US2191375A - Noise suppression apparatus for telephones - Google Patents

Description

Feb. 20, 1940. B. FISHER 2,191,375

NOISE SUPPRESSION APPARATUS FOR TELEPHONES Filed June 2, 1939 Patented Feb. 20, 1940 UNITED STATES NOISE SUPPRESSION APPARATUS FOR TELEPHONES Bergie L. Fisher, Martinsville, Va. Application June 2, 1939, Serial No. 277,106

13 Claims.

This invention relates to a noise suppression system for telephone lines, generally, and in particular to open wire ground return telephone lines in rural communities afiected by power induction and static disturbances. The invention is also applicable to metallic trunk lines between exchanges where the length of the trunk line is comparatively short.

An object of this invention is to provide a relatively inexpensive noise suppression system for telephone lines of the open wire ground return type or the short trunk line type.

Another object of this invention is to provide a noise suppression system for telephone lines adapted to utilize the commercial alternating current as a source of power.

A further object of this invention is to provide.

a noise suppression system for telephone lines adapted to impress the voice currents upon the telephone line at high transmission levels to obtain a high voice to noise ratio without the use of circuits that will produce troublesome sustained oscillations.

Still another object of this invention is to provide a noise suppression system for telephone lines, said noise suppression system employing a high gain amplifier tube for amplifying to a high transmission level the voice currents impressed upon the telephone line, said amplifier tube having the power circuits thereof energized from a commercial lighting circuit or similar power source which is also connected to the telephone line for bell ringing purposes.

A further object of this invention is to provide a noise suppression system for telephone lines employing a high gain amplifier tube for amplifying the voiced currents impressed upon the telephone line to a high transmission level to obtain a high voiced to noise ratio, said amplifier tube being adapted to have the power circuits thereof energized from a commercial power or lighting circuit and being adapted to have its anode or output circuit connected to said power source through a filtering impedance or choke coil which is also adapted to function as animpedance coupling for coupling said anode or output circuit to a telephone or transmission line.

Another object of this invention is to provide a noise suppressor or amplifier adapted for use on a telephone line and employing a vacuum tube amplifier which is adapted to be connected to the telephone circiuit through resistance networks, said resistance network being arranged to prevent sustained oscillations in the circuits of arrangement for the suppression of noise in telephone lines particularly the open wire ground return type of telephone-lines and also in telephone circuits employing comparatively short trunk lines between exchanges. The apparatus of my inventionis'comparatively simple, reliable in operation and inexpensive which makes it feasible for use in rural communities where telephone lines are afiected by power induction and static disturbances. The telephone set employed in accordance with this invention is provided with one or more amplifier tubes having the microphone or transmitter connected to the input and the telephone line connected to the output for amplifying the signal from the microphone to a relatively high level .prior to impressing the signal on the telephone line. Where one tube is employed in the amplifier I prefer to use the pentode type although of course any other type of amplifier tube may be used as will appear from the following specification. The amplifier tube is suitably coupled to the telephone line and a rectifying tube device for rectifying the alternating current derived from the conventional lighting circuit is used to energizethe plate circuit of the amplifier.

The signal is transmitted over the telephone line to the receiver where it is reproduced, It is intended that all telephones on the same line be equipped with this type or substantially this same type of noise suppressor since a telephone set so equipped will not operate with a set not so equipped. However, it would not be necessary to provide every line in the telephone exchange with telephone sets of this high gain type since one telephone line employing such sets may be coupled to a line employing conventional telephone sets through a noise suppressor circuit which also forms a feature of this invention. This noise suppressor circuit also includes a vacuum tube of the pentode type although, of course, other types of tubes well known in the art may be used. Pure resistances, that is, resistance devices consisting substantially only of ohmic resistance, are connected into an electrical network both in the input circuit and output circuit of the noise suppressor tube to permit two directions of transmission through the noise suppressor. These and other features of this invention will be more fully set forth in the following specification and the drawing in which briefly, Fig. 1 is a schematic circuit diagram of the noise suppressor telephone set and Fig. 2 is a schematic circuit diagram of the noise suppressor circuit for connecting various telephone lines.

