US1772551A - Echo suppressor - Google Patents

Description

Aug. 12, 1930.

p. MITCHELL ET AL EGHO SUPPBESSOR Filed Feb. 2o, 1929 llllllli III indicates paling q/all' atta-des,

INVENTORS L/ v ATToNEY Patented Aug. 12, 1930 UNITED STATES -PATENT OFFICE DOREN MITCHELL, 0F NEW YORK, N. Y., AND HAROLD C. SILENT, OF LOS ANGELES, CALIFORNIA, ASSIGNORS TO AMERICAN TELEPHONE AND TELEGRAI'H COMPANY,

A CORPORATION 0F NEW YORK ECHO SUPPRESSOR Application filed FebruaryA 20, 1929. Serial No. 341,542.

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

For a discussion of some ot 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 Aknown 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.

y One object of the invention is to provide for reduction of the sensitivity of the echosuppressor switches in such a manner as to prevent false operation of these devices by noise in the transmission aths.

A second object of the 1nv'ention is to prevent the above-mentioned false operation, Without rendering the echo suppressor insuiciently sensitive to voice waves when no noise is present.

A further object of the invention is the reduction ofthe sensitivity of the echo-suppressor apparatus to noise currents without introducing improper interaction between the two sides of the echo-suppressor system.

In attaining the above stated objects, the applicants, in general, provide means responsive to noise in either side of the four-wire circuit for biasing the echo-suppressor relays, and means for preventing the biasing of the relays in response to voice Waves, and arrange the circuits designed to accomplish the control of the echo-suppressor relays in such a manner that neither side of the system acts falsely upon the other side.

The invention will now be more clearly understood from the following detailed description ot' Yone desirable embodiment thereof. This description is to be read with reference to the accompanying drawing, showing diagrammatically the circuits and apparatus involved.

With reference to the details of the drawing, there is shown a section of a four-Wire circuit having a path L1 adapted for transmission from west to east, for instance, and a second path L2 adapted for transmission from east to West. Connected acrcs the line L1 is an amplifier A1 which has its output connected tothe input of a detector D1. The left hand Winding of a relay R1 is included in the plate circuit of detector D1 and this relay controls other relays which directly accomplish the echo-suppressing function. Likewise there are associated with the line L2 and amplifier A2, a detector D2, and a relay R2, the relay having its left-hand windingc n- `nected in the plate circuit of the detec r. The relay R2 controls other relays which irectly accomplish the echo-suppressing ful ction. The relays controlled by relays R1 a d R2 are fully described in the above-identified article by Clark and Mathes; accordinglylthe apparatus controlled by the two relays in the detector circuits of the applicants arrangement are merely indicated in the drawing.

It is, of course, desirable that the echosuppressor apparatus described above be as sensitive as possible to voice waves in the four-Wire circuit. It is, however, equally desirable that the echo-suppressing apparatus be not operated in response to noise in the four-wire circuit.

The applicants add a right-hand winding to each ot' the relays R, and R2, these windings being so connected that current through them opposes the operating current from the detectors Dl and D2, that is, current in the right-hand windings biases the relays against operation. To control the current through these right-hand windings of relays R1 and R2, and consequently the sensitivity of the relays, the applicants employ the vacuum tubes VTl, RT, VT2, VTS, and VT4. y A relay R', condensers C1 and and resistances r3, m,

f5, and r6 are connected as disclosed in the drawing. The input of tube VT1, which is an amplifier tube, is associated with amplifiers A1 and A2 through a hybrid coil arrangement HC. The resistances 1", and r2, forming parts of the hybrid coil arrangement, are given such values that current passing from ampliiier A1, for instance., to the tube VTI, Will set up no false action in the side of the echo- .suppressor system connected across line L2 of the four-Wire circuit. It is to b understood that any suitable arrangemen known in the art may be substituted for-the hybrid coil arrangement described.

