US2173173A - Television receiver - Google Patents

Description

Se'pt. 19, 1939.

H. M.l I Ewls TELEVISION RECEIVER Filed Dec. 10,v 1936 Patented Sept. 19, 1939 UNITED Is'rATI-:s

2,173,173 j TELEVISION RECEIVER Harold M. Lewis, Great Neck, N. Y., assignor to Haseltine Corporation, a corporation of Deia- WIN Applicationl December A'10, 1936, Serial No.`1l5,068v

Claims.

This invention relates to program receivers for use in carrier-frequency program broadcasting systems and in particular invsystems, such as ytelevision systems, in which each station trans- 6 mits a plurality of components of a uniiled program on separate but related carrier waves, one component, for example the vision component.

being transmitted as a broad band of modulation frequencies, and the other component, such as a l0 sound component, being transmitted as a relatively narrow band of modulation frequencies.

In the transmission of television programs by radio broadcast, it has been proposed that each` program should comprise a broad band oi modulation frequencies of the lorder of three megacycles representing the vision component and centered about itscarrier frequency and a narrow vband of modulation frequencies of the order of one-quarter megacycle or less representing the sound component and centered about a carrier frequency adjacent that of the vision component and spaced only suiilciently to avoid interference between the two signal components. The exacting requirements for accurate definition of the television image, leading to the use of such an extremely wide band of modulation*-frequencies,` increases the diiliculties in the design of the selecting, amplifying, and transmitting circuits of the vision-signal channel of the receiver. Thereso fore, these circuits are designed to transmit uni-v formly a frequency band as wide, but only as wide, as is necessary to obtain reasonably -accurate deflnition of the television image and it is important that the vision-signal channel of the receiver shall be tunedv as accurately as possible to avoid cutting or attenuating any of the sideband frequencies essential for an accurate reproduction of the image. On the other hand, since the signal-selecting-circuits ofthe vision-signal 40 channel pass a wide band of frequencies, there is little or no variation in the intensity of the carrier frequency output of the selector for mistuning of the vision channel -of the receiver up to atleast a one-half of the band width. Since the intensity of the carrier-frequency output of (Cl. P18-7.5)

vision signal is transmitted through the soundsignal channel, resulting in harsh noises, `while a sound signal is transmitted through the vision-- signal channel, resulting in objectionable flashes of. light on, or damage to, the screen of the `vision- 5 f signal 'reproducen It is anv object of the invention, therefore, to provide an improved program receiver for use in Va carrier-frequency program broadcasting system in which a first component of each-pro- 10 gram is transmitted as a broad band ,of modulation frequencies on one carrier wave and a second component is transmitted as a narrow band of modulation frequencies on a related carrier wave, including means for accurately indicating l5 the correct tuning of the receiver, that is, the

condition in which the broad-band signal compo- `when the broad-band signal component is accu- 25 rately centered with respect to its broad-band se- .n lector.

It is a further object of the'inventicn to provide an improved program receiver, for use in a system of the type described, in which' the nar- 80 row-band signal channel is maintained in an inoperativefcondition except when a signal having a predetermined component characteristic of the desiredl broad-band signal is received by the broad-band signal channel.

In accordance withlthe invention, an improved program receiver, for use in a system of the typeI described, comprises a. `nrst signal-translating channel including a broad-band selector for a iirst program component and a first signal-reproduc- 40 ing means coupled thereto; a second signal-translating channel including a narrow-band selector for. the second program component and a second y signal-reproducing means coupled thereto. The

receiver includes also means responsive silelyv tol the signal output of the narrow-band selector for f controlling the signal-transmission eiiiciency ofA the signal channel for the broad-band program' component and means for maintaining said narrow-band signal channel in rative except-when a signal having a predetermined characteristic' nel. Y l

