US2366358A - Television amplifier circuit - Google Patents

Description

Jan. 2, 1945. K. scHLEslNGER TELVISION AMPLIFIER CIRCUIT 4 Sheets-Sheet 1 Filed May 7, 1943` IJIYIHII- OMQ IMU v Jan. 2, 1945. K, scHLEslNGl-:R 2,366,358

TELEVISION AMPLIFIER CIRCUIT Filed May 7, 1945 4 Sheets-Sheet 4 IN V l'N TOR. Kur ,LS/Mayer TTC/@NEX Patented Jan. 2, 1945 2,366,358 Y TELEVISION AMPLIFIER cIItcUI'rY Kurt Schlesinger, West Lafayette, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application May 7, 194s, seria1No.4ss,9sz

(ci. 17a- 7.2)

25 Claims.

This invention relates to an amplifier circuit, and'particularly to that type of anamplifler circuit which finds use in television systems. In its broadest form, the invention may be said to relate to that type of television amplier which is used for providing a composite output television signal which includes not only the video signal, which is derived from scanning a subject by an appropriate form of image scanning or camera tube, but which combines with such video signals other control signals or energy component by which the video signals are caused to b e true representations of the conditions of average background brilliance, which is also known as the so-called introduction of the d. c. component. To such a signal there are also added two other signals, commonly known in the art as the blanking and the sync (also termed synchronizing) signals.

Broadly speaking, the amplifier unit herein to be described is one which operates at a high voltage output level and in which the overall gain approaches unity. As the system will herein be described, it will be found to comprise a plurality of stages of amplifiers, preferably, although not necessarily, pentodes, which are connected in series, and wherein the modulation introduced therein is preferably done in the circuit of the screen electrode. The amplifier is further characterized by making provisions -for overcoming any possibility of so-called cross-talk between the blanking and the picture or video signals, which is accomplished through the use of electronic rectifier devices as coupling elements.

For this purpose, the system herein to be described makes provision for producing a composite signal which includes all of the information necessary to transmit intelligence (such as the complete television and all necessary control signals) over a wire line channel or via a radio transmission channel at high frequency. To this end, the resultant signal output from the system herein to be described will be found to include the changes or variations from which the picture reproduction at the receiver or monitor points will be produced, and, still further, will include a signal inserted to care for the initial loss of the direct current component from the picture or video signal, which loss has come about due to the prevailing trend to use a. c. amplifiers as much as possible for efficient and stabilized operation. With signals of this nature, two other signals, known as the blanking signal and the "super-sync signal, are added, so as to insure that -blanking at receiver or monitor points is carried forward. and likewise to provide a control signal to make certain that receiver or monitor operations are carried out in synchronism with the scanning at the transmitter point, whereat the initial video or picture signals were developed.

It thus becomes a primary object of this invention to provide a television amplifier circuit for the aforesaid purposes, wherein the operations are carried forward with great simplicity and, at the same time, without the introduction of objectionable cross-talk eects.

Other objects of the invention are those of providing a television combining amplifier circuit which is of greater stability than those heretofore used.

A further object of the invention is that of providing a television combining amplifier wherein recourse may be had to the eilicient type of cascaded cathode-follower between the several stages.

Other objects of the invention are those oi overcoming one or more of the defects of the prior art in connection with such forms of combining amplifiers, and, at the same time, of providing an amplifier which is relatively free from complex circuits, but yet efllcient and reliable in its operation.

Still other objects Will become apparent without specific statements thereof when the drawings are read in connection with the following specification.

Referring to the drawings,

Fig. 1 is a series of curves a, b, c and d for the purpose of showing the general wave form of the television signal at various points in the system;

Fig. 2 is a simplified circuit diagram of one suitable form of circuitby which the invention may be practiced, although this circuit primarily is for the purpose of indicating the general operational principles;

Fig. 3 includes several curves a through h, inclusive, to explain further the operation of the cathode-follower amplifier system;

Fig. 4 shows further the general form of interstage coupling; and,

Fig. 5 is a circuit diagram of'a portion of a complete amplifier circuit incorporating the invention.

Now making reference to the drawings, and first to Fig. 1 thereof, a series of curves has been set forth which show the general operational sequence of the circuit of this invention. In any television system, the signal resulting from the scanning operation is one wherein the various values of light and shadow are represented.

Such a signal may result from the scanning of a scene of action by means of a cathode ray type of scanning tube, for example, such as that type known as the Iconoscope" or the Orthicon, as have already been explained in the literature. and, to some extent at least, in the patented art, and require no further detailed reference herein. when such signals are amplified, they are generally amplified in an amplifier of the so-called a. c. type and, as a' result, they lose the so-called d. c. component which indicatesl the `relative loverall brightness of -the scene scanned. Such signals are of the general wave form as those shown by curve "a" of Fig. 1. In this figure, it may be assumed that black in the picture vis that signal value indicated at the pointP, or in a case where a mask is used, for instance, to establish the black level, the value may be determined by reference to an output signal which will be representative of black as indicated by the signal value M of Fig. la.

The signal which results after the d. c. component is re-inserted, is shown by the curve b of Fig. l. From this curve, as Iis known', the signal values now all lie one side of a fixed value which is marked "black level" on the diagram.

