Nov. 5, 1946. R. A. LYNN 2,410,669 PROGRAM CHANGE INDICATÓR Filed Feb. l5, 1945 NB äÉIl. mmfQ b œ.oiSkìnâgse Patented Nov. 5, 1946 2,4l0,669 UNITED STATES PATENT OFFICE 2,410,669 PROGRAM CHANGE INDICATOR Roland A. Lynn, Teaneck, N. J., assignor to Radio Corporation of America, a corporation of Dela Ware 1 Application February 13, 1943, Serial No. 475,728 9 Claims. (Cl. Z50-17) This invention relates to program change in dicators and has particularly to do with a methodof automatically monitoring the modulated wave of a transmitted radio signal and compa-ring it with the original wave at the point of origin. The invention also has other uses, as will be ap parent to those skilled in the art. In the ñeld of broadcasting the possibility eX ists, unfortunately, that the wire channel be tween a studio and the transmitter might be sev ered by an act of sabotage, and at the same time an unauthorized program might be substituted to modulate the transmitter’s carrier. The substi tution of such a program, if of a subversive na ture, could easily create a panic. Program cir a system which departs from practices of the prior art in the following respects: v I have arranged for my alarm device to respond only to unfavorable comparisons between the vol ume level of the studio program and that of the same program as received on a monitoring radio. Heretofore, systems have been tried in which the alarm would be subject to failure if the trans mitter modulation output continued during the absence of studio output. A failure of the alarm also resulted from the application of a relatively high level substitute signal to the transmitter modulator during a period of low volume output' from the studio. Such “one-way” alarm systems 15 are to be avoided in a dependable monitoring cuits between studios and associated transmitters system such as I have disclosed herein. might be subject to such acts of sabotage with One of the Weaknesses of the comparison de out the knowledge of anyone in the studio. Fur vices used in the past arose from attempts to thermore, no dependable means are available at the transmitter whereby a substitute program 20 control the gain in the ampliiiers by half-wave rectiñcation of the signal components. I have could be detected, especially if it appeared to be foundthat full-wave rectification is practically innocuous. essential to a successfully operable device of .this Accordingly, it is an object of my invention to character. provide a radio program monitoring system in cluding a comparison device by which an alarm 25 My invention will now be described in more detail, reference being made t0 the accompany may be sounded, or a visual indicator may be ing drawing, the sole ligure of which represents actuated, Whenever a difference occurs between in diagrammatic form a preferred circuit ar the modulated waves applied to an outgoing line rangement. ' and such waves brought in for comparison by a The terminals I are connected in any suitable radio receiver which is tuned to the associated 30 transmitter. . It is another object of my invention to provide a system of the class described in which an alarm device may be actuated whenever there is a de manner to a studio output circuit on which a modulated audio frequency wave is impressed. The energy is applied across transformer 2 to a load impedance 3 having a center-tap 4 which parture from a normal ratio between the volume 35 connects with a center-tap on the secondary of transformer 2. The impedance 3 may be used as of the studio output signal and the volume of the incoming monitoring signal from the transmit ter. This means, of course, that the alarm will be sounded whenever a pause occurs in one modu a Voltage divider, taps from which are connected respectively with the control grids in two dis charge tubes 5. These tubes are connected for lated wave without a corresponding pause in the 40 push-pull ampliiication. Their cathodes are con nected through a milliammeter E to a suitable point other modulated wave. It means, further, that on a voltage divider l. By this connection, the an increased Volume in one wave without a si normal cathode potential is maintained at a suit multaneous increase in volume in the other wave able value above ground potential, as indicated shall likewise be used to actuate the alarm, According to another object of my invention, 45 atv the left end section of the Voltage divider 1. The amplifier tubes 5 are preferably of the means are provided in combination with the afore pentode type, for the reason that they possess mentioned comparison device for producing a a variable mu characteristic which is of value certain limiting action upon the signal energy in obtaining a wide latitude of control action. component which is derived directly from the studio output circuit and which is applied to the 50 The screen grids are connected through resistor 32 to junction I6 on the voltage divider 1, Each comparison device. The purpose of the limiter suppressor grid is connected to its cathode. The is to compensate for a limited modulation which anodes are connected to the primary terminals is characteristic of the signal as