US2321343A - Television system - Google Patents

Description

K. R. wEND-r 2,321,343

'2 snets-sheet 1 June 8, 1943.

LEVISION SYSTEM Filed oct. 2s, 1940 KarL June v8, 1943, K R WENDT 2,321,343

TELEVIS ION SYS TEM Filed oct. 25, 1940 2 sheets-sheet 2 l E 15 6.2. g

FIG. 3i

BLc/rLL'l/gz' HM n 'n n'n cL/PP//vs LEVEL 146.6. `n/ n H L Patented `lune 8, 1943 TELEVISION SYSTEM Karl lt. Wendt, Audubon, N. J., assignor to Radio Corporation of ware America, a corporation of Dela- Application October 23, 1940, Seria No. 362,312

(ci. 17a-7.1)

10 Claims.

vsignal itself at a given point in the system.

The invention is especially useful in systems wherein a portable transmitter of comparatively small power transmits the television signal to a main transmitter having sufficient power to cover the desired service area. It has been found that insuch systems the signal is no longer in its original condition by the time it reaches the main transmitter. For example, the synchronize ing pulses may have been saturated oil' a certain amount by the portable transmitter. Also, the signal, by the time it reaches Ythe .main transmitter, may contain low frequency errors both as a result of losing low frequency signal components and as a result of acquiring undesired low frequency signal such as 60-cycle hum. Because of this, it may be diilicult to transmit to the receivers a signal which contains synchronizing pulses of the proper relative amplitude for good synchronization of the receivers. It will be apparent that, in a sense, the main transmitterfis functioning as a relay station in the system just described.

The invention is also useful in transmitting television signals originating from a studio,

especially when the `studio and the studio television equipment is located some distance from,

the radio transmitter. In this case, the television signal, a composite signal consisting of picture signal and synchronizing pulses, is transmitted to the radio transmitter either through a long synchronizing pulses to .a suitable amplitude with respect to the picture signals.

A further object of the invention is to provide an improved picture transmitter and, particularly, to provide an improved picture transmitter which is 'being modulatedA by signals originating at apoint remote from the radio transmitter. l

In one embodiment of the invention the composite signal, consisting of picture signals and synchronizing pulses, is transmitted from the portable transmitter, received at the main transmitter station and amplied a suitable amount, and then supplied to a circuit which corrects for the low frequency errors in the signal. The low frequency correction preferably is obtained by means of a' keyed diode direct-current setter or reinserting circuit.

`Next, synchronizing pulses are obtained from the corrected signal and added to the corrected signal at a later point in the circuit whereby, in`

the resulting signal, thesynchronizing pulses have a greater amplitude than they should have whenapplied to the modulator of the transmitter. The next step is to clip oil' the synchronizing pulses at a fixed voltage level beyond cable or through a radio link. In the latter case,

the situation may be similar to that described above where a portable transmitter is employed, although usually the dimculty will not be as great. Where the transmission is through the long cable,"one diiculty encountered is caused by the cable acquiring different D.C. potentials i "black in the picture, that is',.at blacker than black, whereby the desired signal is obtained for transmittion to the receivers. This lasty clipping action .preferably is controlled by a keyed direct-current setter or .reinserter operating on the black level. l

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a circuit diagram illustrating one embodiment of my invention as applied to a television system,

Figures 2 to 6 are curves which are referred to in explaining the invention,

Figure 7 is the plate-voltage plate-current curve of a pentode which is referred to in explaining the operation of one of the D.C. inserters employed in the embodiment of Figure 1, and

Figure 8-is a group of curves which illustrate the final clipping operation in my invention. Referring to Fig. 1, the invention is shown applied to a television transmitter, which in the example villustrated receives a composite signal consisting of picture signals and synchronizing pulses by means of a suitable radio receiver indicated at l0. The received signals may have been transmitted from a portable transmitter or transmitter through a radio link. The received signal is to the type commonly employed in the United States at the present time for television transmission and consists of picture signals, periodically recurring line synchronizing pulses and periodically recurring framing pulses. The peaks of the svnchronizing pulses have been made to go to a fixed voltage level such as a xed number of volts beyond black" in the picture before the direct current and low frequency components have been suppressed. This may be accomplished in various ways, as by varying the clipping level of a vacuum tube in the way described in United States Patent No. 2,192,121, issued February 27, 1940, in the name of Alda. V. Bedford, or by utilizing a suitable mask wit'h a scanning disc so that the signal goes to black at the end of each scanning line.