In the curve c of Fig. 1. there is a representa- 4,tion of the modification that has taken place in the signal when the blanking 'signal is combined with the signal to which the d.-c. component has been added.A In this curve, it will'be seen that blanking modulation has taken place and that the video signal value or swingfre'aches a prescribed minimum value D regardless of whether or not .the picture field at the instantwas black or white. A

After the blanking level has been set and the blanking modulationintroduced, it is then possible to add the synchronizing or sync signals as a further modulation, as is shown by curve d of lIlig. 1. 4With the signal form shown by the several curves of Fig, l1,lit maybe noted that -the various signalsl are shown as positive" signals (in contrast to the so-called "negative" signals) and, as will be apparent from what is to` follow. this form of signal lends itself to amore simple control with the form of amplifier herein to be set forth, and is partiomariy' adaptable for l use withr the grid controlled cathode ray monitor receivers, and yet the form of lsignal set forth and developed by this amplifier unit is stillsuitable to produce the so-oaned "negativemodultion" of a radio frequency carrier in a manner prescribed by standards now existing. l

With such a signal formation to be requiredfrom the television amplifier, itjwill become apparent that the signal must remain unchanged in polarity as 1t posses mrougn the ampuner;

that vthe frequency band shall be passed without loss, for otherwise the 4transmission efficiencyv would be considerably impaired; that the amplifier shall act as alinear device so that the halftone picture distribution remains'unaltered; that the amplitude level' of the so-'called super-syn? y (shown at fS" in curve d of-Fig; 1) and c the blanking pedestal (shown at D by curve c of Fig. 1) shall be maintained rigidly constant and :at all times completely independent of the value of the video or'pieture signal being instantaneously transmitted; that the amplifier unit deliver ,adequate-output power to enable the use of one 'or several monitor units, as weil as toA provide adequate energy to lfeed a transmissioniine or, co-axial cable to connect the unit tothe radio -lated or keyed cathodesfollower circuits of this invention.` This is done, generally speaking, by connecting several cathode-follower stages in series and then keying them by the blanking and the sync control pulses. Bythe use of conductive coupling throughout the ampliner, the d. c.

component is carried through to the .output of the amplifier, and the successive cathodes of 'the system assume increasingly positive levels relative to ground and contain the various modulations per stage in a manner in which no objectionable reaction eifects result.

Reference may now be made to Fig. 2 for a brief-explanation of the general principles of operation of vthe mixer ampliiler. In this arrangement, it may be assumedthata scanning orcameratbe il is of the 'general type known as the Iconoscope or Orthicon for instance, and includes a mosaic target element `I8 upon which an optical image l! is adapted to be proiected by a suitable optical ,system I6. When the opticalimageris cast upon the mosaic target Il, it produces ctrostatic charges (asis known for the typeA of tube),` which may be scanned by a suitable electron scanning beam (not shown) in known manner to release output signal energy to an external circuit, such as that indicated in schematic form' by the amplier unit I1.' The output signalsy are usually developed across some suitable form of a load resistor for the scanning tube, such as the resistor i! of the amplifier `I1.

Generally speaking, the amplier unit I1 is of the so-called a. c. type so `that the d. c. compo- -nent of the output signal is lost, with a result that the signal output from the amplifier unit l1, as it appears at the output terminal 2l, may be considered generally to be of the form shown by Fig. 1, curve a, or Fig. 3, curve a, later to be described. 'I'he general form of amplifier lncluded within the amplifier unit I1 may be of the so-oalled condenser coupled resistance type, and, for example, may be of the form shown and described by my copending application entitled Video amplifier," Ser. No. 485,981, flied May 7, 1943.

For purposes of illustration and for the purpose of furnishing a concrete example of operation of an amplifier unit to aid in the understanding of this invention, it will be assumed,

from the explanation furnished by the several curves of Fig. 3 (later to be described), that the output signalvat terminal 2| is of the order of 30 volts.l Since this-signal lacks the d. c. component, that component of the signal is reinserted by the diode unit 23 which is appropri- `ttely coupled to the amplifier i1 yby the coupling condenser 25. T'his diode-23 is shunted by a very high impedance unit 21 and functions in a manner well understood. f

The output'signal, with the d. c. correction added, is now su'ppliedy to the control electrode circuit of the first cathode-.follower tube'v 29 which presents a low impedance output at the output terminal 83 across the cathode resistor 3|. At this output terminal the signal output wave formation is of the general form shown by Fig. 1 (curve b) and Fig. 3 (curve d) and corresponds tothe wave form as applied to the' control electrodeof the ilrst cathode coupled tube 29. l

Following the yiirst tube 29 there are two addi' l tional cathode-follower tubes l35 and 45, respec' tively, which are directly connected to the preceding tube in a new and novel or characteristic manner which is herein termed "plate mod-v ulated cathode-follower cascade in series con-` nection. In contrast toheretofore known forms of circuits, the tubes 35 and 45 havetheir plate power modulated by the blanking signals,` as derived from the source 39, vmodulating tube' 35, and the sync signals, as derived from the sync signal source 41, modulating the plate of the tube 45. n

