US2085409A - Television system - Google Patents

Description

June 29, 937. A, V* BDFQRD 2,085,409

TELEVISION SYSTEM Original Filed May 28, 1952 2 Sheets-Sheet l Figi. ,m lo

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/ AT oeNEY June 29, 1937. A v BEDFORD 2,085,409

TELEVI SION SYSTEM Original Filed May 28, 1932 2 Sheets-Sheet 2 Patented Junev 29, 1937 TELEVISION SYSTEM Alda V. Bedford, Collingswood, N. J., aasignor to Radio Corporation ot America, a corporation oi Delaware Application May 28, 1932, Serial No. 614,117 Renewed October 22, 1936 A 19 claims.

My invention relates to improvements in television systems embodying a cathode ray tube provided with ray-deecting coils, or other equiva- 1 lent electron device, at either or both the transmitting and receiving stations, and, more particularly, to improved apparatus for deflecting the electrons in such tubes or devices for scanning purposes.

In television systems of the general character referred to, it is important that saw-tooth current waves, at the line-scanning and framing frequencies, respectively, be caused to pass through ray-defiectirw coils. For this purpose, the so-called peak" circuit is used, which comprises a network including a resistance and a condenser, the network being connected across the input circuit of a voltage amplier tube in the output circuit of which is connected one of the sets of deflecting coils.

In operation, there is developed across the network a voltage Wave of the shape required to cause the saw-tooth current wave to pass through4 the inductive circuit including the deflecting coils, and this wave is made up of two components, one being a saw-tooth component which appears across the condenser in the network, the other being an impulse component which appears across the resistance in the network.

While the various systems referred to lend Vthemselves to satisfactory operation, it is a fact that in these systems, as heretofore known, the shape of the current Wave through the deecting coils is not a perfect saw-tooth. This is attributed to any one of the following, or the joint effect thereof: the non-linear plate-current vs. gridvoltage characteristic of the voltage amplifier tube, a non-linear charge characteristic of the condenser in the network, charge current taken by the grid bias resistor of the amplifier tube which would otherwise go to the cqndenser, distributed capacitance across the output circuit of the amplifier tube, and inductive drop in the load circuit of this tube, which circuit includes the deecting coils.

With the foregoing in mind, it is one of the objects of my invention to provide an improved deiiecting circuit or apparatus of the character referred to which not only has the variousadvantages of the deflecting circuits proposed hereto- 50 fore, but which, also, causes a practically perfect saw-tooth current wave to pass through the deecting coils.

Other objects and advantages will hereinafter appear.

In accordance with my invention, a negative (Cl. Z-36) feed-back action is utilized from a circuit of a voltage amplifier tube in the deecting circuit to reshape the voltage wave which would otherwise appear across the input circuit of this tube, the nature and degree of the reshaping being such as to compensate for each of the various actions referred -to above which cause distortion of the 'voltage wave shape from the exact shape required.

Further, in accordance with my invention, a network impedance is connected in one of the circuits of the voltage amplifier tube in the deilecting circuit, and this network is caused to operate to effect a feed-back action to the input circuit of this tube and of such character as to compensate for each of the various actions referred to which would otherwise distort the wave from the shape required.

My invention resides in the system and method of operation of the character hereinafter described and claimed.

For the purpose of illustrating my invention, an embodiment thereof is shownnin the drawings, wherein Figure 1 is a diagrammatic view of a television receiving system embodying my invention;

Figs. 2 and 3 are graphical views illustrative of the principle of operation in Fig. 1,

Fig. 4 shows a number of feed-back networks which might be embodied in Fig. 1, as modications; and

Fig. 5 is a diagrammatic view of a cleection circuit proposed for embodiment in Fig. 1 as a modification.

The particular embodiment of my invention shown in Fig. l comprises a cathode ray tube I0 having its control-grid circuit II supplied with picture signals by way of a connection I2 terminating at an adjustable contact I3 associated with a resistance I 4, the latter being connected across the output of a suitable radio receiver I5.

A cathode ray I6 is caused to scan a uorescent screen I 'I by electromagnetic coils I8 for deflecting the ray horizontally and by electromagnetic coils I9 for defiecting the ray vertically. For this purpose, a horizontal deection circuit, designated generally by the reference numeral 20, generates a saw-tooth current wave at a. suitable line-scanning frequency, for example 4320 cycles, for a picture having 180 lines and for 24 frames a second, and supplies this wave to the coils I8. In like manner, a vertical deiiection circuit, designated generally by the reference character 2|, generates a saw-tooth current wave at the picture-frame frequency, for example, 24 cycles, and supplies this wave to the coils i9.