The radio receiver I demodulates the incoming signal whereby there appears in its output the above-described composite signal which may be further amplified by means of an amplifier indicated at II.

The signal appearing in the output circuit of amplifier Il will have the general character indicated in Fig. 2 where the picture signal is represented at I2, the synchronizing pulses at I3 and the black level or pedestal at I4. `As represented in Fig. 2, the synchronizing pulses I3 are not of suilicient amplitude With respect to the rest of the composite signal for obtaining good synchronization. Also the black level of the picture is not at a fixed voltage level but, instead. follows a level represented by the dotted line I6 which'may represent either the presence of 60- cycle hum voltage or the like, or the loss of a low frequency signal component.

In Fig. 2, the pulses I3 are the line synchronizing pulses. It will be apparent that the drawing has been greatly simplied since, for any one cycle of a low frequency hum voltage, there would be a large number of the pulses I3.

Before describing the circuit of Fig. 1, in detail, it may be helpful first to consider what is accomplished by the circuit, especially with reference to Figs. 2 to 6. First, it will be noted that the output of amplifier II is fed through a. coupling condenser C to an amplifier tube I1 at the input circuit of which black in the picture is brought back to a fixed voltage level as illustrated in Fig. 3. Stated differently, there is D.C.`in sertion and low frequency insertion and/or correction at the input electrodes of the tubeV I'l.

In the plate circuit of amplifler tube I1, the ysynchronizing pulses are increased greatly in amplitude by supplying synchronizing pulses, such as represented in Fig. 4, over a conductor I8 whereby the composite signal of Fig. 3 and the synchronizing pulses of Fig. 4 are added in the plate resistor I9 of the tube I1 to produce the signal shown in Fig. of the drawings. chronizing pulses I3 now have aymuch greater amplitude than they should have in the final transmitted signal.

The combined signal is next passed through a coupling condenser 2| to an amplifier tube 22 of the cathode follower type. The amplifier 22 is caused to clip the synchronizing pulses I3 at such a level as to make them of the desired amplitude in the final signal, such a clipping level being indicated in Fig. 5.

As will be described hereinafter, the amplifier tube 22 is made to clip the synchronizing pulses at the proper level by utilizing a. "keyed direct- The syn-A rect current at the input electrodes of the tube 22.

In the particular example illustrated, this D.- C. reinserter comprises a pentode indicated at 26. Delayed pulses for keying this pentode dur- 'ing the period of black level in the signal are shown in Fig. 6.

The signal output of the amplifier 22, which now contains synchronizing pulses of constant amplitude with their peaks at a fixed voltage level, may be supplied over a cable-indicated at 2l (properly terminated by resistor 28) to the main radio transmitter 29.

l Referring now to the circuit more in detail, the desired D.-C. insertion and low frequency correction at the input of amplifier tube I1 isA rection to be inserted.

The D.-C. reinserting circuit includes an amplifler tube 3| through which the coupling con-l denser C receives a charge, the control grid and the cathode of this tube functioning as a diode. It also includes a diode 32 through which the condenser C may discharge a slight amount during the occurrence of a synchronizing pulsein the event that the amplitude of the synchronizing pulses has decreased. As will be explained later, the tube 3| has a double function, since itfunctions not only as a diode for the D.C. reinserting circuit but also as one of the clipper tubes in a series of such tubes provided for the purpose of obtaining synchronizing pulses free from picture signal.

The condenser C must receive its charge through the grid-cathode impedance of the tube 3I since, in the specific example-being described,

lthe amplifier tube 'I1 has a rather high negative.

, bias on its control grid 34 due to a cathode recurrent reinserting device for reinfflling the di- 7.5

sistor 33 which is given sufficient resistance to make the characteristic of tube I1 linear. Preferably, the resistor 33 is variable so that it may be employed as a gain control. Theuse of the resistor 33 biases the control grid 34 of the tube I1 so far negative that the positive synchronizing pulses applied thereto cannot cause grid-current flow.

The resistor 33 alsoholds the control grid 34 more negative than desirable for proper amplification of the synchronizing pulses. To overcome this objection, the R.-C. network comprising re- -sistor 36 and condenser 31 is connected next in the control grid of the tube 3| whereby there appears across the R.C. network a bias voltage which is of the proper amplitude to reduce the bias on control grid 34 to the. desired value during the occurrence of a' synchronizing pulse. It will be understood that, during the occurrence of a synchronizing pulse, the control grid of the tube 3| is at substantially ground potential because of the low grid-cathode impedance during the I flow of grid current.