Referring to the drawing in detail reference numeral designates a pentode or similar type of high gain amplifier tube having the control grid 3 thereof connected to the secondary 4 of the microphone transformer 5. The telephone transmitter or microphone 2 is connected to the primary 6 of the transformer and is electrically energized from the secondary winding 1 of the transformer 6 through the bridge rectifying circuit 9 and the filter circuit including the choke coil I 0 and filter capacity II. The filament I2 of the amplifier tube I is connected to the secondary I3 of the transformer 8 and the center tap of this secondary I3 is connected to the resistance and capacity circuit I4 to obtain the desired grid bias for the grid electrode 3 with respect to the filament or cathode I2. The circuit I4 including the bias resistor and the bypass condenser is grounded-to the metal chassis or frame upon which the elements of this apparatus are supported and in this way the circuit between the lower terminal of the secondary 4 which is also grounded to the chassis and the filament circuit is completed. The amplifier tube I is also provided with a suppressor grid which is connected to the filament and an additional grid I5 which is connected to the anode I1 through the resistor I6 so that a relatively high positive potential is ap plied to the grid I5. Positive potential derived from the secondary I8 of the transformer 8 and the rectifier I9 is applied :to the anode I1 of the amplifier I through the impedance and choke coil 20. The center tap of the secondary I8 is grounded to the chassis of the apparatus and the two terminals of the secondary are connected to the two anodes 2| and 22 of the tube I9. The filament 23 of therectifier I9 is connected to the secondary 24 of the transformer 8.

The filament of the rectifier I9 is connected to the anode I I of the tube I through the impedance of the choke coil 20 so that the positive potential from the rectified alternating current supplyis applied to the anode of the amplifier. The negative terminal of the rectifier apparatus, namely the center tapof the secondary winding I8 is connected or grounded to the metal frame or chassis as is also the resistor and condenser cir-, cuit I4 so that the negative potential from the rectifier is applied to the filament I2 of the amplifier tube through the circuit I4 and the secondary winding I3. The return circuit for the control grid 3 to the filament I2 is also completed through the transformer secondary winding I3 inasmuch as the secondary 4 of the transformer 5 is also grounded to the chassis as indicated so that it is in effect connected to the rounded terminal of the resistance circuit I4 and in this way the grid 3 is given a direct current biasing potential depending upon the potential difference across the resistor of circuit I4. The potential difference across this resistor will of course be determined by the current flowing through the tube I from the filament I2. A filter condenser 25 is connected to the filament of the rectifier I9 and the choke coil 26 as shown in the drawing. The choke coil 20 is also employed as an impedance for coupling the anode circuit of the amplifier I to the telephone line 36 so that this choke must be designed to prevent the passage of telephonic signals derived from the microphone 2 and amplified by the amplifier I, from being shunted to the ground through the filter condenser 25. These highly amplified telephone signals must be impressed upon the line 36 through a coupling condenser 26 and in this case the telephone circuit employs one metallic conductor 36 and the earth or ground as the return circuit. Thus the metallic frame or chassis of the telephone set is also connected to the earth or grounded in order that this telephone line circuit may be properly completed.

It will be observed thatthe amplifier tube I is of the pentode type having three grid electrodes, namely the control grid 3, the screen grid I5 which is connected to the. anode I1 through a suitable resistance I6 and a suppressor grid which is connected to the filament- It is obvious that amplifier tubes other than the pentode type may be employed and if necessary more than one amplifier tube may be used.