The vertical deflection circuit 2l includes an oscillator circuit 22 comprising an electron tube 23 and a transformer 25 providing an inductive coupling between the plate and grid circuits. A blocking condenser 25 is connected as shown between the grid of the tube 23 and the grid winding 26 of the transformer. An adjustable resistance 21 provides a. leakage path to ground for the electrical charge stored in the condenser 25, as will hereinafter more fully appear.

'I'he manner of operation of the oscillator circuit 22 will now be explained. At the start of the operating cycle, the plate current increases, and the polarity of the grid winding 26 of the oscillation transformer is such that the grid is then driven positive. 'Ihis action is effective to increase the plate current still further until a condition of saturation is reached, after which the plate current begins to decrease. From this point, the plate current decreases almost instantaneously to Zero by reason of the fact that, at the instant the plate current begins to decrease, the polarity of the grid is reversed by the action of the transformer windings with respect to each other. This drives the grid almost instantaneously to a negative potential far below that necessary for cut-oir.

During the action just described, the grid draws current for a very short period and a negative charge is, accordingly, stored in the condenser 25. The charge stored by the condenser is not appreciably affected by the resistance 2l because the latter is relatively high compared to the resistance between the grid and cathode. The negative charge stored in the condenser blocks the tube against further oscillation, be-

cause this charge places on the grid a negative potential much greater than that'required for out-o. This negative charge, therefore, must be reduced by leakage through the resistance 2l before the plate can again begin to draw current to start tbhe next cycle of operation. The time required for this discharge is determined by the ratio existing between the value of the resistance 2l and the capacity of the condenser 25. This ratio is made such that at the end of the complete operating period, which equals a picture-frame period, the charge will not have leaked oi sufciently to reduce the negative bias quite to a potential such that the plate can again begin to draw current. At this instant, however, a sharp framing impulse is received and this is eiective to counteract the bias potential on the grid of the tube 23 sufficiently to permit space current to flow, thus initiating the next succeeding operating cycle.

By exciting the grid circuit from the plate circuit, and by using the proper values of resistance and capacity, the oscillator circuit is permitted to make only one oscillation during each complete operating period. Only a single, sharp impulse of current, therefore, passes through the plate circuit during each complete operating cycle, and this impulse, under the control of the received framing impulse, always occurs exactly at the same point in each complete operating period. In the particular embodiment shown, the capacity of the blocking condenser 25 and the value of the associated resistance 21 are such that the period during which the plate of the tube 23 draws current is about one-flftieth of the entire operating period of the oscillator circuit.

The current impulses developed in the plate circuit of the tube 23, -as just explained, cause a saw-tooth voltage wave to appear across a. condenser 28 and an impulse voltage wave to appear across a resistance 29, the condenser 28 and the resistance 29 being connected in series relation with respect to each other and comprising a network 30 connected in the plate or output circuit of the tube 23. A voltage Wave 3l comprised of the saw-tooth and impulse components, therefore, appears across the network 3U. The sawtooth voltage wave appearing across the condenser 28 is attributed to the fact that the condenser is charged through a relatively higher resistance 32 connected to the potential supply source shown, and to the fact that when the charge reaches a definite level, it is discharged practically instantaneously by the sharp impulse of current which then passes through the tube 23.

Neglecting distorting actions which, however, are always present, the voltage wave 3l would be effective, when applied to the input circuit of a voltage amplifier tube 32, to cause a perfect sawtooth current wave 33 to pass through the inductive circuit including the electromagnetic deiiecting coils I9. However, on account of the distortion actions, the plate current curve appears as though the shape of the voltage wave across the resistor 35 is more like the letter S, as represented by the voltage wave 35. The reason for this is that the upper end portion A of the wave 35 would droop, as indicated in Fig. 2, by reason of the tube plate-current characteristic, the non-linear charge characteristic of the condenser 28, and the fact that an appreciable amount of current, which would otherwise go to charge the condenser 28, passes through the resistor 34.

