US2079446A

US2079446A - Over-modulation protective device

Info

Publication number: US2079446A
Application number: US694383A
Authority: US
Inventors: Alfred N Goldsmith
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1933-10-20
Filing date: 1933-10-20
Publication date: 1937-05-04
Anticipated expiration: 1954-05-04
Also published as: DE658907C

Description

y 1937- A. N. GOLDSMITH 2,079,446

OVER MOD ULATION PROTECTIVE DEVICE Filed Oct. 20, 1935. 2 Sheets-Sheet l "Q 5 x fi Q 3 ye q INVENTOR ALFRED N. GOLDSMITH ATTORNEY Patented Ma 4, 1937 UNlTED STATES OVER-MODULATION PROTECTIVE DEVICE 7 Alfred N. Goldsmith,

New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October 20,- 1933, Serial No. 694,383

' 18 Claims. (01. 179-111) This invention comprises a correction device for preventing the occurrence of excessive signal voltages or over-modulation in communication systems. p

A primary object of the invention is to obtain automatic control of signal apparatus in response to over-modulation effects in a simple and highly eflicient manner.

A further object is to provide a corrective ar- I rangement for signal apparatus which utilizes a deflection of a beam of energy in one or more directions which is proportional or related to the signal voltages for correcting for departures of the signal apparatus from a desired condition.

A still further object is to provide a device for indicating peak voltages in excess of a predetermined value.

Although the foregoing objects are herein described as being accomplished by using a cathode ray tube, it is to be understood that any device may be employed which utilizes an electriccharge-carrying stream or a luminous beam con trollably deflected or modified through the agency of a modulated current, or, for that matter, any rapidly responsive galvanometric indicator of the same current.

A-feature of the present invention lies in the utilization of a particular type of cathode ray tube which obviates the necessity of the use oi steady currents, such as those provided by direct current sources for the electrodes of the devices. Essentially, this type of tube is the same as that described in a copending application of Dr. Irving Wolff, Serial No. 678,208, filed June 29, 1933, which relates to peak voltage visual indicators of the cathode ray type and wherein the tube functions by the application of alternating voltages-in phase to the two anodes and the control electrode of the tube. 'In this arrangement, the control electrode is normally iased negative and the alternating voltage applied to it is given such a value that the negative bias is periodically reduced sufficiently to permit the passage of the electron beam through the tube. This will be described in more detail later'in the specification.

The present invention involves, according to one modification, placing an auxiliary electrode 7 within the cathode tube for efiecting correction, and, in accordance with another modification, placing a photo-electric cell outside the tube at -a point corresponding in the travel of the defiected cathode ray to more than 100% modulation of the peaks. Thus, when either the electrode or the photo-electric cell is energized by the cathode ray as a result of over-modulation, the modulation oi the broadcasting transmitting station is reduced through control of the amplification of the circuit until over-modulation on 5 peaks no longer occurs. This is accomplished either through circuits which may have a large time constant and/or by means of small motor actuating gain control apparatus.

Another feature is the delay circuit employed 0 for retarding the special currents which modulate the transmitter ln order to enable the complete functioning of the over-modulation protective device before the over-modulation currents actually reach the modulation circuits of the transmitter. a

Other features, objects and advantages will appear in the subsequent detailed description of the invention.

In the drawings. Figure 1 illustrates, by way 20 of example only, the novel portions of a complete system -in accordance with the principles of the present invention for preventing overmodulation in'the transmitter of a communication system; Figure 2 illustrates a view looking squarely into the round end of the cathode ray device and shows the particular relation of. the electrode within the tube to the path of travel of the cathode ray beam; Figure B'shows a modification of the arrangement of Figure 1 wherein a photo-electric cell may be utilized externally of the cathode ray tube instead of the internal electrode arrangement shown in Figure 1; Figure 4 illustrates a dashpot arrangement for introducing a time delay at the transmitter in the restoration of the transmitter circuit to normal for avoiding over-modulation at the end of the silent periods during the transmission of message waves; and Figure 5 illustrates a particular type of delay circuit adapted to retard or store the message waves in the transmitter.

Referring to Figure 1 in more detail, there is shown a cathode ray tube I of a well known type' which is commonly used as a receiving tube in television systems and which comprises an 45 evacuated glass envelope 3 for enabling the obtainment of a pure electron discharge between the indirectly heated cathode 5 and the first anode 1.