In the connection. shown, the output-of the tube 29 with the video and d. c. component signals is supplied Vto the control electrode of the tube 35 across the load resistor 13| and through the bias source 31 which is provided essentially to avoid any excessive blanking peaks. After the blanking signals have' been added in the tube 35 b'y way of the modulation of the plate energy, these signals now appear across the cathode yresistor 43 of the tubef35, and aiso at the but put terminal 4I. The general form ofthe signal at the terminal. 4| was above described, but for further reference it may be again mentioned that it corresponds substantially tothe signal forms shown by curves c and f of Figs. 1 and 3, respectively.` This signal, which now'includes the video with the d. c. correction and blanklng added to it', is supplied by the direct coupling 4to, thegrid or control elementof the-tube 45 and, when modulated by the sync4 signals from source 41, causes a resultant output signal at the terminal 49, to correspond to the general wave form shown by curves dand h of Figs. v1 and y3, respectively. 4

If reference is now madeto Fig. 3, the foregoing explanation'will be clearer in the light vof the diagrammatic representations there shown. By Fig. 3, curve a shows the video signal at the terminal 2| (Fig. 2) and curve b shows the same signal ywith the d. c. component added thereto. For illustrative purposes, as was above suggested, thevideo signal may be assumed to be of the order of :i0v volts, peak to peak. Under such cir-v cumstances, reference may be made to curve c which is representative of the first cathode follower action (e. g., tube 290i Fig.' 2).V In this curve, it will be seen that'by drawing the characteristic of the firstl cathode Vfollower tubeas the characteristic a, it will be possible to determine, from what is to follow, the. cathode potential as derived from the static characteristic of the tube. The characteristic a shows the relationship of the plate current to the grid-toground voltage, which latter voltage-is iplotted along the abscissa. The positive peak value of the voltage is represented at the point b as be-4 ing the 'assumed 30 volts. It'will be appreciated y that the cathode follower tubeoperates without energy consumption or grid current only in that -portionof its characteristic which lies to the left of the value where the grid voltage equals zero (point o).

In order to obtain the cathoder potential f for any given grid-to-ground voltage, a load line is drawn from the point c .until it intersects the characteristic av at point d, and from this'point a line is drawn at a constant plate current value -(that is', parallel to the abscissa) tothe point i where it intersects the ordinate at e, and, from this point e, another load line is drawn parallel to the nrst point to point I. where the load line intersects the abscissa. l l

It is clear, from what iis above stated, that thecathode potential in such a circuit will el ways be positive relative to the grid or control electrode as long as thepoint d, where the nrst load line intersects the characteristic, lies to the leftI of the ordinate.y From the diagram it is possible to calculateiat once the minimum value of cathode resistor. which is compatible with negative grid operation at the given grid swing of the assumed 3o volts by merely drawing another load 'line `from the point b (the positivepesk value) to the point g. which is the plate current for zero 'voltage on the grid. The value of the resistor is then evident, but, in order to be certain that'the tube does not, draw grid current, it is usually desirable to .select a slightly larger resistor, whichlwould be that represented by the valueV for the line b-h, which will insure undls' ficant from this curve that the output voltage does not reach zero, even if the grid voltage should happenl to do so. It thus becomes apparent that unless the complete characteristic a is falling into the origin by either keying of! the plate voltage of they tube (such astube 29, for examplelor, in the case of tetrodes ork pentodes, the scrern voltage, the output cannot reach below the positive l threshold voltage 'of the point f, assumed in Fig. 3c to be 6 volts. Thus a first, or blanklng potential is obtained from which to key down to zero.

Considering still further the curves of Fig. l3, the output of' the vilrst cathode follower tube 29 (Fig. 2) is shown by Fig. 8d. This output signal includes the d. c. correctionI and it is conductively coupled to the grid l of tube 35 through the bias source 3 1, which, in the example chosen, may be assumed to be of the order of 10 volts so that excessive blanking peaks are' avoided. The result of this bias is that the characteristic is shiftedto the right, as per Fig. 3e. The

graphicall construction above explained is repeated with the curve of Fig. 3e and the minifor any value ofthe signal voltage under the assumption that there is no grid current.

`The last step inthe operation is explainedwith the aid of the curves'- Figs.- 3a and Again, 4

the koutput from .the cathode follower.l in this case tube '35, is conductively coupled to the grid of the nextstage, such as tube 45, wherein thesync signals are added. From lthe curves it will be seen that thepeaks of the blanking signi-l5. although they reach fground level, are unable to carrythe cathode of the last tube 45 to ground but, rather, keep, it at a level indicated at 8 by Fig. 3a. The remainder of the signal will be seen as oscillating between the assumed 10 volts level, at D, and 'the 30 volt level. at V. From this curve. .the load resistor can again be determined and the sync .signals are introduced by keying the plate power of tube 4l by the sync signals at 41 to reduce the outputto zero for -thefduration of the syncy signals. As will be evident from the diagrams of Fig. 3, it may, with the assumed signal values, be possible to derive an output signal of 27 volts (almost 1 to 1) with the system herein described.

