US2298809A - Low frequency reinserting circuit - Google Patents

Description

LOW FFEQUENCY REINSERTING CIRCUIT Filed May 27, 1959 4 Sheets-She et 1 FIG. 1.

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Zhwentor Henry E. Rhe a Q B15 TIME PICTURE Gttomeg Oct. 13, 1942. H, E, RHEA v 2,298,809

I LOW FREQUENCY REINSERTING CIRCUIT Filed May 27, 1939 4 Sheets-Sheet 3 q plans cwmzflr SEC- fiwzgw. 11v crass PE)? and.

Bnvenior He nry Rhea H. E. RHEA 98,809

LOW FREQUENCY REINSERTING cmcuxw Fil ed May 27, 1939 4 Sheets-Shgt 4 0-1 mf 'VVVV- d f 8 4 g g '3 1 8 :3 3 5 a 8 Y a B B- T 18 p27 260000- l 26000 4- 1 "1G. 10. 46 49 1 7 Imp/a I RECEIVE)? FILTER 1 CIRCUITS Z'mventor ttorneg Patented 13,

UNITED I STATES" P TENT "OFFICE y,

LOWLFBEQUENCY aamsaa'rme cracm'r Henry EJRhea, Medford, N. .L', assignor to we Corporation of America, a corporation of Delaware . Application May 27, 1939, Serial No. evaosz 6 Claims. (01. na -7.3)

My invention relatesto electrical circuits for reinserting low frequency and/or direct current components mica television signal or the like. More particularly, the invention relates to acircult which may be cmployedqto improvethe low. frequency response of an amplifier in either a operation of the separating circuit to be unset- Accordingly, an object of my invention is to provide an improved method of and means for televsion transmitter or a television receiver and a which may also be employedlto provide improved separation of synchronizing signals from picture signals. a

It is well known that, in television systems of the type preferred at, the present time, the direct current component and the low frequency components of a signal may be lost in an alternating current amplifier and reinserted at a later point in the system byntilizingthe synchronizingimpulses to operate the reinserting circuit. In such circuits, the synchronizing im-- pulses havebeen made to go to a definite level, such as "black in a picture before the abovementioned alternating current amplifier has removed the direct current component. Such systems of the prior art are disclosed in Williams a Patent 2,252,746, issued Aug. 19, 1941.

In systems of the above-mentioned character, it has been found that, at or in the origin of the line frequency or horizontal synchronizing frequancy, thereis a dip in the frequency response characteristic of the'amplifler. From one viewpoint, itwould appear that,.with the designs formerly employed, any attempt to make the low frequency inserting circuit operate to reinsert frequencies extending farther toward the high frequency end of theamplifier range resulted in making the amplifier, considering it operating independent of the reinserting circuit, have a low frequency cut-oil. at a higher frequency than formerly. Thuspthere appeared to be two conflicting requirements in the design of an. amplifier of this type. a

Referring to another aspect of the invention,

it has become the usual practice in a television receiver to remove the picture signal from the synchronizing impulses before they are impressed upon the'deflecting circuits. A. separating circuit commonly employed forthis purpose is the 1 one described in Tolson Patent 2,237,839,1ssued July 16, 1910- While a separatingcircuit" of this type isentirely satisfactory providedthe low frequency content of the signal applied to the separating circuit is the same as in the original signal, it has been found that the presence of too much or too little of the iowfrequency com-.

ponents and that the presence of phase shift reinserting low frequencyand/or direct current components in a television signal or the like. More specifically, one of the objects of my invention is toprovide an amplifier to have a substantially fiat frequency characteristic from zero frequency up to the inghestirequency to be amplified. i Y

A further object of my invention is to provide an, improved separating circuit for separating picture signals from synchronizing impulses.

,In the preferred embodiment of my invention, I impress the synchronizing impulses of a television signal upon either a diode or a triode through a condenser which is charged as a result of the current fiow produced by the synchronizing impulses. A dischargingfcircuit is provided for this condenser which, instead of consisting of a fixed resistor as in the case of similar circuits ponent which ordinarily is present in a relaserting circuit of this general type. When a properly adjusted circuit is operated in this manner, the dip inthe frequency response curve formerly present is greatly reduced and, for all in the low frequency components may cause the practical purposes, may be considered eliminated. Theinventicn will he better understood-from 'the following description taken in connectionwith the accompanying drawings in which- Figures 1 and 2 are circuit diagrams or different embodiments of my invention for reinserting low frequency and/or D.-C. components:

