US1740856A - Volume regulation in two-way telephone circuits - Google Patents

Description

Dec. 24, 1929. .1; 'HERMAN Er AL y VOLUME REGULATION IN TWO-WAY TELEPHONE CIRCUITS Filed oct. 15, 1927 www m gx IT NN., m N mf ww m Pl) mv Hy JB Nimby. |L Q wm .\P.wm l .QUI l vided to balance the two-wire line.

Patented Dec. 24, 1929 1 UNITED STATES PATENT OFFICE JOSEPH HERMAN, or 'wi-:STRIRLH AND AsUMNRR B. WRIGHT, or soU'rH ORANGE, NEW

JERSEY, AssrsNoR-s To AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A oon- ?ORATION OF NEW YORK VOLUME REGULATION IN TWO-WAY TELEPHONE CIRCUITS Application led October 15, 1927. Serial No. 226,440.

` This invention relates to two-way signaling systems, and more particularly to twoway telephone systems including a four-wire circuit having one path adapted for transmission in one direction, a second path adapted for transmission in the opposite dinected to one end of the :tour-wire circuit through a hybrid coil, and a network is pro- At its other end the four-wire .circuit may be connected, as in the case of all-wire systems, di-

rectly to another two-wire circuit through a.

hybrid coil. There may, however, be an intervening radio link forming a part of the four-wire circuit. In such a case, the fourwire 'circuit includes apparat-us for sending and receiving .radio signals over the radio link in the two directions of transmission. For the purpose of clear description, the term four-wire circuit will be used hereinafter to denote that section of the two-way system which lies betweena two-Wire circuit and that portion of the four-wire circuit which, in the case of transmission partly by radio, is formed by the radio link.

Since the balance of a two-wire line and the associated balancing network is not perfeet, there is the likelihood that echoeswill occur due to -the reflection from the two-wire line over the transmitting path of the fourwire circuit of currentsflowing to the twowire line over the receiving path of the fourwire circuit. Accordingly, it is usually desirable to employ means for disabling one path of a four-wire circuit while transmission is taking place over the opposite path. A pparatus which operates in response to voice currents in the paths of the four-wire circuit to prevent echoes and also to prevent. singing in the four-Wire circuit has been designed, as is disclosed in an article by A. B. Clark and R. C. Mathes, published in the Journal of the American Institute of Electrical Engineers for June, 1925, pages 618 to 626.

These voice operated devices will operate properly only if the voice currents in the paths of the four-wire circuit are of sufficientv amplitude. It' the voice currents are too weak and the voice operated devices fail to operate, transmission over the two-way system, as will readil be understood, is serious ly interrupted. n the other hand, it will be understood that the volume of the voice current must not be so great as to cause overloading in the system. In order to prevent excess or deficiency of volume, it has been necessary heretofore that a technical operator be constantly in attendance on each fourwire circuit to monitor the circuit and make the necessary adjustments.

The principal object of this invention is to eliminate the requirement of constant monitoring for the adjustment of volume in the paths of the four-wire circuit.

A further object of the invention is to increase the speed with which the volume regulation can be effected.

In the accomplishment of these objects, the applicants connect, at each end of the section of the two-way system which lies between two hybrid coils, a potentiometer into each Apath of the four-wire circuit ahead of one of the amplifiers, provide for thev automatic maintenance of suitable input voltage for the two amplifiers under normal conditions and provide for the automatic reduction of vthe amplifier input voltage in the transmitting path in response to voice currents in that path 'of excessive amplitude and the simultaneous increase in a corresponding amount of the input voltage of the amplifier in the receiving path. The applicants further provide for the automatic prevention of change in the setting of the potentiometer when talking has ceased over the transmitting path and is taking place over the receiving path. f

One of the principal advantages of the applicants arrangement is that it automatically' which is suitable for, the proper operation of echo suppressors and similar voice operated devices in the circuit.

Another advantage of the arrangement is that it provides for a compensating adjustment of the volume in the receiving path of the four-wire circuit, thus preventing` harmul effects of echo currents under certain conditions of noise and balance.

These and other advantages of the invention will be more clearly understood when the following description of one desirable lembodiment of the invention -is read with reference to the accompanying drawing.