radiated by the of a transformer 8. This primary has a center transmitter. . tap to rwhich anode potential is applied from the In carrying out my invention, I have provided 55 right hand end of the voltage divider 1. »A power 2,410,669 4 The common cathode of tube 23 is connected to point A on a balanced bridge circuit consist ing of four resistors 25, 25, 21, and 28. Resistors and for other tubes as Well. 25 and 26 are preferably made equal to each The anodes of tubes 5 are also interconnected 5 other. Resistors 21 and 28 may, if desired, be by an impedance circuit I0 at the center of which of the order of one megohm each. These values, is a connection with the center-tap on the pri however, are not critical. ' mary of transformer 8. One arm of ‘this im Point C on the bridge circuit is >connected to pedance circuit is used as a potentiometer for the mid-tap on the secondary of transformer 8. ywithdrawing a signal voltage component to be Point D is grounded. Point B on the bridge cir used for amplitude limiting. This signal com cuit is diametrically opposed to point A and re ponent is impressed through an impedance which ceives rectified signal potentials from a circuit includes resistor Il, capacitor I2, and resistor i9, presently to be described. the latter being connected to the voltage divider Referring now to the lower portion of the cir 1 at point I8. The control grid in a discharge cuit diagram, I show input leads 4I to which are tube I3 responds to this signal potential as ap applied audio frequency output energy from a plied acrossV resistor I9. This tube is preferably monitoring radio receiver. The primary winding of the duplex-diode-triode type. The triode sec of a transformer 42 is connected to the terminals tion includes the cathode, control grid, and main lil. The secondary winding 3d of transformer anode I4. The diode section includes two anodes e2 feeds energy to a load impedance 44 which is I5 adjacent the cathode. The D. C. potential 20 preferably composed of a potentiometer having of the cathode is fixed by its connection to point a tap connected to the control grid of an ampli I1 on potentiometer 1. der tube 45. One terminal of the secondary 34 The main anode I4 is supplied with a plus D. C. is grounded and is also connected through a potential from a point I5 on the potentiometer 1, cathode resistor ¿it to the cathode of tube 45. this point being connected to one terminal of a Resistor ¿le is preferably shunted by a capaci primary winding in transformer 33. The other tor c1. terminal of this primary is connected directly to supply unit 9 is used to obtain all the necessary potentials for the different electrodes of tubes 5 Although the amplifier tube 45 is shown as a the anode I4. Transformer 33 has a secondary winding, the triode, tubes of other types may be used if de sired. Anode potential is supplied to the tube 30 terminals of which are connected respectively 45 through the primary winding of a transformer to the two anodes I5. This secondary also has .a di! from junction point I6 on the potentiometer 1. center-tap which is connected to ground through Transformer 4B is used for the purpose of cou resistor 22. The control grids in the push-pull pling the output from tube 45 to lthe anode cir amplifier tubes 5 are'biased in part by the po 35 cuit of a double-diode full-wave rectifier tube tential drop across resistor 22. ` e3. The secondary winding of transformer 48 A D. C. potential for normal grid bias is de is center-tapped andV is connected to point C rived from point I8 on the potentiometer 1 and ,on the above described bridge circuit. The ter is impressed upon the control grid of tube i3 minals of the secondary of the transformer 48 are through grid leak resistor I9. connected respectively to the anodes in tube 43. From the above description, it will be seen that the triode section of tube I3 is enabled t'o am plify a signal component, and this amplified component is rectified, causing a negative bias po tential to be impressed upon the control grids in tubes 5 in dependence upon the amplitude of the signal itself. Thus excessive peaks of energy are limited and only signals within a limited The common cathode of tube 43 is connected .to point B on the bridge circuit and also to a control grid in a discharge tube 43 which is used as a control value in a relayv circuit. rI‘wo discharge tubes t9 andäil are suitably connected to be controlled by differences of po tential in different arms of the bridge circuit so that either one of two relays 52 andì53 may volume range are fed across the transformer 3. be cie-energized whenever an unbalance of po The gain reduction is Very rapid. The time con 50 tentials occurs in the bridge circuit. Normally stant value of the gain recovery is dependent upon the relays 52 and 53 are energized by a conduc the values of capacitor 2l and the grid leak re tive state in the two tubes 49 and 5l). The relays sistor 22. These values may, for example, be of 52 and 53 are disposed in the anode circuits of the order of .25 mfd. and 4 megohms, respectively. tubes 43 and 53 respectively. These anode cir The limiter tube I3 serves a very useful