The cathodei and the anode 1 form an electrongun which causes a beam of electrons to move down the axis of the tube 1 and strike, in the ordinary case, a fluorescent screen when the proper voltages are applied. The tube, as shown, diiiers from the ordinary cathode ray device by 55 the absence of the fluorescent material and in the insertion of a pivoted electrode 9 which extends, in an adjustable manner, to a point corresponding to a location immediately outside the ordinary path of travel of the electron beam, if over-modulation does not occur. By referring to Figure 2 there is shown a cross-sectional view of the tube looking squarely into the round end of the tube from the narrow portion thereof, the dotted lines indicated representing the silver coating. The trace of the deflected electron beam is shown by reference characters l, G9, the distance between 41 and 49 depending upon the percentage of modulation. A pivot I9 is provided for the search electrode 9 which enables movement of the free pivoted portion of the electrode 9 for varying its position through the means of a magnet il located externally of the tube, the electrode being so positioned that the electron stream will strike electrode 9 only when the modulation rises above a certain point. It will be understood, of course, that magnet ii is removed when not actually in use for the purpose of adjustably controlling the position of the electrode 9; Alternatively, if desired, a fixed search electrode can be used, and the width of the beam trace 4T, 49 controlled so that it does not reach the search electrode unless the percentage of modulation exceeds a predetermined value.

The inner surface of the large end of the envelope 3 of the cathode ray tube is coated with a metal or conducting material to form a second electrode or anode l3 for accelerating or focusing the beam of electrons.

The customary beam deflecting plates i5 are provided to which the signals or message waves, the amplitude of which is to be controlled, are connected through a suitable input circuit Hi. This may be a source of the modulation signals for a transmitter which, in the present case, is shown connected through a suitable medium I1 which may be either a conductive or inductive pick-up of a portion or all of the modulated transmitter carrier wave. For energizing the electrodes of the cathode ray tube there is provided, in accordance with the preferred cathode ray tube employed, a transformer I!) which supplies alternating voltages to all of the tube electrodes from a source of 6!! cycle current. A low potential secondary winding 2| supplies current to the heater 23 of cathode 5. A high potential secondary winding 25 is connected at one end to the accelerating and focusing anode l3 which is preferably connected to ground as indicated, the other end of the high potential winding being connected to the cathode 5 through either a biasing battery 26 or a resistor 21 which is shunted by a condenser 29. The resistor-condenser combination comprising units 21, 29 is the equivalent of thebattery 26 and causes a negative bias potential to be established on the control electrode, this bias potential being obtained by the rectifier action of the cathode ray tube. A suitable switch 39 may be provided for connecting the cathode to either the resistor-condenser combination or to the biasing battery, as desired.

The secondary winding 25 of transformer I9 is shunted by a tapped potentiometer device 3! which supplies the proper alternating voltages to the control electrode ii and the first anode I through conductors 33 and 35, respectively, and corresponding spaced taps on the device 3|.

The control electrode l I is given a negative bias by means of the condenser-resistor combination so that it never becomes positive, this adjustment being necessary to prevent injury to the cathode. It is preferred to bias the control electrode II to cut-off, except during a certain period when it is the least negative. the expression bias to cut-oil being used in the sense that the control electrode is made so negative that the electron beam is normally prevented from reaching the extreme end of the tube where search electrode 9 is located.

In this manner .there is provided a cathode ray tube wherein there is supplied a comparatively low value of alternating voltage to the control electrode H, which voltage is in phase with the alternating voltage supplied to the anodes 1 and i3, and for this reason the control electrode l I will reach its least negative value with respect to the cathode at the same time that the anodes i and I3 are most positive.

For a more complete description of the operation of this particular type of cathode ray tube reference is made to the copending application of Dr. Irving Wolff, supra.

Although the particular type of cathode ray tube described above is preferred, it is to be understood that for the purposes of the present invention any type of cathode ray device may be employed.

In circuit with search electrode 9 is a time constant circuit comprising a condenser 61 and a resistance 68 which, together with a glow tube 69 normally arranged so as not to pass current controls the action of an alarm 16 and the corrective circuit of the transmitter. The resistance-condenser time constant circuit 61, 68 is arranged to build up a charge for causing the glow tube 69 to function and acts as an integrating device which enables the protective circuit to operate only upon the occurrence of over-modulation for at least a short interval of time; i. e., it permits the over-modulation protective control to operate only if a number of audio frequency cycles of high amplitude have occurred and thus prevents momentary and relatively unimportant high audio frequency modulation from operating the device.