The drawing shows diagrammatic-ally a fore-wire circult (the term denoting that scction 'of the two-way system stated above) 'having a two-wire line connected thereto and having applied to it suitable apparatus for controlling the volume in the two paths in accordance with the invention. `It is to be understood that at the other end-0f the section of the two-way system which lies between the hybrid coil shown and the next hybrid coil to the right, the apparatus employed may be identical with that shown in the drawing and the operation thereof similar to that described'hereinafter.

With reference to the details of the drawing, the -four-wire circuit is connected at its left end through a hybrid coil to a two-wire line which leads to a telephone set T. A network is provided to effect an approximate balance with the two-wire line in the hybrid operation of relay S which places a'short circuit on the receiving path L2. The line L1 is normally disabled by a short-circuit under the control of-'relay S in order to preventl singing. When voice currentsV from T pass over L1, it is necessary that this path be cleared, and accordingly the amplifier AD is designed not only to operate relay S but also to operate S to remove the short circuit.

If no voice currents are passing over the line L1 and a talker at the distant end of this circuit sends vvoice currents into the path L2, these currents operate the amplifier-detector AD and cause. the o eration of relay S which places a. short-circuit on the path L1 and temporarily prevents the operation ofthe amplifier-detector AD. It will be understood, ot course, that it is desirable to insert networks in the two paths of the four-wire circuitin order to provide the necessary de lays to permit the operation of the echo and singing suppressor apparatus before the voice currentsreach theshortcircuit points of the transmission paths; in theabsence ot such delay circuits or networks, clipping of the voice is likely to occur.

The applicants connect a potentiometer P into the transmitting path lL1 ahead of the amplilier A and a secondl potentiometer P into the receiving path L2 ahead of the amplilier A. These two potentiometers, in one suitable arrangement, are concentric, and are otherwise positioned with respect to each other and are provided with steps as clearly shown in the drawing. A slider G associated with P and a slider F associated with P are mounted on a common shaft H and are radially spaced in fixed relation toeach other,

as shown. These two sliders are'insulatedy from each other, as is indicated in the drawing by the showing ofthe shaft H. A third slider E is mounted on the shaft H and is arranged to make contact with a stud c when slider G is on step 30 of P and with'stud d when G is 'on step 0. As will appear hereinafter, the relative positions of the potentiometers and sliders should be adjusted in accordance with the prevailing conditions of static or other noise. The shaft H of the potentiometers is driven through suitable gears by a shunt motor. The field winding of lthis motor is connecteddirectly to a battery B3, while the armature of the motor is connected to this battery through the contacts ofv a relay S3. As will be more fully disclosed below, relay S3 is designed to reverse the armature connections and, as a result, to reverse the direction of rotation 'of the potentiometer sliders. A

It willbe noted in the drawing that a counter-clockwise rotation of the sliders G and F .increase the resistance through that part ot yously provides for a decrease of the input voltage of amplifier A. It is understood, of course, that when slider G is on step 30 of P, the setting provides for the maximum input voltage of amplifier A.

A detector D is connected across the line L1 at they output side ofamplier- A. This detector D is adjusted to operate only when the amplitude of the voice currents in the transmitting path exceeds av 'predetermined safe value, and vcontrolsthe relay S1. Relay S1 in turn controls the operation of the motor through a relay S2 and the relay S3.Y Relay S4 is designed to interrupt the operationof the ly slider E makes Contact with stud c. The' relay S5, also, has the function of interrupting the operation of the motor-in combination with relay Si and under the control of the relay S6, which last relay is. in turn controlled by the amplifier-detector AD, responsive to voice currents in the receiving path L2.

lt is believed that further understanding of the regulator apparatus will be best obtained from the following description of the operation of the regulator.

Tf no voice currents are being transmitted over the circuit, the position of the relays S1 to S6 is that shown in the drawing. The motor armature is connected to battery B3 through a resistance R1. The motor operates slowly to cause the slow rotation of the potentiometer sliders in the counter-clockwise direction. The slow operation ofthe motor is due to the inclusion in the armature circuit of the resistance R1. This rotation of the sliders brings the slider G to step 30 on potentiometer P and causes slider E, which is connected to ground as indicated on the drawing, to make contact with stud c`it being assumedthat the counter-clockwise rotation is not checkedin the meantime. When slider E makes contact with studc, relay S4 operates and opens the armature circuit of the motor to prevent further counter-clockwise rotation of the potentiometer sliders. Under the nornal or non-transmitting condition of the circuit, which has been assumed thus far, the transmitting path L1 of the four-wire circuit will at this vpoint in the operation of the regulator be adjusted for maximum gain, While the receiving path L2 will be adjusted for some intermediate gain, for reasons to be discussed hereinafter. For the 'purpose of illustration, the

sliders G and F are shown in the drawing as radially spaced to bring slider F to step 15 of P when slider G is on step 30 ofP.

