US2486498A - Means for preventing cross talk in sound-vision systems - Google Patents

Description

Nov. 1, 1949. K. scHLEslNGER MEANS FOR PREVENTING CROSS `TALK IN SOUND-VISION SYSTEMS f 6 Sheets-Sheet 1 Filed April 2o. 1945 Nov. 1, 1949. K. scHLl-:slNGER MEANS FOR PREVENTING CROSS TALK IN SOUND-VISION SYSTEMS 6 Sheets-Sheet 2 Filed April 20, 1945 ATTORNEY NV- l 1949- K. scHLl-:slNGER 2,486,498

MEANS FOR PREVENTING CROSS TALK IN SOUND-VISION SYSTEMS Filed April 20. 1945 6 Sheets-Sheet 3 ATTORNEY NOV. l, 1949. K, SCHLEslNGER .2,486,498

MEANS FOR PREVENTING cRoss TALK IN I SOUND-VISION SYSTEMS Filed April 20. 1945 6 Sheets-Sheet 4 VIDEO INPUT 59 BLANKING slQNAL -S`l INPUT V INVENTOR KURT SCHLESINGER.

ATTORN EY Nqv. 1, 1949. K. scHLEslNGER 2,486,498

EANS FOR PREVENTING CROSS TALK IN SOUND-VISION SYSTEIS 6 Sheds-Sheet 5 Filed April '2o. 1945 d.. 022WI n.50 o23 INVENTOR KURT SCH LES! N GER..

ATTORNEY Nov. l, 1949. K. scHLEslNGER MEANS FOR PREVENTING CROSS TALK IN SOUND-VISION SYSTEMS 6 Sheets-Sheet 6 v Filed April 20,

oww m NO. m*

nu* K BY ATTORNEY Pater-lied Nov. 1,1949V i MEANS Fon PREvEN'rlNG cnoss TALK 1N SOUND-VISION SYSTEMS 'Kurt SchlesngerQN-ew York, N. Y.,'assignor to l Radio Corporation of oi' Delaware America, a corporation Application April 20, 1945, Serial No. 589,338

This invention is directed'to signalling systems '11 claims. (o1. 17a-5.8)

and particularly to new and improved television methods and means whereby signals representative of both the image and the accompanying or related sound or any other intelligence effects maybe transmitted over a common carrier by suitable diplexing or multiplexing in such a way that the effect on the observer of the finally reproduced signals shall be as if the independent series of signalling indications were each continuous.

It has already been proposed in the art tov transmit signalling indicationsvof both the sight (video) and the sound (audio) eifects by. what are known as video modulation signals and audio modulation signals, and various ways and means to accomplish such results have already been set forth.

In these systems of the prior art, it has been customary, where a signal carrier is utilized tol radiate signals representative of the sight and the sound, to provide an arrangement whereby the image or video signals modulate the carrier in substantially the usual manner; this modulation is then usually followed by way of the desired control signal, such as sync and/or related or image control signals, and suitable modulation representative of the audio or sound eects. The audio eiects in the transmission are produced by virtue of a, varying duration signal as might result from scanning a variable width or area sound recording on the edge of-a motion picture film, for instance, with the scanning direction being transverse to the lm, or the signals may result from scanning a variable density sound record on the edge ,of the motion picture film, with the scanning direction also being transverse to the film, The Wald Reissue Patent No. 22,025 is illustrative of a system of this character.-

Further than this,it has already been proposed to provide transmission of video modulations and audio modulations in such a manner that the two series of signals are alternately transmitted and used to vmodulate a single carrier. In such systems the audio modulation signals are frequently substantially of the type which are known in the art as theA constant frequency variable duration type.

Signals of this general .type used to modulate a carrier to conveysound intelligence have already been suggested in the art by Kell in U. S. Patent No. 2,061,734, granted November. 24, 1936. Signals of the type where the video andthe audio modulations follow one another, and where the audio modulation signal is of variable duration 'have already beenrepresented in the art by Roosenstein in U. S. Patent No. 2,227,108, granted December 31, 1940. In the Roosenstein patented system, provision was made 'for sequentially transmittingthe sound and video signals on a common carrier where the limit of time ydura- A tion of the sound signal modulation was confined to the time interval of interruption o f the video i signal production. y

In systems of the type herein above suggested,

`general advantages are realized in that it makes possible theconveying of intelligence representative of both optical images of pictures,is`cenes and thelike and sound signal energy (related usually but not essentiallyE so) by the use of a transmitting medium over which but a single carrier is transmitted or radiated. However, disadvantages have been experienced in the prior art arrangements through the inability faithfully to produce the sound in good quality at the receiving Vend of the system and,v at the Sametime, failure to. avoid detrimental reactionary eiects between the two forms of transmissions. Y

In systems of the'type where two completely different forms of signals are to be multiplexed.

or' where a switching mechanism is to be developed to provide for the modulation of the carrier by the two different forms of signals in sequence, the systems of the prior art usuallywere subjected to the serious disadvantage that undesirable noise components are introduced into lthe sound reception under the influence of the operation of the sight transmissions.

It, accordingly, becomes a primary object of this invention Ato provide ways and means by which the audio signals (that is, for instance, relatively low frequency signals such as sound or facsimile) and video signals (that is, for instance,

the signals resulting from image scanning operations in television) may be lswitched or rmultiplexed, one with the other, in such a way that4 the sequence of .transmissions of the" two forms of signals is achieved-without any objectionable cross-talk features being introduced, or without the introduction of noise from one signal channel into the other.l Thus, the invention contemplates a system where two intelligence signals of dif'- ferent character, such as video, with or without special control signals, are multiplexed with other signals, such as audio or facsimile, with eilicient transmission of both without producing interference between the two signals.-

A further object of the present invention is that of providing a system for multiplexing the transmission of two completely different typesv of intelligence signal modulations. For example,

3 the-video signals of a television system, which occupy a relatively wide frequency band when reasonably highdetail is to be transmitted, are mixed with signals occupyilllr arelatively narrow frequency band, as vwould be represented by the :gaudiosignals occupying a relatively narrow frewqu'encysba'ndf or the modulation signals of the second type may originate from facsimile scannings, for instance.

In such a system` provision is made so that neither form of signal exists simultaneously with essaies lthe other and..at the same time, provisions are made to avoid the objectionable effects of mutual cross-tall: which tend to occur at each instant commutation or switching between the two forms of signal modulation is brought about. 1n an illustrative form of system herein to be described Lthe two forms of intelligence signals will be as- "sumed to be television video or image signals on the one hand and sound or audio signals which f are related (although not necessarily so) to the video signal modulation on the other hand.

Under such circumstances the main sources of cross-talk which usually exist are found ina system of this character in the sound-tov-sight f cross-talk effect at the transmission end of the system, Iand in the sight-to-sound cross-talk effect at the receiver end of the system. This is because of the fact that the injection of the souncio'r audio modulation at the transmitter has a tendency to influence synchronization. Accordingly, it becomes an object of this invention to provide ways and `means by which the introduction of sound modulation upon a radiated ca'iriercanbe accomplished without in any way altering, varying, or impairing either or both synchronization and video effects. HA system of this character also ject that of providing for avoiding cross-talk effects in the receiver between the two forms of intelligence signals. Such cross-talk effects frequently result in View of the fact that because the video carrier is radiated at very high frequencies, it is frequently possible for the video signalling energy to pass directly into the sound portion of the multiplex receiver through inter-electrode capacity even in cases where the electronic conductivity of the commutating or switching arrangements is completely suppressed during the video portion of the cycle of operation.

It is an object, therefore, of this invention to 1. y provide ways and means by which this form of capacity ycross-talk or, what might be termed .picture chatter, can be avoided.

Further, it is an object of this invention to provide Ways and means by which greater fidelity of yline `and frame or field synchronization may be obtained in sight-sound multiplex systems. It has been found in such connection that serious difficulties and objections in the sound channel has as its obdeflection for limited time periods.

reason of the frame or neld synchronizing and.' to this end, suitable provision for relay operated means have been made and will herein be described.

A lurther object of the present invention is that of providing ways and means by which suitably interrupted modulations of the carrier, under the influence of the sound signals, may be brought about. According to the present system, provision is made for developing a high frequency sub-carrier and for frequency modulating this sub-carrier under the influence of sound signals only for .a relatively short period during the transmission of the synchronizing pulses. The subcarrier is of approximately the maximum frequency which can be accommodated within the alloted frequency band within which all of the video, the control and the sound signals must be radiated.

Another feature and object of the invention is that of providing a system for the transmission of sight and sound signals wherein adequate provision is made for bringing the receiver operation to a steady state condition in an extremely short period of time or, stated otherwise, it becomes an object of this invention to provide a condition of multiplex operation wherein the periods during which the amplitude tends to increase, shall be of extremely short duration. Thus, the transit time, as it were, must be extremely short as compared to the pulse duration. The result of this is that it becomes unnecessary, as will later be pointed out, to provide any limiting in the receiver used to detect the frequency modulated sound signals.