The telephone receiver 3| is connected to the line 36 through the switch 29 and the tap 32 of the resistor 30. The other terminal of the receiver 3| is connected to the ground. The resistor 30 is employed for the purpose of introducing a transmission loss between the receiver 3| and the line substantially equal to the transmission gain of the pentode amplifier. Therefore the tendency toward the production of sustained electrical oscillations caused by the acousticalmechanical coupling between the receiver 3| and the transmitter 2 is reduced to the same value as in a telephone set in which no amplifier is used. The advantage of this type of telephone set is that the voice currefits are delivered to the line 36 at a high transmission level, traverse the line at a high level and are reduced to the normal telephone transmission level by the resistance pad 30 in the receiver circuit. thereby the ratio of noise level to voice transmission level is decreased by the transmission gain of the pentode amplifier.

/ Both of the switches 28 and 29 are arranged the 110 volt alternating current circuit to the transformer primary 33 is closed through the switch 28 when the receiver 3| is removed from its supporting hook and the telephone set is ready for operation inasmuch as the various circuits described above are energized from the various sec- 4 ondaries of the transformer 8.

The alternating current bell 21 as well as other similar bells connected to the line 36 may be energized from the 110 volt alternating current circuit by closing the push-button 35 which is connected to the bell 21 and line 36. A current limiting resistance 34 is connected between the 110 volt line and the push-button switch 35. The lower terminal of the bell 21 is grounded and in this way it may be energized by closing the switch 35 without closing the receiver operated switch 28. It is of course obvious that if the telephone line 36 employs two metallic lines that the bell However, it will not be necessary to so equip every line in the telephone exchange for by means of the noise suppressor described below it is possible to connect a line equipped with noise suppressor telephone sets to a line using conventional telephone equipment. It is also possible to use the noise suppressor described below on two wire metallic trunk circuits of moderate length.

In Fig. 2 is shown a diagram of a noise suppressor circuit that may be employed to connect a telephone line employing telephone sets such as shown in Fig. 1, to a telephone line employing conventional telephone sets. In this case the telephone line 36 is connected to the terminal 49 of the resistance network a. employing ' resistors 45, 46, 41 and 48. The secondary 44 of the transformer 42 is also connected to the terminal 49 and also to the terminal 5| of this network.

The terminal 52 is connected to the earth or ground in cases where the ground is used as the return circuit or if a metallic return circuit is used then this terminal is connected to such a metallic circuit. This terminal 52 is connected by means of the wire 53 to the terminal 6| of another resistance network 54a. including the resistances 54, 55, 56 and 51. The terminals 58 and 60 of the latter resistance network 5411 are connected to the primary 64 of the transformer 62. The telephonelines 66 and 61 employing conventional telephone equipment are connected to the terminals 58 and 59 of the latter resistance network. The two resistance networks are also connected together through the wire 69, the variable resistance 68 and wire 10.

In the circuit of Fig. 2 transformer 1| supplies the required voltage for the vacuum tubes in the noise suppressor. The full wave electronic rectifier 16, retardation coil BI and condensers 82 and 83 supply the necessary high voltage for the anode 4| of the pentodetube 31. The winding 13 also supplies the necessary cathode current for pentode tube 31. Resistance 14 and condenser 14a are used to obtain the proper po-' tential relation between the cathode and control grid of the pentode tube.

The control grid electrode 39 of the pentode 31 is connected to the secondary 63 of the transformer 62 and to the condenser 65 which is shunted across this secondary. The lower terminal of the transformer secondary 63 is grounded to the metal frame or chassis of the apparatus forming the grid to filament return circuit by way of'the resistance 14 and secondary winding 13. The anode 4| of the pentode amplifier tube 31 is connected to the primary 43 of the output transformer 42. I

Thus it is seen that the telephone signals coming from the lines 66 and 61 are impressed upon the input circuit of the amplifier tube 31 and amplified whereupon they are transmitted to the line 36 at a relatively high level. On the other hand signals coming from the line 36 the familiar bridge arrangement is used to separate the two directions of transmission. The use of this arrangement instead of the usual special type transformers and complicated balancing impedance networks used in previous systems of this natureis made possible by the fact that on ground return circuits of the usual length employed the input impedance is determined to a very large extent by the terminating impedance, which in this case will be largely resistive depending upon the value of resistance 36 in Fig. 1.