-. Let it now be assumed that a person begins to talk at the telephone 'set T. lSince potentiometer P is set for the maximum input voltage of amplifier A, the amplitude of the voiceA currents at the output of A will, at times, be larger than is necessary for satisfactory communication. In the case of aloud talker the amplitude may even be so large as to overload the amplifier A or other amplifiers in the circuit. Therefore, it is desirable to provide for the reduction of the gain in line L1 to a value which will not cause overloading; but, the gain must still be sufficiently great to produce satisfactory operation of the voice operated echo and singing Suppressors. Accordingly, the applicants connect the detector D,

(which is adjusted to operate only when theamplitude of the voice currents is excessive) as shown, to control the relay S1. When the voice currents are of excessive amplitude, this detector operates and causes the armature of relay S1L to go to its right-hand contact. This operation of relay S1 causes the operation of relay S2 which, in turn, causes the operation of S3. Relay S2, upon operation, moves its armatures to the right-hand contacts and causes the reversal of the armature connection of the motor connecting the motor armature directly to battery B3 and disconnecting the resistance R1 from the circuit. A clockwise rotation of the potentiometer sliders is thereby produced, and this rotation, by decreasing the resistance of the part of the potentiometer P which is in parallel with the input of amplifier A, tends to reduce the gain. This clockwise rotation of shaft H continues until the output of amplifier A is reduced to the point at which the detector D ceases to operate and relay S1 is released, this release causing the release of relays S2 and S3.

It is to noted at this point that relay S2 releases slowly because of the presence in this operating -circuit of resistance R3. and condenser C. As a consequence of this slow release feature, this relay S2 will not follow the rapid fluctuations of the voice, as does relay S1, but will remain operated While the armature of relay S1 moves rapidly back and forth between its contacts. Relay S2 will be released only if the armature of relay S1 remains on its left-hand contact for a certain length of time.

As soon as relay S3 has been released by relay S2, the direction ofoperation of the motorwill be reversed and the rotation of the potentiometer sliders will again be in the counter-clockwise direction. The' operation of the motor and the counter-clockwise rotation of the sliders will nov1 be at a speed considerably slower than that at which the opposite rotation took place, since the resistance 1 is once more connected in series with the motor armature. The operation of the motor and the slow counter-clockwise rotation of the sliders continue, causing the Aprovision of a gradual increase in the input voltage of amplifier A, until relay S1 is again operated by the detector D in response to voice currents of excessive amplitude. The operations for reducing the gain in line L1 will then be repeated.

With a suitable adjustment of the detector D, the arrangements described above produce i reaches step 30, relay S4 is operated due to the contact between slider E and stud c, as

explained above, and when the slider reaches step O; relay S5 is operated by the contact made between slider E `and stud d. These two relays are thus used to prevent rotation vof the sliders beyond the limits of t-he p o- Slider moves gradually toward step 30 and will reach that step unless the talking is resumed in the meantime.

If voice currents /of sufficient amplitude arrive over the receiving path of the circuit p after the person at T stopstalking, the amp plifer-detector AD" will e energized. This will cause the operation of relay Se which will in turn operate relays S4t and S5', thereby opening the armature circuit of the motor and preventing any change in the settings of the potentiometers.

The adjustment of potentiometer P in the receiving path of the circuit simultaneously with that of P in the transmitting path is f required becauseof the effects of the echo currents mentioned above. This feature will now be discussed in detail.

Potentiometer P is so designed that the step corresponding to maximum gain (step.