' From the foregoing, it will be apparent that the coupling condenser C is initially charged by current ow through the grid-cathode impedance of tube 3| and that, upon an increase in the amassises plitude of the synchronizing pulses, it receives an additional charge through this same grid cathode impedance. The right-hand or grid side of the condenser C, during the ow of charging current, goes to a potential with respect to ground which is equal to the voltage across the network 36-31. As will be pointed out later, this side of the condenser goes to this same potential upon the occurrence of a synchronizing pulse whether the condenser charges or discharges.

In order to make the diode 32 conducting during the occurrence of synchronizing pulses, vand nonconducting during the picture signal intervals, keying pulses are applied through a conductor 4l,a resistor 82, and a condenser 43 to the plate of the diode 32, there being a resistor 33 connected between the diode plate and ground. These keying pulses are applied to the diode plate with a. positive polarity and are obtained as follows:

The amplifier tube 3| clips oi some of the picture signal, as previously described, and supplies the synchronizing pulses to an amplier'tube 56 with negative polarity. The tube 44 functions as a linear amplier and may be provided with a cathode resistor 46 to make its characteristic more linear.

synchronizing pulses of positive polarity appear on the control grids of clipping tubes 41 and 48, these tubes having va substantial negative bias thereon which is obtained by giving a grid condenser 49 and a grid leak resistor 5l suitable values for producing grid leak biasing, as is well understood in the art. This grid leak bias is made such that no picture signal appears in the plate circuits of the tubes 41 and 48. It has previously been explained that the synchronizing,

pulse output of the tube 48 is fed over the conductor I8 to be added to the synchronizing pulses appearing in the composite signal supplied from the amplier tube I1.

Since the synchronizing pulses appearing in the plate circuit of tube 41 are of negative p0- larity, they are reversed in polarity by means of an amplifier tube 52 which supplies the synchronizing pulses or keying pulses to the conductor 4I. 52 is provided with the proper negative bias by means ofgrid leak biasing. Inv this instance, it is the signal of comparatively long duration on the positive side of the A.C. axis that drives the control grid of the tube 52 slightly positive to pro- 'ducethe grid current which maintains the desired bias.

In the event the synchronizing pulses begin to decrease in amplitude, the condenser C will dis- In the example illustrated, the tubevalues that their timeconstant is longer than the time constant of the circuit including the resistor 38 and the condenser d3 associated with the diode 32. It has been found that this reduces the tendency of the circuit to oscillate.

Next, considering the operation of the D.C. reinserting circuit comprising the pentode 25, this pentode is keyed by means of a delayed pulse which, in the example illustrated, is provided by means of a vacuum tube 6i, which. with its associated elements, forms a nonlinear differentiating circuit of the character described and claimed in my copending application Serial No. 331,158, filed May 25, 1940, entitled Electrical delay circuits, and assigned to the Radio Corporation of America. As described in the said application, the positive rectangular pulse which is impressed upon the grid condenser 62 of the tube 6l is differentiated. in an unsymmetrical manner, the pulse resulting from the front side of the rectangular pulse being of small amplitude because of the low grid-cathode impedance ofthe tube 6I which is maintained by the application of a positive voltage to the control grid, and the pulse resulting from the back side of the rectangular pulse being of comparatively large amplitude, since the differentiating circuit now includes the grid resistor 63. This last-mentioned pulse appears on the grid of the tube 6I with negative polarity whereby the more negative portion of the pulse is clipped off by the tube 6|. Therefore, there appears in the plate circuit of the tube 6| a delayed pulse of positive polarity which'is fed through a coupling condenser 18 to the control grid of the pentode 26. These delayed pulses key the pentode 26 to make it conducting only dur- Y ing the period of a black level in the composite condenser 8 I.

charge slightly upon'the occurrence vof a synchronizing pulse. This discharge circuit may be traced from the condenser C through the plate circuitof amplifier Il to ground, through ground to the plate circuit of tube 52, through the said plate circuit and through conductor 4l to the diode 32 and through a resistor 38 back to the condenser C.

' connected through a resistor 51 to ground. The

resistor 51 isshunted by a condenser 58, this condenser land the resistor 51 preferably having such signal, as will be seen by comparing Figs. 5 and 6.