Likewise on two wire or metallic telephone circuits of open 'wire construction of moderate length the terminating impedance controls to a large extent the input impedance of the line. Therefore, when two of these noise suppressors are connected, one at each end of a metallic trunk line between twoexchanges the bridge arrangement of each noise suppressor is suificiently well balanced to prevent sustained oscillation from being generated.

, A similar bridge arrangement is provided by a pure resistance network 540. at the input of the noise suppressor tube 31 as was described above.

The operation of the two types of suppressors in combination is as follows: C'onsider first transmission from a switchboard to a telephone station. Voice currents from. a telephone line enter terminal 66-61 of the noise suppressor in Fig. 2 and are impressed upon the control grid of pentode tube 31 by input transformer 62. The voice currents are amplified by pentode tube 31 and impressed upon the resistance network 45a by output transformer 42. Resistance network 45a prevents these currents from entering circuit 53-16 but allows them to enter the line 36 at the terminal. After traversing the line they enter resistance 30 of Fig. 1 and after being attenuated by this resistance to a normal telephone transmission level are introduced into receiver 3|.

Transmission in the opposite direction, that is from telephone station toswitchboard is accomplished as follows: The voice currents generated in the carbon button transmitter set of Fig. 1 are amplified to a high level and impressed upon the line as described above. These voice currents traverse the line 36 and enter the terminal of Fig. 2. Upon entering resistance network 450: a part of the voice currents enter the circuit 53-10 and a part are impressed on the secondary of output transformer 1 where they serve no useful purpose. The part of the voice currents entering the circuit 5310 flow through variable resistance 68, which serves the purpose of reducing the currents to the desired transmission level, and then into resistance network 54a where they are prevented from entering input transformer 62 because of the balanced condition of the network, but are impressed across the lines 6661 at the noise suppressor.

The amplified voice currents in traversing resistance networks 54a and 45a. and variable resistance 68 have been attenuated to a normal telephone level.

While I have described this invention in detail with respect to the foregoing embodiments thereof it is of course apparent that various modification may be made therein without departing from the spirit and scope of this invention and therefore I do not desire to limit this invention to the details set forth except insofar as they are defined by the claims.

What I claim is as follows: v

1. Noise suppression apparatus for telephone lines, comprising: a microphone, an amplifier having input and output circuits, connections between said microphone and said input circuit, a telephone line connected to said output circuit for receiving highly amplified signals from said amplifier output circuit, a telephone receiver, and a resistance pad connected between said telephone line and said telephone receiver, said resistance pad being adjusted to attenuate the signals impressed upon said telephone receiver to a lower level sufficient for reproduction of said signals by said telephone receiver so that the undesired noise is reduced so as to have substantially negligible interference effect on the desired signal, a second amplifier at a remote point from said first amplifier, said second amplifier having input and output circuits, a second telephone line connected to said second amplifier input circuit for feeding signals to be amplified to said second amplifier, a resistance network connected between said second amplifier output circuit and said first mentioned telephone line for impressing amplified signals upon said first mentioned telephone line, and an attenuating device connected between said resistance networks for passing telephonic signals therethrough from said first telephone line to said second telephone line at an attenuated level.

2. Noise suppression apparatus for telephone lines, comprising: an amplifier tube having cathode, grid and plate electrodes, a resistance network connected between a telephone line and the plate circuit of said amplifier tube, a second resistance network connected between the grid circuit of said amplifier tube and a second tele-- phone line, means for connecting said second resistance network to said grid circuit for impressing telephone signals to be amplified upon said grid circuitfmeans for connecting said first resistance network to said plate circuit for impressing highly amplified telephone signals from said plate circuit upon said first telephone line, and an attenuating device connected between said resistancenetworks for passing telephonic signals therethrough from said first telephone line to said second telephone line at an attenuated level.

