fig. 4.

Therefore, the gain of both vario-amplifiers and thereby the sensitivity of the associated echo suppressors are adjusted at least to that level represented by the amount of the charge on condenser 1i regardless of the amount of the charge on condenser 10. As described above in connection with Fig. 3, the charge on either condenser 16 or condenser 1| is rendered efi'ective through the mixing circuit 12 to control the gain of both vario-amplifiers, depending on which of the charges is the larger,

The remaining portion of the amplified energy in the output of vario-amplifier VA1 is impressed through circuit 38 on reducer No. 1 which functions to varyingly charge condenser 10 whose charge together with that of condenser 1| serves to control the gain of both vario-amplifiers and thereby the sensitivity of both echo suppressors in the manner hereinbefore described. A more complete description of the functioning of reducer N0. 1 may be had by referring to the patent of B. G. Bjornson, supra.

The portion of the energy diverted into circuit 39, when containing strong parts of speech, will cause the functioning of disabler No. 1 by energizing the relay 56 which operates to open normally closed contact 11 thereby disabling reducer No. 1. This serves to prevent change in the gain of the varioamplifiers when the amplitude of the energy in the circuit 40 incoming from the path WE exceeds a predetermined value. Such predetermined value is the amplitude of the peaks of the loudest noise that can be tolerated on commercial circuits. A more complete description of disabler No. 1 may be had by referring to the patent of B. G. Bjornson, supra.

Finally, the operation of relay 41 opens normally closed contact I68 thereby interrupting the operating circuit of relay 54 in the output 69 of the amplifier-detector 53 included in the echo suppressor ES2. The latter is therefore disabled so long as the subscriber at the west terminal continues to talk.

Now, let it be assumed that while noise is still present in the path EW a subscriber at the east terminal commences speaking thereby producing speech waves that are impressed on the EW path of link C in Fig. 2. As hereinbefore explained, this will cause, first, the operation of echo-suppression point 30 in link C and in due course thereafter the operation of echo-suppression point 28 in link B. Also, as previously described, the operation of echo-suppression point 28 serves to cut off from the control circuit 33 of nogad 23 such noise as originates in the west terminal. Also, as above explained, the effect of this is such that in the absence of the invention the sensitivity of nogad 23 increases rapidly to cause false operations of echo-suppressor point 30 during the interval required for nogad 26 to adjust its sensitivity after echo-suppression point 28 has been released to allow noise from the west terminal to reach control circuit 33. The rapid change in the sensitivity of nogad 23 together with the false operations of echosuppression point 30 are substantially reduced in the manner similar to that set forth above in connection with the sensitivity of nogad. 26

coupled with the false operations of echo-suppression point 20, and will now be explained.

When the speech arriving over circuit EW causes operation of suppressor ESz, relay 68 is operated, opening contact 18 to cut condenser 10 off from circuit WE, leaving condenser 16 charged to a voltage dependent upon the noise present in path WE. Therefore, at least the amount of charge remaining on condenser 16 will be rendered efiective through the mixing circuit 12 in Fig. 3 to bias the grids of both varioamplifiers to control the gain thereof in the manner set forth above.

Thus, when relay 68 operated to open its contact 18 in each of the circuits according to Fig. 4 embodied in both links B and C, it was thereafter unimportant whether the control circuits 3| and 33 of the respective nogads 26 and 23 in Fig. 2 were either exposed or cut off from noise originating at the west terminal. As a matter of fact control circuit 3| remained exposed to such noise while the control circuit 33 was cut off therefrom. The reason for this unimportance is that the disabling of the reducer No. 1 in Fig. 4 renders it impossible for further noise energy in the path WE from affecting the charge on condenser 10 in Figs. 3 and 4.

Therefore, the gain of both vario-amplifiers and thereby the sensitivity of both echo suppressors are adjusted at least to that level represented by the amount of the charge on condenser 10 irrespective of the amount of the charge on condenser 1 I. As previously explained relative to Fig. 3, the charge on either condenser 10 or condenser 1| is rendered effective through the mixing circuit 12 to control the gain of both amplifiers, depending on which of the charges is the larger.