Essentially the apparatus herein to be set forth comprises, in the transmitting end, a suitable form' of sync signal generator which, for the purpose of illustration, may be assumed (for an understanding of this invention) to be a form of arrangement providing for the development of both line, frame and field synchronizing signals, as well as the usual blanking signals. The output of this sync generator is then fed to a suitable wave shaping unit and to a peak power amplifier, the output of which serves to control, at prede- .termined time periods, a start-stop oscillator,

with the start-stop oscillator being set in operation initially under the influence of the pulse signal developed from the sync generator at the end of each scanning line transmission time.

Thus, an arrangement of this general character provides an oscillator for the sound sub-carrier, with the oscillator being inan inoperative state for the greater part of each scanning cycle and then subsequently rendered operative, under the influence of the pulse signals for controlling This condil tion occurs under the influence of the sync pulse result, frequently due to the irregularities of any y synchronizing (commonly termed sync) pulses. if =Even slight variations in the duration of the horlzorntal sync impulses which would normally pass unnoticed to the video receiver instrumentality in systems relying for separate carrier transmission for the sound become pulse irregularities tending to provoke extremely annoying and objectionable sound effects in the receiver. To this end, the present invention contemplates also the provision of a form of sync signal generator in which the objectionable effects of drift are avoided.

Another. obiect of the invention is that of overcoming the effects of disturbances introduced byv output from the syncsignal generator causinga burst of plate voltage which makes the oscillator operative for the duration of the sync signal. The delayed oscillations are then frequency modulated by the sound or audio (low) frequency control signals under lthe influence of a reactance tube of the usual type which is. in turn, controlled by the output from the audio signal amplifier unit after that output has been passed through an limage frequency suppressor. The frequency modulated sub-carrier is then supplied through a. suitable amplifier and, if necessary, frequency multiplier, to a combining unit wherein the sight (video) and the sound (audio) signals are combined for transmission.

The signals are duly received ina receiver inthat this signal can be applied directly to control the modulation of the cathode ray beam developed within a suitable image producing tube where desired. At the same time, the signal may be passed through a suitable filter which will iter out the sub-carrier from the sync signal pulses and permitthe usual form of sync signals, representing the .initiation of the line and the field or frame scannings. to be. separated from one anotherin aA suitable sync signal separator unit of known character. The output from the sync signal separator then controls horizontal and vertical deflection vof the scanning beam of an. image producing tube in known manner to deflect the modulated scanning beam of the image producing tube in exrepresentations andA trolled; and

Fig-.- 7 is 'a series of curves diagrammatidpa'rticularly.*of1th'e.operation of the receiving instrumentality shown by Fig-6.

Referring now to the drawings and first' to the showing ofFig. l, the video-signalsfare-'derived from a camera or image pickup unit conventionv ally shown in block form at "I I. This unit may .include anyof the well forms of camera tubes, such -those `which have lbecome known in the art as' Iconoscopes and Orthicons of various types, Dissectors and the like. v'I'he signals act synchronism with the transmitter scanning operation. ,I y

To separate the sound modulation signals from the video and sync signals, it is desirable to pass the complete received signal through an y inverter stage which then makes it possible to have the sound modulated peaks occur in the positive direction. These signals are then caused to control the operation of a suitable sound gate and relay arrangement which controlsthe operative periods of a gate amplifier to which the complete train of signals has been applied. The

gate amplier is thus limited as in its operative periods to those during which synchronizing impulses are received. Output energy from the gate amplier is supplied to a frequency modulation discriminator circuit, which may be of substantially known character. The detected output of the discriminator unit is then directed through a suitable audio amplifier to a sound reproducing unit.

'I'he foregoing summarizes generally the operation of the transmitter and receivervunits, and it, accordingly, becomes an object of this invention to provide ways and means for achieving the foregoing results with circuits which are stable and efiicient in their operation and which make possible transmissions of signals of which at least a part become usable in existing 'forms of television apparatus without any modification of such apparatus.

Other objects, of course, will suggest themselves and immediately become apparent to those skilled in the art when the following specifications are considered in conjunction with the accompanying drawings, in which: l

Fig. 'l schematically' illustrates one form of transmitter unit;

Fig. 2 schematically ceiver unit;

, Fig. 3 is a schematic representation of a general forrn of transmitter circuit showing particularly the start-stop form of oscillator and one form of frequency modulator therefor;

Fig. 4 is a-circuit arrangement to show particularly the sight-sound injector system for combining frequency modulated sub-carrier energy with the sync pulses;

Fig. 5 is a series of curves to represent the operation exemplified particularly in Fig. 4;

Fig. 6 is a circuit diagram of one form of receiver arrangement to provide for receiving video, audio `and control signals to provide image represents a form of reresulting from any form of scanning operation thus' may be derived from either a storage or a non-storage type ofc'amera tube or pickup unit.

' Further description of the pickup or camera unit is therefore believed to be unnecessary and, accordingly, schematic representation hasvbeen relied uponv for the showing of this figure in that the system herein to be described is adaptable for use in any system wherein video signals are developed as aforesaid.

The resultant output image or Video signals from the camera tube or image pickup unit, connected and arranged in known manner,are amplified also in known manner, in the video amplifier conventionally represented at I3. This amplifier, it will be understood, incorporates all of the general and customarily used arrangements, such as the pre-amplifier, the mixing amplifier, the main video amplifier, and if necessary, the line amplifier. The auxiliary controls such as those to provide for distortion correction, for the introduction of suitable background control voltage, and the like have not been il- .tion of the manner by which the audio or sound signals developed are to be combined with those video signals which are representative of the optical image scanned by the image pickup unit or camera tube I I. For reasons of simplicity, the control of the image pickup unit and the various'- amplifiers, such as the blanking' amplifier assumed to be contained within the unitv I3, operating under the influence of a suitable sync signal generator I1, have not been shown. However, schematic representation is made of all connections between the sync signal generator I'I and the sight-sound injector I5 by way of the connections for introducing the horizontal synchronizing signal pulse represented at the terminal SH, the vertical synchronizing pulsesrepresented as appearing at the terminal Sv, and the blanking vpulses for both the Ivertical and the horizontal represented at the output of the sync signal generator at the terminal marked Brr-v.v

The syncl signal generator, for the purpose of these presently general considerations, may be of purely schematic nature and of the general type used to provide sync signals Vfor developing coaudio signals suitably conv 7 type of sync signal generator is represented by Bedford Patent No. 2,258,943, granted October 14, 1941.

The sound signals which are to be combined with the video signal representations, according to this invention, and which are to be included in the sequential transmission of the two types of signals upon the common carrier, are assumed to be developed in the output of the audio or sound pickup unit, such as that conventionally represented by the microphone I9. although a facsimile scanner may have its output substituted for the microphone I9 and this disclosure shouldbe so understood. The signals from the microphone unit I9 are then suitably amplied in any desired form of amplifier, such as that conventionally represented by the sound signal amplier 2|. An amplifier of this character may be of any desired form, and therefore, has been illustrated in a purely schematic manner.

The output signals from the amplifier 2| are supplied through a filter 213, designated as an image frequency suppressor. This filter consists essentially of a serially connected inductance elements 22 and shunt connected capacity 24. The filter is preferably tuned by initially setting the circuit constants or by variation of either or both capacity or inductance to a frequency corresponding substantially to half the line frequency at which the image to produce the video signals is scanned, which will represent the highest audio frequency for high fidelity transmission.

It can be shown that where the video or image representation is assumed to be formed in a raster of 525 lines, with the complete image frames being repeated 30 times per second, with interlacing as desired, the line frequency of the synchronizing pulses used to control the receiver unit will be 15.75 kc. Accordingly, theside band limit at which sound reproduction with good fidelity can be produced is of the order of '1.5 kc., vso that the cutoff frequency of the image frequency suppressor lter 23 is set at a value justslightly less than half the assumed line frequency of image scanning to achieve highest possible Adelity.

Under these circumstances, there will be developed in the output conductor 25 connected across the termination of the lter 23, sound or audio signal energy of frequencies which vary between and 7.5 kc. for the assumed conditions which must be regarded as illustartive rather than limiting. This signal energy is then supplied by way of the conductor to control the reactance tube 21, later to be described.

At the time that image scanning is taking place under the control of the camera or pickup tube I I, in turn controlled by the sync signal generator l1, output signals or pulses from the sync signal generator, which occur at line lfrequency and which are designated as appearing at the terf minal S'n (also designated as 29) are supplied to the shaping unit 3| by way of the indicated conductor. These pulses should lead slightly with respect to the horizontal sync pulses SH. .The horizontal blanking pulses Bnfare suitable for these keying pulses at terminal 29.

The shaping unit consists essentially of a distorting circuit comprising the capacity element and the potentiometer 31 connected to ground I0, with a variable tap 39 on the potentiometer connecting to the remote end of the capacity 35 and the opposite terminal of the capacity 35 is connected to the control electrode 4I of a peak power amplifier tube 43.