The pentode 26 is` in a nonconductingcondition (that is, it is biased beyond cut-off) between successive keying pulses as a result of a l grid leak bias which is produced by the positive keying pulses driving the control grid 1l of the pentode slightly positive periodically to produce grid current flow. The cathode 12 of the pentode 26 is held substantially at groundpotential forv alternating currents by means of a by-pass condenser 16. However, the cathode 12 is held at a negative D.C. potential with respect to ground and with respect to the pentode plate 13 by means of a suitable voltage source which, in the example illustrated, comprises aA GO-cycle voltage supply 11, a rectifier 18, and a resistor 18 and smoothing This negative D.C. potential is also the level setting voltage. for the D.C. reinsertion circuit.

The plate of the pentode 26 is connected through a conductor 86 and a resistor 81 to the control grid of the output tube 22 and to the grid side of coupling condenser 2|- whereby condenser 2| may be either charged or discharged therethrough for the purpose of maintaining the proper bias on the output tube to clip ol the exaggerated or stretched synchronizing pulses at the proper level with respect to black in the picture, as indicated inA Fig. 8. The resistor 81 is not essential to the circuit operationand may be omitted in some cases.

The suppressor grid of the pentode 26 is preferably held slightly positive with respect to the cathodein order to reduce the pentode impedance during the time that it is conducting and functioning as a diode, as will be explained below. It is during this time that the coupling condenser 2| will get an additional charge if the amplitude of the pedestal (black) has decreased with respect be set at a predetermined voltage level upon the to the preceding pedestal. occurrence 'of each synchronizing pulse.

The plate of the pentode 26 is also connected If the pedestals Il decrease in amplitude (beto ground through a high impedance resistor 8B come less negative), then, upon the occurrence of whereby the coupling condenser 2| may discharge 5 such a pedestal,the condenser 2| receives a-slight `a certain amount therethrough when it is not` additional charg through the pentode 26 acting receiving an additional charge through the pensimilar to a' diode during the time it is rendered tode 2|.` ,conducting by the delayed keying pulse (Fig. 6).

As indicated above, the voltage across the por- This charging circuit is through the conductor tion of resistor 19 between the tap 80 and ground 10 86, the pentode 26, through the lower portion of is both the plate 'voltage for the pentode 26 and resistor 19 (and also through br-Pass condenser 4the level setting voltage. Actually, the tap llili 16) to ground. and through ground" to the plate is adjusted'for the proper level setting and it circuit of tube I1. This additional charge always happens that in the circuit illustrated it` is also brings the grid of tube 22 substantially to the a suitable voltage4 for the pentode plate 13. If 15 voltage of the tap-8 0.

Va. different voltage were required vfor the plate 13, If the pedestals llincrease in amplitude (bethe lower end of the resistor. couldbe connected come more negative), the condenser 2| will conto a positive voltage instead of to ground. tinue to lose more of its charge through resistor y Since there is an A.C..signal on the plate 1 88, until the condenser 2| again receives a slight (impressed thereon through-conductor 86), the;.20.additionalcharge through thepentode 26. D.C. positive voltage on plate 13 mustbe greater It will -be understood that, `at the grid of tube than; the amplitude 0f the A -Q Signat 0n 'the 22, the D.C. insertion must be in accordance with negativeA side of the A.-C. axis. Otherwise, the the height of the pedestal I4 rather than in acplate. 13 would go negative and the pentode 26 cordancewith the height of the Vsynchronizing culd-.not be made conducting' lby the keying 25 pulses I3 becausea s shown in Fig. 5,' the pulses pulses. The, resultantvvoltage on fthe-plate 13 is I3 have been greatly increased in height atthis .very low, -from 'one to-ve volts'for` example, point by thepulsesjadded in` from the conwherebythe ,pentode isoperatingon the steep partv y ductor I8.