No reference has herein been made to the general form of the generator or source from which the sync and/or` blanking signals are developed, for the art is familiar with such types of signals, but, if desired, reference may be made to applicants copending application Serial No. 452,921, filed July 30, 1942, and entitled "Synchronizing signal generator, for one `suitable form of arrangement to develop such signals. Specincally, the generators of such forms of signals formno part of this invention, except insofar as it cooperates with the remaining portions of what is herein set forth and claimed.

While the circuit` hereinbefore described explains one convenient way by which the invention may be practiced, it will be appreciated that it is, in some instances, desirable that less power be utilized for the keying operations, and this can be accomplished through the use of keying circuits, as will herein be set forth, whereby the keying effectsmay take place in the screen, rather than the plate, circuit of the tubes. A keying circuit of this character is set forth by the disclosure of Fig. 4.

Referring now to Fig. 4, signals of the form shown by Fig. 3b (that is, video with the d. c. correction added) are applied at the input terminal 50 to modulate the grid 5l of the cathode follower tube 35. The Vtube 35 is now preferably of the screen grid (or pentode) type. In this arrangement, the screen electrode of the tube 35 is maintained at a suitable bias with as low an impedance to ground as possible, in order that it shall operate properly for its conductive pe riod. However, it should be noted that it cannot be blocked to ground by condensers as such a connection would tend to prevent the necessary high speed keying.

Accordingly, screen power is supplied through another cathode follower, such as the triode 53, which permitsthe adjustment of thescreen bias by means of a potentiometer 59 which is connected at the grid side of the triode 53 to supply voltage through resistor 60. In this way, the screen electrode 54 of the tube 35 can be keyed to ground very rapidly by signals impressed upon the grid 5I of the coupling tube 53. With this arrangement, the screen element 5l appears to have a very low impedance to ground in the absence of any keying pulses on the tube 53. With a pentode of the type of tube 35 connected as disclosed, the video signals are transmitted without affecting the frequency response. The resistor 52 in the cathode circuit of the tube 53 is so selected as to provide an adequate value of the plate current through the tube to permit ythe transconductance to obtain its full value.

In the operation of the circuit of Fig. '4, the

` blanking signals are supplied through condenser I8 (at the hereinbefore assumed 10 volt level) from the input terminal point 81 so as to act in positive polarity on the rgrid of the tube 65 with the resistor 'Il being utilized to prevent excessive grid swing and thus in the capacity of a limiter and clipper against too high a level of the incoming pulses applied at terminal B1. Suitable bias for the tube 55 is built up automatically across the grid resistor 10 by grid current. The plate resistor 13 of the tube 85 is made as large as can be .to be compatible with the steepness of the pulses to be amplified. The result is that across the plate resistor 13 strong negative pulses are developed when signals are applied at the input terminal 61, and these negative pulses may be of the order of volts, which is more than adequate to drive the tube 53 well beyond its cutoff point and thereby reduce the screen voltage on screen element 54 to zero for the duration of the pulse applied for blanking at the terminal 61.

For ready reference to the operation hereinbefore mentioned, thewave forms adjacent different portions ofthe drawings are for the purpose of exemplifying the general form of signal at the point indicated. It also will be apparent that the tubes 6| and 65 may readily be combined within a single envelope, where desired, and, where such practice is followed, it is suggested that a convenient type of tube to-use under such circumstances is that known in the art as the 6F8 or a 7F?, for instance.

From what has been set forth above, it is apparent that the circuit disclosed by Fig. 4', while generally satisfactory in operation, can be improved to some extent in a further modification, as will be disclosed by Fig. 5. With the arrangement of Fig. 4, it is a-pparent that, under some conditions, there could be a sort of inter-action between the picture signals and the synchronization. This may result because if the screen of the blanking tubescreen element 54 is modulated to zero by the injection of the blanking signal through the tube.53, the cathode of the tube 35 cannot promptly follow to ground where there are positive signals on the grid 5I thereof. The coupling triode (which precedes the tube 35) is sufficiently powerful to deliver current into the first grid 5l: of the tube 35 as soon as this grid is positive relative to the cathode thereof. This condition would occur as soon as the plate of tube 35 is keyed off. Usually, of course, in the desired operation the cathode is positive relative tothe grid, but during the blanking interv val the condition reverses, as was explained. Therefore, the circuit of Fig. 5 is essentially so set up that the coupling between the various modulated cathode followers is conductive as long as the 4tubes are supplied with plate power, but the coupling is made non-existent whenever the plate -power is keyed off and the cathodes drop to ground potential. Thus, a diode electronic coupling element, which is a unilateral conducting device, is disclosed in the modification of Fig. 5, and it prevents any possibility of reaction within the amplifier unit. As will be explained further, the diode coupling element is provided as the coupling instrumentality between the several cathode follower stages.

Referring now to the circuit of Fig. 5, it will be seen that the video signal input is supplied, as before, through the coupling condenser element l25, and that the tube 23 serves as the d. c. re-inserter. The tube 23, in the modification of Fig. 5, is shown as a triode, although it has its grid and anode connected together so that it functionsI in the same manner as the diode of Fig. 2. The showing of Fig. 5 is merely illustrative and is made in order that it may be evident the tube 23 and the first cathode coupled tube 29 may be contained within a single envelope and be a tube of the type known in the art, for example, as the 6F8. It is ythus seen that in this circuit the control signal applied to the grid of tube 29 is the video signal with the d. c. correction applied by the tube 23. In this modification,`the cathode output of tube 29 `is now connected through the diode 8| (which, for example,

may be one-half of the tube 81, later to be described, in the form of the 6H6, as an illustration) .to the grid or control electrode 'of the cathode follower stage 35 wherein the blanking signal is added.