8 With the arrangement as provided, the line frequency pulses appearing at the output terminal 29 of the sync signalgenerator I1 are suitably amplified in the amplifier 43 and, at the same time, by virtue of the shaping unit 3|, the high frequency components of the wave pulses have been emphasized and the lower frequency components suppressed.

The connection provided between the output of the peak power amplifier tube 4| and the plate or anode electrode 45 of a start-stop oscillator tube 41, is made through the coupling conf denser 49 so that at time periods when line signailing pulses appear in the output of the sync signal generator I1 an exaggerated plate voltage pulse will be supplied to the plate or anode of the oscillator tube 41. The result is that with the plate voltage normally supplied to the oscillator tube 41 by way of the connection to a suitable source, not shown, but indicated generally by the plus terminal connected to the terminal 50, there is developed in the oscillator a rather substantial voltage peak which is perfectly coordinated and synchronized in time with the leading edge of the pulse Sn derived from the syncsignal generator I1. This supplied high voltage pulse then initiates in the oscillator 41 suitable oscillation frequencies which occur in the parallel resonant circuit inculding the inductance element 5| and the capacity element 53. The initiation of oscillations in this circuit is thus controlled completely under the influence of the sync signal generator I1 so that each sub-carrier first starts with like phase conditions because all are excited from the same plate pulse as derived from the line sync pulse. The exponential build-up of the sub-carrier frequency output of the oscillator 41 then naturally follows from general laws of transient conditions in oscillatory circuits.

The suppression of the oscillations in the cir-.- cuit of the start-stop oscillator tube 41 is controlled through the suitable adjustment of the shaping unit 3| which is such that energy transferred between the output terminal 29 of the sync signal generator |1 for each line sync pulse and the tube 43 is cut off just slightly prior to the termination of the line synchronizing pulses S'n. The, result is that the oscillations developed from the start-stop oscillator, whose frequency is determined incidentally by the adjustments of the capacity and inductance elements 53 and 5I, are existent only for the duration of S'n and exhibit maximum amplitude at the center of this interval. These oscillations are then supplied to the reactance tube 21 in generally known manner. At the same time, the signal output from the sound signal amplifier 2| is continuously being supplied to the grid or control electrode 55 of the reactance tube by way of the coupling condenser 51 and the R. F.choke 59. Plate voltage is supplied to the reactance tube 21 continuously from a source of positive voltage, not shown, connected to terminal 6| and4 fed through the R. F.choke 63 to the plate or anode 55.

The oscillating circuit is shunted by the reactance tube by way of the coupling condenser 51. The grid of the reactance tube 21 is controlled with almost phase-lag through resistor element 52 and condenser 54. The inductance 56 has the effect of introducing slight regeneration into the parallel circuit 5I, 53, so that its losses are minimized, With this arrangement it is apparent that modulation of the frequency developed from the start-stop oscillator 41 `can occur only at limited times in the complete line scanning cycle, as represented in the time difference between the scanning 'of two suc-r the conductor 89, for instance, is as if the sound signals were of an interrupted character.

Due to the fact that it is desirable to make the' l sound sub-carrier as high as possible, and due to the fact that the presently allotted wave band for television transmission is only' megacycles, it is apparent that the sound sub-carrier may preferably be of the order of 6 megacycles as-the center frequency (less the maximum deviation).

With these conditions and due to the fact that the reactance-tube 21 usually will not provide substantial deviation under ordinary operating conditions, and that these deviations may be considered, for illustrative purposes at least, as being of the order of 50 kc. it is desirableto. provide a frequency tripler stage 1i in order -to provide a greater or wider frequency deviation which may approach 150 kc. The output from the frequency tripler stage 1i is then supplied by way of conductors 19 to apower output amplifier 15 which, in turn, is used to energize the sightsound injector i by way of the conductors 11. It was above stated that theoutput from the video amplifiers i3 and the sync signal generator i1 also were addedin the sight-sound inlector element I5 so that it functions, thereby, as a combining circuit to control transmitter modulation or modulation of a carrier frequency in any sort of a transmission channel.

With the coordination between thel development of the sound modulation signals and the time of production of the synchronizing signals in the sync signal generator, it is apparent that the desired relationship between the time of development of the video signals for modulat-A ing a suitable transmitter and-the audio signals .is adequately and properly controlled so that the various independent signals, representative of sight-sound synchronization and` blanklng, may, through suitable controls 1in the sightfsound injector, be applied to an output transmission line 19, for instance, to control or modulate therefrom a suitable carrier frequency for transmission. The carrier frequency which is to be modulated by the output signals appearing as the composite sight-sound signal in the transmission line 19 is constant, during amplitude modulation, with the sight-sound multiplex signals and, accordingly, it is possible to modulate into one and the same very high frequency carrierv more than one complete sight-sound program, provided separate intermediate veloped.

Reference may now be made to Fig. 2 for an understanding of the conventional elements of a receiver instrumentality wherein images are produced in accordance with the video signal modulation developed in the transmitter unit described in Fig. 1, and wherein sound signals are reproduced in accordance with the accompanying sound or audio signal modulation as transmitted in multiplexed fashion on the main video carrier.

The receiver arrangement of Fig. 2 provides a common amplifier 85. for the video and audio signals which are received as carrier module@ tions upon a suitable transmission channel or antenna system connected to deliver the received Signals to appropriate input terminal points 86 and a1. The general form of this ampliner is the frequency carriers are dei usual well known type of wide band amplifier so as to be capable of taking care of the combinedy amplification of the sight and sound signal modulations (covering a wide frequency range) impressed on the input terminals 88 and 81 without frequency loss. 'l

In this way it will be apparent that within the amplier unit 85 there are common R. F.. I. F. detector and local oscillator stages,` as well as various video frequency stages, which are not shown for reasons of simplicity. In this connection no rejector or wave trap circuits are necessary and, as a result, the variable channel band width can be betterutilized so that an improve- .ment in definition is obtainable in the final received image.

At the output of the common video and audio ampliiier unit 85 a signal, representing all of the video, the video control and the sound or audio accompaniments, appears in conductor 89. This is shown in diagrammatic and schematic form by the wave adjacent to the conductor 89, with portion 98 indicating, for instance, in schematic fashion, the blanking and synchronizing pulses for each line of the image; the portion 9| indicating the blanking and synchronizing pulses occurring at the end of each field of scanning, and the portions 92 representing video signal `modulation extending ,between a level marked black and a level marked white. The sound signal modulations are conventionally represented at 93 and occur as frequency modulations, later to be described in more detail, which are superimposed upon the synchronizing signals, which` are, in turn, placed upon the blanking signals according to established practice.

It will be apparent'from the diagrammatic showing of the wave form in the conductor .89 that all of theA blanking, the synchronizing and the sound signals occur in a direction such that the amplitude thereof extends further in the black direction than do the signalsv represented by black level in the produced image. Signals of such amplitude are frequently vreferred to as being blacker than black" in thatthey extend in a direction which is more 'negative than that corresponding to black or, in other words, in a direction beyond a cutoff value in the image producing tube. z

contrasted with the control and sound signals, the video or image signal modulation represented at 92 extends Within an amplitude range between that marked black'and that marked "white," so that with these signals being applied to control an image producing tube, modulation of a cathode ray beam may readily occur as it traces the usual scanning raster.

The signal modulationappearing in the conductor 89 is passed then through a conductor 95 and also conductor 96 to control thepotential on the grid or control electrode 91 of a cathode ray image producing tube |80 inv which an elec# tron beam, conventionally represented at 98, is

|02 and |03 and the horizontal deilecting coils4 synchronizing pulses, such as those represented.

at 90, from the eld or frame synchronizing pulses, such as those represented at'9l. These pulses, when separated, are then fed by way of conductors and to control the operation of suitable horizontal deflection control circuits The sound gate, in general, must be considered as comprising three essential parts which are regarded as the synchronizing stage |25. the electronic relay stage |23, and the keyed amplifier stage |2|'. The output signals from the keyed ampliiler stage |2| all appear as interrupted frequency modulated sound signals and, according- ||2 and vertical control circuits ||3 whichV are then respectively connected by appropriate conductors with the horizontal and vertical deflection coils hereinabove mentioned. 'I'he horizontal and vertical deflection control .circuits ||2 and ||3 are ofgenerally conventional character and need not therefore be discussed herein in further detail.

The same signals that appear in the conductor 35 and which are used to control the modulation on the grid or control electrode 91 of tube |00 are also passed through theconductor ||5 to an inverter stage ||1 for the purpose: of causing the development of the sound signals in the sound reproducer element |20, which, obviously, maybe a facsimile recorder under conditions when the low frequency signals are of the facsimile modulation type. 'I'he signal wave form appearing in the conductors 89, 95 and ||5 carries the 'sound in the negative direction. In order to operate a grid-controlled sight-sound separator without image signal chatter, positive sync pulses arev needed. Therefore, it is apparent that )an inverter stage I1 is necessary in order to pl'ace the sound' signal modulation 93 in the positive direction on the sync'pulses 90 and 9|.

