US1951465A - Echo suppressor - Google Patents

Description

Maxl'ch 20, 1934. s. B. WRIGHT ET AL ECHO SUPPRESSOR Filed Aug. 19, 1930 2 Sheets-Sheet l 'lill'.i.-

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l INVENToRs f. @f/ Mdc/ed BY L ATTORNEY March 20, 1934- s. B. WRIGHT Er AL ECHO SUPPRESSOR Filed Aug. 19, 1930 2 sheets-sheet 2 .EO 5.3% Q EN www:

. L' lll INVENTORS 5.29. mgmmfa'/wa ATTORNEY Patented Mar. 20, 1934 UNITED STATES ECHO SUPPRESSOR Sumner B. Wright, South Orange, N. J., and Doren Mitchell, New York, N. Y., assignors to American Telephone and Telegraph Company, a corporation of New York Application August 19, 1930, Serial No. 476,422

4 Claims.

This invention relates to two-way transmission circuits, and more particularly to four-wire circuits and the arrangements often associated therewith for suppressing echoes.

5 For a discussion of some of the fundamental problems connected with the presence of echoes in two-way transmission circuits and for a description of echo-suppressing devices which are among those better known in the art, reference may be had to an article by A. B. Clark and R. C. Mathes, Journal of the American Institute of Electrical Engineers, June 1925, pages 618-626.

1n United States Patent No. 1,826,196, issued to L. G. Abraham October 6, 1931, there is disclosed the so-called terminal echo suppressor, which accomplishes generally satisfactory suppression oi echoes in long transmission circuits without the requirement of a hangover time of such length as to be undesirable.

In United States Patents Nos. 1,814,017 and 1,814,018, issued to S. B. Wright and D. Mitchell July 14, 1931, and in United States Patent No. 1,772,551 issued to D. Mitchell and H. C. Silent August 12, 1930, there is disclosed the so-called noise desensitizing echo suppressor, in which noise is rendered ineffective to operate the echo suppressor relays and yet the sensitivity of the device to voice currents remains unimpaired when no noise is present.

One of the objects of the present invention is to combine in a satisfactory echo suppressor arrangement the advantages of the terminal and the noise desensitizing Suppressors.

A further object of this invention is to provide an arrangement for suppressing echoes which is at once free from. false operation and excessive hangover and sufciently reliable to be satisfactory for use on transmission circuits having long four-wire sections.

These objects are attained by utilizing the terminal echo suppressor principle, employing one or more noise desensitizing circuits and employing a novel arrangement of relays and associated circuits, as will appear clearly from the ."2 description of the apparatus and its operation The disclosure of Fig. 1, in addition to showingthe iirst arrangement, helps to bring out the purpose,

advantages and arrangement of the modied system of Fig. 2. Wherever such designation appears helpful to the reader of the diagrams, like refer-v ence characters are used in the two figures of the drawings to designatecorresponding elements.

With reference to the details of the drawings, and first with particular reference to Fig. 1 thereof, there is shown a four-wire circuit having its upper two-wire path adapted for transmission from west to east, for instance, and its lower two-wire path adapted for transmission from east to west. tits west end the four-wire circuit is connected through the hybridV coil HC1 to the two-wire line L1, this line being balanced by the network N. At theY east end the four-wire circuit is similarly connected through the coil HC1 to the two-wire line L2, which is balanced by the network N. It will be understood that# the echo suppressing apparatus is located at or quite near the west end of the four-wire circuit in the one case, and at or near the east end of the fourwire circuit in the other case, and'that the circuit rnay be very long, with no intermediate echo suppressing devices.

In the transmission line L there are disclosed two one-way ampliers A1 at the west end and As at the east end; likewise, in the line L there are found the one-way amplifiers A1 at theV east end and A3 at the west end. It will be noted.

that on the output side of each of these four ampliers there is an arrangement of resistances, which may be quite small, and repeating coilsl and that the apparatus associated with thesev four amplifiers is also associated with the inputs of one-way ampliers A2, A4, Az and A4, respectively.