With this arrangement, as long as there isv plate current'flowin-g in the tube 35 the cathode will be positive relative to the grid. This bias is then supplied to the coupling diode element 8| which connects to the cathode of tube 35 by means of the resistor 83. Under these circumstances, if the absolute value of the cathode potential of the tube 35 is more positive than the average .bias of the signal source (that is, the cathode (as was explained in connection `withFig. 4),v

the cathode potential on the tube 35 tends to drop to zero, with a result that the anode voltage applied to the diode 8| is now less than the synchronizing stage the signal in the conductor 85 reaches the diode 81 at a substantially fixed level (that is, the blanking signal level) so that the tube 81 acts more as a clipper against excessive amplitude of sync pulses than inthe sense Y explained for the tube 8|.' The tube 81 might be replaced by a conductive coupling but, to do so, requires the addition of suitable voltage dividers to obtain the effect of the diode 81 since to obtain the clipping effect a carefully and properly chosen grid bias level must be established.

The synchronizing stage 45 is screen modulated on its screen electrode 88 by way of the' control tube 96 in' a manner very similar to that explained for the control of the blanking tube. To

` this end thesync signals, which have been developed in any suitable form of sync signal generator, are supplied at the sync input terminal 88 and through the coupling condenser 88 and the limiting resistor 82v to the tube 83. kWhere the pulse input energy atterminals 61 and 88 vis freefrom any undesirable amplitude modulavoltage applied to its cathode from 'the signal source. This thenpermits the cathode element of the tube 35 to reach ground potential 20 atA a rate which is limited only lby the electrical inertia of the associated circuits. This'fo'rm of coupling permits faithful reproduction of all half-tones, as well as the full frequency spectrum to which the amplifier isfto respond. It is such as to permit faithful synchronization for all conditions of picture content. However, care should be taken to avoid the possibility of any shunt capacitance across the diode element 8| which might tend to cause the re-introduction of crosstall: effects.

Continuing with consideration of the circuit of Fig. 5, it will be seen that the blanking tube is controlled, as before, by the application of the blanking pulses at the terminal 61 from which they are finally supplied as pulses of substantial negative polarity (as shown bythe Wave form sketched) on the screen electrode 54 of the tube 35. This, as above noted, causes the tube to cut off, and a signal wave form, substantially like that sketched adjacent the conductor 85, is supplied by that conductor to the second diode element 81, from the output of which the tube 45 is controlled in its grid circuit.

The voltage divider arrangement to set the screen potential 54, described in connection with Fig. 4, is again provided by the potentiometer 58, although this may be determined and set once and for all to a suitable value which' is a maximum compatible with tube ratings and an absence of distortion. So that the signal energy which is delivered to the diode tube 81 shall be a signal with a suitable blanking pedestal, a variable gridbias for the first modulator 35 is provided by the adjustable resistorr element 84, which is shunted by a suitable capacity element, as indicated.y This corresponds to the bias-source shown at 31 in Fig. 2.

In connection with the diode tube 81, it should be pointed out that the action is not, in all respects, like that of the tube'8l, but, to the contrary, since the dilculty of cross-talk exists only once in connection with blanking modulation where the input signal is at a variable and ar-` bitrary level at the instant 'of keying, in the opposite polarity is available at output terminalv tion, the limiting resistors 1| and 82 need not be included in the circuit. l

Suitable bias for tube 83 is built up by means of the resistor element 8| (as `explained for resistor 18). In this connection, the sync signal is applied in positive polarity at the terminal 88`and then. when amplified in tube 83 and impressed upon the tube 86 by way of the condenser 85,`and across the load resistor 84. it will serve to bias the tube 88 to a cutoff state in'a manner practi-L cally like that explained in connection with the control of tube 53. 'I'he result isthat now the screen 88 of tube 45 is keyed at the rate of, and in accordance with, the supplied sync signals', so that a final output signal Aof the Vgeneral wave form shown by curve h of Fig. 3 is available in vpositive polarity at the output terminal. |83, Vwhen the signal is developed across the cathode load resistor, or a signal of the same 'form but of the |8I, when the signal is derived across the output or load resistor |82 connected to the -plate or anode lelectrode of the sync signal inserter tube y45. In' the operation of the sync signal inserter,

the voltage divider combination 81 andthe gridleak resistor 88 function inthe same Vgeneral manner as explained with regard to the blanking tube and further detail need not be given.

In connection with the circuit disclosed, it will be seen that a short coaxial cable unit is also connected at the output terminal |83. This for'm of connection is particularly suitable for the installation of a monitor receiver which may be in the form of a cathode ray scanning tube (not shown) which may have its control circuit elements, such as the grid and cathode, connected to the terminal points' |85 at the end of the cable |88. To provide adequate peaking of the higher frequencies, a suitable peaking coil |81, which may act in series resonance with the cable capacity, is provided. A transmission cable so'y connected should be of limited length and the capacity thereof should'be maintained at reasonably low limits.