3. Noise suppression apparatus for telephone lines, comprising: an amplifier tube having cathode, grid and plate electrodes, a substantially balanced resistance network connected between a telephone line and the plate circuit of said amplifier tube, a second substantially balanced resistance network connected between the grid circuit of said amplifier tube and a secondtelephone line, said substantially balanced resistance networks being connected between their respective circuits so that telephone signals are normally transmitted through said amplifier from input to output and amplified to a relatively high transmission level to obtain a high signal to noise ratio, and an attenuating device connected between said resistance networks for passing telephonic signals therethrough from said first telephone line to said second telephone line and attenuate said signals to a lower transmission level at the same time keeping the high signal to noise ratio obtaining 'on said first telephone line.

4. Noise suppression apparatus for telephone lines, comprising: an amplifier tube having an input circuit and an output circuit, a pair of resistance bridge networks, one of said resistance bridge networks being connected to said amplifier input circuit for impressing signals from a telephone line upon said input circuit to be amplified and the other of said resistance bridge networks being connected to said amplifier output circuit for impressing amplified signals from said output circuit upon a telephone line, said resistance bridge networks each having a pair of points having substantially zero potential therebetween and means for connecting the corresponding zero potential points of said bridge networks together whereby signal transmission in the direction opposite to that normally accomplished through said amplifier tube from the input to the output thereof may be accomplished around said amplifier tube.

5. Noise suppression apparatus for telephone lines, comprising: an amplifier tube having an input circuit and an output circuit, a pair of resistance bridge networks, one of said resistance bridge networks being connected to said amplifier input circuit for impressing signals from a telephone line upon said input circuit to be amplified and the other of said resistance bridge networks being connected to said amplifier output circuit for. impressing amplified signals from said output circuit upon a telephone line, said resistance bridge networks each having a pair of points having substantially zero potential therebetween, means for connecting the corresponding zero potential points of said bridge networks together whereby signal transmission in the direction opposite to that normally accomplished through said amplifier tube from the input to the output thereof may be accomplished around said amplifier tube and signal attenuating means in said last mentioned means for adjusting the level of signal transmission through said last mentioned means to that desired.

6. Noise suppression apparatus for telephone lines, comprising: a pentode high gain amplifier tube having an input circuit and an output circuit, a pair of resistance bridge networks, one of said resistance bridge networks being connected to said amplifier input circuit for impressing signals from a telephone line upon said input circuit to be amplified and the other of said resistance bridge networks being connected to said amplifier output circuit for impressing highly amplified signals from said output circult upon a telephone line, said resistance bridge networks'each having a pair of points having substantially zero potential therebetween and signal attenuating means for connecting the corresponding zero potential points of said bridge networks together whereby signals transmission in the direction opposite to that normally accomplished through said amplifier tube from the in-' put to the output thereof may be accomplished around said am lifier tube. 1

7. Noise 8111356581011 apparatus for telephone lines, comprising: an amplifier tube having an input circuit and an output circuit, a pair of resistance bridge networks, a telephone line connected to one of said bridge networks for impressing a telephone signal thereon, a transformer for coupling said bridge network and said telephone line to said tube input circuit, for impressing signals from said telephone line upon said input circuit to be amplified, a second transformer for coupling the other of said resistance bridge networks to said amplifier output circuit for impressing amplified signals from said output circuit upon a second telephone line, said resistance bridge networks each having a pair of points having substantially zero potential therebetween and means for connecting the corresponding zero potential points of said bridge networks together whereby signals transmission in the direction opposite to that normally vaccomplished through said amplifier tube from the input to the output thereof may be accomplished around said amplifier tube.