The output signals from the inverter ystage ||1 are represented by the wave form H8 appearing adjacent to the output conductor] Il). These signals are then applied to a sound-gate system ly, inorder to produce the control on the sound reproducer element |20,.such signals are impressed upon a suitable frequency modulation discriminator circuit conventionally represented at |29', which may be of known character in order e toA detect the said signals and to transform them into amplitude modulated peaks.

By reason of the-small ratio of signal to repetition period low frequency amplification may frequently benecessary in order to control the operationofthe sound reproducer element |20 under' the inuence of the detected sound signals appearing'in the output of the frequency modulation discriminator |29. For this purpose, any suitable audio amplifier |3| may be provided intermediate the output of the discriminator unit |29'and'the sound reproducer unit |20, although itwill be observed that no limiter is required or shown' inl` the frequency modulation system.

Further description of the general form of the sound.l gate hereinabove represented by the elements |2 |23 and|25 will be made in the reference to Fig. 4, wherein there has been disclosed ,a suitable circuit for establishing the `control hereinabove referred to and diagrammatically represented for purposes of illustration.

The circuit shown by Fig. 1 shows various v schematic representations of an arrangement to provide for, developing the frequency modulated soundv sub-carrier and modulating it under the influence of developed soundsignals.

l By making reference now made to Fig. 3, further vdetails 'and some modifications of the arrangement-of Fig. 1 will be noted. In this connection,' i`t will be appreciated that one of the im- `portant problems to be considered in developing the' sound modulated sub-carrier is that of properly-controlling its phase as to the beginning or l initiation of the pulses and aS t0 the termination which includes three units-HI', |23 and |25.V It

serves the purpose of eliminatingthe cycle frame signal noise. The separator unit is the v gate amplifier |2| which is suitably controlled under the operation of a gate vrelay unit |23, which is, in turn, controlledv by the gate sync control |25 that is, in turn, energized under the influence of the incoming signal by way of conductor |21. As will hereinafter be explained in more detail, the sound separator' |2| is usually a pentagrid tube which is subjected to a dual control. It may be opened only yif positive pulses from the input ||8 and from thegate relay |23 are present simultaneously. The gate relay |23 will al'sobe described later in more detail but at the present time, the complete gate system may -be considered as being for the purpose of elimior endthereof.

` `t was explained above, in Fig. 1, that the oscillator ltube 41 is in the nature of a start-stop oscillator which is controlled, as to its operative periods, under the influence of the output signals from the sync signal generator |1.

` If the oscillator were to be permitted to oscillate continuously, instead of being operated as an interrupted element, undesirable noises and beat freqeuncies would be produced during the time periods of injection of the sound modulation unless the keying of the sound sub-carrier is at an absolutely constant rate. Accordingly, with the' arrangement of Fig. 3, and also that of Fig. 1, the output'from the sync signal generator I1, when it is passed through theishaping unit circuit, comprising the potentiometer 31 and the condenser 35, will porvide the keying pulse energy for the oscillator unit.

The shaping unit is soldesigned that the condenser shunted volume control element 31 permits a variable amount of high frequency emphasis in the output of the sync signal generator, so that the fiat top pulses, diagrammatically represented as appearing at the output terminal 29 (the diagrammatic showing is not to scale but merely illustrative), are transferred into a signal with a steep Wave front and a decreasing amplitude, This phasing adjustment results in 13 placing the energy maximum of the burst of oscillators atthe center of the Sync interval, while the sub-carrier amplitudes of the ends of this interval are kept small. This form of energy distribution offers the advantage of not being susceptible to irregularities of the sync generator.

Thus, all high frequency energy bursts which are impressed upon the peak power ampliiler tube 43 by way of the coupling condenser |35 are started with the same phase. .At the same time. this form of oscillator keying insures that the oscillator will always start its oscillations from zero.

Where the sync pulses from the sync signal generator i1 occur prior to the time normally to be expected, the oscillator 41 will start its oscillation with the same advance or delay, and the relative phase of the sub-carrier and the sync pulses will automatically be identical.

The oscillations of the oscillator tube 41 are stopped by removal of the energizing pulses so that the plate energy transmission stops before the sync signal time duration has passed. "ilo accomplish this result, the peak power amplifier may be considered as a high-pass lter and peaking amplifier arrangement such that it essentially comprises a choke fed tube which will be fed, in the instance shown, by way of the inductivev element |31 connected seriallywith theresistor |38 to a, source of positive voltage (not shown) connected at terminal |39. This form of connection causes the load on the second half gif the tube 43 to become predominantly induc- The current flow throughthe second half of the tube 43 is normally substantially constant, and the first half of the tube is caused to become conducting during periods when positive pulses S'n are derived from the output'terminal 29 of the sync signal generator |1. This develops negative pulses in the plate circuit of the ilrst half of the tube 43 during the period of the sync pulses Sn and'this, in turn, produces a strong positive peak voltage across the output impedance of the second half of the tube. The input circuit of the second half of the tube-43 isconnected to the output of the rst half of the tube by way of the parallelly connected resistor and condenser combination |44 and |46. yThe second half of the .tube thus acts generally as a phase inverter and amplifier. The-anode of each half of the tube is vsupplied with positive voltage from the indicated source |39. The signals applied exceed a zero value only for a fraction of the total duration of the horizontal sync pulse output passed to the input or control electrode |4| through the wave shaping unit 35, 31, 39. It is advisable to clip the keying pulse S'H which starts somewhat in advance of the ordinary horizontal sync pulse Sn. This gives the sub-carrier oscillator some time to get started. It was found that the horizontal blanking pulses Bn are very suitable for this purpose of keying the sub-carrier oscillator.

The signal wave form appearing inthe conductor |42 from the output of the tube 43 is essentially of the wave form diagrammatically shown adjacent to the conductor. Thisvsignal is then supplied across the resistor |43 through the coupling condenser 49 to control the operation of a-cathode follower tube |44 by way of the signal pulses applied to the control grid |45 thereof. Output energy from the tube |44 is derived across the cathode load resistor |46 and appears asa strong signal pulse with a low impedance to ground through the condenser |41 andthe anode ele- 14 ,y ment 45 of tube 41. y plate pulse initiates the oscillationsintube 41. A .y

Average plate voltage for tube 41 is adjusted through a leakage resistor element |49, which is of a reasonably high impedancevalue so that the tube 41 will not undergo oscillations unless s. strong positive pulseis applied to the plate 45 by way of the condenser |41. The tube |44 acts as a cathode follower in the arrangement shown, and the cathode supply resistorl|48 is made of such high value that the cathode to ground 1mpedance, of itself, does not permit the tube 41 to undergo oscillations. However, at time periods when the energy pulses (such as those diagrammatically shown adjacent to conductor |42) are applied tothis cathode follower stage |44, it permits the cathode follower to draw current and it becomes a small impedance to ground which then makes it possible for the oscillator section 41 to undergo oscillations at extremely high frequency. Usually tubes |44 and 41 may be within a single envelope (as indicated) and one type of tube which may be utilized in the combined operation is that known in the art as the 6F8.

The oscillator coil |50, which is shunted by the condensers itil and |52 (the latter condenser being shunted by a resistor E54 connected between the tube cathode and ground), is connected, it

will be seen, so that the tube 41 functions essentially as the well known type of Colpitts oscillator, although the oscillator arrangement which has been shown by Fig. l is of a somewhat dierent variety. A

If it is assumed, for purposes of illustration, that a six megacycle band is allowed for the television signals, and if. it is further assumed, for purposes of illustration, that the sub-carrier on which the sound frequency modulations are to be produced also approaches a value of 6 megacycles (for example, 5.8 megacycles) then it becomes apparent that the frequency at which the oscillations aredevelopedfrom the tube 41 may, under the example illustrated, be considered as being of the order of 2 megacycles (actually slightly under 2 megacycles in the assumed example) since the output is to be passed through a frequency tripler stage 1|. This is because of the fact that, as above explained, it is generally desirable, in an arrangement of the type herein disclosed, to provide the sub-carrier from ,a master oscillator oscillating at a sub-harmonic of the final sub-carrier frequency. r'This is done to obtain a suiliciently wide frequency deviation by any subsequent frequency multiplication. The output from the oscillator 41 is thus subjected to action of a reactance tube 21 controlled in the manner set forth in the description above made of Fig. 1.

With this arrangement the reactance or quadrature tube 21 receives its speech or audio frequency input through the low-pass filter circuit conventionally represented at 23, so that the audio or sound signals are applied to the grid or control electrode 55 through the grid inductance59. The reactance tube 21 thus is continually connected to the audio amplifiers but l the signal input thereto can produce a variation 15 is lfrequently able not only to reduce any losses which are introduced by virtue of the quadrature tube 21, but frequently even able to overcome such losses.