In a preferred embodiment of the invention, for

use ina television broadcasting system in which 56 is being'received by the broad-band signal chanl the vision signal component includes a characteri'stic component such as a synchronizing-signal component or other periodically recurrent modulation-signal component and in which the sound-signal component comprises only non-recurrent signal components, the broad-band signal-reproducing means comprises a cathode-ray tube including one or more control electrodes normally biased to suppress the cathode ray. Coupled to the output of the narrow-band selector is the sound-reproducing means and means for deriving a bias voltage proportional to the amplitude of the carrierrequencycutput of the narrow-band selector, the range of proportionality being limited by a tube operating beyond its upper and lower saturation points for signals above a predetermined value, and impressing it positively upon an electrode o1" the cathode-ray tube to initiate a ray therein. By means of this arrangement, the vision-signal channel is normally maintained inoperative until the sound-signal channel has been accurately tuned, as evidenced by the developing of a bias voltage of suicient value to initiate and maintain the cathode ray. By virtue of the relatively extreme selectivity of the narrow-band selector, whenever the receiver is tuned suiliciently accurately tc obtain a substantial signal output from the narrow-band selector, it will be tuned to center the vision signal with respect to the broad-band selector with suincient accuracy to ensure the transmission of the essential modulation frequencies for an accurate reproduction of the image, so that an indication of the proper tuning of the receiver' is given in accordance with the illumination of the cathoderay tube.

In order to ensure that the receiver will be maintained inoperative except when each signal channel is receiving its characteristic type of signal, there is coupled to the output of the broadband selector, means for deriving a bias voltage dependent upon a predetermined characteristic thereof, such as the synchronizing impulses of a vision signal or other periodically recurrent modulation-signal component. This bias is impressed y positively on an electrode of a tube of the soundsignal channel preceding the point from which the mst-mentioned bias is developed, thereby overcoming a normal negative bias supplied such electrode and rendering the channel operative.

It is understood that the term carrier frequency, as used herein, refers either to a radio frequency or an intermediate frequency and that the selectors of the receiver may operate at either radio frequencies or intermediate frequencies in accordance with whether the receiver is of the tuned radio frequency or superheterodyne type.

Also, the expression tuning of the receiver as used herein refers to the adjustment of the resonant frequencies of the radio-frequency selectors' or the local oscillator, or both.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. i is a schematic diagram of a complete television receiving system embodying the invention',v while Fig. 2 is a chart illustrating the relation between the program components in a television system oi the type which the system of Fig. 1 is designed to receive.

Referring now more particularly to Fig. l of the drawing, there is illustrated, partially schematically, a complete television receiver of the superheterodyne type embodying the invention and including an antenna circuit II coupled to a radio-frequency amplier I2, which is connected, in cascade, with an oscillator-modulator' I3, a video-intermediate-frequency amplifier and selector Ili, a video-frequency detector I5, a video` frequency amplifier and selector Id, as indicated. II, constituting the vision-signal channel of the receiver. An automatic amplication or contrast control (A. C. C.) may be connected from the output of the detector I5 to the control electrodes of one or more of the tubes of the intermediatefrequency amplifier and selector Id, as indicated. The elements described may be of conventional construction, so that a detailed illustration and description is considered unnecessary herein.

The cathode-ray tube II comprises the usual envelope I8 containing an electron gun comprising a cathode heater I9, a cathode 20, a main or video control grid 2|, an accelerating anode 22 for generating and accelerating an electron stream, and a focusing anode 23 for focusing it into a beam. The envelope is also provided with a second anode 24 usually comprising a conductive coating on the interior surface and the usual fluorescent screen 25 formed on the end oi' the tube. Line-frequency and frame-frequency scanning or deflectingicoils 27 and 2G, respectively, are disposed about the neck of the tube I'I for developing a eld normal to the axis of the tube, for deecting the cathode ray.

The video-frequency amplifier i6 is coupled to the control grid 2|' of the tube Il by meansof a coupling condenser 28, a blocking resistor 29. and a by-pass condenser 3|, while the scanning coils 2l' and 26 are energized from the line-frequency generator 33 and frame-frequency generator 32, respectively, also coupled to the video-frequency amplifier to receive therefrom synchronizing lmpulses transmitted with the video signal, thus synchronizing the scanning of the cathode-ray tube with the scanning of the image at the transmitting station. For supplying operating potentials to the various electrodes oi' the tube I 1, there is provided a voltage divider 34 adapted for connection to a suitable direct current source (not shown) and having a tap 84a for supplyinga bias to the control grid 2| through an isolating resistor 35 and taps 34h, 34o, and 34d connected to the anodes 22, 23, and 24, respectively. Certain of these'taps may be manually adjustable for regulating the initial bias voltages applied to the tube electrodes and for controlling the operation of the system.