Figure 3 is a curve showing the type of television signal which may be with my invention; 7

Figures 4 and 5 are curves or diagrams illusutilized in connection tratingtheeffect of low frequency components on thetelevision signal; I

, Figures 6 and 7 are diagrams which are referred to in explaining theinvention;

Figure 8 is a frequency response curve of my improved amplifier; I

Figure 9 is a circuit diagram of another embodiment of my invention for reinserting low frequency and/or D.-C. components; and

' Figure is a circuit diagram showing my invention as applied to a separating circuit for removing picture signal from synchronizing im- Pulses.

Referring to Fig. 1, I have shown my invention as it may be applied either to a television transmitter or a television receiver. It will be assumed, for the purpose of explanation, that it is located in a transmitter where its output is supplied directly to the transmitter modulator tube 8. The input signal which is fed through the condenser l0 may be a signal such as that shown in Fig. 3, which has passed through amplifiers indicated at 9 designed to have good gain and stability at the expense of their low frequency response. The particular circuit illustrated includes two amplifier tubes II and 12 which may be triodes, as illustrated, or screen grid tubes, such as those of the pentode type. The amplifier tubes H and I2 are resistance-Coupled in the conventional manner, the tube I I having in its input circuit the usual coupling condenser l3 and grid leak resistor H. For the purpose of showing a variation of the circuit connections, a cathode resistor I6 is shown connected between the cathode of tube H and ground.

Referring to the low frequency inserting portion of the circuit, the diode I! may be employed with its anode connected to one terminal of the condenser I0 and its cathode connected to ground, whereby a synchronizing impulse of positive polarity appearing at the input terminal will cause a flow of diode current into the condenser ID to give it an additional charge. This is assuming, of course, that the relation of the synchroniz- 'ing'impulse amplitude and the charge of the condenser lll is such that the synchronizing impulse can cause a further charge of the condenser. As will appear later, the relation may be such that the condenser ID will discharge through its discharging circuitinstead of receiving a charge.

The discharging circuit includes a variable impedance device which preferably is a vacuum tube 18 having a control grid 19. It is found in practice that it usually is preferable to include in series with the vacuum tube IS a resistor 2|. Its use is optional, however, and would serve no useful purpose if the tube I8 had suitable characteristics.

The tube I8 is connected in the discharging circuit in the proper direction so that the condenser l0, assuming that the tube 18 is not blocked by a high negative potential on its grid l9, can discharge through the resistance of the preceding amplifier circuit indicated at 22 and through the tube l8 and the resistor 2 l It generally will be found desirable to make the anode of the tube 3 positive a certain amount with respect to ground by means of a voltage divider 23 or the like, although, as will appear later, in some cases this anode may be connected directly to ground.

The tube [8 is normally biased beyond cut-ofi, a suitable negative voltage being applied to the grid [9' from a voltage divider 24 through a grid leak resistor 28. This tube is made conducting Referring to Fig. 4, there is represented the teleperiodically by applying synchronizing impulses of positive polarity to the grid l9 through a coupling condenser 21. These synchronizing impulses may be obtained from the output circuit of the amplifier tube l2 and supplied through a conductor 28 and a switch 29 or, if preferred, they may be taken from across the cathode resistor vision signal which occurs during a period of slightly more than one vertical deflection, where the signal includes a 60-cyclecomponent, as indicated. This component may be one that has been introduced by the picture shading. It will be noted that the composite signal consists of picture signal, line frequency or horizontal syn chronizing impulses and the vertical synchronizing impulses, as indicated by the legends. In Fig. 5, this same signal is shown after it has passed through the amplifier indicated in Fig. 1, which has removed the 60-cycle component. Obviously, the 60-cycle component will be reinserted if the synchronizing impulses are pulled up to the original horizontal level. In this respect, my 1m.- proved circuit functions substantially the same as similar systems of prior art.

