US2361585A

US2361585A - Radio warning system

Info

Publication number: US2361585A
Application number: US432549A
Authority: US
Inventors: Henry W Appel
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-02-27
Filing date: 1942-02-27
Publication date: 1944-10-31
Anticipated expiration: 1961-10-31

Description

Oct. 31, 1944. H. w. APPEL Y RADIO WARNING SYSTEM Filed Feb. 27. 1942 Patented @ein 3i, lifascites anni@ wma sys'rawr Henry W. lippel, Kew Garden Hills, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 27, 1942, Serial No. 432,549

13 Claims.

My present invention relates to radio warning systems, and more particularly to an improved and highly efficient form of alert receiver capable of emitting an intermittent siren signal upon reception of a predetermined sub-audible control frequency.

There has been disclosed by H. B. Deal, in application Serial No. 403,736, filed July 23, 1941, ,February 1, 1944, as U. S. Patent No. 2,340,798, a radio Warning system wherein sub-audible modulation was superimposed upon normal modulation at the transmitter, and responsive relays were employed at the receiver. These relays were so arranged that one sub-audible modulation tone controlled the receiver reproducer circuit, while another sub-audible modulation tone rendered reproduction ineffective. In that patent selective reeds were utilized.

It has also been proposed by G. L. Beers, in application Serial No. 432,607 filed February 27, 1942, to use only a single sub-audible control tone which is transmitted during an alert period.`

A warning signal is provided by permitting the audio amplifier to oscillate whenever the control tone is being transmitted. An electronic device is employed to silence the audio amplifier in the absence of the sub-audible modulation tone, While another electronic device is employed to render the first electronic device inoperative upon the reception of the sub-audible modulation tone.

It may be stated that it is one ofthe important objects of this invention to provide a radio warning system of the type using modulation tones of different sub-audible frequencies for rendering operative or inoperative an audio oscillation network employing a device for producing an intermittent siren effect. l

Another important object of my invention is to improve the audio feedback circuit of the aforementioned Beers system and to provide a feedback path between the voice coil of the reproducer and the input circuit of the first audio amplifier, the feedback path being designed to produce an audible signal in the low frequency portion of the-audio range. t

Another object of my invention is to provide in Still other objects of this invention are to improve generally the simplicity and efficiency of radio .warning systems and more especially to provide such systems in a compact, economical and readily-assembled manner.

The novel features which I believe to be char acteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown the circuit detailsof a radio receiving system which includes the improved net- Works and devices. In general, the receiving system comprises receiver circuits designed to receive modulated carrier waves such as are transmitted from standard broadcast stations. For example, the receiving system can be the usual type of superheterodyne receiver employed in the broadcast band of 550 to 1700 kilocycles (kc). However, it is to be clearly understood that the present invention is not limited to that band and the receiver, by proper choice of tuned circuits, may operate in the short wave bands. Furthermore, the invention is not restricted to amplitude modulated carrier waves, since it may be used in connection with frequency modulated, or phase modulated, carrier waves.

Considering rst the networks of the receiver, the numeral I denotes a converter stage which "may be a combined first detector-local oscillator collector device may be of any desired type. The

an alert type of receiver a self-generated siren which comprises an audio feedback circuit'employing a device which is capable of varying the tone of the siren at a predetermined low freintermediate frequency (I. F.) energy produced in the output circuit of the converter is transmitted to an I. F. amplifier. The latter is designated by the numeral 2 and is provided with a resonant output circuit 3' tuned to the operating I. F. value. The following resonant circuit 4 is magnetically coupled to circuit 3, and both circuits may be tuned to the aforesaid I. F. value. The latter may be of the order of 400 kc., or any other desired value. Reference is made to the aforesaid Beers application for a disclosure of circuts which may be employed in networks l and The numeral designates a tube of the diodetriode type. The common cathode cooperates with the auxiliary anode 6 to provide a demodulator. or second detector, device. The anode 6 is connected to the high potential side of. input circuit 4, the low potential side of the latter is connected to ground through a path which includes a resistor element 1 and a potentiometer resistor 8. The path 1-8 functions as the diode load. Direct current voltage developed thereacross is utilized for automatic volume control (AVC) and the AVC bias is applied over lead 9 to the signal grids of the converter and I. F. ampliiier tubes. Pulsatlon voltage filter resistors I6 and l0' are inserted in the AVC lead. The action of the AVC circuit is well known.