- of itsv characteristic when it becomes conducting. FromFig. 8, it isapparent'that the undesired I Thus, referring to Fig. 7,.the pentode 26 loperates 30 part of the synchronizing pulses I3vis" clipped oi on the portion'ofthe characteristic curve A below b y the output tube 22, the pedestal. I4 being held the dotted line ,the plate impedancefof'the ata 'xed point with respect to the tube charpentode beingvery lowin this region. "The curve j acteristic. Therefore-the .tube 22 supplies to the 1 A of Fig. 7'-is for, the case where' the suppressor transmitter 29 a composite signal having .black -gridof rpentode 26 is positive with respect-tothe 35 in the picturefat a fixed level asa result ofthe A' -cathode, as previously described. It will be ap- .D.C. insertion and low frequency correction and, `r.parent that, in .the region oflowv platevltage, alsophavingsynchronizing pulses 'of thev desired the curve A is lmuch steeper (indicating. lower xed amplitude. f

plate impedance) than .is the curve'V B, which The output tube 22 need not be of the cathoderepresents the'tube characteristic when the sup- 40 Vfollower type `but, instead, may have the usual 1 pressor grid is at cathode potential. plate output. fThe type illustrated is conveniently .Considering the D.C. insertion actionV of 3f-employed when'the tube is feeding into a'cable as pentode 26 more in detaiLthe resistor-88 isfgiven: the impedances are easily matched. a `resistance value `such that the coupling Y con- For the purpose of illustrating a specific-exmit the potential on the control grid coftub e..22`A and microniicrofarads.

v denser. 2| will discharge just enough betweenig-'45 ample,vthe valuesof` various-,circuit elements 'are' successive horizontal'synchronizing pulses to"-per-r- 4'indicated in Fig. 1 in ohms; mgohms, microfarads to follow` the low frequency variationsfor'whicn f From. the foregoing, lt -will be apparent-.that it is desired to correct. It is evident that'in this various modifications may be made in my inventype of circuit the time constant for the discharge 50 tionwithoutdeparting from the spirit and scope path of the condenser 2| is slow.. On the other thereof.

hand, the time constant of the charging circuit I claim as my invention:

Vthrough the pentode 26 is fast. y 1.`V Picture transmission apparatus for l thel To follow through a cycle of operation, first transmission of a composite signal comprising considering the circuit when there is no incoming 55 picture signals and periodically recurring synsignal, the coupling condenser 2| has been chronizingpulses, said apparatus comprislngan charged-up to .the plate voltage of ampliner. tube 1 .inputfcir'cuit for Yreceiving said, composite signal.

il since there is a D.C. path-fromlthe platesup-'rfmeas for j' applying: direct currentrand low fre-` ",--piyqof-tube i1 through-the resistors andv oonfiquenfcyggccrrection.t0 Said'rCeiVed-Signal, means :ductor sstothegridsideof thejcondenserxzi. 6o Wedding t0the:Synchrnimg"pulses/0f` Said :Now Vassume a .Sgnalisreceived 'Sinc'e the orreted mpsitesignal additional 'pulses t0 synchronizmgpuisesiis and pedestaisuyir'ig; 5)' increase "the" "amplitude onine' synchronizing 1f `are of negative .polarity at the plate yo ftum, n,

f .Sychronizing pulse. -A1so, the greater thel ampli-. i

f tude of. the pedestai u. withrespeottdthe n.41.V 'airis, the moieofitsfcharge'the;condenser.gli,syqill;

.Jfi".11ose.- :l L The -chargebf condenser z l'will vary'iri Vac g fcorda'ncewith the'heightof'the pedestals|4jfrom-` theAlLcLliaxisithe .keying of pentode 2s; being .zpulsesjin'the-resulting composite signal toa value greaterY ha': isfdesiredin the iinalsignal, and means 'fonlclipping off the tops of the synchronizingjjpulses inA said resulting;` gnal atafiired level with' respect` to-"fblacknin :the picture.

the condenser obviously will losea certain amount transmission of a composite 'signal comprising forfapplying direct current-and: Vlowirequency i only; during pedestal),this being iatcircuit action' ,f correction to saidfreceivedsignal, meansffor' prothat must-take'place if the grid of .tube 22 isjtc 'Idrvdihg a-*signal comprising'control `pulses, which 2, vPicture'.transmission' apparatus -flf the Y'pictural signals and periodically recurring' syn# chronizing pulses, said apparatus having an input t circuitfor receiving' said 'composite signal, means i id occur simultaneously with the synchronizing pulses in said corrected signal, means for adding said corrected composite signal and said lastmentioned pulses to increase-the amplitude of the synchronizing pulses in the resulting composite signal to a value greater than is desired in the ilnal signal, and means for clipping ou the tops of the synchronizing pulses in said resulting signal at a fixed level with respect to black in the picture.A