The grid coil |53 of the reactance tube 21 is equivalent to a variable inductance with losses and by rotating and retarding the phase of the grid voltage (that is, the high frequency voltage) by the coil |53 the result is that a negative resistance'appears in parallel to the variable tube inductance. This results in more or less complete cancellation of the damping caused by both the reactance tube and its associated circuit. This also gives regeneration which depends upon the natural frequency of the grid network comprising resistor |56, grid coil |53 and the gridto-cathode condenser |58. Consequently, the coil |53 is changed until the net effect is best. If the value of the inductance is too high the tube may tend to oscillate. This effect is somewhat desirable because it will tend to reduce and often cancel any amplitude modulation or a signal burst and this effect is otherwise difficult to obtain due to the short time of the burst which makes the ordinary types of clippers for frequency modulation methods where pulsed frequency modulation of the type herein explained is utilized.

With the modification of Fig. 3, the sound or audio frequency signals are assumed to originate, as in Fig. 1, at a sound pickup microphone, conventionally in Fig. 3 represented at I9', and then are fed through a suitable cable or the like |55 into the first stage |51 of a multi-stage resistance coupled amplifier 2| of the usual character. The amplifier may comprise any desired number of stages and preferably vincludes the use of screen grid tubes connected substantially as shown.

So that flexibility of operation may be provided and so that the signals representative o! sound effects may also originate from recordings, a sound pickup head |59 is also arranged to connect with a second conductor element |6| which may, in turn, connect into the second stage |63 of the resistancecoupled amplifier 2| by way of a switch instrumentality |65. The switch, in the position shown in solid lines in Fig. 3, is so arranged as to feed the output from the sound pickup microphone I9' through the complete amplifier, but with the switch in the lower position, indicated by `dotted lines, the input originating from the pickup head |59 will be utilized. Under these conditions the output from the pickup head |59 is fed across the volume control potentiometer |60 and the cable |6| so that the resultant signal is supplied to the input of the amplier |63 so that the signal is amplified to the exclusion of any signals developed at the pickup' point I9'.

The'output signals from the tube |63 are then supplied by way of coupling condenser |66 to the grid of the output tube |64, from which a cathode output is derived, and supplied to the filter unit 23.

The output signals from the multi-stage amplifier 2| are then suitably supplied, by way of the lter 23, hereinbefore described, to control operation of the reactance tube 21. In this operation the output signals from tube |63 feed to tube |64 from which'the lter unit 23 connects in the cathode circuit. As suggested by Fig. l, the filter 23 comprises series inductance elements 22 and shunt capacity elements 24 with a terminating resistance |10. An output signal is obtained by suitably tapping to the resistor |18 volume control adjustment at |88 on the resistor |89. The output energy from the oscillator tube 41, as suitably controlled and modulated in frequency by the reactance tube 21, is then supplied, as shown by Fig. 3, for instance, through the conductor 69 and the coupling condenser |66 across resistor |61 into a tube 1| conventionally representing the hereinabove discussed frequency tripling stage.

The frequency tripling, it will be seen, serves to raise the frequency of the sub-carrier from the assumed value in the general region of 2 megacycles and produces the maximum deviation of the assumed 150 kc. in either direction from the assumed center frequency, it being, of course, understood that in the proposed arrangement the deviation normally introduced by way of the reactance tube 21 shall fbe considered as of the order of 50 kc. in either direction. Of course, where desired, the frequency of the oscillator 41 may be whatever sub-carrier frequency is desired and the frequency deviation of the desired amount may be directly obtained without the use of the multiplier stage comprising the tube 1|.

The tube 1|, as provided, is of the ordinary variety of frequency multiplier, except for the fact that the grid bias as applied to the control 'electrode |69 is produced by virtue of a cathode drop appearing in resistor |1| -under the influence of limited plate current. The resistor |1| provides bias due to the fact that the screen bias is on at all times so that a` D. C. component is available Aat all times to provide necessary tube bias regardless of the applied pulses. Conventional methods of producing grid bias by grid current are not to be preferred in an arrangement of this character, due to the fact that there is no grid input for most of the time, so that bias production from grid current rectification would be extremely ineilicient. This absence of control voltage naturally results from the fact that the oscillator 41 is keyed on only at the times when the sync pulses at horizontal or line frequency appear on .the output terminal of the sync generator 29. At other times the oscillator, as above stated, is inoperative.

The power output amplifier, conventionally represented at 15 in Fig. 1, is shown in Fig. 3 by a tube of like number. This tube receives its input signal on its grid, for instance by wayvof the resistor |16 connected to the transformer coupling provided through the transformer |19 connected in the output of the frequency tripler stage 1|. The primary and secondary windings of this transformer are each appropriately tuned 17 y In the arrangement shown, there is included also a double diode tube |83 which is connected to the local power supply source (assumed to be connected at terminals |85) by way of the transformer-` |86. The double .diode |83 provides for the development of constant grid biasY voltages which are supplied by way of the conductor |81, with the tube |83 being arranged as a full wave rectilier. The diode `|83 may frequently be replaced advantageously by an ordinary copperoxide rectier. y

The tapping adjustment provided by the slider |88 on the potentiometer |89 will readily provide for the adjustment of the sub-carrier center frequency, as above noted. Output signals for the purpose of -monitor control or the like may read.- ily be obtained at either of the output terminals |90 or |9| indicated, these signals being representative, in one instance, of the sound or audio signal representations and, in the other instance,

of the bias voltages obtainable.

In the broad reference to the circuit of Fig. l, the sight-sound injector I has been referred to asthe element wherein the separate signals representing the video, the synchronizing effects, the blanking controlsv and the frequency modulated sound sub-carrier are to be mixed and combined. Also, in the broad and more general reference to Fig. 1 to give a more generalized description of the system, it was stated that this sight-sound injector element would be further explained in reference to the showing of Fig. 4.

Now making reference to Fig. 4 and bearing in mind the'i'act (as explained with Fig. l) that the video signals whichreach the sight-sound.

injector I5 from the video amplifier I3 are stripped of all of the D.C. components in view of the fact that it is customary, in television operations, to use the so-called A.C. amplifiers, it becomes necessary in the sight-sound injector unit I5 to complete the video signals, so to speak,

. by the addition of the D.C. component, the

blankingor pedestal pulses and the synchronizing pulses. The latter of these two controls is derived from the sync signal generator I1 connected into the sight-sound in the manner diagrammatically shown by Fig. l.

The particular apparatus shown by Fig. 4 for forming the sight-sound injector unit comprises an input terminal |9| to which, it may be astwin triode amplifier tube las by way af the usan coupling condenser |94 and grid resistor |95.

The first half of the twin triode |93 serves to amplify the incoming pulses to about 10 times the impressed voltage whilethe second half of the tube serves as an inverter and clipper, after which the signals are passed through the triode coupler tubel |91 which is connected as a cathode lll 'second conducts through, under zero bias conditions by a resistor |98 of several megohms.

The output signals derived across the cathode loadresistor |98 of the tube I 91 are yshown conventionally by curve B of Fig. 5, where again the vertical sync pulses are represented as v and the horizontal sync pulses as h, although amplified in character from those impressed at the input terminal I9I.

The resistance condenser network I--202 is connected at the line 203 serving as the output conductor for supplying the sync signals to the injector unit keying tubes 205 .and 206. This resumed, the horzontal and the vertical synchroi no curves shown are to be considered as drawn strictly to scale), represented also by the wave reference immediately adjacent to the termi-l If reference is made at this point to curve A of Fig. 5, it will be seen that the horizontal and vertical sync pulses, as initially received at the terminal I9I, consist of pulses which are positive in polarity with respect to ground. As is evident from Fig. l all of the pulsesare of approximately equal amplitude. with the designations h representing the horizontal pulses, and the designations v representing the vertical pulses, for instance. 'I'hese pulses are similar to the form of sync pulses recommended by the FCC and RMA for present standards, exceptinsofar as the omission of the so-called preparatory or equallzing pulse isconcerned and except for the provision of a second-slot of 0.04H width (see Fig. 5A at S2). The produced signal series is impressed upon a sistance condenser network provides appropriate bias for the output voltage which appears irrthe conductor v203, as indicated by the wave form of curve B. of Fig. 5 heretofore mentioned. The conductor 203 then connects to the screen electrode 201 and 208, respectively, of the keying tubes 205 and 206, with the connection to the screen electrodes being made in parallel.

