US2111736A - Automatic volume control with noise suppression - Google Patents

Description

22,1938. wff/AN E. ROBERTS 2,11%735 AUTOMATIC VOLUME CONTROL WITH NOISE SUPPRESSION original Filved-Dec. 19, 1928A f6 Y a: alalr-J v20 cordance with the amplication. For instance, trol coil 4 is less than acertain amount and a Nin-:o STA-Tes Param* cries AUTOMATIC' VOLUME CONTROL WITH NOISE f SUPPRESSION Walter van B. Roberts, vPrinceton, N. J., assignor to Radio Corporation of America, a

corporation of Delaware Application December 19, 1928, Serial No. 326,990 Renewed September 18,l 1931 29 Claims. Y (Cl. Z50- 20) It is well known that in radio receivers having In Fig. 4 is shown schematically a complete automatic yvolume control an annoying hiss ocradio receiver embodying the present invention, curs when, in tuning, the dial passes over freas applied specically to Fig. 2. quency bands containing no signal energy. This In the following matter which is given solely 5 is due'undoubtedly, to the fact that in such defor purposes of clearly describing the invention 5'VV vices, amplification automatically becomes very and not for purposes of limitation, speciiic exgreat in the absence of signals with the result amples will be given in order that a clearer unthat static and other noises are amplified to a derstanding of the invention will be possible; great degree. f however, in no case is it to be taken that the par- This, of course, is inherent due to the very naticular. values chosen or given by way of illustra- 10 vture and way that the said volume controls option areintended as a curtailment on the scope erate, that is, the less energy coming into the re- Y of the invention, except as set forth in the claims. ceiver, the greater the tendency to amplify. In Figure 1, device 3 represents diagrammati- In some automatic volume control arrangecally a magnetron tube of well known type havments, such, for example, as disclosed in patent ing an anode surrounding an electron emitting 151 of T. A. Smith, entitled An Automatic gain confilament within an enclosure and Il represents its trol, Patent No.l 1,999,668, issued April 30, 1935, external control coil. This tube is so constructthe plate current supplied 'to the tubes in the ed that it presents a relatively low direct current radio frequency amplifying` system varies in acinternal resistance when the current in its conin certain particular devices of this type the curmuch greater internal resistance when the current supply to the radio frequency tube is about rent in the said control coil exceeds a certain 15,mils when very Weak signals are received, amount; that is, the device is constructed so as somewhat more than 15 mils when no signals are to be very sensitive to changes in current passing present, and less than 15 mils when loud signals through the control coil. The coil Il, through are present; that is, approximately' `inversely leads I-I, is connected in the plate current cirproportional to the strength of the signal. This, cuit of the radio frequency ampliiier of the reof course, is true only within certain limits. ceiving apparatus, not shown. Tube 2 represents The broad system of noise suppression in autothe audio output tube of the radio receiving apmatic gain control systems is disclosed and paratus and as is evident from the drawing the 30 claimed in an application filed by Arthur F. Van plate supply 5 of the audio tube is in series with Dyck et al., Serial No. 465,852, filed July '7, 1930. the magnetron tube 3 so that the magnetron tube The purpose of the present invention isv to conacts as a current limiting relay or control valve struct a device for eliminating the above undefor the plate supply of tube 2. A loudspeaker or siredV effects without in any way impairing the indicator B and a by-pass condenser l are con- 35 operativeness and effectiveness of the volume nected in the output 'circuit in the manner shown. control device. It is thus seen that as the magnetron tube 3 is Another purpose of the invention is Vto utilize in series with the plate supply 5 of the audio a vacuum tube in its various capacities for acamplifier output tube 2 it very substantially de- 40 complishing the desired results. creases the plate current of the audio output 4o Still other purposes will appear in the descrip- When the current in winding 4 exceeds a certain .tive matter which is to follow when read in conamount, say in this case 15 `mils, and that it nection with the drawing in whichf allows normal current to flow to the plate of tube Figure 1 illustrates diagrammatically one em- 2 When the current through i is less than 15 mils bodiment of my invention in which a vacuum and that by suitable adjustments the current lim- 45 tube of the type known as a magnetron is used iting control may be made quite critical. The for the control resistance, current in winding 4 being in the plate current Figure 2 illustrates diagrammatically a mode supply of the radio frequency tubes has its value of accomplishing the objects of my invention by determined by the automatic volume control. the use of a triode tube and suitable biasing The operation of the device in Figure 1 is as 5o means; follows: When no signal is present the volume Figure 3 diagrammatically illustrates another control tends to increase the amplification which embodiment of my invention in which an osclresults in great amplification of static and other lation circuit is made use of for the control undesired noises thus causing more current to go means. through coil 4 which in turn controls tube 3 in 55 Ill) such a way that the audio frequency end of the circuit is made substantially inoperative whereby the undesired noises will not be heard through the phones 6.