The sound-signal channel comprises the antenna circuit II. the radio-frequency amplifier I2, and the oscillator-modulator Il in common with the vision-signal channel and, coupled thereto in cascade, lan audio-intermediate-frequency amplifier and selector fan audio-frequency detector 31, an audio-frequency amplier 38, and a sound `reproducer 3,9. An automatic amplification or volume control..(A.vV. CJ may be oonnected from the output of the detector 81' to the control electrodes of one or more of' the tubes of the intermediate-frequency amplier and selector 36, as shown.-

The systemof Fig. 1, as thus far described, is

conventional in construction and operation so by. the antenna circuit II are selected and amplided in the radio-frequency ampliiier'iz and sup- Signals are converted Iinto intermediate frequen The intermediatecies in the usual manner. frequency amplier and selector I4 Aselectively .i amplifes the vision-modulated intermediate-fre Vquency carrier and supplies it to the detector I5 where the video-frequencies of modulation are derived and subsequently amplified by the ampli. er I6 and supplied in the conventional manner :to the control grid 2I of the cathode-ray tube I1 and to the scanning generators 32 and 33. With proper operating potentials supplied to the electrodes of the tube I1, a stream of electrons is generated, accelerated, and focused`by the electron gun comprising the elements I9, 20, 2|, 22,

and 23, which strikes the fluorescent screen 25 of the tube. The line-frequency and frame-frequency generators 33 and 32,-respectively, generate periodic currents of saw-tooth Wave form y synchronized with the Scanning circuits of the transmitter and supply them to the deflecting coils 21 and 2 6. The magnetic fields produced by the coils 26 and 21 of saw-tooth wave form are eiectiveto cause the cathode ray to scan the screen 25 rectilinearly to reproduce the' trans-Y mitted image.

Simultaneously, the amplier and selector 36 selectively ampliiies the sound-modulated intermediate-frequency car- Y yrier and transmits it to the detector 31 where the audio frequencies of modulation are derived and subsequently amplied in the amplifier 38 and reproduced by the loud-speaker 39.

In order to ensure that-the vision-modulated l: carrier is accurately` centered with respect tothe intermediate-frequency selector and amplier I4', there is provided an arrangement for4 controlling the operation of the cathode-ray tube I1 in accordance with the accuracy of tuning of the sound-signal channel, the selector of which, as stated above, passes a band which is extremely narrow relative to that of the vision-signal channel. The arrangement includes a vacuum-tube repeater 40 coupled to the output of the linterij mediate-frequency amplifier 36 byv means of a coupling condenser 4I and a grid leak 42. The

`output circuit of the repeater 40 is coupled by means of an intermediate-frequency selector 43 to ay diode vrectifier 44. The selector 43 has a selectivity of the same order as, or sharper than, lthat of the selector36.- Suitable operating potentials are provided for the tube 40, as indicated by +Sc and +B. The rectifier* 44 is provided with a' load circuit comprising a resistor 45 by,

passed by a condenser46,. while its cathode is' effectively grounded for intermediate frequencies by condenser 41. The load resistor 45 of the rec- Y tier '44 is included in series in 'the connection ally suppresses, the cathode ray of the tube I1,

so that the screen 25 is not illuminated. When the receiver is accurately tuned, that-is, when the sound-modulated carrier is approximately centered with respect to the selector and a signal is beingV received, th signal outpptLof the selector 36 is of substantial amplitudesapd, a. `substantial intermediate-frequency chronizing component.

bias voltage is developed across the resistor 45 which is superimposed upon that supplied by the tap 34D, the sum of the two voltages being of suflicient magnitude to initiate and maintain the cathode ray in the tube I1 to reproduce the transmitted image in the conventional manner. In order to minimize the effect of normal variations in the signal output of the selector 36, due either to an inexact tuning of the sound-signal channel lor to variations in the strength of the signal input, the characteristics of the repeater 40 are so selected with reference to the magnitude of its -input voltage and its circuit constants that it becomes saturated for .signal inputs of a predetermined value somewhat less than the normal signal output of the selector 36. With such an arrangement, the bias voltage developed by the rectifier 44, for values of signal output of the selector 36 substantially less than normal, is proportional to the signal-input to repeater 40 but is insuicient to initiate the cathode ray in the tube I1, while increases in the signal output of the selector 36 substantially beyond this normal value do not effect appreciable increases inthe bias voltage developed by the rectifier 44, which might affect the intensity of the cathode-ray beam and thus the average background illumination of the reproduced'image.