This action is indicated in Figs. 6 and '7, Fig. 7 indicating the action over one complete vertical deflection. In these figures, the diode characteristic is shown at 30. The operation is indicated somewhat more clearly in Fig. 6, where the A.-C. axis of the signal and the amount of bias produced by the reinserting circuit are indicated. It will be understood that, if the height of an impulse with respect to the A.-C. axis increases, the charging current for the condenser II through the diode l'l increases, whereby the bias or voltage from the diode to ground increases slightly. If the height of the synchronizing impulse with respect to the A.-C. axis decreases, a reverse action takes place, the condenser l0 discharging somewhat, whereby the next occurring impulse of lower amplitude may cause a fiow of charging current through the diode. r

The above general statement of the circuit operation is true for the operation of circuits previously employed, as well as for applicants circuit. One of the main differences in operation. however, is that whereas, in the circuits previously employed, the condenser Ill discharged a certain amount between the occurrence of successive synchronizing impulses as indicated in Fig. 6, in applicant's circuit the condenser I. discharges only a slight amount or not at all between successive synchronizing impulses. It will be apparent that this is true if it is remembered that the tube i8 is biased beyond cut-off and is conducting only for the duration of a synchronizing impulse. Therefore, my improved circuit has eliminated the line frequency sawtooth component with the result that, as indicated by the solid line curve in Fig. 8, the frequency response curve for the amplifier is substantially flat, rather than having a very substantial dip at or near the line frequency, as indicated by the dotted line.

It will be understood that, during the occurrence of a synchronizing impulsethe condenser Ill may either charge or discharge, depending upon whether the height of the synchronizing impulse with respect to the A.-C. axis has increased or decreased. If the height has decreased, the voltage as represented by the height of the impulse will not be sufficient to drive the anode of the diode l1 positive to charge the condenser Ill and, at the same time, if the voltage across the condenser I is larger than the synchronizing impulse voltage, the condenser will discharge a certain amount through its discharging circuit 22, I8, 2|, since the tube l8 is conducting at this time. It will be understood that 22 represents the output or plate circal deflecting frequencies were 13,230 per sec- 0nd and 60 per second, respectively. Also, the

tube type "numbers have been indicated, by way cuit resistance of the amplifier tube preceding the preceding amplifier 9. Obviously, if it is desired to modulate with the D.-C. component in addition to the lower frequency components, the amplifier ll, 12 may be designed as a D.-C. am-

plifier, the coupling condenser l3 and the other coupling condenser between tubes II and I! being omitted in such a redesign.

In Fig. 2 there is illustrated another embodiment of my invention which, in operation, is substantially the same as that shown in Fig. 1. However, it is designed to operate with negative synchronizing impulses applied to its input instead of positive impulses, as in the case of Fig. 1, and it is also designed to reinsert the D.-C. component, as well as the low frequency components. In the two figures, like parts are indicated with the same reference numerals. 'It willbe noted that Fig. 2 differs mainly fromuFig. 1 in that the connections of the,diode electrodes have been reversed, the same being true of the tube II. The other difference is that the amplifier tube 32 has a biasing battery 33 provided between its cathode and ground to provide suitable bias to the grid of the tube 32 through the D.-C. connection. Y 1

It will be understood that the output lead of the amplifier shown in Fig. 2 may be applied-to of example. It will be understood. of course, that these circuit-constants may be varied over a wide range. In particular, the value of the resistor II is not critical. It may be omitted entirely although it was found that under some operating conditions a value of from 50,000 ohms to 200,000ohms seemed to make the circuit more stable; However, onepoint is to be noted with respect to the circuit constants, and that is that the series condenser--10 in-the reinserting circuit has very small capacity, whereby its charging time constantisvery fastg.

Considering now 'the use of my invention in a separating circuit, this use is illustrated in Fig. 10. Referring to Fig. 10, there is illustrated a television receiver, comprising a suitable radio receiver portion 40 which supplies to the amplifier tubes "and" and the cathode ray tube 40 the demodulated video signal of the type shown in Fig. 3. for example. This signal is also supplied from the output circuit of the amplifier tube 41 where the synchronizing impulses are of positive polarity tothe separating circuit, which is the same as the low frequency reinserting circuit shown in Fig.i 9. In the two figures, like parts are indicated-bylike reference numerals. In this circuit, the first tube ll functions as the separator tube and it separates out thepicture signal rather than passing it, as in the case the grid of a modulator tube. as indicated. or, where the reinserting circuit is employed in a television transmitter, this output lead may connect to the control electrode of the cathode ray tube. With. the latter connection, the circuit functions to provide automatic background control for the receiver.