The resistor element 1 is shunted by a pair of condensers and l i' while the cathode of tube 5 is grounded and connected to the junction of condensers il and Il'. The audio voltage developed across the diode load resistor is applied to the audio signal grid |2 of tube 5. 'I'.ie grid l2 is connected to the adjustable contact element i3 of potentiometer resistor 3.. This connection path includes condensers |4 and |5 and resistor I6. The junction of condensers I4 and |5 is Iconnected to ground by resistor. l1. This last mentioned network, between adjustable element I3 and grid I2, effectively prevents the transmission of V,any carrier component to the audiolgrld.

The grid return path for the audio section of tube 5 may be traced through the path comprising resistor I8, lead i9, the secondary winding 26 of audio output transformer 2|, lead 22, resistor 23 and resistor 24 to ground. At this point it may be stated that the voice coil 25 of the loudspeaker reproducer 26 is connected in shunt across secondary winding 26. 22 is by-passed to ground by condenser 22. It will, therefore, be observed that I have provided an audio feedback path from the voice coil of the reproducer to the grid circuit of the audio ampliiler fed by the demodulator.

Returning to the normal path of signal transfer through the audio amplifier network, the amplified audiovoltage is developed across plate load resistor 21. The numeral 2B denotes the audio power output tube. For the sake of compactness tube 28 is shown as one ofthe type including within a common envelope the electrodes `of a beam power tube, and the electrodes of a diode rectier. Specifically tube 28 is of the '10 L'IGT type. The signalinput grid 29 is corinected to ground by grid return resistor 35. The cathode 3| is connected tofground through two paths. One of these paths may be traced from lead 32 to the contact point 33 of a control switch, the flexible contact element 34 thereof, lead 35 and biasing resistor 36 to ground. The by-pass condenser 31 by-passes resistor 36 for audio frequency currents. Hence, resistor 36 provides normal bias for grid 29. The second path from cathode 3| to ground consists of lead 32, lead 38, switch 39, lead 40, lead and biasing resistor 36. Hence, it will be seen that when switch 39 is open, then the cathode circuit of the power amplifier tube has been opened. This is the alert position of the receiving system. Closure of switch 33, 34, due to reception of the proper sub-audible modulation control tone, will then close the cathode circuit of the second audio am plifler circuit. When switch 39'is in its closed position, then normal reception of program modulation on the received carrier waves is had.

The lead ,sary to describe the construction of the beam amplier section of tube 28, since those skilled in the art are fully aware of such construction` The plate 43 thereof is connected to the upper end of the primary winding of output transior'mer 2i. The cathode 44 of the diode section of tube 28 is connected to an intermediate point 45 on the primary winding, While the anode 46 is connected through a resistor to the ungrounded side of the power input line 41. As schematically represented, the filaments of the various tubes are shown included between ground and 41 so that the laments are heated from the power line. W

The grounded side of the power line includes a volume control switch to permit control over the energlzation of the entire system. The primary winding of transformer 2| is shunted by the by-pass condenser 48. The intermediate tap 45 is connected by lead 49 to the voltage supply potentiometer which comprises the resistor sections 50, 5| and 52 arranged in series between ground and lead 49. Appropriate by-pass condensers to ground are employed at the potentiometer sections. .It will, therefore, be seen that the

rectiler

46, 44 acts to rectify the alternating current from the power line, and that the rectified voltage, after proper iiltration, is employed to supply the various potentials for the system. Thus, the plate circuits of the tubes in stages l and 2, tube 5 and plate 43 are all supplied with positive potential from the lead 53.