purpose cuits also preferably include milliammeters 5l for under ordinary circumstances. It operates to convenience in observing the operation of the rapidly establish a lowered gain characteristic system. upon the advent of an audio wave form of ab The power supply unit 9 furnishes current for normally high peak value, but does not disturb the , operation of the relays 52 and 53 through the established operating bias of the tubes 5 during space paths of tubes 49 and 5@ respectively. The the presence of signals within a normal range 60 relay circuits are Completed through the cath of amplitude variation. The desired degree of odes of tubes il@ and 53, through the two arms the limiting action may be selected by adjustment of a potentiometer `54%, and through series re of the tap on potentiometer I0. In certain cases sistors 55 and 55 to ground. Resistor 56 is pref the limiter tube might be dispensed with and a erably made adjustable and the tap on potenti conventional automatic gain control circuit may ometer 5d may also be adjusted for balancing be vconnected to the input circuit of the amplifier the amplitudes of the currents through tubes 4:9 tubes 5. , and 50. The limited modulation wave output from In order to facilitate the starting up or re tubes 5 is fed across transformer 8 to- the anodes setting of the apparatus after an alarm and par of a double-diode full-wave rectifier tube 23. A ticularly in order to energize relays 52 and 53, desirable attenuation of the higher audio fre it is advantageous to momentarily short-circuit quencies is obtained by the use of capacitors 24 at least a part of the impedance of elements 55 whichi are shunt-connected across the anode andv 56. This is accomplished by actuating a leads of tube 23. Grounded by-pass condensers 7.5 key (not shown) which closes contacts ‘51 land 38 may also be provided for the same purpose. 5 2,410,669 thereby places resistor 58 of relatively low re sistance value in shunt with resistors 55 and 59. 'I'his operation enables the tubes 49 and 50 to draw more than a normal now of current and thus to energize the relays 52 and 53. Under normal operating conditions, the current flow through tubes 49 and 55 is sumcient to hold up the armatures 58 and 19, but may be insufficient to attract them across their gaps. Any one or more alarm devices may be actu ated whenever either of the relays 52 and 53 is 6 of such movement being dependent upon the de gree of unbalance between the rectification com ponents from tubes 23 and 43. Whenever point A is driven sufficiently nega tive with respect to ground so that the imped ance through the space path in tube 50 becomes too great to permit relay 53 to hold up its arma ture 10, this armature is released and causes the alarm indication to be made in such manner as 10 to show an abnormal output from the transmitter with reference to the studio output. Also when point B on the bridge circuit is rendered suiiî' blocking the associated tube 49 or 59 respectively. ciently negative with respect to ground, then the The armature 60 of relay 52 is associated with a impedance of tube 49 becomes too great to allow back contact 6l, the latter being connected to' 15 armature 60 on the relay 52 to be held up. The sufliciently de-energized by at least partially one terminal of an energy source 63. l The other alarm indication is then made so as to indicate terminal of this source is connected through a a failure on the transmitter side of the compari switch 64 to common return circuit 65 for all of son device. the alarm devices. If the alarm circuits which include pilot lamp Illustratively, four different types of alarm de 20 1S, buzzer 11, and annunciator 18 are actuated, vice are indicated in the diagram. These are then it is known that an abnormal condition has lamps 66 and 16, buzzers 61 and 11, annuncia occurred in which the output from the trans tors 68 and 18, and a monitoring speaker 40. mitter cannot be accounted for as originating at The circuits for the different alarm indicators the studio. The reverse condition is indicated '66, B1, and 68 are preferably in parallel. The 25 by the actuation of pilot lamp 66, buzzer 61, and indicator units 16, 11, and 18 are also connected annunciator 68 when the tube 49 becomes at least in parallel. Although the individual indicator partially blocked and relay 52 de-energizes. The units may be separately cut in and out of service - amplitude of the output from the loud speaker 40 by individual switches, it is a simpler matter to will be greatly increased -upon setting off either provide one switch 64 for opening them all at 30 side of the alarm circuit, provided there is any once after the alarm has been noted. signal at all which the monitoring radio receiver Relay 53 has an armature 1B associated with a back contact 1l for controlling the parallel con nected circuits through indicator lamp 15, buzzer 11, and annunciator 18. These alarm circuits are likewise actuated whenever the armature 10 is re leased by relay 53, that is to say, when the tube 50 becomes sufficiently blocked. can pick up. For the purpose of smoothing the rectified po tential output from tubes 