Under normal operational conditions, the wave form, as shown by curve B of Fig. 5, is such that the pulses extend within a voltage range from approximately 150 volts negative in the interval between successive pulses. This variation in voltage thus provides ways and meansv by which the keying tubes 205 and 206,draw no current except during periods of synchronization, by reason of the f act that the negative portion of the impressed voltage wave tends always to drive the tube beyond a cuto state.y

The sound or audio sub-carrier coming from the output of the power amplifier 15 (Fig. l), for instance, is impressed upon the injector unit by way of the cable or conductor 11, as above mentioned in connection with the description of each of Figs. l and 3. The sound or audio-modulation signals, as vwas above explained, are in the form of interrupted frequency modulations of a subcarrier and consist of bursts of waves of a frequency approaching the maximum frequency in the allotted spectrum, with the accurate timing and initiation of the carrier having been effected under the control of the horizontal blanking pulses BH which occur at line frequency which are applied at terminal I9I. `These pulses lead the horizontal sync pulses by about 2 of a line period and thus the start-stop oscillator has (with present standards) over one microsecond more time to start oscillations which will conserve full strength` during the sync interval., Thus, the sound modulated sub-carrier pulses `which appear in the cable 11 and which are further diagrammatlcally represented by Fig. 5 (curve C) are coincident with, and interior to, the horizontal sync pulses represented in each of curves A and B of Fig. 5 which initially appear at the input termfrom 2|2 on the transformer secondary through the resistor 2|3, and shunting condenser 2|4. On the other hand, the unmodulated pulse form derived from the conductor 203 is applied to the two keying tubes 205 and 200 in parallel. The result is that, under these circumstances, the tank circuit of tubes 205 and 205, comprising the primary winding of transformer 2I5, with the variable condenser 2|6 across the same (the condenser usually being shunted by a resistor 2I1) is not excited by the unmodulated sync pulses as applied to the tubes in parallel by way of the conductor 203, but only by the incoming subcarrier energy, Fig. 5-C, and this is only for the duration of the sync pulses of Fig. 5-B. There is also included a resistor 22| across which there appears the keying voltage alone without the sub-carrier modulation, as shown by the drawing.

This gives a negative pedestal to the sub-carrier oscillations so that the sync position of the two components results 'in a curve form such as shown by curve Fig. 5-D. However, the keying tubes 205-206 can only become conducting as far as the incoming modulated sub-carrier appearing in the conductor 11 isconcerned at time periods when sync signals are present in conductor 203 and applied to the tube grid elements 201 and 208. At times when output energy representative of the incoming sub-carrier is passed from the tubes 205-206, it will be applied to the circuit of the diode 2|9 by way of the secondary of the transformer 2|5, which secondary is appropriately tuned by the tuning condenser 220, for instance. These transfer pulses of modulated sub-carrier energy then are of the general form of thosev shown by Fig. 5--D after they are combined with the sync pulses which appear in negative polarity across resistor 22|.

It will be noted, however, that under these circumstances a transient signal, such as that marked on curve D of Fig 5 at x, tends to be developed. This transient eiIect is probably due to the fact that the back-kick from the controlling sync pulse occurs before the sub-carrier oscillations have died down completely. A transient of this type is most undesirable and it is important that it be eliminated before reaching the main transmitter output stage consisting, for example, of tubes 223 and 224, later to be described both as to their connections and operations. The presence of transients of the type indicated at :c on curve D of Fig. 5 would tend to produce, in the iinally viewed image representation at the receiving point, the effect of a sound modulated high frequency pattern which, of course, would be objectionable and conspicuous.

So as to clip these signals, the vdiode tube 2|9 which, for example, may be of the 1V type, is provided between the superimposed sub-carrier voltages appearing across resistor 22| plus the secondary of transformer 2 I 8 vand the final transmitter output tube combination 223 and 224. The

load resistor 225 is so adjusted that clipping occurs at an appropriate intermediate voltage shown, for instance, at curve D of Fig. 5, as the level indicated by the dotted lines. The result Vis that there is obtained across the load resistor 225 a complete intermodulation of the sync pulse energy and the sound sub-carrier energy without transients. This effect has been exemplified, for instance, by curve E of Fig. 5 and diagrammatically represented by the conventional wave form indicated on Fig. 4 by the curve adjacent to resistor 225.

These signals then are transferred to the final modulator tubes 223 and 224 by way of the coupling condenser 221. Appropriate bias on the electrodes 220 and 229 to which the signals are supplied may be set and adjusted through the use of a resistor 23|, which may even be variable, where desired, which is connected to the positive voltage supply line 232 receiving positive voltage from a terminal source (not shown) connected at the point 233. The bias obviously can be set in such a manner that the final transmitter modulator tube combination is 100% modulated by the peaks of the sound sub-carrier and approximately 90% modulated by the peaks of the unmodulated sync pulses alone. With a setting of this character, the multiplex transmissions may be carried forward without any undesirable clipping or limiting action which contributes greatly to the overall fidelity of the sound transmission.

Since it is desired that the video signal modulation should be combined with the produced sound signals appearing as frequency modulatedl pulses superimposed upon the line sync impulses, provision is made in the portion of the circuit shown in Fig. 4 to the right of the transmitter output tubes 223 and 224, for introducing the video signal and combining it with the frequency modulated sub-carrier for the audio signal, in the manner described.

The video circuit shown is, in many respects, like that described in presently pending application for United States Letters Patent led by this applicant on May 7, 1943, and identified as Serial No. 485,982, now U. S. Patent No. 2,366,358, and entitled Television amplier circuits. In a system of such character, the video modulation signal is impressed at the video input terminal 239 and supplied to the amplifier 24| through the coupling condenser 243. The grid resistor 245 is connected across the grid cathode circuit of the amplifier 24| to ground I0.

As explained in the mentioned copending application, a diode 241 may be connected between the grid or control electrode of the amplifier 24| and ground |0 with the cathode of the diode connected to the grid or control electrode and the anode thereof to ground in order to provide ways and means, as also explained in the copending applicatiomby which the so-called D.C. component may be added or restored in the video signal supplied to the amplifier 24|.

Output signals from the cathode follower coupling stage 24| are derived across the cathode load resistor 249 therefor and are supplied, in turn, through a diode 25| which serves as a coupling element to supply the resulting signals upon the control electrode 253 of the amplier 255.

With a circuit of this character, it is the function of the diode 25| to provide a conductive path between the tube 24| and the amplifier 255 whenever the cathode element 251 of the amplifier 255 becomes more positive than the average bias of the signal source, which will be measured usages 21 by the cathode output from the tube 2H. At such times the diode 25| becomes conducting and signal voltage is then applied through the diode to the grid or control electrode 253 of kthe tube 255 and the grid isfthuscaused to'follow the output of the tube 24|.'

A circuit of the type described also provides an arrangement whereby provision is made for keying the output tube 255 at certain time intervals, which will be assumed herein to constitute those intervals during which blanking pulses, identiiled as Bv-n on Fig. 3, for instance, are developed from the sync generator |1 and are applied, in the arrangement shown, to the blanking signal input terminal 259.

In the manner also explained in the copending application, the applied blanking pulses are passed through the coupling condenser 260 and conductor 26| to the grid or control electrode 262 of a keying control tube 263 from the out- 'then supplied across the conductor 28| and nally appear as a mixture of the various groups of signals at thev output terminal 283 connected to the terminating resistor 285 of the conductor. .It will be apparent that by adjustmentof the tap 286 across the cathode feeder 219, the grid bias of the power stages 228-223 may be adjusted, whereby a control of the adjustment of the height of the synchronizing pulses carrying sound modulation in the form or the interrupted frequency modulation has been made possible at the cable input. The amplitude of the combined signal then may also be controlled by'an adjustment .of the tapping point 286 for connecting the output terminal 283 to the terminating resistor 285 of the load conductor 28|. Output signals at terminal 283 are then fed to any desired form of transmitter channel (radio or wire 4line) from which they can be supplied to an put of which signals are applied to the keying tube. f

The tube 263 serves to control the operative periods of the tube 265 by'way of the'connection of the control electrode 266 thereof to the output load resistor 261 through the coupling condenser 268. Accordingly, it becomes evident that thev cathode 268 'of tube 265 will follow exactly the blanking pulse periods as applied to the input terminal 258 and will, accordingly, control the operation of tube 255 by virtue of the connection of the outer or screen' electrode 21| thereof to the cathode element 268 of tube 265 by way of the conductor 212. Thus, provision is made whereby the keying periods of the tube 255 to transfer the video signal impressed at terminal 239, with the D.C. component added thereto and the blanking control also supplied,

into the diode element 215' which has its anode connected to the cathode 251 of -the keying tube 255.

The arrangement so far described in connection with the application of the video signal essentially constitutes a mixer amplier which produces a blanking pedestal within the video signal but not yet the sync pulses as such. These are injected within the stages 223-224 together with the sound sub-carrier modulation. It is a cascade of cathodefollower stages with` each output cathode connected-to the next input grid either galvanically or through electronic valves.

The diode 215 essentially provides the output coupling for the system while the diode serves as the input coupling. The output of diode 215 is connected to the No. 1 grids 216. and

` 211 of the output tubes'223 and 224 and-feeds across the resistor 218 which connects at an apfpropriate point to the load resistor 219 for the f composite signal..