With reference to the above-identified disclosures of the terminal echo suppressor and the noise desensitizing echo suppressor, it will be seen that the arrangement of Fig. 1 of the present application combines the advantages of these two echo suppressing devices. When currents arrive at the west end of the circuit from the east end over' line L', relay R2 connected in the output of detector D2 will operate to placev a short circuit on the two-way line L unless earlier arriving cur-v rents flowing in the line L have broken the operating circuit through relay R2 through the action of detector D1 and relay R1. Likewise, it will be seen that the desensitizing circuit S functions to prevent the operation of either the line shorting relay R2 or the auxiliary control relay Rr in response to noise currentsr in either side of the four-wire circuit.

At theeast end of the four-wire circuit the echo suppressing apparatus is similar to that more fully disclosed in Fig. 1 as connected at the west end of the circuit. The detectors D1 and D2 corresponding to detectors D1 and D2 at the opposite end, are shown only schematically, and, likewise, the desensitizing circuit S', schematically shown, will be understood to correspond to the desensitizing circuit S at the opposite end.

, The most satisfactory understanding of the arrangement of Fig. 1 will be obtained, it is believed, from the following description of the operation of the circuits of that gure:

Transmission from line L1 at the west end of the four-Wire circuit to line L2 at the east end, passes through the amplifier A1 and on over the two-wire line L. At the output of amplifier A1 a voltage drop is caused in the small resistances connected in the line. This potential is stepped up through the repeating coils, is added to the voltage drop across the line,and. is applied to the input of amplier A2. It will be understood that these transformer arrangements at the output of the line amplifiers are such that any transmission passing into the lower branch of the four-wire circuit, due to slight unbalance through the hybrid coil HC1 will not affect amplifier A4.

From the output of amplifier A2 the energy passes into the detector D1 and into the desensitizing circuit S through the hybrid coil HC2. This coil, however, is so arranged that the output energy of amplifier A2 cannot pass through the coilin any appreciable amount to aifect detector D2. Detector D1 without the intervention of the other elements would, of course, operate in response to either voice or noise energy of sufcient intensity passing through amplifier A2 to actuate R1. The winding W1 of relay R1 is included in the output circuit of detector D1, and if. this winding is energized without the energization of either one of ,the other windings W2 and W3, the relay will operate to break the output circuit of detector D2 through winding W2 of relay R1 and winding W4, the operating winding of relay R2; Accordingly, given voice currents without considerable noise energy passing fromy West to east over line L, the disabling of the echo suppressor at the west end of the circuit is assured, and the talker connected with line L1 hasproper control of the transmission circuit. It Will be understood, of course, lthat if before the voice currents from L1 reach lamplifier A1, voice'currents passing in the opposite direction have reached ampliiier A3, detector D2 will have operated to energize winding W4 of relay R2, thus placing a short circuit on line L, and will have energized the winding W2 of relay R1 to bias this relay against operation by any current entering detector D1.

VIf the energy passing through amplier A2 from the west end of the circuit or A4 from the east end of the circuit is sustained noise, it causes the operation of the desensitized circuit S, described in some detail in the above-identified patent to Mitchell and Silent. It will be sufficient to state here that this circuit comprises the vacuum tubes VI1, VT2, VTs and VT4, the plate lament circuit of the last tube VT4 including the biasing windings Ws of relay R1 and W5 of relay R2. The condensers C and C1 and the corresponding resistance arrangement r and r1 operate in response to noise to cause an increased current ilow in the output of tube VT4, and the relays R1 and R2 are biased against operation by'detector D1 or detector D2, respectively. It will be understood that if the energy is .voice energy as distinguished from noise, the desensitizing circuit S operates in a manner to prevent the building up of a bias on the relays R1 and R2.

At the east end of the four-wire circuit the echo suppressor operation is quite similar to that described above in connection with the west end of the circuit. Voice currents arriving over line L and reaching the suppressor point before the arrival of voice currents from L2, operate detector D2, energize the operating winding W4 of relay R2 and the biasing winding W2 of relay R1, and, Without opposition, short-circuit the line L at the east end of the four-wire circuit. If voice currents from L2 reach the suppressor point first, detector D1 is operated, and the relay R1 operates to break the circuit through relay R2; thus control of the transmission circuit is assured to the talker connected with line L2. Noise reaching the hybrid coil HC2 from either direction operates the desensitizing circuit S to bias relays R1 and R2 against false operation.

