US2165770A - Electrical control apparatus - Google Patents

Description

July 11, 1939. WA. TOLSON 2,165,770

ELECTRICAL CONTROL APPARATUS Filed April 28, 1934 3 Sheets-Sheet 1 I. film"- a w, w p: '93 u, 9 r WI I HII 7.3

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Patented July 11,1939

PATENT OFFICE 2,165,770 ELECTRICAL coN'raoL APPARATUS William A. Tolson, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 28,

8 Claims.

My invention relates tovelectrical control apparatus and more particularly to methods of and means for maintaining synchronous operation in television systems and the like.

In television systems utilizing a cathode-ray receiver tube, it is the general practice to synchronize the transmitter and receiver by transmitting a horizontal synchronizing impulse at the end of each scanning line and a vertical synchronizlng or framing impulse at the end of each picture frame. At the receiver, the horizontal synchronizing impulses and the framing impulses are separated from the picture signals and from each other and impressed upon saw-tooth wave generators which supply the deflecting current or voltage for the cathode-ray receiver tube. In such a system, the saw-tooth wave generators oscillate under the control of the synchronizing impulses.

In order to facilitate the separation of the horizontal synchronizing impulses from the vertical synchronizing or framing impulses, it has been found desirable to give them different wave shapes whereby the framing impulses contain considerably more energy than the horizontal synchronizing impulses. Such a system is disclosed in application Serial No. 565,953, filed September 30, 1931, in the name of R. D. Kell, and assigned to the same assignee as this application.

Experience has shown that vertical framing impulses contain sufiicient energy to make the action of the vertical saw-tooth wave generator dependent upon the amplitude and shape of a framing impulse. This is especially objectionable in television systems employing interlaced scanning since the scanning lines of one picture frame should fall between the lines of a preceding frame with all the lines equally spaced apart.

The difference in the amplitude and/or shape of the successive framing impulses may be due to various causes, but in an interlaced scanning system the main reason for such diiference is the fact that alternate framing impulses have the adjacent horizontal synchronizing impulse a difierent time interval away in order to produce the interlaced scanning efiect. As the apparatus for transmitting a picture having interlaced scanning is no part of my invention, no description of it will be given. Unless special precautions are taken, the result of alternate framing impulseshaving different wave shapes is that the interlaced scanning lines at the receiver tend to pair, Accordingly, either the lines are prominent in the picture or picture detail is lost.

It is, therefore, an object of my invention to 1934, Serial No. 722,843

provide an improved method of and means for maintaining synchronism in television systems.

It is a further object of my invention to provide an improved receiver for use in a television system of the interlaced scanning type.

It is a further object of my invention to provide a system in which the operation of an oscillator controlled by a plurality of periodic impulses is substantially independent of the amplitude and/or wave shape of said impulses.

A still further object of my invention is to provide a television receiver in which positive synchronizing action is obtained without the use of manually operated adjusting devices.

A still further object of my invention is to provide a television receiver in which the saw-tooth wave oscillators need not be constructed within close limits for satisfactory synchronization.

A still further object of my invention is to provide improved means for reshaping an electrical impulse.

In practicing my invention, instead of impressing the framing impulses directly upon the oscillator of the vertical deflecting circuit at the receiver, I interpose between the said oscillator and the synchronizing input what will be referred to as a driver tube. The driver tube is so connected with .an associated circuit that it will operate to produce a sharp voltage impulse in response to the application of a suitable voltage impulse to its control grid. In a preferred embodiment of my invention, the driver tube is so connected that it will start to oscillate in .response to, and only in response to, the reception of a synchronizing impulse. Also, the adjustment is such that after each synchronizing impulse, the driver tube assumes its normal inactive state. Thus, each framing impulse causes a sharp voltage impulse to appear in the output circuit of the driver tube for controlling the oscillator of the deflecting circuit, this voltage impulse containing much less energy than the framing impulse and so little energy that changes in its amplitude and/or wave shape will have substantially no effect upon the oscillator.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawings in which:

Fig, 1 is a circuit and a block diagram of a television receiver embodying my invention,

Fig. 2 is a set of curves which are referred to in explaining the operation of my invention, and

Figs. 3 and 4 are circuit diagrams of other embodiments.