Atthe output terminal 288, signals representing the combined video blanking and sync pulses, with the audio modulation' added as a frequency modulation of the sub-carrier, become available,

although it should be apparent that the screen electrodes 228 and 229 oftheoutput tubes 228 `and"224- do not receivethe simple super sync pulses directly butrather the combined super sync and sub-carrier more schematically represented by curve E of Fig. 5 and also by theI diagrammatic Wave form conventionally demonstratedadjacent to the output` resistor 225 of the diode 2|8. The resultant output signals as shown by curve F of Fig. 5 (and also by the wave form adjacent to conductor 28| with the sound portion of the signal indicated as s) .are

in Fig.r 2 then also set forth the general and.

schematic arrangement whereby the incoming signals lwere fed through a suitable sub-carrier lter to actuate or control the sync signal separator circuit. These same signalsv were also then applied to an inverter stage from the output of which signals were applied to the sound gate relay system and thence to the sound reproducer.

yIn the arrangement of the Vcircuit of Fig. 6,

further details of theA receiver instrumentality havebeen shown in such a way as to make clear the time division basis of operation. As faras the receiver per se is concerned, it will be appreciated that one of the important features thereof is the sound gate arrangement schematically represented by Fig. 2. The purpose of this sound gate is to eliminate the cycle frame signal noise which is otherwise produced by the vertical sync signal. lTo this end an electronic switch is needed which connects the sound receiver to the input at regular line intervals and constant duration, whether or not there isy a frame signal being transmitted. To accomplish this result, a so-called sound` gate, which will be described below, is provided. In the arrangel ment as schematically shown by Fig. 2, it will separator which is 'so controlled as to provider` for elimination of the frame signal noise. To

this end the relay unit is made to respond to each j incoming'horizontal or line sync pulse, and the relay unblocks the gate amplifier or separator' for"its,holdi ng time, which is made adjustable and preferably slightly less than the line sync period. Elimination of the frame signal noise is only accomplished by rigorously holding the f time constant of operation stabilized and of substantially xed duration during the pulse input. The sound gatey system should be triggered by the wave front of the fpul'seinput and should .respond to these pulses but only to every second pulse. Frequency selection of this character thus becomes important in the operation of a combined sight-sound multiplex system.

The circuit arrangement of Fig. 6 thus constitutes a sound gate relay system which is extremely regular in its operation and which does not introduce or contribute any additional noise of its own to the sound reproduction. Where` the gate relay system |23 is closed, no video signals should be able to pass through the keyed gate amplier or separator stage into the sound reproducer |20. It is the function of the complete sound gate circuit toprovide operation of the system in such a way that the actual periods of sound reproduction from the sound reproducerv element shall be limited only to those periods when sound bursts Occur as frequency modulations superimposed upon the line sync pulses and lasting for a. fractional portion of the time duration of each. The ear of the observer then .must be relied upon to fill in, `so to speak, the remaining sound which actually is not transmitted. l

Referring now, in more detail, to the circuit shown by Fig. 6 of the drawings, the composite signal which was assumed to be present at the output or load terminal 283 (see Fig. 4), or which was present in the cable 28|, is app'ied in the output of the cable 28| across the terminating resistance 285 (these portions of Fig. 6 are numbered equivalent Ato Fig. 4, for convenience of reference) and is then fed through the resistor element or volume control unit 281 to the cathode element 289 of an amplifier tube 290.

The tube 290 thus constitutes a cathode driven amplifier with the volume control established by virtue of the variable resistor 281 which, like the resistance 285, is of relatively low value and usually of the general order of 100 ohms each. Plate voltage for the amplifier tube 290 is supplied from a source, such as the known power supplies not shown, connected with its positive end at the terminal 29| and its negativeend customarily connects to ground at I0. The voltage is then r supplied to the tube anode by way of the filter network including the series resistances 292 and condenser 295, as well as the plate impedance, represented by the resistor 293 in series with the inductive peaking element 294.

The control electrode of tube 290 assumes a bias which equals the negative peak of the swing of the cathode 289. This bias is maintained for at least the time constant which is measured by the R-C network comprising resistor 282 and capacitor 286. Bias is produced by the grid current flow during negative peaks of the cathode voltage. Resistor 288 is of low value and serves primarily as a suppressor against undesirable oscillations which the tube 290 is apt to undergo in a. system of this type where this tube would act as an oscillating ycathode follower.

By virtue of the grid current flowing in the tube 290 when it is cathode driven, the output from the tube which will appear in the load connection made through conductor 296 is an amplifled sight-sound (video-audio) multiplex signal of the character conventionally represented adjacent to cable 28| in Fig. 4, with the proper D.C.

component added thereto and without polarity .for the image producing tube |00.

. 24 inversion. Therefore, in theoutput connection from the tube 290, which is supplied tothe ampliiler tube unit 291 by direct connection to the control electrode 298 thereof, there is apositive video modulation upon the grid or control. electrode 298.

In the arrangement shown, the indutance element 294 constitutes a shunt peaking. element. While it is represented in the connectionparticularly shown by Fig. 6, that the tube 290 is a' cathode driven tube, this particular form of connection is only one of several possibilities and is herein-represented by reason of the fact that, for convenience of illustration, the input is represented in the nature of a cable such as that shown by Fig. 4, and cathode driven amplifiers can be used to advantage in such connections because they yield an output without polarity inversion. However, where the output of the transmitter system, such as that shown by Fig. 4, is applied to a radio link vso that the complete signal including the video, the sync and' the sound modulation appearing upon the sub-carrier are all used to modulate a radio carrier in 'negative modulation, fthen the conventional grid controlled type of video amplier may well be substituted for the repeater element 290, if desired.

The signal output from the repeater element 290, as it appears in the conductor 296, is then fed through the amplifier and inverter stage 291 so that the input signal can be repeated without polarity inversion and ata low output impedance value at the cathode, and with inversed polarity at the plate. The output signal for controlling the cathode ray image producing tube |00 is supplied to the conductor 96 by way of the con-y nection from conductor 299 as the signal is derived across the cathode load resistor 30| for the tube 291 is shown.

At the point 303 the cathode output signal of the tube 291 is supplied into two branches, of which one branch leads to the control electrode 91 of the image producing cathode ray tube |00 by way of the series peaking coil 304, and of which the other Aconnection leads into the subcarrierlter |01 which was also described in connection with the reference hereinabove given to Fig. 2 and which will later be further explained in connection with the descriptionof the sync signal separator and deflection control circuits At the. moment, however, it may be sufllcient to state that the signal output from the filter |01, as it appears in the conductor 30.5,is a signal withoutvthe sound modulation and negative peaks of sync pulses. It has substantially the conventional wave form of a presently lrecognized standard form of television video signal without any sound sub-carrier, but has no preparatory pulses -and double slotted frame signals, so that, consequently, it is of the general form of Fig. 5A.

Provision is also made for deriving from the inverter stage 291 a second signal which will be obtained at the plate or anode element 301' thereof by way of the conductor 308 and coupling condenser 309. This signal will be applied. as explained in connection with the schematic representation of Fig. 2, to control the combined sound gate relay unit which includes the synchronizing element |25, the relay element |23 and vthe amplifier or separator unit |2|.

Accordingly, the sound receiver portion' of the circuit isdriven from the plate circuit of the inverter stage 291 where there are' produced inverted sight-sound multiplex signals with positive peaks. The load resistor 3II of the inverter stage 291 is preferably shunted by a small variable condenser 3|3. Adjustment of the condenser 3 I3 provides Ways and means by which the high frequency gain may be reduced. This is desirable in circuits of this nature because the capacitive portion of the load across the cathode, as it appears at point 303. results in an emphasis of the high frequency components across the plate or anode load resistor 3| I. If no counteracting features are introduced and the effects of the high frequencies nullied, cross-talk will appear in the sound channel leading to the sound reproducing element |20 and the sight-sound separation would not be complete.

Generally speaking, it is desirable that the adjustment of the condenser 3 I3 be such that there is a one-to-one ratio of high and low frequency amplification across the line 3 I5 connected to the terminal of the condenser 309 which is remote from the tube 291. This provides proper relationship between the sub-carrier and video signal amplification.

It is, of course, to be' appreciated that the condenser 3| 3 may be adjusted once and for all in its operation in which case the sound gate mechanism of the tubes 320, 330 and 340, later to be described in more detaiLshall operate as desired.

In considering thisoperation, the sound gate circuit which is fed from the output of the inverter stage 291 functions in such a manner that the final keyed-separator tube 340 is keyed by both the sound and the control signals. In this way the output from the plate of the ytube 291 is applied by way of the coupling condenser 309 and the conductor 3|5 to the No. 3 control grid 3I1 of tube 340 and also is applied through the same coupling condenser 309 and the connection 3I9 to the control grid 32| of the gate synchronizing tube 320, as indicated.