For controlling the transmitter circuit and responsive to the operation of the glow tube there is provided a motor which is arranged to be continuously operated and which has a frictional cylindrical extension 5! on its shaft. A frictional drum 52 is arranged to drive a belt 53 which passes over a pulley 54 in engagement with a sliding contact 55 which rests on a resistance 56, 51, the sliding contact being moved over the resistance when pulley 54 is rotated. The resistance combination 56, 51 is a simple form of volume control for the telephonic transmitter arrangement shown. This transmitter arrangement may comprise a microphone 58 supplied with a direct current through battery 59, the audio frequency output of the microphone passing through primary and secondary windings 60 and Si, respectively, of transformer 52. The volume control arrangement is adapted to control the voltage on grid 63 of a vacuum tube 64, the output of which may be supplied to a suitable amplifier and radio frequency oscillator circuitiiil from which the message waves are transmitted to a radio channel. The grid cathode circuit of the vacuum tube 64 is determined by resistance-condenser combination 65, 66.

The operation of the circuit arrangement of Figure 1 will now be described.

The application of message waves to the plates I oi. the cathode ray device I over the signal input circuit I6 will cause the electron beam for a predetermined interval of time.

The application of the electron stream to electrode 9 will make the electrode more negative and thus cause the grid of glow tube 69 to become more positive dueto the presence of condenser I00. The negative bias on the grid will thus be overcome and the glow tube caused to strike and pass current. A condenser 10 and a resistance 1| are in circuit with the grid of glow tube 69 and comprise, in effect, a time delay circuit which cooperates with the resistance-condenser arrangement 31, 68 for retarding the initiation of the flow of current through glow tube 69. Upon the striking of the glow, in the tube. relay 12 in the output of the glow tube is energized, thus closing contacts 13, 13 and actuating relay coil 1 I5 over an obvious circuit. If desired, an alarm 16, which may be a bell or any other arrangement. may be caused to function, as indicated. The energization of coil 15 will attract its magnet 16 and cause the rotation 01' bar 11 with armature I6 around the pivot 18. consequently raising the frictional drum 52 into contact with cylindrical extension 5| on the shaft of the motor 50, the operation of which will cause belt 53 to rotate contact over resistance 56. 51 by means of pulley 54 and thus control the volume of the transmitted message waves impinging upon miof amplification may crophone 58 through the medium of the grid 83 of vacuum tube 64. In this manner. the degree be reduced until overmodulation on peaks no longer occurs.

In order to permit the grid of the glow tube to regain control of the circuit once the glow has started, there is provided a circuit breaker or interrupter IOI in the anode circuit, the interrupter functioning in well known buzzer fashion to open the anode voltage sup ly path. Due to the presence of an appreciable charge in the timeconstant circuit connected to the grid, the low will strike several times until the charge is dissi-.

pated which will occur when over-modulation ceases. Electromagnet 12 will remain operated, however, during the rapid fluctuations in the anode circuit caused by the interrupter until the tube ceases to pass current for more than a momentary lapse of time.

Figure 3 shows another modification of the invention which may be utilized instead of the time delay condenser-resistance combination 61, B8 and the glow tube circuit 69 of Figure 1. In

' this circuit if modulation becomes excessive, light will fall through one of the two apertures in mask 4| 43 onto the photo-electric cell 44, whereupon the relay 12 is made operative through the passage of current in a three-element vacuum tube 45. These apertures are symmetrically located relative to the central position of the cathode ray spot. The operation of relay 12 closes contacts 13 14 for controlling the protective device in the same manner shown in '01 the over-modulation 3 Figure 1. For energizing the vacuum tube 45 there is provided a 60 cycle alternating current source which heats the filament through the transformer 46. The potential of the filament is determined by tap 40 on resistance 18 which is bridged across a direct current supply

source having terminals

19 and 80. There is provided a grid circuit resistance 8I for properly biasing the grid of tube 45.