Under circumstances where longer cable lengths are to be energized from the output signals in which both the blanking and sync signal components are combined withv the video signalto which d. c. correction has been supplied, a modication of the output termination is provided. This connection is made by way of the connector termination ||5 to which the signal output is supplied by way of a high frequency correcting will usually be terminated by some suitable low .resistance |25which generally is of the order of about 100 ohms and preferably variable, as indicated.

The outputof the coaxial cable |I0, as it appears across the resistor |25 is then supplied to the cathode element |21 of the cathode driven amplifier tube |29. With reasonably chosen circuit constants, it is easily possible to provide an incoming voltage of the 'order of 4 volts, so that again of only the order of 10 is required to provide adequate voltage to operate a cathode ray image reproducing tube (not shown) of the type known as the Kinescope, for instance, which generally requires a control voltage of the order of 40 volts.

With the circuit shown, it will be apparent that Y the ampliiier |29 provides a convenient means whereby the output energyl from the cable IIO may be amplied without polarity inversion. In

the form in which the amplier is shown, the use vis chiey in connection with a low impedance source (such as the cable) and for voltage ampliilcation, rather than power amplifica- Output energy is derived from the amplifier |29 vfrom its plate or anode element I3I and fed to the input control electrode or grid |33 of a cathode follover coupling tube |34 (the purpose of which will later be explained). A conductor element |35 connects from the video signal output |38v to a terminal |36, at which a suitable sync signal separator circuit (not shown) of well known character is connected to separate the syncsignals from the combined video and sync.

' bined capacities of the Kines'copef connected to terminals |39, and the separator'circuit |39, from I loading the amplifier.

`The coil or inductonce |40 serves to provide efficient 'series peaking of the high frequencies which are supplied to the output terminals |38. Instead, or in combination, shunt peaking may be carried out in the driver stage |29, as shown by the shunt peaking coil or inductance 4| in series with the load resistor |42.

Having now described the invention, what is claimed is:

1. In combination, a plurality of thermionic vtubes each having a cathode, an inner and an outer grid electrode and an anode, means to supply control signal energy upon the inner grid electrode of one of the tubes, a cathode resistor for each tube cathode, av connection from the cathode of one tube to the inner grid electrode of the other tube, means for deriving a plurality of different keying energy pulses, means to lkey the outer grid electrodes of the said tubes individually by the said keying energy pulses so that a predetermined current ilows through each tube during periods of the presence of the said keying energy pulses, and a load circuit connected to the cathode of the last of the tubes.

2. A combining circuit comprising a thermionic tube 1n vcascade follower connection, said tube having a cathode and a plurality of cold electrodes of which at least one is a grid electrode, a, signal input source to supply signal energy to the grid electrode adjacent the tube cathode, a keying pulse source for supplying keying pulses to another cold electrode of the tube to establish a predetermined current flow through the tube, a cathode resistor, and a load circuit connected to the tube cathode.

3. A combining circuit comprising a thermionic tube in cathode follower connection, said tube having a, cathode, a control electrode and an anode, a signal input source to supply signal energy to the control electrode, a keying pulse source for supplying keying pulses to the anode to establish a predetermined current iiow through the said tube, a cathode resistor, and a load circuit connected to the tube cathode.

Y 4. A combining circuit comprising a thermionic tube in cathode follower connection, said tube having at least a cathode, an inner control e1ectrode, a screen electrode and an anode, a signal input source to supply signal input energy to the inner control electrode, a keying pulse source to supply keying energy upon the screen electrode yto establish a. predetermined current flow in the said tube, a resistor in the cathode circuit of the tube, and a load circuit connected to the cathode 0f the Said tube.

5. In combination, a plurality of thermionic tubes each having a cathode, an inner and an outer grid electrode and an anode, means to supply signal energy control to the inner grid of one of the tubes, a cathode resistor for each of the tubes, a connection from the cathode of each tube to the inner control grid of the next tube of the plurality, a plurality of sources of keying energy pulses, means for supplying energy from each source of keying energy to the outer grid electrodes of the tubes individually to reduce the tube current to a substantially zero level during keying periods, and a load circuit connected to the cathode of the last tube of the plurality to receive the combined signal energy andV keying signal pulses.

6. An electronic mixer circuit for producing multiple modulation of an input signal at various predetermined amplitude levels comprising a plurality of thermionic tubes each including a cathode, an anode and a control electrode, a cascade of cathode follower stages including a connection from the cathode of each of the tubes to the control electrode of the next succeeding tube, and means for applying modulating signals to one of the other of the electrodes of each of said tubes of the successive stages.

7. An electronic mixer circuit for producing multiple modulation of an input signal at various predetermined amplitude levels comprising a series of thermionic tubes in cathode follower connection each including a cathode, an anode, an inner control electrode and an outer control electrode, a connection from the cathode of each of the tubes to the inner control electrode of the next succeeding tube of the series, means for applying modulating signals to the outer control electrode of each of said tubes, and terminal means for deriving a modulated output at predetermined signal level from each of the tubes.