30) is the step at which it is desired to operate this branch of thecircuit. However,

this is not always possible, especially if pol it will cause false operation of this device and will thereby prevent satisfactory communication. As has been indicated above, the amplifier-detector AD is adjustedto operate only in response to currents of a certain or a greater amplitude. Just as voice currents of suiiicient strength will operate the amplifier-detector while voice currents of a lower amplitude will not,'s'o noise currents- (especially static) of a certain or a greater amplitude will operate the amplifier-detector while small noise currents will not. As long as the noise currents, static currents for instance, are of suiiicient amplitudeto operate this amplifier-detector AD, the relav S" -will operate and the transmission pathL,

will be blocked; accordingly, the echoes of the noise currents, which echoes are returned from the two-wire line over the path L1, cannot reach the amplifieredetector AD. There may, however, be smaller nolse currents entering path- L2 which willnot operate thev amplifier-detector AD and the relay S controlled thereby. If, now, these noise currents are echoed back over the transmission path L1 and if the volume regulation in the path L1 is such that there is. a great amplification in that ath, it may well be that the echoed and amplliiied noise currents will operate the amplifier-detector AD, since the path L1 is not blocked by the relay S. This false operation of the amplifier-detector AD may be prevented if the currents in the receiving path L2 are kept suiiciently low in amplitude.so that the amplication in the ath L1, when thev currents are reflected back over that path from the two-wire line, will not bring the reflected currents to the amplitude which is required for the operation ofthe amplier-detector AD. Accordingly, whenever the gain in the transmitting path L1 is increased beyond a certain value, the

gain inthe receiving path L2 must be decreased a corresponding amount, in order to keep the amplitude ofthe reflected currents at the input to the amplifier-detector AD at a safe value. v

Because of the fact that the amplitude of static (or other noise) varies from time to time, the relation between the gainv settings of the two paths of the circuit must also be varied. The diagram shows a relation such that step on the potentiometer P corresponds to step on the potentiometer P. If the amplitude of the static should increase it would be necessary to change the relative positions ofrings P and P or of sliders G and F so that step 20 on P corresponds to a step lower than 25 on P. Simi-4 larly, a decrease in the static permits of a relative position of the two potentiometers such that steps 20 on P corresponds to a step higher than 25 on P.

The description thus far has beenl on the assumption that the relative ositions of P and P and the sliders are etermined entirely by the amplitude of the static. This would be true if the condition of balance between the two-wire line and its balancing 1 dition of balance must be assumed, unless a c hange in adjustment is made from time to time as the balance changes, and the relative positions ofthe two potentiometers will then be determined only bythe' amplitude of the balance is assumed, a larger margin against static. Obviously, if the worst condition of false operation from static will result when the balance is good than when it is poor.

Except for this setting based upon the amplitude of static, 'which must be made manually Whenever the vadjustment of the receiving voice operated device AD is altered, the operation of the regulator will be entirely automatic, as Will be clearly understood from the above description.

While the invention has been described for th-e purpose of illustration as embodied in one specific form which is deemed desirable, itis to be understood that it is capable of embodiment in many other and -Widely different forms Within the scope of the invention as defined by the appended claims.

What is claimed is:

1. In a two-Way telephone system, a circuit having a irst path adapted for transmission in one directionand a second path adapted for transmission in the opposite direction, means in each of saidpaths for controlling the amplitude of voice currents therein, and means responsive to voice currents in said first path of amplitude in excess ofl a prede.- termined value for effecting a reduction of the gain in sai'dpath and simultaneously effectling an increase ofthe gain in said second `pat 2. In a'tWo-Way telephone system, a circuit having a first path adapted for transmission in one direction and a second path adapted for.. transmission in the opposite direction, means in-each of said paths for controlling the amplitude of voice currents therein, means responsive to voice currents in said first path of amplitude in excess of a predetermined value for effecting a reduction of the gain in 'said path and simultaneously effecting an increase of the ,gain in said second path, and means responsive to voice currents in-said second path for preventing change of gain in either path. 3. In a two-Way telephone system, a circuit having a first path adapted for transmission in onedirection'and a second path adapted .for transmission in the opposite direction, -means associated with each of said paths and responsive to voice currents therein for disabling the opposite path, means in each of said paths or'controlling` theamplitude of voice currents therein, and means responsive to voice currents in said first path of ampli tude in excess of a predetermined value for eii'ecting a reduction of the gain` in said path land simultaneously effecting an increase of the gain 'in said second path.