The operation of the applicants circuits `for controlling the sensitivity of relays R1 and R2 will now be considered. If voice waves act on tube VTI, the tube RT,which is a rectifier tube, is operated, and by reason of the connection shown and the action of condenser Cl and resistance r3 a negative po tential is impressed on the grid of the vaccuin tube VT2, which functions as a blocking tube. This negativepotential serves to reduce the current in the plate circuit of the tube VT2 and the condenser C2 begins to ydischarge through resistances r4 a uil T5. Since the voice Wave is rapidly fluctuating, the discharge of condenser C2 will usually be checked by a decrease of the amplitude of the input wave, which causes an increase of plate current in the blocking tube of VT2 and a re-charging of condenser C2. However, as the condenser C2 discharges through iesistances r4 and n, the grid of tube VT3 becomes less negative. It will be noted that the plate circuit of this tube VTa includes the aiasing windings of relays R1 and R2 and the normally closed contact controlled by relay R. With relay R unoperated, it is evicent that the driving of the grid of tube VT3 less negative increases the plate current in the tube and tends to bias relays R1 and R2 against operation by detectors D1 and D2, respec Lively. Accordingly, voice waves which represented somewhat sustained energy might, through the operation of the applicants system` cause a slight reduction of sensitivity in relays Rl and R2, which reduction is'not desired in response to such waves.

In order to prevent the above-mentioned false reduction of sensitivity in relays R1 and R2, the applicants employ the tube VT4 associated with a resistance re, the condenser C2, and the relay R. inoperative, due to the negative biasing voltage on the grid. 'Voice Waves of considerable amplitude, however, will drive the grid of the tube considerably less negative and the tube will pperate, causing the operation of relay R. This relay Ioperation breaks the circuit through the biasing windings of relays Rl and R2, and plate current in tube VT?` will have no effect on the sensitivity of relays R1 and Rg. l

If, however, the waves coming in from the four-wire circuit are not fluctuating voice waves but relatively steady noise, the condenser C2 will discharge slowly and fully through resistances r., and 1'5, and plate cir` rent will tend to How intube VT3 not only while the circuit is troken at relay R through operation of tube VT4 but also after tube VTl has ceased to operate through restored negative biasing voltage on its grid and the circuit has been re-closed at relay R. Thus it will be seen that while the fluctuating The tube VT4 is normally voice waves have no effect whatsoever on the sensitivity of the echo-suppressor relays, noise which is sustained will, after a certain time, cause the full biasing of the echo-suppressor relayis, and accordingly operation of the echosuppressor apparatus in response to noise is substantially prevented.

It will be noted that the biasing windings of relays Rl and Rg are so connected in the system that noise on either side of the fourwire circuit causes reduction of sensitivity in both sides of the echo suppressor system, and accordingly false operation df the echo suppressor by' noise is prevented.

While the invention has been described in one embodiment which is deemed desirable, it is to be understood that this description is merely for the purpose of illustration and does` not limit the scope of the invention, which is determined by the appended claims.

'hat is claimed is:

l. In a system for suppressing echoes in a two-way circuita voice-operated device associated with each side of the circuit, a relay controlled thereby and adapted to block the oppositeside of the circuit, and means responsive to noise in either side of the circuit for biasing both the relays to prevent operation thereof by the noise.

2. In a system for suppressing echoes in a two-way circuit, a voice-operated device associated with each side of the circuit. a relay controlled thereby and adapted to block the opposite side of the circuit, means responsive to noise in either side of the circuit for biasing both the relays to prevent operation thereof by thenoise, and Ineans for preventing the biasing of the relays in response to voice waves.

3. In a system for suppressing, echoes in a two-way circuit, a voice-operated device associated with each side of the circuit, a relay controlled thereby and adapted to block the opposite side of the circuit, means responsive to noise in either side of the circuit for biasing both the relays to prevent operation thereof by the noise, means for preventing the biasing of the relays in response to voice waves, and means for preventing interaction between the two sides of the echo-suppressor system due to the association there? with of the biasing means.

In testimony whereof, I have signed m name to this specification this 18th day ot' l `ebruary` 1929.

DOREN MITCHELL.

In testimony whereof, I have signed my name to this specification this 13th day of

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