8. An electronic mixer circuit for producing multiple modulation of an input signal at various predetermined amplitude levels comprising a plurality of thermionic tubes in cathode follower connection each including a cathode, an anode and a pair of control electrodes including a connection from the cathode of each of the tubes to one of the control electrodes of the next succeeding tube, terminal connections for deriving a plurality of separate modulation signals,means for supplying the said signals to one only of the other of said electrodes of said tubes, and terminal means for deriving the output signals of each of the tubes.

9. An electronic mixer circuit for producing multiple modulation of an input signal at various predetermined amplitude levels comprising a series of thermionic tubes in cathode follower connection each including a cathode, at least two grid electrodes and an anode, means to apply video modulation signals to one of the grid electrodes of the first tube of the series, a connection from the cathode of each of the tubes to the controlelectrodes of the next succeeding tube which corresponds to the modulated electrode of the first tube, means for applying modulating signals to one only of the other electrodes of each of said tubes of the successive stages for combining with the said initial modulation, and

terminal means to derive output energy representing the combined signals.

10. An electronic mixer circuit for producing multiple modulation of video input signals at various predetermined amplitude levels representative of blanking and synchronizing signals comprising a series of cathode follower tubes each including a cathode, an inner control electrode, an outer control electrode and an anode, means to supply video signals upon the inner control electrode of the first tube of the series, a connection from the cathode of each lof the tubes to the inner control .electrode of the next succeeding tube, means for applying blanking and synchronizing signals to the other control electrode of the next succeeding tubes of the series respectively to combine the independent modulating signals in sequence, and terminal means for deriving output signal energy representing the combined signals.

1l. The'circuit claimed in claim 10 comprising, in addition, a rectifier unit connected as a uni-directional coupling element between successive modulated cathode follower tubes of the series to render the blanking independent of the instantaneous video signal energy source.

12. The circuit claimed in claim. comprising, in addition, a rectifier element connected to supply the video modulation signals to the rst keyed cathode follower tube, and means to render the rectifier inoperative to passsignals during periods of blanking pulses applied to said modulated tube, whereby the blanking control is independent of the instantaneous value of the video signals. 1

v13. A mixer circuit comprising a rst thermionic tube, a second thermionic tube connected as keyed cathode followers, a first terminal means for supplying video signal energy to the input of said first tube, a second terminal means whereat energy to be mixed with 'said video signals appears, means to key the output energy of the second tube by said signals at the second terminal, and a cathode connected load circuit for the second tube to provide mixed vsignal output of the signals originating at each of the first terminal and of the second terminal.

14. A mixer circuit for a television system to produce a mixed series of video. blanking and synchronizing signals Where the blanking signals follow a predetermined time duration of video signals and the synchronizing signals are added tothe blanking signals, comprising a series of' three cascade cathode follower connected thermionic tubes, a source of blanking signal energy and a source ofsynchronizing signal energy, means to energize the input circuit of the first of said cascade of cathode follower connected tubes by the video signals, means to control the output current of the second and third of the said tubes by the blanking and synchronizing signals, respectively, thereby to reduce the output energy of each of the said tubes to a predetermined signal level independent of the video signals and to combine thereby the said series of control signals, and output `terminal means for deriving the combined series of signals.

15. A mixer circuit for combining video and a blanking signal comprising a multi-grid tube having at least a cathode, a pair of control electrode elements and an anode, a cathode load resistor connected to the cathode and an output terminal connected to derive output energy in accordance with the current flow through said tube as flowing through the cathode resistor, means to supply video signal energy ywith d. c. correction included to innermost control electrodes, and means to supply control signals of predetermined signal level recurring at a predetermined rate to the second of the control electrodes to establish a predetermined output signal level in said tube independently of the presence and absence of signals at the first of said control electrodes, whereby a predetermined control reference signal level is established at the output terminal during periods of the presence of the said control signals.

16. The circuit claimed in claim 15 comprising, in addition, a unilateral electronic coupling element connected in the circuit including the control electrode to which thevideo signals are supplied for establishing the blanking level independently of the instantaneous value of the video signal input.

17. A combining circuit comprising a plurality of thermionic tubes in cathode follower connection, each of said tubes having a cathode and a plurality of cold electrode members of which at least one is a grid electrode, a signal input source to supply signal input energy to the grid electrode adjacent the cathode of the first tube of the plurality, a keying energy pulse source for supplying keying pulses to another cold electrode of the first of said tubes to establish a predetermined current ow in the said tube, a cathode resistor for said tube, a direct connection from the cathode of the first of said tubesand the grid electrode adjacent the cathode of the next of said plurality of tubes, a second source of keying energy pulses connected to supply pulses to another cold electrode of said second tube to establish apredetermined-current flow therein, a cathode resistor for the second v tube, a uni-directional coupling connection from the cathode of each tube to the grid electrode adjacent the cathode of the next succeeding tube. means for supplying keying energy upon another cold electrodeof each stage. and a load circuit connected to the cathode of said tube.