Referring to Fig. 1, the invention is shown applied to a television receiver in which a radio receiver and an amplifier 3 receive and amplify incoming television signals and impress them upon the control-grid 1 of a cathode-ray receiver tube 5. Both picture signals and synchronizing impulses are impressed upon this control grid.

The synchronizing impulses are separated from the picture signals by means of a suitable separating circuit indicated at 9 before being applied to the deflecting circuits. This separating circuit also separates the horizontal synchronizing impulses from the framing impulses, the horizontal synchronizing impulses being supplied to a saw-tooth wave generator indicated at H for supplying saw-tooth current to the horizontal deflecting coils |3 of the cathode-ray tube 5. A

separating circuit of this type is disclosed in the above-mentioned Kell application.

The framing impulses are supplied through a conductor I5 and a potentiometer H to the vertical deflecting circuit. The saw-tooth wave generator for supplying saw-tooth current to the vertical deflecting coils IQ of the cathode-ray tube 5 comprises an oscillator 2|, an impulse tube 23 and an output tube 25. The oscillator 2| is illustrated as a blocking oscillator although it should be understood that in place of a blocking oscillator, other types of oscillators, such as oscillators of the Dyna-tron or multivibrator types, may be employed. The oscillator 2| comprises an electric discharge tube 21 of the suppressor grid type which has a cathode 23, a control grid 3|, a screen grid 33, a suppressor grid 35, and an anode 31. The control grid 3| is connected through a fixed grid leak resistor 39 and a variable grid leak resistor 4| to ground, and through ground to the cathode 23.

The grid circuit of the oscillator also includes a grid condenser 43, the secondary winding 45 of the transformer 41, and a section of a potentiometer 49, connected in series between the control grid 3| and ground. The anode 31 is connected through the primary 5| of. the transformer 41 to a suitable source of positive potential (not shown) for feeding energy from the plate circuit into the grid circuit. The coupling thus provided between the plate and grid circuits is such that, as the plate current through the primary winding 5| increases, the control grid 3| is made more positive. The resulting positive potential on the control grid 3| causes a flow of grid current which charges the grid condenser 43 in a direction such that it tends to make the control grid negative. The plate current reaches a maximum value and then decreases, at the same time reversing the direction of the induced voltage in the grid circuit whereby the control grid is made so negative that the tube is biased beyond cut-off.

The grid condenser 43 has been charged suflicienily to bias the control grid 3| beyond the cutofi' point in the absence of any other voltage in the grid circuit. Therefore, no current will flow again in the plate circuit until the charge has leaked off the condenser 43 through the grid leak resistors 33 and 4| to lower the negative bias on the control grid a sufiicient amount. By properly adjusting the values of the various elements in the oscillator circuit, and, in particular, by properly adjusting the values of the grid condenser and the grid leak resistors, the oscillator may be made to oscillate at the desired picture or frame frequency. g

In practice, the oscillator is adjusted to as in cillate at a frequency somewhat lower than the desired frequency in order to permit synchronization. For example, if 60 picture frames per second are desired, the oscillator may be so adjusted that it tends to oscillate at 57 cycles per second.

In accordance with my invention, the synchronizing signals are impressed upon the oscillator through a driver tube 53 which may be of the suppressor grid type including a cathode 55, a control grid 51, a screen grid 59, a suppressor grid BI, and an anode 63. As will appear from the following description, the driver tube 53 is so connected that it would function as a blocking oscillator except for the fact that it is normally biased beyond cut-off.

The control grid 51 is connected through a grid-leak resistor and a biasing battery 61 to ground and through ground tothe cathode 55. The grid circuit also includes a grid condenser 69 which is connected in series with a winding 1| of a transformer 13 and a section of the potentiometer l1.

The plate circuit of the driver tube 53 is coupled to its grid circuit through a winding 15 of the transformer 13, the anode 63 being supplied with positive potential through this transformer winding. The coupling between the windings 1| and 15 is such that, except for the biasing battery 61, the tube would oscillate as a blocking oscillator the same as oscillator 2|. The potential applied to the control grid 51 by the biasing battery 61 is suiiicient, however, to bias the driver tube beyond the cut-off point whereby it is inactive until a synchronizing impulse is impressed thereon.