In Figure 4 there is shown a dashpot arrangement which may be used to eflect rapid control protective device and slow restoration of the volume control arrangement back to normal. In this arrangement the pulley 54, belt 53 and contact 55 are the same as those disclosed in Figure 1. Attached to the pulley 54 is a shaft to which there is secured a wound up spring I30, one end of which I32 is attached to the shaft and the other end of which MI is fixed externally in a manner not shown. At the end of the shaft opposite the pulley 53 is a gear I38 which meshes with a rack I33, the lower portion of which is attached to a dashpot. shown in cross section. comprising a cylinder I35 in which moves a piston I33. and attached to the piston is provided a valve I31 which is retained in place by springs I33. A viscous medium I39, such as air, oil or other suitable material, is arranged in the manner indicated to prevent too rapid and undesired movement of the piston I36 in one of its directions of travel. Rack I33 is arranged so that it can be driven downward rapidly, but will return upward under the action of spring I30 slowly and at a. rate determined by the flow of the viscous material I39 through the small holes in the valve I31. Accordingly, the device will function as follows:

Assuming that over-modulation occurs. contact 55 will be suitably displaced rapidly and rack I34 will be driven downward rapidly, thus winding up spring lation occurs the rack I34 will rise slowly, gradually carrying contact 55 towards settings corresponding io higher modulation. If the modulation then becomes excessive the process will be repeated. It is preferred that the rate at which-rack I34 rises should be quite slow, for example. several tens of seconds. in order to cause a change in the position of contact 55 corresponding to a marked change in the percentage of modula ion. The time delay thus In the cylinder introduced by the dashpot should be considerably longer than incidental silent periods in the program for avoiding over-modulation at the end of such periods.

In Figure 5 there is shown a delay circuit adapted to retard the signals in the transmiter for a time sufiicient to enable the correcting circuit to adjust the sensitiveness of the transmiting modulation circuits just before the arrival of the signals in the transmitter. The circuit provides an adjustable delaying device which retards or stores the message waves without appreciably distorting the music or speech current for a time suificient to enable conact 55 of the volume control arrangement shown in Figure 1 to be. moved to lower modulation points just before the modulation currents reach the modulation circuit of the transmitter from vacuum lube 64 of Figure 1. In the arrangement shown. there is provided a telegraphone wire I58 which is driven by pulleys I50 and I 5I and passes between pole pieces I53 and I51 of energizing coils ed to be energized by the message waves through terminals I52 and I53 which may be connected in any suitable manner by transformer or otherwise to the circuits associated with the anode and cathode of vacuum tube 64 of Figure 1. The magnetized steel wire I58 is arranged to carry the sound record to the pole pieces I64 and I65 associated with coils I62 and I63, respectively, and to induce in these coils the speech or music currents recorded on the wire. The output terminals I60 and I6! in circuit with coils I62 and I63 are connected to the input of the remainder of the amplifiers of the modulation system which is shown in Figure 1 diagrammatically by box 80, the latter, of course, not being connected to the point of signal input I6 in this instance. The output of tube 66 will be connected to both the input coils I52 and I53 of the telegraphone and also to the point of signal input I6. Coil I66 is a powerful electromagnet and serves to magnetically clean the wire 50 that it may again be used after passing under coils I66 and I65. It will be understood, of course, that the separation between pole pieces I56 and I51 and I64 and I65, together with the rate of travel of the wire I58 determines the retardation of the message wave.

Alternatively, instead of the telegraphone there may be used, in the same way, well known speech or music retarding filter circuits of the non-distorting type such as are employed, for example, in certain types of trans-oceanic radio reception and transmission systems.

It will thus be seen that there are utilized in the present invention two types of time delay circuits; namely, a type of delay circuit such as 61, 68 which permits the over-modulation protective control to operate only if a number of audio frequency cycles of high amplitude have occurred, and also a type of delay circuit such as the telegraphone arrangement which is used to delay any modulation of the transmitter by the audio frequency wave in question until after the control for correcting over-modulation has been able to act.

To some extent, the functioning of the complete system described approaches closely in operation to what would be done by a skilled control room operator. In the operation of the device the volume control is operated and modulation considerably reduced just before overmodulation might occur. The automatic device then cautiously and gradually increases modulation until traces of over-modulation are imminent, whereupon the process is repeated. Overmodulation is thus avoided, but the modulation is maintained at high values by a mechanism which foresees possible over-modulation and prevents it.

I claim:

1. In combination with a device for producing a sharply defined electron stream in the form of a beam of energy and means for deflecting said beam in accordance with the amplitude of the signal waves applied thereto, of additional means responsive to the deflection of said beam a predetermined amount for reducing the range of amplitude of the signal waves applied to said device.

2. The combination with a cathode ray device, of a broadcasting transmitter for transmitting modulated message waves to said cathode ray device, a circuit responsive to said modulated message waves for deflecting the electron beam of said device, and means responsive to the movement of the beam of said cathode ray device outside a predetermined limit resulting from overmodulation of the message waves to control the modulation of the transmitter.