Relay R2 and relay R2 should each have a small hangover time provided, corresponding to the time required for echoes to pass over short terminal circuits which may be connected to the four-wire circuit and which may be unequipped with echo suppressors. It is one of the chief advantages of the terminal echo suppressor arrangement, however, that such hangover time isvery short in comparison with that heretofore required. v While the arrangement of Fig. 1 described above will in all probability meet the echo suppressing requirements of the greater number of circuits, there is the possibility that certain troubles in the operation of the suppressor apparatus will arise due largely to the fact that the control of the West suppressor, for instance, through the associatedV desensitizing circuit will be broken when noise is cut off by the operation of the suppressor relay at the east end. The desensitizing circuit during this quiet period willffall back to a condition in which no appreciable biasing current ows in the windings of the relays. Accordingly, when the short circuit is removed at the far end of the circuit yand noise again comes on, the echo suppressing relay at the West end may be momentarily operated-before the desensitizing circuit has had a chance to build up again the biasing current in the relays, and the result will be a brief mutilation of speech. With this possibility of diculty in mind, the applicants disclose in Fig. 2 of the drawings a modified arrangement which will remove the difficulty;

With particular reference to Fig. 2 of the drawings, at the West end of the four-wire lcircuit the transmission lines L1, L and L', the hybrid coil HC1, the balancing network N, the one-way ampliers A1, A2, A3 and A4, and the transformer arrangements at the outputs of A1 .and A3, are in arrangement identical with that shown in Fig. 1 and discussed hereinabove. It will readily be understood that at the east end of the four- Wire circuit the arrangement of transmission lines and amplifiers and the arrangement of echo suppressor apparatus are similar to those disclosed in detail at the West end. Just as in Fig. 1, the two ends of the circuit correspond in these respects. Accordingly, the description of the arrangement and operation of the echo suppressor devices and'circuits will, in general, be conned to the west end of the circuit without confusion to the reader.

It will be noted that in the arrangement of Fig. 2 there are two desensitizing circuits S1 and S2, the former .one being associated with the amplifiers A2 and A4 through the peculiar hybrid coil device HG2. In addition there are employed a greater number of relays, with connections and functions to be described hereinbelow. As indicated above, the main object of the modification of the circuit oi Fig. 1 is to permit the desensitizing circuits at the west end of the four-wire circuit, for instance, to remain at a setting determined by the value of steady noise which has been arriving at this west terminal during a period when the transmission from the east end is cut ofi due to the echo suppressor action at the east end. In other words, when the noise comes on again, the echo suppressor at the west end is found in the condition in which it was placed by the noise from the east end at the time of the break in the transmission, and so there is prevented the false operation of the west echo suppressor by a renewed transmission of noise. It will be noted that this is the only respect in which the action the desensitizing circuit, as described above and in the patent to Mitchell and Silent, is changed or interfered with.

in view of the discussion of the circuits of Fig. 1 given above, it is believed that the arrangement and action oi the circuits of Fig. 2 will be readily understood from the following description of their operation:

Transmission outgoing from the west terminal of the four-wire circuit passes through ainpliiier A1, and energy is drawn off and passed through amplifier A2 to detector D1 and the upper desensitiz. ig circuit S1, it being understood that the hybrid coil HG2 has the function of preventing energy coming to it from either direction from passing beyond it in any substantial amount. Thus, in the case now being considered, the lower lesensitiaing circuit S2 and the detector D2 are unaffected.

When detector D1 operates, the operating winding W1 o relay R1 is energized. The energy passing into the desensitiaing circuit S1 has the action described above in connection with Fig. l;