There will now be described the control relays which function to determine whether the siren tone shall be reproduced or not. Two electromagnetic relays are employed. One of them is the o" relay, while the other is the "on relay. The iirst named relay comprises a yoke 54 which is provided with a magnetizing winding 55'. One end of winding 55 is connected bylead 56 to a metallic contact 51. The opposite end of winding 55 is connected'by lead 58 and a resistor 53 to the energizing lead 45. The contact 51 is a 'part of a switch whose mobile element is a reed 59 constructed to vibrate solely at 24 cycles. Asshown in the drawing, the upper end of the reed terminates in a contact point which completes the electric circuit through ground upon contact between

elements

51 and 59. The lower end of the reed is xedly secured to the, support Gil which is horizontal, and the latter is carried by a vertical spring element 6| whose lower'end is rigidly secured to chassis of the system. An L'shaped armature 62 has the right end of its horizontal section rigidly secured to the exible member 6|. The armature 62 is adapted to be attracted toward the electromagnet 63, which may be constructed in the manner of theelements 54, .55. The energizing winding of electromagnet 63 has one end thereof connected to the junction of potentiometer sections 5|, 52. The opposite end of the winding is connected by lead 64 to the plate 65 of one of the triodes of twin-triode tube 66. The tube 466 is included in a noise gate network which will be described aaeaeee frequency of 36 cycles, and is denoted by the numeral 61. As in the case of reed 59, the reed 61 has its upper end functioning as a switch element for the contact stud 68. 'I'he stud 69 is connected by lead 69 to one end of the energizing winding 1li of the magnetic yoke 1l. The opposite end of winding is connected through resistor 59 to the energizing lead 69. It will now be seen that closure of

switch

61, 69 energizes winding 19 which causes magnetization of yoke l l. v

The yoke 56 has pivotally associated therewith a metallic armature 12 which is normally biased away from the yoke by spring 18. It is believed that further description of the relay elements is unnecessary, since it is of well known construction and it is suiiicient for the purposes of this application schematically to represent the relay. The yoke 1I is similarly provided with a pivoted armature lll which is spring-biased by' spring 15 to position away from the yoke. The free ends of armature 12 and M are so relatively constructed that when the free end of armature 18 is pulled into contact with yoke 1l-,. the free end of armature 12 will ride over the free end of armature 19 due to the biasing action of spring 13. of a device to guard against accidental release of armature 14 during such periods when the yoke 1l is supposed to be energized. Oficourse, when yoke 59 is energized to pull armature 'l2 towards it. the armature 1 6 will readily be capable of release.

The armature 14 is constructed so as to control three switches. One of these switches is 33, 34. and the latter is the middle one of a bank of switches. The lowermost switch in the bank comprises the fixed contact 8l] which cooperates with the flexible contactor 8l. The uppermost switch comprises the xed contact 82 which cooperates with the flexible contactor 83. The ends of each of contactors 83. 34 and 8l are fixed on one side. while the opposite ends are xedly secured to spaced points of an insulation member 84. The lower end of insulation member 86 is rigidly secured adjacent to the free end of armature 14. Conseouently armature 19 and the spaced contactors 83, 34 and 3| move as a single unit. This is a well known form of relay switching construction and will be well understood by those skilled in the art.

Considering now the electrical circuits to each of the switches, contacter 83 is connected byI a lead 85 to the ungrounded side of the power line, while Contact point 82 is grounded. An outlet plug 86, adapted for any other control service.

may be included in the line 85. 1t will, therefore, be seen that the

switch

83, 82 is provided for the purpose of an auxiliary indicator'service which may be energized through the outlet plug 86. It has already been explained that switch 36. 33 functions to control the effectiveness of the cathode circuit of the amplifier section of tube 28. The contactor 8l of the lowerx'nost switch is connected by lead 61 to the feedback lead i9. The contact point 89 is connected through a switch 88 to ground. The

switches

88 and 39 are represented as arranged for concurrent closing or opening. It will therefore be seen thatl switch 80, 8|, upon closure, completes a path through lead 81 and switch 88 to ground, if switch 88 is closed. This path will act to short-circuit the audio feedback path Hence, armature 12 functions in the manner are selectively operated by magnet 63.

should the listener throw switches 98 and 89 into the closed receive positions.