The transformer is provided with a third winding 11 which has the potentiometer 49 connected thereacross for impressing a portion of the voltage output of the driver tube upon the blocking oscillator circuit. In order to prevent an undesirable amount of reaction from the blocking oscillator 2| upon the driver tube 53. the blocking oscillator is connected across only a small percentage of the potentiometer 49 as indicated by the potentiometer values which are given in the drawings by way of example.

The above-described circuit operates to produce voltage impulses in the blocking oscillator circuit 2| which are under the control of the synchronizing impulses. As previously mentioned, this control is obtained by adjusting the oscillator 2| to operate at a frequency slightly lower than that of the synchronizing impulses whereby a synchronizing impulse (reshaped by driver tube 53) increases the voltage on the control grid 3| above the cut-off point at an instant just before this voltage change would be made by the oscillator itself.

The voltage impulses generated by the blocking oscillator 2| are impressed upon the tube 23 which is provided for the purpose of so altering the shape of the voltage impulse that a perfect saw-tooth current will be supplied to the deflecting coils l3. This change in wave shape is obtained by means of a condenser 19 and a reslstor 3| connected in series in the output circuit of the tube 23.

A voltage wave, having a saw-tooth shape. appears across the condenser 19. while a voltage impulse wave appears across the resistor 8|. the two waves combining to give the proper wave shape for causing a pure saw-tooth wave of current to flow through an inductance coil containg a certain amount of resistance.

The deflecting coils I9 are connected to the output circuit of the output tube 25 through a direct current connection including the variable tap 83 and resistors 85 for properly centering the cathode-ray beam on the fluorescent screen of the tube 5, and through an alternating current or impulse connection which includes a bypass condenser 81. I

The manner in which a synchronizing impulse containing a large amount of energy is converted into a synchronizing impulse containing a relatively small amount of energy will be better understood by referring to the curves in Fig. 2 which indicate wave shape and phase relation but not relative magnitude. In this figure, the voltage across the potentiometer I1 of the framing impulse circuit is represented by the curve 89, while the voltage appearing in the grid circuit of the driver tube 53 is represented by the curve 9|.

Referring to the curves 89 and 9|, it will be seen that at the time h the voltage on the control grid 51 of the driver tube 53 is that supplied by the fixed bias battery 81 and is below the cut-off point of the driver tube. At the time t2 the grid potential of the driver tube 53 has been raised by the incoming synchronizing impulse to the cut-off point, at which point plate current begins to fiow and the cycle of operation is begun as shown. This cycle of operation is a typical blocking oscillator cycle except that at the time is when the charge has leaked off the grid condenser 69, the cycle does not repeat but, instead, the circuit is maintained inoperative because of the fixed bias on the control grid 51. The time from t2 to is may be referred to as the active period of the oscillator. The cycle of operation is notrepeated again until the next synchronizing impulse occurs.

It will be apparent by comparing the area under a framing impulse shown in curve 89 with the area under that portion of the driver tube voltage shown in curve 9| above the zero axis that there is much less energy in the synchronizing output of the driver tube 53 than in the framing impulses. For this reason, slight changes in the output of the driver tube will have practically no effect upon the blocking oscillator 2|. In particular, it has beeniound that by employing a driver tube, as described, there is no appreciable tendency for the scanning lines to pair due to changes in the relative spacing of horizontal synchronizing impulses and framing impulses in interlaced scanning systems.

Attention is called to the fact that the driver tube 53 is so adjusted that, with the fixed grid bias removed, it will oscillate to produce positive votage peaks which have a smaller time interval between them than do the vertical synchronizing impulses. That is, the interval between t2 and ta (Fig. 2) is less than the interval between framing impulses. The reason for this adjustment is that it permits all the charge to leak ofi the grid condenser 69 and so return the driver tube to its original state before the next framing impulse occurs. This prevents the action of the driver tube during the reception of one framing impulse from being influenced by a preceding framing impulse.