In order to prevent the occurrence of clisturbing light ashes on, or damage to, the screen of the cathode-ray tube I1 uponl thel reception of signals other than television signals and the vision-signal channel, there is providedan arrangement for maintaining the sound signal channel inoperative except when a signal having a component characteristic of a. television signal is received bythe vision-signal channel. arrangement includes a vacuum-tube repeater 50 coupled to the output of the video-frequency amplifier I6 by means'of a coupling condenser 5I and a grid leak 52. The output circuit of the repeater 50 is coupled to a diode rectler 54 by means of a selector 53 sharply` tuned to one of the characteristic modulation components of a television signal, preferably, a periodically recurrent signal component such as the line-frequency syn-I tentials are provided for the tube 56, as indicated by -I-Scand +B. `The rectifier 54 is provided with a load circuit comprising a resistor 55 bypassed by a condenser 56. A control circuit is provided for one or more of the tubes of the audio-intermediate-frequency lamplifier 36, including a source of negative-bias potential 51 and the load resistor 55 ofthe rectifier 54 connected with such polarity as to 'oppose the bias source 51.

In the operation of the last described control amplier 36 is normally biased to an inoperative condition'by the battery 51 which action, in turn,

This

Suitable operating po- .arrangement, the audio-intermediate-freouency removes the signal input to the auxiliary control A age, for normal -received signals, being sufficient to overcome the negative -bias of thezbattery 51 and render conductive the tube or tubes of thev audio-intermediate-frequency amplifier 36. In

order to minimize the eect of the normal varla- I as a narrow band of modulation frequencies on tions in the amplitude of the predetermined signal component in the output of the video-frequency amplifier i6, due to variations in the strength of the signal input or other causes, the

characteristics of the repeater tube 50 are so selected with reference to the magnitude of its/ inputI voltage and its circuit constants that it becomes saturated for signal inputs of a predetermined value somewhat less than the normal amplitude of the predetermined component of the signal output of the amplifier i6. With such -an arrangement, the bias voltage developd by the rectifier 54 for all values of signal output ofthe amplifier I5 substantially less than normal is inj sufficient to overcome the bias 5'! and render the sound-signal channel operative, while increases in the signal output of the amplifier I6 substantially beyond this normal value do not eiect appreciable increases in the bias voltage developed by the rectifier 54, which might ailect the transmission eiliciency of the sound-signal channel and, thus, thelevel of the sound output thereof. Fig. 2 is a chart of the relation between the vision modulation band, the sound modulation band and the radio-frequency or intermediatefrequency carriers as it has been proposed to relate them for each television-broadcast station. It will be noted that the vision modulation band a is a. band approximately 5 megacycles in width centered about a carrier frequency lv, which is spaced by 2-.75 megacycles from one edge of the allotted signal channel, while the sound modulation band b comprises an extremely narrow band centered about a carrier frequency at the other edge of the signal channel. The band b may be extremely narrow` and still provide adequateA transmission of the sound modulation frequencies. As a maximum width this band may be of the order oi" 0.25 megacycle in order to allow for reasonable drift of the local oscillator or the carrier frequency. Thus, whenever the receiver is tuned suiliciently accurately that thesound signal is even approximately centered with respect to the pass band of the selector 16 so as to produce a substantial control voltage on the accelerating anode 22 of the tube i1, the vision modulation band must necessarily be centered with respect to the pass band ofthe selector i4 with such accuracyas to transmit and reproduce the essential modulation frequencies for an ,accurate reproduction of the transmitted scene. There is thus given an indication of the proper tuning of Vthe receiver in accordance with the illumination of the cathode-ray tube I1. At the same time,

the entire receiver is rendered inoperative except modications as fall within'the true spirit andV scope of the invention.

What is claimed is:

1. In a carrier-frequency Program broadcasting system in which a rst component of each program is transmitted 'as a broad band of modulation frequencies on one carrier Wave and a second component of each program is transmitted another carrier wave related in frequency to said rst carrier wave, a program receiver comprising a rst signal-translating channel including a broad-band selector for said rst program component and a cathode-ray signal-reproducing tube coupled thereto, said cathode-ray tube having a control electrode normally biased to suppress the cathode ray, a second signal-translating channel including a narrow-band selector for said second program component and a second signal-reproducing means coupled thereto, and means for giving an indication oi the tuning of the receiver in accordance with the illumination of said cathode-ray tube comprising a repeater tube coupled to the output circuit of said narrow-band selector, said tube being so designed relative to the constants of its associated circuit that it saturates for input voltages above a predetermined value. rectliier means coupled to said repeater tube for deriving a bias voltage proportional to the input' to said repeater tube for inputs below said predetermined value, and means for impressing said bias voltage positively on said control electrode to initiate and maintain the cathode ray only when the signal input to said repeater approximates said predetermined value.