23 and 43, I preferably place a delay circuit in shunt with resistors 25 and 26. This delay circuit includes capacitors 29 and resistors 30. The junction between the two capacitors is connected to point C on the bridge The winding of a relay 31 may, if desired, be circuit and also to the anode circuits of the rec inserted between the common leads 3B and 65 of 40 til-ler tubes 23 and 43. the alarm indicator cricuits. Relay 31 has conn It will be seen from the above description that tacts which are closeable to short-circuit an at my automatic program change indicator is of tenuating resistor 39 in the voice circuit of a value not only in detecting subversive acts which monitoring loud speaker 4l). Thus the relatively would interfere with the Iproper operation of a low-level input to the speaker 49 may be raised ‘ broadcasting system, but it is also of value for to a high level whenever either of the alarm re giving an alarm whenever an abnormal trans lays 52, 53 releases. mission condition occurs. The loss of normal The operation of my program change indicator established levels due to partial or complete fail is further explained as follows: ure of any of the equipment in the paths be The voltage developed across resistor 25 in the tween the points of origin and reception of the bridge circuit is dependent upon the amplitude of the signal component which is rectified by the tube 23. Likewise, the voltage developed across resistor 2S is attributable to the amplitude of the signal component rectified by tube 43. When the voltage drop in resistor 25 equals that in re sistor 26, then equal biases are applied to the grids of tubes 49 and 59 and these biases are of such value that emission takes place in both tubes, and they normally draw sunicient current to main tain the relays 52 and 53 in an energized state. Under this balanced condition, regardless of the signal amplitudes, points A and B are both at substantially ground potential. When the rectmed signal potential in tube 23 exceeds that in tube 43, then the difference be tween the potential drop across resistor 25 and across resistor 25 is such that point A on the bridge circuit is rendered positive with respect to ground and point B is rendered negative. This is true because the values of resistors 21 and 28 are equal and point D is grounded. Therefore, the ground potential applied to point D is one with reference to which the voltages of points A and transmitted signal makes itself evident in the operation of the alarm system. Failure may oc cur in the wire lines, at the equipment of the transmitter, at the transmitter itself, the trans mitter antenna, the propagation space path, or in the radio receiver. Abnormally high levels of signal reception as well as abnormally low levels on a comparative basis will result in giving an alarm. Various alarm devices, not only such as shown, but of other types as well may be placed at any convenient points and those which are parallel connected with each other may be dis posed- at different points remote from one an other, if desired. Various modifications of my invention Will, doubtless, suggest themselves to those skilled in the art in View of the foregoing description. Such modifications would, however, be compre hended within the spirit and scope of the inven tion. I claim: l. A program change indicator system for use in connection with a broadcast » studio output circuit and with a monitoring receiver, said sys B\move equally in opposite directions, the extent 75 tem comprising separate full-wave rectiñers, one 2,410,669 .7 ` being coupled to said studio output circuit and the other being coupled to the output side of said receiver, means including a iour-armbridge circuit for comparing the instantaneousenergy components delivered by said rectiñers, one arm of said bridge circuit being in series with the rectiñer which delivers the studio energy com ponent, a second arm of said bridge circuit being in series with the rectiner which delivers the monitoring receiver energy component, the third and fourth arms being each grounded at one end thereof and being connected at the other ends to the iìrst and second arms at the respec tive' junctions of the latter with the cathode elec to the ~junction between the iirst and third im' pedances, a connection from the cathode elec trodes of the other said rectifier to the junction between the second and fourth impedances, cir cuit means connected to the junction between the nrst and second impedances and branched to the anode electrodes of both rectiiiers for pro ducing potentials across the first and second im pedances in accordance with the relative ampli tudes of the waveenergy from the different sources, a ground connection to the junction be tween the third and fourth impedances, a pair of electronic tubes each havingV an input circuit connected between its ground-connected cathode and its control-grid and having a load'inits trodes oi said reotiñers, conductive circuits in output circuit, a connection from the control grid terconnecting the anode electrodes of both rec of one tube to thejunction between the nrst tiiiers with the junction between the