8. Noise suppression apparatus for telephone lines, comprising: an amplifier tube having an input circuit and an output circuit, a pair of resistance bridge networks, a telephone line connected to one of said bridge networks for impressing a telephone signal thereon, a transformer for coupling said bridge network'and said telephone line to said input circuit for impressing signals from said telephone line upon said input circuit to be amplified, a second transformer for coupling other of said resistance bridge networks to said amplifier output circuit for impressing amplified signals from said output circuit upon a second telephone line, said resistance bridge networks each having a pair of'points having substantially zero potential therebetween, means for connecting the corresponding zero potentials points of said bridge networks together whereby signals transmission in the direction opposite to that normally accomplished through said amplifier tube from the input to the output thereof may be accomplished around said amplifier tube and signal attenuating means in said last mentioned means for controlling the level of signal transmission through sai ilast mentioned means. 7

9. Noise suppression apparatus for telephone lines, comprising: a microphone, an amplifier tube having input and output circuits, connections between said microphone and said input circuit, a telephone line connected to said output circuit for receiving amplified signals from said amplifier output circuit, a telephone receiver, and a resistance pad connected between said telephone line and said telephone receiver for attenuating the signals impressed upon said telephone receiver from said telephone line and said amplifier output circuit.

10. Noise suppression apparatus for telephone lines, comprising: a microphone, an amplifier tube having input and output circuits, a transformer for connecting said microphone to said input circuit, a transformer having a plurality of secondary windings for energizing circuits of said microphone and said amplifier tube, a telephone line connected to said output circuit for receiving amplified signals from said amplifier output circuit, a telephone receiver, a switch operable by said receiver, connections between said switch, a sourcenf current supply and the primary of said second transformer for connecting said primary to said source of current supply when said receiver is lifted for use, and a resistance pad connected between said telephone line and said telephone receiver for attenuating the signals impressed upon said telephone receiver from said telephone line and said amplifier output circuit.

11. Noise suppression apparatus for telephone lines, comprising: a microphone, an amplifier tube having input and output circuits, connections between said microphone and said input circuit, a transformer-having a plurality of secondary windings for energizing circuits of said microphone and said amplifier tube, a telephone line connected to said output circuit for receiving amplified signals from said amplifier output circuit, a telephone receiver, a switch operable by said receiver, connections between said switch, a

, be energized from said source of current supply,

said bell being connected to said telephone line and a switch for connecting said source of ourrent supply to said telephone line for energizing said bell and similar bells connected to said telephone line.

. 12. A telephone system adapted for high level signal transmission to reduce the eflects of noises produced in telephone lines, comprising: a microphone, an amplifier tube having input and output circuits, connections between said microphone and said input circuit, a telephone line connected to said output circuit for receiving highly amplified signals from said amplifier output circuit, a telephone receiver, a. resistance pad connected between said telephone line and said telephone receiver for attenuating the signals and noise impressed upon said telephone receiver to a lower level suflicient for the reproduction of the signals by said telephone receiver, a second amplifier having its output circuit connected to said telephone line at a remote point for impressing high level signals on said line atsaid remote point for transmission to said telephone receiver, a resistance bridge network connected between said second amplifier and said telephone line, a second telephone line adapted for lower level signal transmission connected to the input of said second amplifier and a circuit for impressing the signals from said first telephone line on said second telephone line at an attenuated level.

13. Noise suppression apparatus for telephone lines, comprising: a microphone, an amplifier having input and output circuits, said microphone connected to said amplifier input circuit, a transformer having a primary connected to an alternating current power source, said transformer having a plurality of secondary windings,

a rectifier and filter connected between one of line, a resistance pad connected to said telephone line, a telephone receiver connected to said resistance pad, a second amplifier having an output circuit connected to said telephone line at a point remote, from said first amplifier, and a second telephone line connected to the input of said second amplifier for feeding signals atrelatively low level to said second amplifier input, said sec- 0nd amplifier being adjusted to feed high level signals to said first telephone line to obtain a high signal to noise ratio so that when the high level signals are attenuated at said resistance pad to a level suitable for said telephone receiver the noise will be substantially eliminated. I

BERGIE L. FISHER.

Images