If desired, a supplementary biasing direct current through 4 may be supplied to make the magnetron cut off at the desired eld strength, or a separate control coil may be preferably provided for the biasing current.

In Figure 2, I 2 is a triode having biasing means such as a grid battery I0. The plate current of the volume controlled radio frequency tubes (not shown) goes from B+ through resistance I4 to the plates as shown by the arrow. Resistance I4 is arranged so as to be in the input circuit of the triode I2 so that changes in potential drop across the ends of the resistance I4 due to an increase in the plate current of the radio frequency tubes will affect the bias upon the grid of tube I2. Battery I0 is of such voltage that it just shuts oi plate current through resistance I3 when the radio frequency plate current owing through resistance I4 has the assumed critical value of 15 mils. Tube I5 is the last audio stage of the receiver, and terminals Iii-I6 are the input terminals to the audio transformer I'I. A suitable grid bias source, such as battery I9, a plate current supply I8 and a telephone receiver device I'I are connected to the tube I5 as shown.

In this modication it is seen therefore that a normal flow of current (15 mils or less) through I4 will cause no change whatever in the receiver. However, suppose that due to the absence of signals the automatic volume control functions in such a way that a current say in excess of 15 mils flows through I4. The potential drop thus produced across resistance I4 opposes the grid biasing battery lil potential and causes the grid to become sufficiently less negative to permit some current to ow through resistance I3. This in turn causes more bias on the grid of audio tube I5 which, may be any one or all audio tubes of the circuit, (only one being shown here for convenience). Due to the amplifying action of the tube I2, an increase in the radio frequency plate current to 16 mils, for example, can readily be caused to increase the negative bias of tube I5 sufciently to cut off all the plate current. A decrease, say to 14 mils however, would make no difference as there is no current at all through I3 except when the radio frequency plate current is greater than l5 mils.

In the modification shown by Figure 3, 25 is a triode having its input and output coupled in any well known manner, for example by the coupling 22, to produce oscillations under certain conditions. 'Ihe plate current for the radio frequency tubes (not shown) goes through from B+, resistance 24 to the plate electrodes of the radio frequency tubes in substantially the same manner as shown in Figure 2. Coil 2|, coupled to control grid coil 22 through the anode coil 22', rectifier 2, resistance 28 and filter 29 constitute a rectifier circuit having its input coupled to the oscillatory circuit 23 of tube 25. The rectier is shown diagrammatically as a crystal 21 although any type of rectifier may be employed. The current smoothing circuit or lter unit 29 is included so that a smooth direct current is derived from the rectier. Resistance 28 is supplied in the rectier circuit in series with the grid circuit 30-30 of one or more of the audio frequency tubes of the circuit. A source of plate voltage 26 is included in the plate circuit of tube 25.

l' It is evident from the figure that when the radio frequency amplifier plate current becomes excessive, say above 15 mils, more current will flow through resistance 24 thereby increasing the potential drop, or voltage, across said resistance making the grid of tube 25 become less negative due to the fact that the potential across the resistance 24 overcomes the eifect of battery 23 when the current through 24 reaches a certain value. When the effect of battery 23 is overcome, strong oscillations will be initiated in tube 25. These oscillations are picked up by coil 2l, rectied by rectifier 2'I and smoothed out by the lter 29. The resulting direct current is passed through the resistance 28 thus giving a biasing voltage which is applied in series with the grids of one or more of the audio frequency tubes which in turn will shut off the output, as in the case of Figure 2.

WhenV this current is properly proportioned with respect to the different biases the audio frequency plate supply will be entirely, or substantially cut off, with the result that the undesired noises are precluded from being sent through the telephones, or other sound reproducing devices.

In the three modifications shown, the audio output should be shut off, or greatly diminished, onlyrwhen the plate current of the radio frequency tubes increases above a certain predetermined amount (say 15 mils) in average value averaged over a reasonable time.

To insure this, in any of the modifications, a low pass lter may be inserted in the connection between the radio frequency plate circuit and the means for shutting off the audio frequency tube output, as shown by way of illustration at Figure 2, by network II which device will allow any temporary rise in current resulting from an unusually great amplitude modulation, as for instance, in the case of a loud note in broadcasting a musical selection, to be by-passed.