3. Picture transmission apparatus i'or the transmission of a composite signal comprising picture signals and periodically recurring synchronizing pulses, said apparatus having an input circuit for receiving said composite signal, means for applying direct current and low frequency correction to said received signal, means for producing pulses substantially free from picture signal which occur simultaneouslywith the picturesignals and periodically recurring control pulses which are set on pedestals of greater width than said pulses, said apparatus comprisiing an input circuit for receiving said composite signal, means for applying direct current and low frequencyv correction to said received signal,

means for producing control pulses and for addwidth than said pulses, said apparatus comprising an input circuit for receiving said composite signal, means for applying direct current and low frequency correction to said received signal, means for producing control pulses and for adding them to said corrected composite signal to increase the amplitude of the control pulses in the resulting composite signal to a value greater than is desired in the iinal' signal, and means for clipping oil the tops of the control pulses in said resulting signal at a iixed level with respect to "black in the picture, said last means comprising a vacuum tube through which the said resulting signal is passed, a source of keying pulses each of which occurs later than the preceding control pulse `and during the occurrence of a pedestal, and further comprising a direct current reinserting circuit for varying the bias of said vacuum tube in response to said keying pulses.

5. Picture transmission apparatus for the transmission oi' a composite signal comprising picture signals and periodically recurring control pulses which are set on pedestals of greater Width than said pulses, said apparatus comprising an input circuit for receiving Vsaid composite signal, means for applying direct current and low frequency correction to said received signal, means for producing control pulses and for adding them to said corrected composite signal to increase the amplitude of the control pulses in the resulting composite signal to a value greater than is deing them to said corrected composite signal to increase the amplitude of the control pulses in the resulting composite signal to a value greater than is desired in the nal signal, and means comprisan output tube for clipping oil the tops of the control pulses in said resulting signal at a fixed level with respect to black in the picture, said last means comprising a direct current reinserting circuit including an electric discharge tube which is rendered conducting only after the occurrence of a control pulse and during the occurrence of a pedestal.

7. In apparatus for the transmission of a composite signal comprising picture signals and periodically recurring control pulses, a signal ampliiier channel comprising an ampliiier tube having a control electrode upon which said composite signal is impressed with the control pulses of positive polarity, means including a second ampliiier tube for adding to the control pulses of said corrected composite signal additional control pulses to increase the amplitude of the control pulses in the resulting composite signal to a value greater than is desired in the final signal,

said second amplifier tube having a control electrode, said two amplifier ,tubes-having their input circuits connected in parallel relation with4 each other and in series with a coupling condenser, means for biasing the control electrode of the first amplier tube sufliciently negative to prevent the control pulses applied thereto -from causing grid current lilow, Vand a resistor connected adjacent to the control electrode of the second amplifier tube and shunted by a condenser, said resistor and condenser Vhaving a time constant such that a bias voltage appears thereacross due to grid current iiow in the second amplifier tube, means providing a discharge path (tor said coupling condenser only between picture signal intervals whereby the received signal is provided withdirect current and low frequency correction, and-means for clipping oil the tops of the control pulses in said resulting sig- -nal at a fixed level with respect to black in the picture.

8. Apparatus for direct current and low frequency correction of a composite signal comprising picture signals and periodicallyrecurring control pulses, said apparatus comprising an ampliiler tube having a control electrode, a coupling condenser through which said signal is applied to said control electrode with the control pulses of positive polaritya diode having its cathode connected to said control electrode-and having its plate connected through a resistor -to ground, means for impressing keying pulses through a second condenser upon said diode plate, the keying pulses occurring in substantially the same time relation as said control pulses, and a second diode having its plate coupled through said second 'condenser to the plate of the first-diode and having its cathode connected through a resistor to ground, and a condenser connected in shunt to said resistorl the time constant of said last resistor and condenser being shorter than the time constant of the circuit including said first resistor and said second condenser.

9. In combination, an electric discharge tube having a control electrode at which thc direct Y means for holding the suppressor grid of the 10 mentioned pulses.

pentode positive with respect to its cathode, a

grid leak resistor Aconnected -between said control electrode and ground, and means for making said pentode conducting periodically whereby saidcoupling condenser may receive a charge.

v10. The invention according to claim 9 wherein said last means comprises means for supplying positive polarity keying pulses to the control grid of the pentode, each of which occurs immediately after -the occurrence of oneof the rst- 'KARLR WENDT.

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