Also, it will be seen that the time interval between i2 and ta should be great enough to prevent a single framing impulse from causing the driver tube to break into oscillation more than once In general, this means that the time interval between is and t3 should be greater than the time from the beginning of a framing impulse to its end. I

By referring to Figure 1,,it will be seen that there-ls only a small amount of couplingbetween the driver tube 53 and the blocking oscillator 2|, this coupling being obtained through the 200 ohm section of the 10,200 ohm potentiometer 49. The coupling is made small since the blocking oscillator reacts on the driver tube and a large amount of coupling between the two tubes would result in oscillation of the driver tube circuit under the control of the blocking oscillator. With small coupling between the driver tube circuit and the oscillator circuit, the operation of the deflecting circuit is perfectly satisfactory when the blocking oscillator is properly adjusted. In practice, this adjustment, which is referred to as a speed control adjustment, is made by varying the grid leak resistor 4| until the oscillator has a normal oscillating period close to the desired picture or frame frequency. If the adjustment of the oscillator 2| is such that, when free from the control of the driver tube 53, it will oscillate at a frequency widely differentfrom the frame frequency, the output of the driver tube will'not pull the oscillator into synchronism.

In some television receivers, it may be desirable to eliminate a speed control or frequency adjustment of the vertical deflecting circuit in order to simplify the operation of the receiver. This may be accomplished by employing a circuit, such as illustrated in Fig. 3, in which an amplifier tube 93 is employed for preventing reaction of the oscillator on the driver tube. While the circuit shown in Fig. 3 and the vertical deflecting circuit shown in Fig. 1 may be identical except for the addition of the coupling tube 93, several variations have been illustrated. In the two figures, like parts have been indicated by the same reference numerals.

Referring to Fig. 3, the driver tube may comprise an electric discharge tube 95 having a cathode 91, a control grid 99 and an anode IOI. The control grid 99 is connected through a gridleak resistor I03 and a biasing battery I05 to ground and through ground to the cathode 91, the biasing battery I05 applying a negative bias to the grid 99 for biasing the tube beyond cutofi. A grid-leak condenser I01, the secondary I09 of a transformer III and the lower section of the synchronizing input potentiometer I1 areconnected in series between the control grid 99 and ground.

The plate circuit of the tube 95 is coupled to the grid circuit by means of the primary II3 of the transformer III. As in Fig. 1, this coupling and the constants of the driver tube circuit' are so adjusted that the driver tube would oscillate to supply voltage impulses if it were not for the biasing battery I05.

The blocking oscillator comprises an electric discharge tube II5 having a cathode H1, a control grid H9 and an anode I2I. The control grid H9 is connected through a winding I23 of a transformer I25 and a grid-leak resistor I21 to ground, and through ground to the cathode III. The grid-leak resistor I21 is shunted by a grid condenser I29 which charges up to apply a negative potential to the control grid II9 during a portion of the operating cycle as explained in connection with the blocking oscillator shown in Fig. 1. The plate circuit of the tube is coupled in the proper phase relation to the grid circuit by means of a winding I3I of the transformer I25.

The voltage impulses appearing in the output circuit of the driver tube 95, are impressed upon the blocking oscillator circuit through the electric discharge tube 93, which has a cathode I33, a control grid I and an anode I31. The anode Ill of the driver tube 95 is coupled to the control grid I35 of the coupling tube 93 through a coupling condenser I39. The control grid I35 is maintained at a negative potential sufficient to bias tube 93 beyond cut-off by means of a biasing battery I which has its positive terminal connected to the cathode I33 through ground and its negative terminal connected to the grid I35 through a grid leak resistor I43. The output circuit of the coupling tube 93 is coupled to the blocking oscillator circuit through a third winding I45 on the transformer I25. It will be understood that the winding I45 is wound or connected in such a direction that a synchronizing impulse in the output of the tube 93 makes the grid II9 of the blocking oscillator II5 less negative.

While it is preferred to bias the tube 93 beyond cut-off whereby only the positive voltage peaks of the driver tube output are impressed upon the blocking oscilator II5, the tube 93 may be biased to function as an ordinary class A amplifier. By biasing the tube 93 beyond cut-off, however, extraneous voltages which might interfere with proper synchronization of scanning are eliminated.