2,. In a carrier-frequency television broadcasting system in which the vision-signal component of each program is transmitted as a broad band of modulation frequencies on one carrier wave and the sound-signal component of each program is transmitted as a narrow band of modulation frequencies on another carrier wave related in frequency to said rst carrier wave, a program receiver comprising a first signal-translating channel including a broad-band selector for said vision-signal component and a cathode-ray signal-reproducing tube coupled thereto and provided with a control electrode normally biased to suppress the cathode ray, a second signal-translating channel including a narrow-band selector, ior said sound-signal component and a loudspeaker coupled thereto. and control means coupled to, and responsive solely to, the signal output of said narrow-band selector for modifying the bias of said control electrode to initiate and maintain the cathode ray, said control means including means for limiting the controlling eifect on said control electrode to a substantially constant value for outputs of said narrow-band selector above a predetermined value, whereby the illumination of said cathode ray tube affords an indication of the tuning' oi said sound-signal channel and is unaffected by said control means during normal operation.

3. In a carrier-frequency pro broadcasting system in which a first compon nt of each program is transmitted as a broad bandpf modulation frequencies on one carrier wave and a second component ofeach program is transmitted as a narrow band of modulation frequencies on another carrier wave related in frequency to said rst carrier wave, a program receiver comprising a rst signal-translating channel including a broad-band selector for said first program component and a rst signal-reproduclngmeans coupled thereto. a second signal-,translating channel including a narrow-band selector for said second program component and a second signal-re'" producing means coupled thereto: means responband selector for controlling the signal transmission efficiency of said rst signal channel, and.

means responsive to the reception by said first signal-translating channel of a signal having a predetermined characteristic component for controlling the transmission efficiency of said second signal-translating channel vat a point preceding the output of said narrow-band selector.

4. In a carrier-frequency program broadcasting system in which a first component of each pro'- gram istransmitted asa broad band of modulation frequencies on one carrier wave and a second component of cach program is transmitted asl a narrow band of modulation frequencies on another carrier wave related in frequency to said first carrierwave, a program receiver comprising a first signal-translating channel including a broad-band selector for said first program component and a rst signal-reproducing means coupled thereto, a second signal-translating channel including a narrow-band selector for said second program component and al second signalreproducing meansv coupled thereto, means responsive solely to the signal output of said narrow-band selector for preventing `operation of said rst signal-reproducing means whenever said signal output is less than a predetermined magnitude, means for biasingfsaid second signal-translating channel to an inoperative condition, and means responsive to the reception by said first signal-translating channel of a signal having a predetermined characteristic component for overcoming said biasing means to render said second signal-translating channel operative.

ing system i'n which a` first component of each program is transmitted as a b road band of modulation frequencies on one carrier wave and a seccoupled thereto,

ond component of each program is transmitted as a' narrow band of modulation frequencies on row-band selector for controlling the signal transl mission efficiency of said first signal channel, control means coupled to the output of said broad-band selector and including a. selector sharply responsive at the frequency of a predetermined characteristic component of a desired broad-band'signal, and means responsive to the output of said last-named selector for` controlling the transmission efficiency of said second signaltranslating channel at a point preceding the output of said narrow-band selector.