nrst and and third impedances, and another connection second bridge arms, a pair of relays each having from the control grid of the other tube to the its winding in circuit with the space path of junction between the second and fourth imped an individual discharge tube, means for control 20 ances, whereby the unbalancing oi potentials ling said tubes in opposite senses in response to across the iirst and second impedances varies unequal potentials developed across the ñrst and the output of said tubes in opposite senses. Y second bridge arms, and indicating means selec 5. A. circuit arrangement in accordance with tively actuable by said relays. ` claim e in which each said load is constituted 2. In a. program change indicator which com 25 as'the winding of a relay, and said circuit >ar pares audio-irequency potentials from two dif rangement includes an alarm indicator operable ferent sources normally possessing the same wave - upon de-energìZation of one of said relays, and envelope, the method of actuating a relay-con a second alarm indicator operable upon de trolled alarm upon the reception of unlike wave energization of the other of said relays. envelopes from said sources, said method com 30 6. A circuit arrangement in accordance with prising separately amplifying and rectiiying a full-wave signal component from each source, claim 4 in which each said load is constituted as the winding of a relay, and said circuit’ ar rangement includes a plurality of alarm indi value in response to the rectiiication of said sig cators operable upon release of a relay armature, nal components so long as they follow the same 35 certain of said indicators being individual to each wave envelope, causing the amplitudes oi said relay and at least one indicator being common producing two currents of substantially equal currents to vary in opposite senses in response to to both relays. ‘ the reception of unlike wave envelopes,- causing 7. A circuit arrangement in accordance with a limiting action to be produced with' respect claim 4 in which each said load is in circuit with to the ampliñcation gain factor which is applied 40 the winding of a common relay andsaid circuit to the signal components from at least one of arrangement includes a load speader the voice said sources', and causing said alarm to be actu circuit of which includes an attenuating imped ated by a relay circuit closure which takes place ance, and means operable by said relay for short when the disparity between said current ampli 4. circuiting said attenuating impedance thereby to tudes reaches a predetermined value. increase the output of sound from said loud 3. In a program change indicator which com pares audio-frequency potentials from two differ ent sources normally possessing the same wave envelope, the method of actuating a relay-con trolled alarm upon the reception of unlike wave envelopes from said. sources, said method corn speaker. _ , 1 8. In a system for comparing the modulation envelopes in two trains of signals normally hav ing a common source, one of said trains being applied to and derived from a space path of radio propagation and the other or“ said trains being applied to and derived from an essentially prising separately amplifying and rectifying a full-wave signal component from each source, metallic circuit, separate means for amplifying producing two voltages oi substantially equal 55 and rectifying the signal energy in each of said ’Value in response to the rectiñcation of said signal components so» long as they follow the same wave envelope, causing the values of said voltages to vary in opposite senses in response to the reception of unlike wave envelopes, causing a limiting action to be produced with respect 60 to the ampliiication gain factor which is applied trains, a four-arm bridge circuit arranged to be fed with rectiñed energy components from said separate means, and an alarm circuit respon sive to a condition of energy unbalance in cor responding arms of said bridge in the event that the characteristics of said modulation en velopes in the two said trains diiier by a predeter to the signal components from at least one of mined amount. ' v ` said sources, and causing said alarm to be actu 9. A system according to claim 8 and including ated ‘oy a relay circuit closure which takes place 6 two indicators operatively connected to said alarm when the disparity between said voltages reaches circuit, one of said indicators being responsive a predetermined value. to an unbalance of potentials in said bridge cir 4. An alarm circuit arrangement comprising cuit which results from an abnormally high en two full-wave rectiiiers each transformer-coupled ergy content of the signal train derived from to a diiïerent source of audio frequency wave radio propagation, and the other of said indica 70 energy, the two said sources normally possessing tors being responsive to an unbalance of poten the same wave envelope, a first, a second, a tials in the opposite sense. third and a fourth impedance constituting the four arms of a Wheatstone bridge, a connection from the cathode electrodes of one said rectifier ROLAND A. LYNN. ' u.