18. A combining circuit comprising a plurality of' thermionic tubes in cathode follower connection, each of said tubes having a cathode and a plurality of cold electrode members of which at least one is a grid electrode, a signal input source to supply signal input energy to the grid electrode adjacent the cathode of the first tube of the plurality, a keying energy pulse source for supplying keying pulses to another cold electrode of the nrst of said tubes to establish a predetermined current ow in the said tube, a cathode resistor for said tube, a direct connection from the cathode of the rst of said tubes and the grid electrode adjacent the cathode of the next of said plurality of tubes, a second source of keying energy pulses connected to supply pulses to another cold electrode of said second tube to establish a predetermined current flow therein, a cathode resistor for the second tube, and a load circuit connected `to the cathode of said tube.

19. A combining circuit comprising a plurality of nthermionic tubes in cathode follower connection, each of said tubes having a cathode and a plurality of cold electrode members of which at least one is a grid electrode, a signal input source to supply signal input energy to the grid electrode adjacent the cathode of the rst tube of the plurality, a keying energy pulse source for supplying keying pulses to another cold electrode of the ilrst of said tubes to establish a predetermined current flow in the said tube, a cathode resistor for said tube, a direct connection from the cathode of the first of said tubes and the grid electrode adjacent the cathode of the next of said plurality of tubes, a second source of keying energy pulses connected to supply pulses to another cold electrode of said second tube to establish a predetermined current iiow therein, a cathode resistor for the second tube, a uni-directional coupling connection from the cathode of each tube to the grid electrode adjacent the cathode of the next succeeding tube, means for supplying keying energy upon another cold electrode of each stage, and a load circuit connected to the cathode of the last of said tubes, where n is the number of tubes in the plurality.

2U. A mixer circuit for mixing d. c. corrected video signals and predetermined keying signals comprising a combining tube including, in the order named, a cathode, a pair of control electrode elements and an anode electrode, a diode means to supply the video signals to the rst of the control electrodes of said tube, a load resistor connected to the cathode of said tube and an output termin-a1 whereat signal voltage measured by the current owing through the tube and its cathode resistor appears, means to supply control signal energy to the second of said control electrodes at predetermined time periods to reduce the signal output energy flow in the said tube to a predetermined signal level, and means to bias the said diode to an inoperative state during periods oi' the presence of control signals so that the keying signal output level is independent of the instantaneous video signal amplitude.

21. The circuit claimed in claim 2.0 comprising, in addition, a keying circuit including a voltage amplifier tube and a cathode follower coupling tube connected to the second of said control eiectrodes, and means to supply keying signals to the said coupling tube to bias it to a cutoff state abruptly with the presence of keying signals to substantially instantly produce the desired predetermined signal level in the combining tube.

22. 'I'he circuit claimed in claim 20 comprising, in addition, a second combining tube having an input circuit, diode means for energizing the second combining tube by the output of the tirst combining tube, means for supplying synchronizing signals occurring duringr the periods of the ilrst named keying signals to said last named control tube to bias said tube to cutoff and thereby reduce the output thereof to a zero level, and a load circuit connected to the last named combining tube-to be energized by the combination signal representative of the video signal combined with the control blanking signal and the synchronizing signals.

23. A mixer circuit for combining video an-d a plurality of control signals comprising a series of multi-grid tubes each having at least a cathode, a pair of control electrode elements and an anode, cathode load resistors connected to the cathode of each tube, interstage connections between each innermost control grid and the preceding cathode to energize the said grid in accordance with the current flow through said preceding tubes as flowing through the cathode resistor, means to supply video signal energy with d. c. correction included to. that control electrode nearest the cathode of the ilrst tube, and means to supply the different keying signal sequences recurring at predetermined rates to the second control electrode of the plurality of stages in sequence to cause the establishment of predetermined output signal levels for each of the y keying modulations in the common output independently of the presence or absenceof signals at the first of said control electrodes, whereby a predetermined control reference signal level is established at the output terminal during periods of the presence of the said keying signals.

24. A mixer circuit for mixing d. c. corrected video signals and arbitrary keying signal sequences comprising a plurality of combining tubes in cascade each including at least a cathode, a

' pairof control electrode elements and an anode each spaced in sequence, rectifier elements to provide unilateral coupling between each cathode and the innermost control electrode adjacent the cathode of the next succeeding stage, load resistors connected to each cathode of said tubes and an output terminal whereat combined signal voltage appears containing the video signals and each keying at predetermined signal amplitude levels, and means to bias the said rectier elements to an inoperative state during periods of the presence of control signals.

25. In an electronic mixer circuit for producing multiple modulation of an input television video signal at various predetermined amplitude levels for combining therewith keying, blanking and synchronizing signals by the use of a plurality of thermionic tubes which each include a cathode, an anode and at least two control electrodes and are connected in a cathode follower series connection which in cludes a connection from the cathode of each of the tubes to one of the control electrodes ofthe next succeeding tube, the method which comprises modulating the control electrode adjacent rthe cathode of the nrst tube of the series by video signals, applying the blanking signals to one of the other of the electrodes of the said modulated tube, supplying the resultant signal from the cathode of the modulated tube to the innermost grid of the next tube of the series, simultaneously modulating the said second tube by another control signal upon another of the electrodes thereof, and deriving a combined output signal wherein all of thel several video and keying signals appear at different output signal levels.

KURT scrmEsINGER.

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