In Fig. 9, there is shown a low frequency and a D.-C. reinserting circuit which, except for slight differences in design, is the same as the circuit shown in Fig. 1. In Figs. 1 and 9, like parts are indicated by the same reference numerals. One of the changes in design is that, in place of a diode, there is employed a'triode M in which the grid and cathode elements perform the function of the diode in the preceding circuit. It will be noted that the triode ll of Fig. 9 takes the place of the diode l1 and the amplifier tube H in Fig. 1. Also, it will be noted that the reinserted D.-C. component is present at the plate of the triode 4|, whereby the signal may be taken 01! at this point and supplied directly to the grid of the circuits previously described, because the amplitude of the; video signal at itsgrid is made so high that only the synchronizing impulses can pass through the tube 4|, the picture signals being beyond the cut-off point of the tube. This result may be accomplished either. by merely providing sumcient amplification ahead of the separator tube 4| or by applying a very low voltage to the plate of the separator tube H as compared with the rated plate voltage of the tube. In practice, of course, both additional amplification and lowering of the separator plate voltage may be utilized for obtaining good separation.

I claim as my invention:

1. Inc. circuit for reinserting lost frequency components of a composite signal comprising picture signal and regularly'recurring control impulses of greater amplitude than the picture N signals of like polarity, the combination of a condenser and rectifier means connected in series of said impulses as measured from the alternatof the modulator tube; to the grid oi? the oath-' ode ray tube, if the invention is being app ied to a receiver; or to any other point in a circuit where the presence of the low frequency components or of the D.-C. component is desired.

to form a charging circuit for said condenser, means for charging said condenser through said charging circuit in. accordance with the height in: current axis of said composite signal, a discharging circuit for said condenser, said discharging circuit includlng an electric discharge tube, and means for so applying said control impulses to said tubethat the impedance thereof is loweredfor the duration of said impulses.

7 that said condenser may receive a charge in 2. In a circuit for reinserting lost frequency components of a composite signal comprising picture signal and regularly recurring control impulses of greater amplitude than the picture signals of'like polarity, the peaks of said control impulses having been brought to a fixed predetermined level before said components were lost, the combination of a condenser and rectifier means connected to form a charging circuit for said condenser, means for 'so applying said composite signal to said charging circuit accordance with the height of said control impulses as measured from the alternating cur rent axis of said composite signal, a discharging circuit for said condenser, said discharging circuit including an electric discharge tube, and means for so applying said control impulses to said tube that the impedance thereof is lowered for substantiallyonly the duration of said control impulses.

3. The invention according to claim 2 characterized in that the electric discharge tube through which said condenser may discharge has a control electrode which is driven more positive periodically by the application of said control pulse thereto. a

4. In a circuit for reinserting lost frequency components of a composite signal comprising picturesignal and regularly recurring control impulses of greater amplitude than the picture signals of like polarity, the combination of an amplifier tube having a control grid and a cathode, a condenser connected in series with the grid-cathode circuit of said tube, means for applying said composite signal across said series circuit with the control impulses of positive polarity at said control grid whereby said condenser may receive a charge through said gridcathode circuit, and a discharging circuit for said condenser comprising a variable impedance device, and means for lowering the impedance of said device periodically in response to the application of said control impulses thereto.

5. In a circuit for reinserting lost frequency components of a composite signal comprising picture signal and regularly recurring control impulses of greater amplitude than the picture signals of like polarity, the combination of an amplifier tube having a control grid and a cathode, a condenser connected in series with the grid-cathode circuit of said tube, means for applying said composite signal across said series circuit, a rectifier connected in series with said condenser with respect to the source of said composite signal, said rectifier being connected in such direction that the applied control impulses may cause said condenser to receive a charge, a discharging circuit for said condenser which circuit has a high impedance between the occurrence of said control impulses, said discharging circuit including an electric discharge tube, and means for applying said control impulses to said tube to reduce the impedance of said discharging circuit periodically to a comparatively low value at substantially the time of occurrence of said control impulses.

6. In a television receiver of the type having a picture signal channel and a separating circuit channel, the combination of a condenser in said separating circuit channel, means for charging said condenser in accordance with the height of synchronizing signals as measured from the alternating current axis of a composite signal containing them and for applying the voltage resulting from said charge to the control grid of a separating tube in said separating circuit, a discharging circuit for said condenser comprising a variable impedance device, means for lowering the impedance of said device periodically in response to the application of said synchronizing impulses thereto, and means for applying said composite signal to said separating tube with the proper amplitude and polarity to cause substantially only the synchronizing impulses to appear in the output circuit.

HENRY E. RHEA.

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