As has been stated heretofore, the audio oscillations produced due to the audio feedback are intermittently interrupted. The intermittent operation is provided by a neon oscillator circuit which includes the neon tube 9i) one of whose electrodes is connected to ground through the resistor 9|, while the opposite electrode is connected by lead 92 to the positive voltage line 53. The grounded electrode of neon tube 98 is further connected through condenser 99 and lead 95 to the junction of

resistors

29 and 26. Rel sistor 9i is shunted by condenser 93. The network 9|, 93 provides a time constant circuit for the neon oscillator. Since the laments of the various receiver tubes are arranged in series with the-

rectifier

66, 99, failure of any of these filaments will be indicated by neon tube 99. Hence, the neon tube functions to perform several operations. In the first place, it acts to produce interruption of the siren tone; secondly, it acts to visually indicate whether the receiver tubes are energized, and lastly, it indicates intermittent operation by virtue of its flashing on and off at the period of interruption of audio feedback.

As explained previously, the.

reeds

59 and 61 In order to energize magnet 63 it is merely necessary to derive the modulation control voltage from the second vdetector circuit, and use such voltage to energize the windingof relay 63, 62. However, there is provided in circuit with the demodulator load4 resistor a noise gate which guards relay 63. 62 from operation by noise impulses. This noise gate circuit is not a part of my present invention, and, therefore, it is believed that a general description of this circuit will be sufficient for the purposes of this application.

The rectified sub-audible tones are taken off bv lead 96 from the anode end ofk resistor 1. Filters. of the appropriate design, reduce the undesired program'audio components, and apply to the grid 91 of the first triode section the sub- 'audible tone with any superposed noise pulses. The grid 91 is coupled to lead 96 by condenser 98. the grid return resistor 99 connecting the grid 91 to ground. 'Ihe cathodes of both triode sections of tube 66 are connected to ground. The plate |99 of the rst triode section is connected to the positive voltage supply source through a voltage reducing resistor lill. The output of the first triode is applied to the grid 65' of the second triode through coupling condenser |92. The plate 65 of the second triode is connected through lead 6d to the 'energizing winding of elcctromagnet 63.

The noise pulses present at the grid 91 are derived from the demodulator diode with a negative polarity, and on aonlicaton to grid 91A produce plate current cut-off in that tube. The effective .positive voltage on plate |00 is made small to make the plate current cut-off take place at a low value of grid voltage, and also to limit the `plate current 'of this tube. A double limiting action is 'thus produced. The second triode `of tube 66 funct'ons as an amplifier of the limited voltage. The output current flowing from plate 65 through lead 66 energizes the d'ivirvg coil of electromagnet 63 and this sets the armature 62 in motion. This in turn vibrates the reeds. Of course, only that reed will vibrate which is tuned to the frequency of the current flowing through the driving coil. The diode output voltage may vary linearlywith percent modulation, but the limiter output is substantially constant above 25 modulation. The reed actuating voltage developed at plate 65 follows the same variation as the output voltage at plate indicating a constant output above 25% modulation. -A reed voltage of 12 volts (peak to peak) is just sufficient to operate the relay system. This is much less than the maximum output applied to the reeds. 'I'his wide tolerance is desirable to take care of 'possible manufacturing variations in tuned reed frequency.

'I'he reeds may vibrate slightly under noise interference impulses. To prevent contact in the case of each of the reed contacts, the distance'between each pair of contacts must be made large enough. A distance of @fof an inch was chosen as a satisfactory compromise between the reed -motion required for contact and the spacing necessary to avoid noise actuation. This eliminates the possibility of false signaling. A 24 cycle reed is chosen to operate the off" relay, while a 36 cycle reed is selected for the on relay. The reeds are very sharply tuned to resonate at their specified frequencies, and have a selectivity factor Q" of about 250. The use of the armature type drive keeps the reeds out of the magnetic circuit. This prevents any-influence of the magnetic field of 63 on the tuned frequency of each reed.

, It will be understood that each of members 60, 6| and 62 are metallic whereby the closure of the reed contacts provide energizing circuits for each of

relay windings

55 and 10. A constant direct current in the reed winding of relay 63 is necessary for proper operation of the vibrating armature 62. This is supplied by the plate current of the output section of tube 66. The broadcast sub-audible tone, taken off from the detector, causes that reed to vibrate which is tuned to the tone frequency. The reed amplitude builds up to a large resonant swing, and the reed contacts are closed, or sparked, at every swing of the energized reed. A capacitor H0 is connected in shunt with relay winding 55, and a capacitor is connected in shunt with winding 1li. each capacitor being-charged up from the power supply capacitor-every time its respective reed contact is made. Each relay capacitor discharges through its associated. relay winding at a peak current intensity of approximately 38 mlliamperes. 'I'his current is sufficient for positive relay action. f