- that is, if it is voice energy it has no appreciable effect on the relays, while if it is noise energy it operates to bias the relays against actuation. Let it be assumed ior the moment that the energy passing through amplier A2 is largely speech energy. Relay rs1 operates and closes a circuit through the winding oi a rela7 R5. This relay is given a suitable slow release time to prevent it from falling baci: between syllables. "in the rrawings t is shown accomplished by the action of the condenser C3 and the resistance 1'3. The function or" relay R5 is to remove, and hold removed for the proper period, the ground connection of the circuit including the winding of the suppressor relay proper R4, which, incidentally, is provided with a slow release time to take care oi" delayed echo in circuits connected to L1. The operation of relay R5, addition to removing ground from relay R4, causes the discharge of a condenser Cr and the consequent operation of a relay Re. relay Re is given a slow release time which equals 2d, when d represents the time required for an echo to travel the length of the four-wire circuit plus the length of the connected two-wire line L2 and any circuits connected thereto. The operation of relay Re removes one ground connection from the operating circuits of relays R7 and R8 associated with the desensitizing circuits S1 and S2, respectively. It will be noted that, as discussed more fully hereinbelow, a second ground is furnished for these relays R7 and R8 through the armature of relay R3; relay Re in removing the ground throughits armature provides for the release of relays R7 and R8 during the periods when noise energy from the east end ofthe circuit is cut off by the echo suppressor operation at the eastV end, and the lower desensitizing circuit S2 is consequently not receiving energy from the line L.

rIhe relays R7 and Rs are arranged so that nornially they are in operated condition and through their armatures provide for the normal connection of condensers C1 and C2 with the resistance arrangement r1 and T2 in the one case, and the normal connection of condensers C1 and C2 with resistances ri and r2 in the case or" the lower circuit. When, however, rrelays R7 and Rs are in any way released, these condensers are disconnected from their respective resistances, and, as will be understood by those skilled in the art, the voltages on the grids of the tubes VT4 and VTC; will be maintained at t eir last values. Accordingly, while these two relays are released no change of any appreciable amount can be made in the biasing current furnished by the two desensitizing circuits to the relays.

It has been stated above that the operation of relay Re removes one ground from the operating circuits of relays R7 and R8, and that the other ground is furnished through the armature oi relay R3. Relay Ra has two windings W7 and We. Winding W7 is included in the output circuit of tube VTi ci the lower desensitizing circuit and causes a bias to be applied to the relay which is normally not quite sufficient to release the armature and break the ground contact. Winding We is included in the output circuit of the lower detector D2 and is normally suijciently energized to hold relay Re operated. It should be stated at this point that, preferably, provision is made for the operation of relay R3 when no noise whatever is present in the line L; as will be understood by those skilled in the art, this bias may be provided by suitable voltage on the grid of the detector tube D2, or by any other suitable means.

Let it be assumed now that noise is coming over the line L from the east end oi the fourwire circuit. This Vnoise energy passes through amplifier A4 and enters both of the desensitizing circuits. Through the operation or" the circuit S1 a bias is built up in the winding W3 of relay Ri and winding W5 of relay R2. through the operation of the circuit S2 the biasing action described above it set up in windings W7 of relay R3. These noise currents, of course, affect detector D2 as well as the desensitizing circuits, and the current in winding We of relay Rs will be sufficient to hold that relay operated. The current in the other winding of the D2 output circuit, W4 of relay R2, however, will not be suiiicient to cause the operation of that relay. When speech energy passing from west to east over line L reaches the east end of the four-wire circuit, the echo suppressing relay (corresponding to R4) will disable the line L', and the noise energy passing toward the west end of the four-wire circuit will be momentarily cut off. Relay Re, however, has been operated as described above and is held in operated condition by the hangover action described above; accordingly, the right-hand ground for the operating circuits of relays R7 and R8 is removed. When the noise in line L ceases, the current from detector D2 will cease immediately. The current from VT4 will not drop down so quickly, and

Likewise,

tent, because the action of detector D2 and relay R3 takes place in a time which is short compared to the time required for the charging of condensers C1 and C2.

From the above discussion, and from a consideration of the hangover time of relay Rs, it will be understood that during a period of time equal to the longest quiet interval which can occur due to the suppression of noise by the speech passing over line L, relays R7 and R8 will be re leased, and the proper bias to prevent subsequent false operation-of the echo suppressor by noise will be maintained on the relaysVK at the west end of the four-wire circuit.