The remaining portion of the amplified energy in the output of vario-amplifier VAz is applied through circuit 36 on reducer No. 2 which functions to charge condenser 1| whose charge together with that of condenser 10 serves through the mixing circuit 12 to control the gain of both vario-amplifiers and thereby the sensitivity of both echo suppressors as hereinbefore set forth. A more complete description of the functioning of reducer No. 2 may be had by referring to the patent of B. G. Bjornson, supra.

The portion of the energy diverted into circuit 31, when containing strong parts of speech, will cause the operation of disabler No. 2 by energizing the relay 63 which then operates to open normally closed contact 84 thereby disabling reducer No. 2. This serves to prevent change in the gain of the vario-amplifiers when the amplitude of the energy in the circuit 42 incoming from the path EW exceeds a predetermined value. Such predetermined value is the amplitude of the peaks of the loudest noise that can be tolerated on a commercial circuit. A more complete description of disabler No. 2 may be had by referring to the patent of B. G. Bjornson, supra.

Finally the operation of relay 54 opens normally closed contact 91 thereby interrupting the operating circuit of relay 41 in the output 61 of the amplifier-detector 46 included in the echo suppressor ESz. Consequently the latter is disabled so long as the subscriber at the east terminal continues to talk.

Thus, it is seen that when a nogad having two reducer circuits is embodied in each of at least two four-wire links in a transmission system the highly sensitive echo suppressors are utilized to 75 disable that reducer whose input originates at a terminal opposite to that from. which the input of the disabling echo suppressor comes. Accordingly, as previously described in detail, echo suppressor ESl operated by energy originating in the west terminal serves through theoperation of relays 4i and 66 to disable reducer No. 1 whose input comes from the east terminal while echo suppressor ESz operated by energy originating in the east terminal serves through the operation of relays 5d and 68 to disable reducer No. 1 the input of which comes from the west terminal.

Therefore, one side of each nogad is always effective for adjustment due to noise in one oneway path of the transmission system and one side of each nogad is always disabled when a subscriber is talking. Obviously, this enables each nogad to function a much larger percentage of time since only one side thereof is disabled with a hangover during the interval when a subscriber is talking.

Moreover, it is seen that when a reducer is disabled the gain of both vario-amplifiers together with the sensitivity of the echo suppressors is held at least to a certain amount for the period of the disability. This is so for, as previously explained, the charge on either condenser 10 or H, depending on which reducer is disabled, remains substantially constant and so long as the particular charge is rendered effective through the mixing circuit '12 for controlling the gain of both vario-amplifiers the latter will be held to a gain depending on the amount of such charge.

Consequently, when the disabled reducer is again rendered effective for controlling the charge on the condenser associated therewith, the change in the charge due to noise energy will be with respect to the previous amount of charge and in ordinary commercial circuits such change would be relatively small. Accordingly, the change in gain of both vario-amplifiers and the change in sensitivity of the echo suppressors associated therewith would also be correspondingly small thereby requiring a relatively short interval for accomplishing such changes. Hence, there is materially reduced, if not entirely eliminated, the possibilities for the echo suppressors to make a relatively large number of false operations of the echo-suppression points 20 and 38 during the interval required for the nogads to adjust their sensitivities after the release of the respective echo-suppression points 2| and 28 as hereinbefore explained.

What is claimed is:

1. In a two-way telephone system comprising a source of voice waves, two or more four-wire links connected in tandem, each of the links embodying two oppositely directed one-way paths subject to interfering noise waves, a wave device connected to the one-way paths in each link and. responsive to waves therefrom to short-circuit the opposite one-way paths therein, a wave amplifier connected between each one-way path and the wave device operatively associated therewith, and means for maintaining for a substantial portion of time an automatic control of the gain of the amplifiers in each link in response to Waves impressed thereon comprising means connected to the output of each amplifier for producing voltages representative of the amplitudes of the waves therein, a control circuit for combining the controlling effects of the produced voltages in such manner that a single control voltage is eiiectively applied to the amplifiers, and means operably connected to each wave device to disable the voltage producing means whose input comes from. the one-way path which is opposite to the one-way path Whose Waves caused the operation of the wave device.