3. The combination with a cathode ray device for producing a sharply defined electron stream in the form of a beam, of means to apply signal waves of varying amplitude to said device, and means in the path of the beam when deflected by a signal whose amplitude exceeds a predetermined value for controlling said first means to reduce the amplitude of said signals applied to said device.

4. In combination, an electron discharge device having a cathode, an anode, and a control electrode, said cathode being arranged to transmit an electron beam in the direction of said anode, a circuit for applying a modulated signal to the said control electrode for deflecting said electron beam, and means responsive to a predetermined deflection of said beam for controlling the degree of modulation of the signal in said circuit.

5. In combination, an electron discharge device having, within an evacuated container, a cathode for producing a beam of electrons, a control electrode for deflecting said beam in response to the application of modulated signals to said control electrode, and a search electrode within said evacuated container normally outside the path of travel of said beam, external means in circuit with said search electrode and responsive to the impinging of electrons on said electrode for controlling the degree of modulation of signals applied to the control electrode whenever the beam due to the action of said signals upon said control electrode departs a predetermined amount from its normal path of travel.

6. In combination, an electron discharge device having, within an evacuated container, a cathode for producing a beam of electrons, a control electrode for deflecting said beam in response to the application of modulated signals to said control electrode, a mask external of the evacuated container and containing apertures located normally outside the path of travel of said beam to permit the passage of energy from the electron beam whenever the beam departs a predetermined amount from its path of travel, and a photo-electric cell responsive to the passage of energythrough the apertures of said mask for enabling the control of the degree of modulation of the signals applied to the control electrode.

7. The combination with a cathode ray device,

of a transmitter for transmitting high frequency modulated signals to said cathode ray device, and a circuit including an electric discharge device responsive to the action of said cathode ray device whenever modulation corresponding to more than 100% modulation of the high frequency by the peaks of the signals occurs, to control the modulation of the high frequency by the signals at the transmitter.

8. In combination, an electron discharge device having, within an evacuated container, a cathode for producing a beam of electrons, a control electrode for deflecting said beam in response to the application of modulated signals to said control electrode, and a search electrode within said evacuated container normally outside the path of travel of said beam and in circuit with a condenser and a glow tube for controlling the degree of modulation of signals applied to the control electrode. whenever the beam due to the action of said signals upon said control'electrode departs a predetermined amount from its normal path of travel, said condenser being arranged to receive a charge for applying the same to said glow tube when electrons of said beam impinge upon said search electrode, said glow beingarranged to strike when the charge on said condenser reaches a desired value.

9. In combination, an electron discharge device having, within an evacuated container, a cathode for producing a beam of electrons, a control electrode for deflecting said beam in response to the application of modulated signals to said control electrode, and a search electrode within said evacuated container normally outside the path of travel of said beam and in circuit with a time-constant circuit and a glow tube for controlling the degree of modulation of signals applied to the control electrode whenever the beam due to the action of said signals upon said control electrode departs a predetermined amount from its normal path of travel, said time-constant circuit comprising a condenser connected in parallel with a resistance, said condenser being arranged to receive a charge for applying same to said glow tube when the electrons of said beam impinge upon said search electrode, the glow in said tube being arranged to strike when the charge on said condenser reaches a desired value.

10. In combination, an electron discharge device having, within an evacuated container, a cathode for producing a beam of electrons, a control electrode for deflecting said beam in response to the application of modulated signals to said control electrode, a mask external of the evacuated container and containing apertures located normally outside the path of travel of said beam to permit the passage of energy from the electron beam whenever the beam departs a predetermined amount from its path of travel, a

' photo-electric cell in line with said apertures and responsive to the passage of energy therethrough, a vacuum tube in circuit with said photo cell and arranged to pass current upon the energization of said cell, an output circuit for said tube comprising a relay whose contacts are adapted to close when said relay is operated, a transmitter for transmitting the modulated signals to said cathode ray device, and means responsive to the closure of said relay contacts for controlling the degree of modulation of the signals. sent out by said transmitter.

11. A modulation correction circuit having, in

combination, a cathode ray tube provided with deflecting means for deflecting the ray in said tube in circuit with a-signal input, a photo-cell responsive to the ray in said tube located outside the path of travel of said ray when no signals appear in said signal input, but in the path of the ray when the latter is deflected by a modulated voltage in said signal input above a certain value, and corrective means responsive to the action of said photo-cell for controlling'the character of the signal applied to said signal input.