It should be noted that if noise energy .passes from west to east over' line L, it cannot cause the release of relays R7 and Rs by itself, even though relay Re has operated to remove the righthand ground from the operating circuits oi relays R7 and Rs: Ythis is true because no substantial amount of this noise energy can pass the coil HG2 and enter the lower desensitizing `circuit YS2 to release relay R3 and remove the left:- hand ground. This point makes it clear why it is desirable to use the two desensitizing circuits: if the circuit S1 were made to periorm the function of Sz' as Well as its own, it would be possible for noise currents lto build up a bias in the Winding W7 of relay R3 which would cause the release or" that relay during the operation of relays R1, R5 and Re. This would result in the release of relays R7 and Ra while relay R5 was held operated by the noise currents in L, and the ground connection over the armature of relay R5 would be broken and the operation of the suppressor relay R4 prevented in response to the voice currents in L'.

Furthermore, it should be noted that noise en ergy passing from east to west in line L' will never cause the operation of relay R1 because the hybrid coil HG2 prevents this energy from entering the detector D1. Accordingly, even though relay R3 is released because of the momentary decrease of noise, relays R7 and Re will be held operated through the released condition ci? relay Re (controlled by relay R1 through relay R5) which maintains the right-hand ground connection for relays R7 and Ru.

It remains to point out one other feature of the arrangement shown in Fig. 2. If the four-wire circuit shown is connected through the two-wire circuits to other four-wire circuits, it may be necessary to make the slow release time of relay Re quite longfor instance, .5 second. In such a case it may be desirable to provide for the canceiiation of the slow release period of relay Re whenever the speech energy or the noise energy passing from east to west over line L has an eiTect on the relays at the Westend of the circuit which is stronger than that due to the biasing current built up by the desensitizing circuits. This cancellation may be accomplished with the arrangement shown in the drawings, whereby an extra armature and an extra contact are added tol relay R2. When relay R2 operates due to the conditions just discussed, this right-hand contact places a short circuit around the resistance r4' forming a part of the slow release circuit of relay Re.

While the invention has been disclosed in two specific embodiments, the second being in the nature of a modification of the rst, it is to be understood that such specific disclosure is not intended to limit thescope of the invention, which is determined by the appended claims.

What is claimed is:

l. In a four-wire circuit including paths adapted for transmission in opposite directions, an echo suppressor relatively near each terminal of the four-wire circuit adapted to disable the outgoing path in response to voice currents in the incoming path, means responsivev to noise currents for desensitizing said echo suppressor, and means responsive to the disabling of the incoming path by echo suppressor operation at the distant end thereof for maintainingon the near echo suppressor the desensitized effect produced by said desensitizing means.

2. In a four-wire circuit including paths adapted for transmission in opposite directions, echo apparatus comprising means for disabling the suppressing and associated apparatus relatively near each terminal of the four-wire circuit, said outgoing transmission path in response to voice 'currents in the incoming path, controlling means responsive to voice currents in the outgoing path for preventing the operation of said disabling means by voice currents in the incoming path of later arrival, means responsive to noise currents in either path for desensiti'zing said disabling means and said controlling means, and means responsive to the disabling of the incoming path by echo suppressor` operation at the distant end thereof for maintaining on said disabling means and said controlling means the desensitized effect produced by said desensitizing means.

3. In a four-wire circuit including paths adapted for transmission in opposite directions, the method oi transmission control which consists in causing voice currents in the incoming path at either end of the circuit to disable the outgoing path at that end, provided said voice currents are or" earlier arrival at that end than voice currents in the outgoing path at the same end,

causing noise currents in either path to set up a counter action to prevent false interruption of transmission, and maintaining the counteracting effect at either end of the circuit during a period when transmission is cut oi at the distant end thereof.

Li. lin a four-Wire circuit including paths adapted for transmission in opposite directions, an echo suppressor relatively near each terminal of the four-Wire circuit adapted to disable the outgoing path in response to voice currents in the incoming path, means responsive to noise currents for desensitizing 'said echo suppressor, means responsive to the disabling of the incoming path by echo suppressor operation at the distant end thereof for maintaining on the near echo suppressor the desensitized eiiect produced by said desensitizing means, and means responsive to substantial increase of amplitude of the incoming currents for terminating the operation of said maintaining 14 means.

SUMNER B. WRIGHT. DOREN MITCHELL.

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