The lower end portion B of the wave 35 would also be rounded or drooped away from the desired perfect saw-tooth shape by reason of the distributed capacitance across the output circuit of the tube 32, the inductive drop in the load circuit of this tube, and the plate current characteristic of this tube.

For the purpose of compensating for the influences referred to which distort the voltage wave to the S-shape of the wave 35, I propose to connect an impedance network, designated generally by the reference numeral 35, in the cathode circuit of the tube 32, as shown. The values of resistance, inductance and capacity in this network are so chosen that the voltage wave 3l which appears across the same is generally S- shape, but the end portions thereof are of greater curvature, or droop more than the end portions A and B of the wave 35. The network 36 eiects a negative feed-back action through the cathode of the tube 32, so that the voltage wave 38, which appears between the grid and cathode of the tube 32, is graphically the difference between the waves 35 and 3l. The wave 38 is also S-shape, as indicated, biit the end portions thereof are distorted or bent in the opposite directions to the end portions of the Waves 35 and 3l. This is due to the fact that the end portions of the wave 3l are more sharply bent or rounded than the end portions of the wave 35, as explained.

The shape of the current wave in the load circuit of the tube A32 will be distorted from the shape of the wave 3B, on account of the platecurrent characteristic of the tube 32, the distributed capacitance across the load circuit of this tube, and the inductive drop in this load circuit. The shape of the voltage wave 38, as determined by the nature of the network 36, is such, however, that the distortion is just sufricient to straighten out the curved ends of the wave 38, whereupon a practically perfect sawtooth current wave is caused to ilow through the load circuit including the coils I9. In -other words, the voltage wave 38 is distorted in anticipation of other distorting actions further on in the circuit, and in such manner that the latter distorting actions compensate for the previous distortion to obtain the perfect saw-tooth current wave desired.

The saw-tooth current wave 33 occurs at a frequency oi 24 cycles, as determined by the received framing impulses occurring at the rate of 24 a second and which are applied to the grid circuit of the tube 23 by the connection 39.

The horizontal deection circuit 25 is con- YVnectecLand operates inthe same manner as the vertical deflection circuit 2l, the various values or resistance, inductance and capacity being so chosen, however, that a saw-tooth current wave to is caused to flow through the coils i8 at a frequency of 4320 cycles. 'Ihis frequency is maintained by the received synchronizing impulses occurring at the rate of 4320 a second and which are applied to the circuit 20 by a connection di. The two frequencies given, for example,

provide for 2d picture frames a second, each having 180 lines.

In describing the manner of operation of the embodiment of my invention disclosed in Fig. 1, it has been assumed, for example, that the picture signals and the synchronizing impulses are being trammitted in the same channel, with the impulses at a substantially greater amplitude than the picture signals. Both the horizontal and vertical impulses are transmitted at the same amplitude, but are of diierent shapes so that these impulses have substantially different steepness of wave fronts. The horizontal impulses are tigansmitted at the end of each picture line, and'occur at the rate of 4320 a second, while the vertical impulses are transmitted at the end of each frame and occur at the rate of 24 a second.

At the receiving station, the picture signals and the synchronizing impulses appear across the resistance it in the form of a single wave. This wave is supplied to the input circuit of a suitable filter apparatus 63 which blocks the picture signels but passes the synchronizing impulses which are of substantially greater amplitude than the latter, as explained. The apparatus 43 is effective to distinguish between the horizontal and vertical impulses, and passes the former to the horizontal deflection circuit 20 by way of the connection M, and passes the latter to the vertical deection circuit 2| by way of the connection 39.

With reference, more particularly, to Fig., 3, the reverse curvature of the right-hand end portion of the wave 38 is attributed to the nductance shunt M in the network 36, and the reverse curvature of the left-hand portion of this wave is attributed to the capacitance shunt 45 in the network. This effect of the inductance shunt 44 becomes more pronounced if the design is such that the magnetic core more nearly approaches saturation as the current through it reaches the peak value. Both end portions, or only one end portion, of this Ne may be reversed, or the existing curvature may be made more nearly straight, as might be required. Other networks for this purpose may be used, such as are shown in Fig. 4. In Fig. 4, the networks in the upper row and the rst network in the lower row include an inductance shunt and a capacitance shunt, and each would therefore be eiective to reverse the curvature of both end portions of the wave. The second and third networks in the lower row include only an inductance shunt, and would be effective to reverse the curvature of only the right-hand end portion of the wave 38. The fourth and ifth networks in the lower row include only a. capacitance shunt, and would be effective to reverse only the left-hand end portion of the wave 33.