Fig. 4 is a schematic diagram of a complete receiver embodying the invention, and it is to bc clearly understood that the diagram is purely conventional, or functional, in nature. The input of the radio frequency amplier includes a tuner, and the latter is symbolized by the usual variable condenser representation. The detector is fed with the amplified radio frequency energy; it detects the energy, and transmits the detected energy to an audio frequency amplifier.

The block designated Automatic volume control symbolizes the arrangement for automatically regulating the gain of the radio amplifier. As stated above, its specic form may follow the teachings of the aforesaid Smith patent. The audio frequency amplifier control, or background noise suppressor, is functionally shown as being fed by a portion of the radio amplifier output; the output of the suppressor being employed to control the amplification efficiency of the audio amplifier. The schematic representation of the background noise suppressor generally illustrates the functions of each of the modifications in Figs. 1, 2, 3. The resistor I4 is the resistor shown in the grid circuit of tube I2 of Fig. 2. As shown in the aforesaid Smith patent, the automatic volume control functions to vary the flow of radio frequency amplier plate current through resistor I4. The latter is to be replaced by coil 4, or resistor 24, when the system is used in connection with the arrangements of Figs. 1 and 3 Arespectively. f

reproduction of undesired background noise impulses, which impulses tend to be greatly amplified by the radio amplifier whenever the collected signal energydecreases in intensity below the aforesaid minimum level.

Having thus described my invention what I desire toprotect by Letters Patent is as follows:

1. In a radio receiving circuit having an automatic volume control device, an intermediate space discharge device circuit having its input,

coupled to the output of one of the radio fre-- quency amplifier stages of said receiving circuit, and its output coupled to the input of an audio frequency vamplifier stage thereof, means in said intermediate ,space discharge device circuit for normally maintaining'said device in an inoperative state, means for overcoming said first named means upon an abnormal increase in output current' in said radio frequency amplifier whereby said devicel is "rendered operative and means controlled thereby for limiting the amplifying characteristics of said audio frequency amplifier stage.

2. In radio receivingapparatus having automatic volume control means, an electron disi charge device having an input circuit and an output circuit, said input circuitforming a portion of the radio frequency amplifier output of said receiving apparatus and said output circuit forming a part of the audio frequency input circuitof said receiving apparatus, biasing potential means for said device and means insaid input circuit for overcoming said biasing vpotential when the current in said radio frequency amplifier output'v eX- ceeds a certain predetermined value only when i sustainedv over an appreciable'length of time.

3; In a radio receiving circuit having aradio frequency amplifier an audio frequency amplifier,

and an automatic volume control device, a tube circuit having its input resistively coupled to the plate supply of said radio frequency amplifier through a low pass filter and its output resistively coupled to the input circuit of said audio frequency amplier and a biasing device in said tube input circuit for normally maintaining said tube inoperative. l

4. In a multi-stage receiving circuit having an automatic volume control device, auxiliarymeans for coupling any stage thereof to any succeeding other of said stages comprising a normally inop-V erativespace vdischarge device-circuit having an input circuit comprising a biasingmeans anda resistance in series, and an `output circuit comprising a resistance and a plate current supply, said resistances being utilized for coupling said Space discharge device circuit, between said other stages respectively and means whereby when a current exceeding a predetermined value ows through said rst mentioned resistance for a sustained predetermined time interval said space discharge device will be rendered y operative whereby a current will flow through said second resistance and means controlled thereby for controlling the output of the stage connected thereto.

5. A signalling system comprising a high frequency amplifier circuit, means for tuning said circuit through a range of frequencies, a gain controlV device for regulating the sensitivity of said amplifier, electron discharge tube means for utilizing the amplifier output `and additional means comprising an electrondischarge device for automatically decreasing the utilizing efficiency of said last means when said amplifier is adjusted to maximum sensitivity but is tuned through a point of said range where less than a predetermined amount of high frequency energy is received.

6. A signalling system comprising a high frequency amplifier circuit, means for tuning said circuit through a range of frequencies, a gain control device for regulating the sensitivity of said amplifier, means for changingv the frequencies amplified at high frequency to a low frequency, a low frequency amplier-andadditional means including a space discharge relay for automatically impairing the transmission einciency of the low frequency amplifier when the high frequency amplifier is adjusted to'maximum sensitivity but is tuned through a point of the range where no high frequency energy is received.