4. In a two-way telephone system, a circuit having a first path adapted'for transmission in one direction and a second path adapted for transmission in the opposite dlrectlon, means associated with each of said paths and .responsive to voice currents therein for disjf'abling' the opposite path, means in each of said paths for controlling the amphtulde of -each of said voice currents therein, means responsive to voice currents in said first path, of amplitude in excess of a predetermined value for effecting a reduction of the gain in said path and simultaneously eiiecting an increase of the gain in said second path, and means responsive to voice currents in said second path for preventing change of gain in either path.

5. In a two-way telephone system, afourwire circuit having a first path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction, means in each of said paths for controlling` the amplitude of voice currents therein, and means responsive to voice currents in said first path of amplitude in excess of a predetermined value or effecting a reduction of the gain in said path and simultaneously effecting an increase of the gain in said second'` wire circuit having a first path adapted for 'transmission in one direction and a second path adapted for transmission inthe opposite direction, means in each of said paths for controlling the amplitude of voice currents therein, means responsive to voice currents in said first path of amplitude in excess of a predetermined value for effecting a reduction of the gain in said path and simultaneously effecting an increase of the gain in said second path, and means responsive to voice currents in said second path for preventing change of gain in either path. a

7. In a two-way telephone system, a fourwire circuit having a first path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction, means associated with each of said paths and responsive to voice currents therein for disabling the opposite path, means in paths for controlling the amplitudepf voice currents therein, and means responsive to, voice currents in said first path of amplitude in excess of a predetermined valueI for, effecting a reduction of the gain in said path and simultaneously effecting an increase of the gain in said second path.

8. In a two-way telephone system, a -four- Wire circuit having a first path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction, means associated with each of said paths and responsiveto .voice currents therein for disabling the opposite path, means in each of said paths for controlling the-am- Y .plitude of voice currents therein, means remeans responsive to voice currents in said second path for preventing chan e of ain 1n elther path. g

` 9. In a two-Way telephone system, a fourwirecircuit, a two-Wire line connected thereto, said four-wire circuit having a first path adapted for transmission from the two-wire line and a second path adapted for transmission to saidline, means in each of saidp'aths for controlling the amplitude of voice currents therein, and means responsive to voice currents in said vfirst path of amplitude in excess of a predetermined. value for eecting a reduction of the gain in said path and simultaneously etecting'an increase of the in in said second path.

10. In a two-,way telephone system, a fourwire circuit, atwo-wire line connected thereto, said four-.wire circuit having a first path adapted for transmission `from the two-wire line, a second path adapted for transmission to. said line and means in each of said paths for controlling the amplitude of voice currents therein, means responsive to voice currents in said first path of amplitude in excess of a predetermined value for eecting are-` -duction of the gain in said path and simultaneously eectingan increase of the gain in said second path, and means responsive to voice currents in said second /path for preventing change of gain in either path.

11. In a two-Way telephone system, a fourwire circuit, a two-Wire line connected thereto, said four-wire circuit having a first path adapted for transmission from 'the two-Wire line, a second path adapted for transmission to said'line and means in each of said paths for A.controlling the amplitude of voice currentsV therein, means associated with each path of the four-Wire circuit and responsive l to voice currents therein for disabling the opposite path, and means responsive to voice currents in said first path of amplitude in excess of a predetermined value for effecting a reduction of the `gain in said path and simultaneously effecting an increase of the gain in said second path.

12. In a two-way telephone system, a fourwire circuit, a two-wire line connected thereto, said four-wire circuit having a irst path adaptedI for transmission from the two-wire line, a second path adapted for transmission 'to said line and means in each of said paths for controlling the amplitude of voice currents therein, means associated with each path ofthe four-wire circuit and responsive to voice currents therein for disabling the opposite path, lmeans responsive to voice currents in said first path of amplitude in excess of a predetermined value for eiecting a reduction of the'gain in `.said path and'simultaneously eiectingan increase of the in said' 'sccond'path, and means res nsive to voice currents in said second pat for preventin change of gainin either-path.

13. -a two-waytelephone system, a fourwire circuit, a two-wire line connected thereto, said four-wire circuit having a first path increase in the input voltage ofthe amplier in the second path, and means responsive to voice currents in the second path for preventing change of gain in either path.