2. In a two-way telephone system comprising a source of voice waves, two or more four-wire links connected in. tandem, each of the links embodying two oppositely directed one-way paths subject to interfering noise waves, a wave device connected to the one-way paths in each link and responsive to waves therefrom to short-circuit the opposite one-way paths therein, a wave amplifier connected between each one-way path and the wave device operatively associated therewith, and means in each link for automatically controlling the gain of the amplifier in response to the waves causing the operation of the wave device therein comprising means connected to the output of each amplifier for producing voltages corresponding to the amplitudes of the noise and voice waves flowing therein, means for combining the controlling effects of the produced voltages in such manner that the larger one is effectively applied to the amplifiers, and means operably connected to each Wave device to disable the voltage producing means whose input comes from the one-way path which is oppositeto the oneway path whose waves caused the operation of the wave device.

3. In a two-way telephone system comprising a source of voice waves, two or more four-wire links connected in tandem, each of the links embodying two oppositely directed one-way transmission paths subject to interfering noise waves, a wave device connected to the one-way paths in each link and responsive to waves therefrom to short-circuit the opposite one-way path therein, a wave amplifier connected between each oneway path and the wave device operatively associated therewith in at least one link, and means in the one link for maintaining for a substantial portion of time an automatically continuous control of the effects of noise waves on the wave devices embodied in the one link regardless of the operation of the wave devices in the other links while retaining the sensitivity of the amplifiers to voice waves comprising means connected to the output of each of the amplifiers and supplied with waves therefrom for producing voltages corresponding to the amplitudes of such waves, a control circuit for combining the con trolling effects of the produced voltages in such manner that the larger of the two voltages controls the gain of the amplifiers, and means operably associated with the wave devices in the link embodying the amplifiers to disable the voltage producing means whose input comes from the one-way path which is opposite to the one-way path whose energy caused the operation of the wave device.

4. In a two-way telephone system comprising a source of voice waves, two or more four-wire links connected in tandem, each of the links embodying two oppositely directed one-way gtransmission paths subject to interfering noise waves, a Wave device connected to the one-way {.paths in each line and responsive to waves therefrom to short-circuit the opposite one-way path therein, a wave amplifier connected between each one-way path and the wave device operatively [associated therewith, and means for maintaining *for a substantial portion of time an automatically continuous control of the eifects of noise waves on the wave devices embodied in each link while retaining the sensitivity thereof to voice waves comprising means connected to tlieoutput of each of the amplifiers and supplied with waves therefrom for producing voltages corresponding to the amplitudes of such waves, means for combining the controlling effects of the produced voltage in such manner that the larger thereof is effectively applied to the amplifiers to control the gain thereof, means operably connected to each wave device to disable the voltage producing means whose input comes from the one-way path which is opposite to the one-Way path whose energy caused the operation of the wave device, and means connected to the input of each amplifier and unresponsive to noise waves but responsive to voice waves of at least a predetermined amplitude to disable the voltage producing means connected to the one-Way path transmitting both noise and voice waves.

5. The two-Way telephone system according to claim 4 in which each voltage producing means includes a condenser charged in an amount depending on the amplitudes of the waves in the respective outputs, and the combining means embodies means for rendering eifective the larger of the two charges and rendering ineiTective the smaller of the two charges.

6. The two-way telephone system according to claim 4 in which each voltage producing means includes a condenser charged to an amount depending on the amplitudes of the waves in the respective outputs and the charge on the condenser in the disabled voltage producing means remains substantially the same during the period of such disability.

KINGSBURY H. DAVIS.

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