Considering now the operation of the system, it will be understood that the manual switches 39, 88 are actuated to open position to place the receiver in the alert state. The power supply switch has been closed, and 'hence the filaments are all heated which means that the receiver has been conditioned for the control frequencies. At the. broadcast transmitter there will be applied either the 24 cycle or 36 cycle modulation tone to the carrier. Assuming that the 36` cycle modulation tone has been applied to the carrier, the collected modulated carrier wave is received. converted to the I. F., amplied at the I. F., and then detected. Sub-audible signal sensitivity of the receiver is considered to be the minimum antenna input necessary to operate a relay with 5% modulation at 36 cycles. The 5% level represents a conservative rating for sensitivity. It is expected that broadcast stations will modulate in the region of twenty or thirty percent. The detected 36 cycle component is fed over lead 96 to the noise gate network.' Any noise impulses will be eliminated, as explained previously. The 36 cycle frequency current flowing with constant amplitude in the circuit connected to plate 65 will energize the driving winding of relay 63. As a result armature 62 will be vibrated with the consequent actuation of reed 61 suiiicient to close the electrical circuit through relay winding 1,6.

Energization of winding 10 results in attraction of armature 14 against the bias of spring 15. This results in closing of each of switches 83, 82, 34, 33 and 8|, 80. concurrently, the free end of armature 12 will ride over the free end of armature 14 thereby acting as a keeper forthe latter. Accordingly, the Operator at the transmitter need only apply the 36 cycle modulation to the carrier for a short period of time, say 5 seconds. This follows from the fact that at the receiver attraction of armature 14 will cause the free end of armature 12 positively to lock armature 14 in place.

Closure of

switch

83, 82 permits plug to be used. Any alarm device not requiring more than three peak amperes may be plugged into 86. 'I'his provision will be useful for installations requiring a visual warning signal, or an audible signal at locations remote from the receiver. Closure of switch 34, 33, as explained previously, closes the cathode circuit of the power amplifier tube whereupon audio feedback from the voice coil to the grid circuit of the first audio amplifier is permitted to provide the audio oscillations which are reproduced by reproducer 26. A distinctive siren tone of the order of some 300 cycles results. By proper choiceof the coupling elements in the audio circuit, such as condenser 4I and the inherent low frequency resonance `of voice coil 25 produces a typical siren wail The audible tone thus generated is varied in frequency by means of the neon tube oscillator which is permanently in circuit. The siren tone shifts up and down at a period of approximately 0.5 cycle per second. This intermittent low pitched siren effect is distinctive and highly desirable for devices of this character. l

It is necessary to have a switching provision to out off the alarm signal in order to hear voice modulation from the speaker. Hence, the manually

operable switch

39, 88 is provided so that the operator can throw the switch into receive position. As explained previously, closure of switch 88 short-circuits the audio feedback path, and effectively removes the siren.` Hence, the

.reproducer will be able to reproduce the voice modulation of the broadcast station, with any instructions that it is desired to transmit. Hence, it will be seen that switch 39. 88 provides a means for receiving voice modulation without having to radiate from the transmitter the 24 cycle off tone. When the 24 cycle tone is radiated, and after ldemodulation at the receiver, reed 58 will be vibrated and ycause energization of winding 55. Armature 12 will be pulled from its keeper position. Accordingly, the armature 14 will be pulled away from yoke 1I due tospring 15. Of course, it is only necessary to apply the 24 cycle vmodulation to the carrier for a short period of assise vention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. A method of signaling which includes modulating a carrier wave with a control frequency, transmitting the modulated wave, deriving the control frequency from the transmitted wave at a reception point, producing oscillations of audible frequency at said point in response to reception of said control modulated carrier wave, and interrupting: the production of oscillations at a relatively low period.

2. A method of signaling which includes modulating a, carrier wave with a sub-audible control frequency, transmitting the modulated wave, deriving the control frequency from the transmitted wave, producing oscillations whose frequency is in the low portion of the audiol range in response to said control frequency, generating auxiliary oscillations of a relatively low frequency, and utilizing the auxiliary oscillations to interrupt said first oscillations.