12. A modulation protective device having, in combination, a cathode ray tube provided with deflecting means for deflecting the ray in said tube in circuit with a signal input, and means located outside the path oi! travel of the ray when no signals are present in said signal input but in thepath of travel when the latteris deflected by modulated voltage in said signal input above a certain value, and additional means responsive to the impinging oi the ray on said first means for controlling the degree of modulation of said modulated voltage.

13. In combination, a cathode ray tubehavina a cathode and a search electrode arranged in spaced relation to each other, an anode electrode adjacent to the cathode, a control electrode interposed between said anode and cathode and an accelerating anode adjacent to the search electrode, means for deflecting said electron beam, a signal input circuit connected with said deflecting means, a potentiometer device, means for supplying alternating current to the terminals of said device, a circuit connection between one terminal of said device and the cathode and between the control electrode and a first tap point adjacent to said terminal on said potentiometer device, means in circuit between the control electrode and the cathode for establishing on said control electrode a predetermined negative bias potential, a circuit connection between the first anode and a second tap point on the potentiometer device, and a third circuit connection with the said potentiometer device for the accelerating anode whereby said anodes and the control electrode receive alternating potentials in phase,'said search electrode being in the path of the ray when deflected by a modulated voltage above a certain value.

14. In combination, a cathode ray tube havin a cathode and an output end arranged in spaced relation to each other, an anode electrode adjacent to the cathode, a control electrode interposed between said anode and cathode, an accelerating anode adjacent to the output end, and means for deflecting said electron beam, a signal input circuit connected with said deflecting means, a' potentiometer device, means for supplying alternating current to the terminals of said device, a circuit connection between one. terminal of said device and the cathode and between the control electrode and a first tap point adjacent to said terminal on said potentiometer device, means in circuit between the control electrode and the cathode for establishing on said 'control electrode a negative bias potential sumcient normally in the absence of signals to prevent the application of the electron beam to the output end, a circuit connection between the.

first anode and a second tap point on the potentiometer device, and a third circuit connection with the said potentiometer device for the accelerating anode, whereby said anodes and the control electrode receive alternating potentials in phase, a mask external of said tube and having an aperture therein positioned adjacent to the output end, said aperture being in the path of the beam when deflected by a modulated voltage above a certain value, a photo cell in line With said aperture responsive to the passage of energy therethrough for enabling the control of the signals applied to said input circuit.

15. In a cathode ray tube system, a transmitter of modulated signals for controlling the path of the ray of said tube, a relay adapted to be energized by the ray of said tube for closing a plurality of contacts, and means including a time delay device responsive to the closureof the contacts of said relay for controlling the degree of modulation of the signals sent out by said transmitter, said time delay device functioning to insure that the degree of modulation of the transmitted signals is controlled before the initiated signals reach the modulation circuits of'the transmitter.

16. In a transmitter provided with over-modulation control, a source of message waves, a vacuum tube amplifier having an input circuit coupled to saidsource, a device comprising an electric charge carrying stream coupled to the output circuit of said amplifier, means for deflecting said stream in accordance with the character of said message waves, a grid controlled glow discharge device having an output circuit, means for rendering said glow device responsive to the deflection of said stream when the amplitude of said waves is in excess of a predetermined value, and electromechanical means coupling the output circuit of said glow tube and the input of said amplifier and responsive to the action of said glow tube for decreasing the gain of said vacuum tube amplifier.

1'7. In a transmitter provided with over-modulation control, a source of message waves, a vacuum tube amplifier for said source having an input circuit, a device comprising an electric charge carrying stream coupled to said source, means for deflecting said stream in accordance with the character of said messa e waves,'a grid controlled glow discharge device having an input and output circuit, means in the input circuit of said glow device for biasing said glow device at a potential below the critical value necessary to cause the .glow in said device to strike, and additional means in the input circuit of said glow device responsive to the deflection of said stream for raising the biasing potential of said glow device at least to said critical value for causing the glow in said glow device to strike, and electromechanical means for coupling the output circuit of said glow device to the input circuit of said vacuum tube amplifier for decreasing the gain of said amplifier whenever the glow in said grid controlled device strikes.

18. The method of preventing over-modulation in communication systems which comprises transmitting modulated message waves, receiving said waves and controlling the amount of deflection of a concentrated electron stream thereby, producing a the degree of modulation of said waves exceeds a predetermined value, and utilizing said voltage impulse to reduce the amplitude of the transmitted message waves.

ALFRED N. GOLDSMITH.

voltage impulse whenever