The blocking oscillator H5 is coupled to the impulse tube 23 for supplying a voltage wave to the input circuit of the output tube 25 of the proper wave form for producing a saw-tooth current in the vertical deflecting coils. The blocking oscillator .I I5 may be coupled to the impulse -tube 23 in the way shown in Fig. 1, or it may be coupled thereto by means of a direct conection from the grid II9 of the blocking oscillator tube to the grid I41 of the impulse tube 23 through a conductor I49, as shown in Fig. 3. The main distinction between these two coupling connections is that in Fig. 3 the control grid of the impulse tube 23 derives a suitable biasing potential from the grid circuit of the oscillator tube II5 instead of from its own grid-leak resistor as in Fig. 1. As in Fig. 1, the bias on the impulse tube 23 is suflicient to bias it beyond cut-off. It will be noted that because of the voltage drop in the plate resistor I46, a grid voltage insufficient to bias tube Hi to cut-off does bias tube 23 to cutoff.

I have found that by utilizing the coupling tube 93 between the driver tube and the deflecting oscillator, it is not necessary to make any close adjustment of the oscillator frequency in order to insure proper synchronization. For example, assuming that the synchronizing impulses are occurring at the rate of 60 per second, in order to produce 60 picture frames per second, the oscillator may be adjusted to oscillate at a frequency as low as cycles per second and it will be pulled into the synchronism by the synchronizing impulses. Not only does this permit the elimination of a speed control" adjustment, but it also insures that a receiver will synchronize properly even through some slight change or variation has been made in the oscillator circuit. Such changes or variations may result from replacing a defective oscillator tube or they may occur during the manufacture of the receivers as an unavoidable result of quantity production methods.

As previously mentioned, oscillator circuits of other types may be utilized in place of a blocking oscillator circuit in practising my invention. In Fig. 4, there is illustrated a circuit of the Dynatron type in which the tube is properly biased to function as a driver tube for a saw tooth wave generator. In Figs. 1 and 4, like parts are indi cated by the same reference numerals.

Referring to Fig. 4, the driver tube comprises an electric discharge tube I50 having a cathode I5I, a control grid I53, 2. screen grid I55, and an anode I51. The anode I5! is connected through an inductance coil I59 to a source of potential such as a battery I6I while the screen grid I55 is connectedto a point of higher potential on the battery I6I such that the tube would oscillate as a Dynatron relaxation oscillator if the grid I53 were connected directly to the cathode I5I.

The grid I53, however, is negatively biased by a biasing source such as a battery I63, the bias being suflicient to bias the tube I50 to cut-oil. Therefore, the tube I50 will start to oscillate only in response to the appearance of a synchronizing impulse across the resistor I1 and will reach an inactive state before the occurrence of the next synchronizing impulse. As in' the case of the circuit of the blocking oscillator type, the tube I50 converts a framing impulse into a sharp voltage impulse containing a small amount of energy.

From the foregoing description, it will be apparent that various modifications may be made in my invention without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim as my invention:

1. In combination apparatus for converting a plurality of electrical impulses having a certain characteristic and occurring at regularly spaced time intervals into a plurality of electrical impulses having a different characteristic, said apparatus including an oscillator of the self-oscillatory type comprising an electric discharge tube of the high vacuum type having a plurality of electrodes, means for causing said oscillator to oscillate and then become inactive in response to the application of a certain potential to one of said electrodes, the time interval between the start of said oscillation and the instant the tube becomes inactive being less than said regularly spaced time intervals and being greater than the duration of each of said first impulses, means for applying said certain potential to said one electrode in response to the occurrence of each of said first impulses, means for producing recurring electrical waves of saw-tooth shape in response to recurring electrical waves being impressed thereon, and means for impressing the impulses produced by said apparatus upon said last means.

2. In combination, apparatus for reshaping an electrical impulse, said apparatus comprising an electric discharge tube having a cathode, a control grid, and a plate, a condenser and a grid leak resistor connected between said control grid and said cathode to form a grid circuit, said tube having an output circuit including said plate, means for so coupling said output circuit to said grid circuit that said grid is made less negative in response to an increase in plate current, means for so biasing saidgrld that said tube is maintained biased beyond the cut-off point until said impulse is impressedupon said grid circuit from an outside source, means for impressing said impulses upon said grid circuit, and an oscillator which oscillates freely in an uncontrolled condition, said apparatus and said oscillator being so.

coupled that said reshaped impulses control said oscillator.