6. In a carrier-frequency program broadcast-l ing system in which a first component of each program is transmitted as a broad band of modulatiorr frequencies on one carrier wave and a second component of each program is transmitted as a narrow band'of modllation frequencies on 'another carrier'wave related in frequency to said first c arrier wave, a program receiver comprising a first signal-translating channel including a broad-band selector for said flrst program cornponent and a first signal-reproducing means a second signal-translating channelincluding -a narrow-band selector for said ,second program component and a second signal- 5. In a carrier-frequency program broadcast--A reproducing means coupled thereto, means responsive solely to the signal output of said n arrow-band selector for controlling the signal trans-l mission eiilciency of said first signal channel, a repeater tube coupled to theloutput circuit of said broad-band s'elector, said tube being so designed relative to the constants of its associated circuit that it satur'atcs for input voltages a'bove a predetermined value, a selector circuit coupled to the output of said repeater tube and sharply responsive at the frequency of a predetermined chai'- acteristic4 component of a desired broad-bard v signal, rectifier means Afor deriving from the outsignal input to said repeater approximates s aid predetermined value. y 2o '7. In a carrier-frequency television broadcastling system in .which the vision-signal component of each program is transmitted as a broad band of modulation frequencies on one carrier i wave and the sound-signal component of each program is transmitted as a narrow band of modulation frequencies on another carrier wave related in frequency to said first carrier wave, a program receiver comprising a first signal-translating channel including a broad-band selector 30 for said vision-signal component and a cathoderay signal-reproducing tube coupled thereto and provided with a control electrode normally biased to suppress theA cathode ray, a second signaltranslating channel including a narrow band selector for said sound-signal component and a loud-speaker coupled thereto, means responsive solely to the signal output of said narrow-band j selector for modifying'thebias of said control electrode to initiate and maintain the cathode ray when said signal output exceeds a predeter- 'mined value, whereby the illumination of said vcathode ray affords an indication of the tuning of said sound signal channel, means for biasing said second signal-translating channel to an inop- 45 erative condition at a point preceding the output of said narrow-band selector, and means responsive to the reception by said first signal-translating channel .of a vision signal including a synchronizing-frequency component for overcoming said biasing means to restore said `second signaltranslating channel to normal operating condition.

i 8. In av carrier-frequency program broadcasting system in which each of a pair of components 55 of each program is transmitted as a band of modulation frequencies on a carrier wave having a predetermined frequency relation to the carrier wave of the other component of the same program, one of said program components having a characteristic modulation component, a program receiver comprising a pair of signal channels, each of said [channels including signalselecting, y-amplifying and -detecting means and a signal-translating device coupled thereto, the 65 signal-selecting means of said channels' being individually responsive to frequencies having said vpredetermined relation to each other, and means for determining the allocation of each program component to its proper channel comprising 70 means selectively responsive to said characteristic modulation component coupled to its corresponding signal channel for controlling the signal-transmission efficiency ofthe other signal channel, and means responsive to' the signal ampli'tude at a point in said other signal channel subsequent to the point at which it is controlled for controlling the signaltransmission efficiency of said first-named signal channel at a point subsequent to the point to which said selectively responsive means is coupled.

9. nra carrier-frequency program broadcasting system in whicha first component of each program is transmitted as a carrier Wave modulated with a recurrent periodic signal component and a second component oi each program is transmitted as a carrier Wave modulated with a non-recurrent signal component and having a predetermined frequency relation to said firstnamed carrier, a program receiverr comprising a pair of signal channels, each of said channels including signal-selecting, -amplifyingand detecting means and a signal-transmitting device coupled thereto, th signal-selecting means of said channels being individually responsive to frequencies having said predetermined relation to each other, and means for determining the allocation of each program component to its proper channel comprising means selectively responsive to said recurrent signal component coupled to its corresponding signal channel for controlling the signal-transmission eiiiciency of the other signal channel, and means 4responsive to theY signal amplitude at a point in said other signal channel subsequent to the point at which it is controlled for controlling the'signal-transmission eilciency of said first-named signal channel at a point subsequent to the point to which said selectively responsive means is coupled.

10. In a carrier-frequency program broadcasting system in which a rst component of each program is transmitted as a carrier wave modulated with a recurrent periodic signal component and a second component of each program is transmitted as a carrier wave modulated with a non-recurrent signal component and having a predetermined frequency relation to said firstnamed carrier, a program receiver comprising a pair of signal channels, each of-said channels including signal-selecting, -amplifying and-detecting means and a signal-translating device coupled thereto,- the signal-selecting means of said channels being individually responsive to frequencies having said predetermined relation to each other, means for normally biasing to cut oi a portion of the signal channel for said rst pro gram component subsequent to its detecting means, means for normally biasing to cut o a portion of the other signaltchannel, and means for determining the allocation of each program component to its proper channel comprising means for deriving from the detecting means of said first-named channel la bias voltage dependent upon said recurrent signal component and applying it to overcome the cut-off bias of the other signal channel, and means for deriving from said other signal channel a bias voltage dependent upon the signal amplitude therein subsequent to the biased portion and applying it to overcome the bias o! said first-named channel.

HAROLD M. LEWIS.

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