7. A receiving system comprising a signaltranslating tube device, a radio frequency amplifier coupled to said device, means for regulating the sensitivity of said amplifier inversely with received signal energy level and means comprising an electron discharge relay for disabling said tube device when said amplifier-is adjusted to maximum sensitivity by said regulating means and no signal energy is received.

8. In combination, a radio receiver comprising,

a radio frequency amplifier and a low frequency amplifier, means for controlling the amplification factor of the radio vfrequency amplifier and an electron discharge relay arranged to be actuated by variations in the output of said radio frequency amplifier for reducing the transmission efficiency of the low frequency amplifier when the gain of the radio frequency amplifier is adjusted to a maximum and no carrier wave is received. 9. In combination, a radio receiver comprising, a radio frequency amplifier, a frequency changing means and a low frequency amplifier, means for controlling the gain of the radio frequency amplifier and an electron discharge device arranged to be actuated by variations in the output of said radio frequency amplifier for reducing the transmission efliciency of the low frequency amplifier when the gain of the radio frequency amplifier is adjusted to a maximum and no carrier wave is received, said electron discharge device being connected between the radio frequency amplifier and the said low frequency amplifier.

10. In a signalling system which includes an amplifying circuit and a gain control device for regulating the sensitivity of the amplifier with tube means for utilizing the amplifier output, in

combination, an electron discharge device, re-

sponsive to a predetermined energy variation in the amplifying circuit output, for automatically decreasing the utilizing eciency of the last tube means when said amplifier is adjusted to maximum sensitivity and where less than a predetermined amount of energy to be amplified is availdischarge relay, responsive to a predetermined energy variation in the high frequency amplifier output, for automatically impairing the transmission efficiency of the low frequency amplifier when the high frequency amplier is adjusted to maximum sensitivity but is tuned through a point of the range where the intensity of the incoming high frequency energy is below a predetermined value.

12. In combination with a radio receiver including a high frequency amplifier, a detector, a lower frequency amplifier, and an arrangement for automatically regulating the gain of said high frequency amplifier, a device for automatically suppressing the reproduction of undesired background noise impulseswhen the sensitivity of said high frequency amplifier is increased above a predetermined level by said regulating arrangement, said device comprising a circuit connected to said low frequency amplifier, said circuit including means, responsive to said increased sensitivity, for controlling the bias of an input electrode of said low frequency amplifier.

13. In a radio receiver, in combination, an audio frequency amplifier, a detector connected to the input of the amplifier, a high frequency network connected to the detector input, an automatic volume control circuit for regulating the transmission efficiency of said network in such a manner that the radio frequency input to the detector is substantially constant regardless of signal carrier intensity fluctuations in the input of said network, and means responsive to variations in the magnitude of t'ne received signal energy for adjusting the bias of an input electrode of said audio amplifier in a sense such that the audioamplifier output is greatly diminished when the signal energy magnitude decreases below a predetermined intensity level.

14. In a radio receiver, in combination, a radio frequency amplifier, means for automatically adjusting the gain of said amplifier, said receiver additionally including a rectifier and a low frequency amplifier in a network following said radio amplifier, and an auxiliary means, operative upon adjustment of said radio amplifier to maximum sensitivity and when signal energy below a minimum intensity level is impressed on said radio amplifier, for automatically biasing a control electrode of said network to cut-off.

15. A radio signalling system of the type including a resonant high frequency transmission network, tuned to a desired frequency, a demodulator and an audio amplifier, means for automatically regulating the transmission efficiency of said network in such a manner that the signal energy input to the demodulator is at a substantially constant level, and additional means, responsive to a decrease of the said signal input below a minimum level, for automatically biasing the audio amplifier in a sense such that the transmission efiiciency of the latter is substantially impaired.

16. In combination with an audio frequency amplifier having a control electrode and a source of audio signal frequency energy therefor, a space discharge device having at least one electrode electrically connected with said source in such a manner that a decrease in intensity of said energy below a minimum level results in a predetermined variation in direct current potential of said electrode, and anadditional electrode in said device connected to a transmission control electrode of said amplifier, and responsive to said potential variation, for impairing the transmission efficiency of said amplifier when said energy decreases below said level.

17. In combination with the audio frequency amplifier of a radio receiver, a background noise suppressor tube having its input electrodes connected to a point in said receiver preceding the input of said amplifier, the output electrodes of the tube being connected to the input circuit of said amplifier, means in the said input circuit for establishing a normal potential difference between the amplifier input electrodes such that the amplifier isl eflicient when audio signal energy is impressed thereon, additional means, connected to the output electrodes of said tube, for providing a potential difference between said amplifier input electrodes adapted to bias the amplifier to cut off, and means connected to the suppressor tube input electrodes for normally biasing the suppressor tube to inoperative state as long as said signal energy is impressed on said amplifier.