3. A method of radio warning signaling with a radio receiver of the type including a demodulator and an audio amplifier followed by aireproducer, which includes transmitting to the receiver a modulated carrier wave having included in its modulation a sub-audible modulation tone, demodulating the modulated carrier wave, transmitting the demodulated Wave energy through the audio amplifier, producing oscillations of audible frequency in response to the said modulation tone, providing interruption oscillations of a relatively low period, and utilizing the latter oscillations to interrupt the production of said audible oscillations.

4. A method of signaling air raid warnings which includes receiving a carrier wave modulated with audio frequency components, said wave including a control frequency of a predetermined 5. In a radio warning system, a modulated carrier wave transmission network, a demodulator, a first audio amplifier, a second audio amplifier, a reproducer having a voice coil coupled to the output of the second audio amplifier, an audio feedback circuit connected between said voice coil and the input electrodes of said first audio amplifier to provide oscillations .of an audible frequency, a neon tube oscillator circuit operatively associated with said feedback circuit intermittently to interrupt said oscillations, and means responsive to the existence of a control signal of a predetermined frequency in the modulation on the received Wave for controlling said feedback.

6. A method of signalling which includes modulating a carrier wave with a first control frequency, transmitting the modulated wave. derlv ing the control frequency from the transmitted wave at a reception point, producing oscillations of audible frequency at the said point in response to the derived control frequency, interrupting the production of oscillations at a relatively low period, and replacing the firstl control frequency by a different control frequency to render said oscillation production ineffective.

7. A metnod of signalling which includes modulating acarrier wave with a nist sub-audible control frequency, transmitting the modulated wave, deriving the control frequency from the transmitted wave, producing oscillations whose frequency is in the low portion of the audio range in response to said derived control frequency, generating auxiliary oscillations of a relatively low frequency, utilizing the auxiliary oscillations to interrupt said first oscillations, and preventing said oscillation-production in response to a frequency change of said control modulation.

8. A method of radio Warning signaling with a radio receiver or' the type including a demoduiator, an audio amplifier and a reproducer; com prising transmitting to the receiver a modulated carrier wave having included in its modulation a Iii-st sub-audible modulation tone, demodulating the modulated cariier wave, transmitting the demodulated wave energy through the audio ainpliiier, producing oscillations of audible frequency in response to tne said modulation tone, providing interruption oscillations of a relatively low period, utilizing the latter oscillations to interrupt said audible oscillations thereby to provide an undulating audible signal, and replacing said first sub-audible tone with a second sub-audible tone of different frequency at the transmitting step thereby automatically to stop said audible signal.

9. A method of signaling an air raid warning which includes modulating a carrier wave with a sub-audible control frequency, transmitting the modulated wave, deriving the control frequency from the transmitted wave at a receiver, producing oscillations of audible frequency and of the order of 300 cycles in response to the derived control frequency, and interrupting the production of oscillations at a relatively low period thereby to provide a siren effect.

10. A method of signaling which includes modulating a cariier wave with a sub-audible control frequency, transmitting the modulated wave `to a. receiver, deriving the control frequency from 4 the transmitted wave, producing oscillations at the receiver whose frequency is in the low portion of the audio range 4in response to said control frequency, interrupting the oscillations at a relatively low frequency, and replacing said subaudible control modulation by a different control modulation of different sub-audible frequency to stop said oscillations.

11. A method of radio warning with a radio receiver of the type including a demodulator and an audio amplin'er followed by a reproducer, which includes transmitting to the receiver a modulated carrier wave having included in-its modulation a sub-audible modulation tone, demodulating the modulated carrier wave, transmitting the demodulated wave energy through the audio amplier, producing oscillations of audible frequency in response to the said modulation tone, providing interruption oscillations of t a relatively low period, utilizing the latter oscillations to interrupt the derivation of said audible oscillations, and transmitting to the receiver a to replacement of said modulation frequency by a diiferent modulation frequency.

13. A receiver provided with atleast a demodulator, audio channel and sound reproducer; the improvement comprising normally ineffective means for providing regenerative feedback in said audio channel, a low frequency oscillator circuit for intermittently preventing said feedback, and means, responsive to a modulation control tone in demodulator output, for rendering said feedback effective.

I-ENRY W. APPEL.