14. In a two-Way telephone system, Wire circuit, a two-wire thereto, said four-Wire circuit having a rst path adapted for transmission from the twowire line, a second path adapted for trans mission to said line and a one-Way amplifier in each path, means associated with each path of the four-wire circuit and responsive to voice currents therein for disabling the opposite path, and means responsive to voice currents in said iirst path of amplitude in excess oa predetermined value for effecting a reduction in the input voltage of the ampliier in said path and simultaneously eiiecting an increase in the input voltage of the ampli- Iier in the second path. A

-15. In a two-Way telephone system, a foura fourampliiier in' line connected wire circuit', a two-Wire line connected there- 1f the amplifier in the second path, and means responsive to voice currents in the second path for preventing change of gain in either path. .Y

- 16.` In a two-way telephone system, a four- Wire circuit, a two-wire line connected thereto, said four-wire circuit having a first path adapted for't'ransmission from the two-wire line, a second path adaptedfor transmission to said line and a one-wa amplifier in each path, means for normal y eiecting an increase in the input voltage of the amplifier in said first path and simultaneously effecting a reduction in the input voltage of the amplifier in said second path, and -means responsive to voice `currents in said first path of amplitude in excess of a predetermined value .for checking said voltage changesand effect'- mg amplifier inthe first path and simultaneously effecting an increase in the input voltage of the amplifier in thev second path.

a reduction in the input voltage of the 17. In a two-Way telephone system, a fourwire circuit, a two-wire line connected thereto, said four-Wire circuit having a first path adapted for transmission from the two-wire line, a second path adapted for transmission to said line, and a one-Way amplifier in each path, means for normally effecting an increase in the inputy voltage of the amplifier in said first path and simultaneously eecting a reduction in the input voltage of the amplifier in said second path, bmeans responsive to voice currents in said first path of amplitude in excess of a predetermined value for checking said voltage changes and effecting a reduction in the input voltage of the amplifier in the first path and simultaneously effecting an increase in the input voltage of the amplifier in the second path, and means responsive to voice currents in the second path for preventing change in the input voltages of the amplifiers other than changes due to variations of voice strength.

18. ln a two-Way telephone system, a four- Wire circuit, a two-Wire line connected thereto, said four-Wire circuit having afirst path adapted for transmission from the two-Wire line, a second path adapted for transmissionto said line and a one-Way amplifier in eachv path, a first potentiometer connected into said first path ahead of the amplifier therein, a second potentiometer connected into said second path ahead of the amplifier therein, means for normally effecting an increase of the resistance of that part of said first potentiometer Which is in parallel with the input of the amplifier in the first path and simultaneously effecting a decrease of the resistance of that part of said second potentiometer which is in parallel with the input of the amplifier in the second path, and means responsive to voice currents in'said first path of amplitude in excess of a predetermined value for checking saidehanges of resistance and effecting changes in the opposite direction.

19. In a two-way telephone system, a fourwire circuit, a two-Wire line connected thereto, said four-wire circuit having a first path adapted for transmission from the two-Wire line, a second path adapted for transmission to said line and a one-Way amplifier in each path, a first potentiometer connected into said first path ahead of the amplifier therein, a second potentiometer connected into said second path ahead of the amplifier therein, means for normally effecting a gradual increase o the resistance of that part of said first potentiometer which is in parallel with the input of the amplifier in the first path and simultaneously effecting a gradual decrease of the resistance of that part of said second potentiometer which is in parallel with the input of the amplifier in the second path, and means responsive to voice currents in said first path of amplitude in excessof a predetermined value for checking said gradual changes of resistance and effecting changes in the opposite direction, said last stated changes being gradual but more rapid than the normal changes.

20. In a two-Way telephone system, a four- Wire circuit, a two-Wire line connected thereto, said four-Wire circuit having a first path adapted for transmission from the two-Wire line, a second path adapted for ktransmission to said line and a one-Way amplifier in each v path, a first potentiometer connected into said first path ahead of the amplifier therein, a second potentiometer connected into said second path ahead of the amplifier therein,

.means for normally effecting an increase of the resistance of that part of said first potentiometer which is in parallel with the input of the amplifier in the first path and simultaneously effecting a decrease of the resistance of that part of said second potentiometer which is in parallel with the input of the amplifier in the second path, means re- JOSEPH HERMAN. SUMNER B. WRIGHT.

October, 1927.

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