18. In a radio broadcast receiver provided with a plurality of electron discharge tubes included in successive stages of the receiver, means for tuning the receiver through the broadcast tuning range, automatic volume control means for maintaining the audio output level of the receiver substantially uniform regardless of signal carrier fluctuations, an auxiliary control arrangement adapted to operate when said tuning means is adjusted to a setting at which broadcast signal energy below a minimum intensity level is collected, said control arrangement including a biasing means, responsive to increased receiver sensitivity below said minimum level, for biasing at least one of said tubes to cut-off.

19. A method of radio frequency signal reception which includes the steps of collecting signal energy, amplifying the amplitude of the collected energy, regulating the amplification of said collected energy with a portion of the amplified energy in a direction to oppose variations in the intensity level of the carrier of the collected signal energy, detecting the amplified energy, amplifying the detected energy, and regulating the amplification of the' detected energy with a portion of the said amplified collected energy in a direction opposite to that of said first regulation when the said level decreases below a desired minimum. value.

20. A method of radio frequency signal reception which includes the steps of collecting signal energy, amplifying the amplitude of the collected energy, regulating the amplification of said collected energy with a portion of the amplified energy in a direction to oppose variations in the intensity level of the carrier of the collected energy, rectifying the amplifier energy, amplifying the rectied energy, and regulating the amplification of the rectified energy with a portion of the said amplified collected energy in a direction opposite to that of said first regulation when the said level decreases below a desired minimum value.

21. In a multi-stage radio receiver having an automatic volume control device, an auxiliary coupling circuit comprising an electron discharge device having its input coupled to the output of one of the stages of the radio frequency amplifier and its output coupled to the input of one of the stages of the audio frequency amplifier of said receiver and means upon the attainment of a predetermined condition in the output of said radio frequency stage for rendering said audio frequency stage inoperative.

22. In a radio receiving circuit, an auxiliary space discharge device circuitl having its input coupled to the radio frequency amplifier output and its output coupled to the audio frequency amplier input of said radio receiving circuit, said audio input being also coupled to said radio frequency output, means in said radi'o frequency output for affecting the conductivity of said auX- iliary space discharge device and meansv controlled thereby for controlling the output of said audio frequency amplifier.

23. In a radio receiver, automatic volume control means acting automatically to decrease the sensitivity of the receiver with increase in signal energy, a uni-lateral current device in the signal path of said receiver having at least two electrodes, and means controlled in operation by the automatic volume control means for biasing the electrodes of said uni-lateral device in a direction toV decrease the sensitivity of the receiver when the signal energy is below a predetermined intensity level.

24. In a radio receiver, a uni-lateral current device in the signal path thereof having at least two electrodes, means for biasing the electrodes relative to each other in a direction, and to an extent, such as to prevent the production of undesired noises by the receiver, and means responsive automatically to received signal energy for varying said bias in a direction to increase the sensitivity of the receiver to the received signals.

25. In a radio receiver, automatic volume control means, a uni-lateral device in the signal path of theV receiver having at least two electrodes, means for biasing the electrodes relatively to each other, and means controlled by the rendering the transmission of detected signals through the network inefficient when the received carrier amplitude decreases below a predetermined value.

27. In the operation of a high gain wave transmission system,'the method of preventing the transmission of currents inadvertently arising Within the system which comprises normally biasing said system to suppress transmission therethrough, and automatically removing said suppression bias when the received waves attain a predetermined critical value.

28. In an electrical wave transmission system, the combination with a plurality of radio frequency and audio frequency stages cascaded to form a transmission line, means for controlling the gain of said stages to maintain the signal voltage across predetermined points in said line at a substantially xed value over a wide range of received signal voltages, and means operative when the received signal voltage drops to a critical value to impress upon an audio frequency stage following said predetermined points a bias Vvoltage which blocks transmission through said .following stage.

29. In a radio receiver, a carrier-current amplier, a non-rectifying modulation-current amplier responsive to signals from said carrier-current amplifier, means dependent vupon the intensity of said carrier current for automatically maintaining the output of said receiver substantially constant over a range of received carriercurrent intensities, and means dependent upon the intensity of said carrier-current for rendering said modulation-current amplifier ineiective, thereby reducing the output of said receiver substantially to zero, when the intensity of said carrier-current falls below a predetermined level below said range.

WALTER VAN B. ROBERTS.

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