'They keyed separator tube 340 is preferably ofthe so-called pentagrid type. One tube type which has proven very satisfactory in this use is that known in the art as the 6L'1 in which two successive grids are provided for dual modulation of a common electron current. The No. 1 grid is usually designed for remote cut-off, but the No. 3 grid has a sharp cut-off and, therefore, requires relatively small cut-off voltage. Accordingly, the separator'tube 340 is preferably keyed in such a manner that the signal input, as shown, for instance, by curve A of Fig. 7, is applied to the No. 3 grid 3I1 by way of the conductor 3| 5 and the gate relay pulses, later to be disclosed and described in more detail, are applied to the No. 1 grid 325 in a manner also later to be described from the output of the relay tube 330.

The synchronizing stage 320 has broadly two functions, of which the first is to keep the voltage peak at the input cable or conductor 3I9 at zero potential, which is desirable to maintain both satisfactory separator action in the stage 340 and proper sync clipping in the stage 320 over a wide range of signal amplitudes. D. C. insertion is obtained by the grid current which flows in the stage 320 which operates with a veryhigh grid leak resistor 321 connected between the grid 32| and ground where the cathode of the tube 320 is connected, as indicated. The second function of the synchronizing stage 320 is to provide appropriate trigger pulses for the relay stage 330.

clipping bias for plate current may be adjusted by quency selection be shown if the input frequency is to be doubled by the slot signals.

The plate voltage output from the synchronizing tube 320 is differentiated by means of the small capacitor 332 and resistor 331. The differentiated pulse appears at point 333 (see curve B yof Fig. 7), as it appears across the load resistor 331. From curve B ofFig. 7, it can be seen that all of the regular horizontal sync pulses appear at equal amplitudes and are stripped of their subcarrier modulation by reason of the capacity 335 appearing across the relatively high resistance 331. Pulses, however, which occur at double the line frequency during the slots c (see curve A of Fig. 7) in the frame signal, rexperience some reduction in their amplitude and this is brought about by the very high value of plate resistor 33| which is unable to supply the normal plate voltage at the necessary rate. The system is so designed that once a signal is passed, the complete voltage on the tube 320 drops noticeably and it takes approximately one-half a line period (of the v general -order of 30 microseconds or more with Various forms of tubes may be used for the pur-- pose of this relay, but one tube which hasv been found satisfactory is that known in the art as 6N?. A tube of this character in thecircuit with which it is associated essentiallyr provides and fullls three conditions of which the first is that one output pulse should be provided for each regular sync input pulse, and, second, that the duration of the output pulse should be made adjustable and completely regular, and, third, that there should be no response from the relay system to any input which' occurs sooner than at line frequency intervals. In the arrangement shown,

the circuit for the relay tube 330 is intended to lfulfill the fioregoing conditions.

The gate width is adjusted by virtue of adjustment of the condenser element 34| andtheV resistor element 343-, while the selection of pulse frequency is provided by the blocking system in the formof the resistor 345 and the condenser 341. In this arrangement the cathodes of the two sections of the relay tube 330 are connected together and are connected to ground I0 by way of two resistor elements 349 the latter is customarily several times the size of the former. The circuit provided by way of the condenser 34| and the resistor 343, which merely serve to adjust the gate width, is connected so that the signal pulses appearing at the point 333 as the output of the sync tube 320 may be fed through the series connections shown and the condenser 34| 354 of the second resistor 343 tem.- In the operation the system is so adjusted that the second half of the relay tube 330 is nor- As the grid-to-ground peak voltage is xed by I D. C. insertion provided by the flow of grid current and by means of a high resistor 321, the

mally constantly conductive in that with the arrangements shown theA grid 354 is conductively connected to the cathode at all times through the variable resistor 343 but on the other hand,

and 35|, of which to the grid or control electrode 27 with the grid 355 of the first half of the relay tube being connected to the cathode through the resistance portion 345 of the pulse frequency integrating circuit and through the resistor 349, it becomes evident that the resistor 349 will serve to provide a permanent blocking bias on the first half of the tube 330 during all periods when the second half of the tube draws current. It will be understood, of course, that, where desired separate tubes may be substituted for the double purpose tube herein illustrated but the operation is, in all instances, the same.

If, now, reference is made to the curve B of Fig'. '7 and it is assumed that a negative wave front is received through the differentiating network provided by way of the coupling condenser 353 and the plate resistor 355 of the rst half of the tube, then a feed back action within the relay system takes place. The negative pulse is transmitted to the grid or control electrode 354 of the second half of the relay tube 330 with the result that emission in the second half of the tube is momentarily interrupted. The cathodes of each half of the tube are shown as being connected so that both cathodes perform a voltage drop' when the grid 354 goes negative but the grid element 355 is unable to follow this effect by reason of the grounding provided by way of the condenser 3-41.

Consequently, within the first half of the relay tube 330, plate emission takes place rst, and second, the grid condenser 341 receives a negative charge due to grid current following in the first half of the tube and the plate current pulse in the triode section of the tube reinforces the incoming signal as it is impressed across the resistor 355 so as to make the plate still more negative. Consequently, feed back action takes place which speeds the relay response up so much that the gate opens. so to speak, with a delay of only a minute fraction of onesound sub-carrier period. The holding time of the circuit is determined by the circuit including the plate or lead resistor 355 serving as a par-t of the differentiating network with the condenser 34| and the resistor 343 which connects the grid 353 to the tube cathodes. The entire arrangement and its holding time can be adjusted through adjustment of the resistor 343, forinstance. Generally speaking, the time constant values should be so adjusted that the holding time of the relay is slightly less than one.horizontal sync pulse duration, or with double slotted pulses, slightly longer than one horizontal sync pulse which, according to presently recommended standards, necessitates the time constant being adjusted to the order of seven per cent of a line period with a 525-line image transmission, which means a time period according to presently-recognized standards of the order of 4.4 microseconds.

It was above stated that the pulse frequency selection which was made by the relay 330 was determined by the resistor 345 and the condenser 341 and also stated above was that the condenser 341 received a grid current pulse during periods when the first half of the tube 330 draws grid current. In the operation of the system, the resistor 345 is so arranged that the charge across the condenser 341, as represented, for instance, by curve C of Fig. 7, will require approximately '70% of the line period (shown as H in Fig. 7) to leak away through the resistor 345. If this is done, .the time constant of the resistance condenser combination 345-341 is chosen equal t or longer than 1,5 the line period as represented where Tg is time constant of the gate and H the line period. Thus, it will be evident that the relay itself will be unable to respond to any pulse which is impressed thereupon at a period prior to one occurring at a time period equal to one line period of the transmission subsequent to the last preceding pulse which effected a control operation. Where any pulse arrives prior to such a time, as do the slot signals S2 (see Fig. 5, curve A) within the vertical or framing signal pulse, for instance, the received pulse will be impressed upon the relay unit when the grid or control electrode 355 of the rst half of the tube 330 is unusually negative and although the cathoc-.s of the tube sections under the control of the incoming wave do change their potential and drop in the negative direction, Athere is no plate current available within the first half of the tube.

The net result is that the keyed amplifier stage 340 is being unblocked by regular pulses shown in Fig. 3, curve D, obtained across resistor 355' and is fed to the first grid 325 of the keyed separator 340. This stage, however, is not unblocked for intermediate pulses or for conditions corresponding to slots in the framing or vertical signal because there is no emission taking place within the rst half of the relay tube 330 and, therefore, no signal at its output circuit. Consequently, there is obtained across the tuned -plate circuit 359 of the amplifier tube 340 and under control from both grids 325 and 3|1 a plate current wave form which has been schematically represented by curve E in Fig. 5 in which therev is an absence of any traces of any transients due to the occurrence of the frame signal as an interruption in the regular sequence of line signais.

These keyed pulses occur at time periods when the keyed amplifier stage 340 is unblocked by reason of a cessation of current flow in the first half of the relay tube 330 so that a positive potential is applied to the No. 1 grid 325 of tube 340 through the coupling condenser 36|. Simultaneously, the audio modulations, as hereinbefore represented. are supplied to the No. 3 grid 3|1 of the keyed amplifier stage 340 by way of the conductor 3I5. With the time constant of the network 355, 34| and 343 being adjusted, as above noted, so as to be longer in time than l/2 aline period the slot effect in the vertical signal does not open the keyed gate amplifier stage 340. The result is, accordingly, that the tube 340 can draw no plate current for the slow portion of deilection cycle but is limited to current flow during the snap back or return trace period by reason of the unblocking of the tube. By the potential applied to grid 325 there is an electron current to modulate and during this period, the modulated sub-carrier is applied to the grid 3|1 so Ithat the output as supplied from'the plate or anode 363 to the tuned circuit 359 is an audio modulated signal which is substantially free from any video cross-talk effects.

In one conventional system where the multiplexing is applied to television operations for the image production results in a traced image composed of 525 scanning lines, for instance, repeated at 30 frames per second the line period for each of the scanning lines thus is of the order of 63 microseconds. If, now, the selector circuit which controis, as above named, the time periods at which

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