3. The method of operating an oscillator 01' the self-oscillatory type comprising an electric discharge tube of the high vacuum typehaving a control electrode and so connected as to generate voltage impulses having a certain period, which method consists in applying a constant biasing potential to said control electrode such that the tube is biased beyond, cut-off until control im- 4. In a television system of the type in which horizontal synchronizing impulses occurring at a certain frequency and having a certain energy content and vertical synchronizing impulses occurring at a lower frequency and having a greater energy content are transmitted for synchronizing the scanning at the transmitter with that at the receiver, the method of synchronizing which comprises separating said horizontal synchronizing impulses from said vertical synchronlzing impulses at the receiver by a filtering action whereby said vertical impulses only may,

be impressed upon a self-oscillatory vertical defleeting circuit at the receiver, next converting said vertical synchronizing impulses into impulses of shorter duration and'having a smaller energy content, and impressing said converted impulses upon the self-oscillatory vertical defleeting circuit for synchronizing the scanning at the receiver with that at the transmitter.

5. In combination, at least two cascade-connected oscillators, the first oscillator having a natural period higher than a predetermined frequency and the second oscillator having a natural period lower than the said predetermined frequency, means for normally biasing the first oscillator torender it inactive and means for periodically supplying control impulses at the predetermined frequency tothe first oscillator to counteract the bias, whereby the second oscillator is constrained to operate in synchronism with the said impulses.

6. In a television system, a saw-tooth wave generator which includes a self-oscillatory oscillator, means for converting synchronizing impulses containing a certain amount of energy into impulses having an amplitude at least as great as the amplitude of the first impulses but containing less ener y, said means including an electric discharge tube of the high vacuumptypefhaving a plurality of electrodes and including means for, causing said tube to oscillate momentarily and then become inactive in response to the appliflcation to one of said electrodes of one of said synchronizing impulses, and means-for impressing said converted impulsesupon said. oscillator whereby said saw-tooth waves are produced in synchronism with said synchronizing impulses.

7. In a television system of the type in which certain frequency and having a certain energy content and vertical synchronizing impulses occurring at a lower frequency and having a greater energy content are transmitted for synchronizing the scanning at the transmitter with that at the receiver, said receiver being of the type including a vertical deflecting circuit having a self-oscillatory circuit, the method of synchronizing which comprises separating said horizontal synchronizing impulses from said vertical synchronizing impulses at the receiver by a filtering action, next converting said vertical synchronizing impulse into impulses of shorter duration and having a smaller energy content, and impressing said converted impulses upon said self-oscillatory circuit for synchronizing the scanning at the receiver with that at the transmitter.

8. Apparatus for converting periodically recurring electrical impulses each having a certain amplitude and energy content into periodically recurring electrical impulses each having an amplitude at least as great as said certain amplitude and an energy content less than said certain energy content, said apparatus comprising a high vacuum electric discharge tube having a plate, a cathode and a control grid, a transformer having a primary and a secondary, a grid circuit connected between said grid and said cathode and including said secondary, a plate circuit conductive to direct current connected between said plate and said cathode and including said primary, said plate and grid circuits being coupled through said transformer in such phase as to produce oscillations, a condenser in said grid circult in series with said secondary for storing a charge to block said tube, a grid resistor and a biasing voltage source in series with each other and in parallel to said condenser whereby a charge may leak off said condenser, said biasing source being connected in the correct polarity and the biasing voltage being of sufficient magnitude to block said tube in the absence of a charge on said condenser, the capacity of said condenser and the resistance of said grid resistor having such relative values that in the absence of said biasing voltage said tube produces voltage impulses which recur at time intervals longer than said first-mentioned impulses, and circuit connections for impressing said first-mentioned impulses upon said control grid in a direction to make it less negative.

WILLIAM A. TOLSON.

. l0 horizontal synchronizing impulses occurring at a I

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