In Fig. 5 is shown a vertical deilection circuit which' may be substituted for the circuit 2| in Fig. 1. In this circuit, an oscillator circuit 45 is used, comprising the tube 41 which operates to discharge a condenser 48, the latter being charged through a resistance 49 fromnthe source of supply Y shown. The oscillator 4G is self-oscillating on account of a feed-back connection 5B, through a condenser 5I, from the output circuit of a voltage ampliiler tube 52 to the input circuit of the tube il. The adjustments are such that the oscillator d5 is self-oscillating at a frequency slightly below the required framing frequency, which in this case would be 24 cycles, the received framing lmpulses applied to the input circuit of the tube 4l by the connections 53 and 5d operating to drive the oscillator and maintain the same at exactly an operating frequency of 24 cycles. The framing impulses are applied to the connections 53 and 55 if they are positive. If these impulses are negative, they are applied to the connections 55 and 5d.

A network 56, corresponding to the network 36 in Fig. 1, is connected as shown. in the load circuit of the output tube 52, and a connection 51 leads therefrom to a resistance 58 connected to the grid of the tube 52.

In operation, a voltage wave 59, corresponding to the wave 31 in Fig. 1, appears between point P and ground. A voltage wave 60, corresponding to the wave 35 in Fig. l, appears across the condenser 48. The wave 59 is in the nature of a negative feed-back to the input circuit of the tube 52, and is subtracted from the wave 60 to develop the reverse voltage wave 6I, corresponding to the wave 38 in Fig. 1. The reverse distortion of the wave 6l is just suficient to compensate for the distorting eiects caused by the platecurrent characteristic of the tube 52, the distributed capacitance across the output circuit of this tube, and the inductive drop in the load circuit of this tube. The result is that when the voltage wave 6I is applied to the input circuit of the tube 52, a practically perfect saw-tooth current wave is caused to pass through the coils I9.

The impedance of the network 56 is relatively low as compared with that of the tube 52, so that it does not directly or appreciably aiect the current in the load circuit while supplying the negative feed-back voltage to the grid circuit.

Since the voltage waves shown are not accurately to scale, they do not show that a denite loss of exciting voltage available for the amplier grid is caused by the negative feed-back. Though this loss is not desirable, it is not serious, as in actual practice the available signal is more than ample to make up for the loss.

The various values of resistance, capacity, inductance and voltage designated in the drawings have been found to provide for very satisfactory operation. However, these values are not critical in any strict sense of the word, and may varied overV a substantial range. Electrostatic deection may be used 'in lieu of electromagnetic deflection. Furthermore, in this connection, va-l frequency, utilizing a feed-back action from a circuit of said tube to reshape the voltage wave, the reshaped wave being such that it is eiective to cause a substantially saw-tooth wave to pass through the inductive circuit, and utilizing the reshaped voltage wave to cause the saw-tooth current wave to pass through the inductive circuit at said frequency.

2. In a circuit for deflecting an electron beam along a substantially saw-tooth path, an electron tube in the output circuit of which a substantially saw-tooth.electrical wave appears at a given frequency, an electrical network connected in a circuit of said tube and operating to effect a feedback action to the input circuit of said tube of such character as to reshape, at said frequency, the voltage wave which would otherwise appear across said input circuit, the reshaped wave being such that it is effective to cause a substantially saw-tooth electrical wave to appear in said output circuit, and a second tube forming part of said detlecting circuit for causing said deflecting circuit`to oscillate.

3. In a circuit for deecting an electron beam along a substantially saw-tooth path, an electron tube in the output circuit of which a substantially saw-tooth electrical wave appears at a given frequency, an impedance network connected in the common grid cathode-plate cathode circuit of said tube and operating to eiect a feed-back action to the input circuit of said tube of such character as to reshape, at said frequency, the voltage wave which would otherwise appear across said input circuit, the reshaped wave being such that it is effective to cause a substantially saw-tooth electrical wave to appear in said output circuit, and a second tube forming part of said defiecting circuit for causing said defiecting circuit to oscillate.

4. In a circuit for deflecting an electron beam along a substantially saw-tooth path, an electron tube in the output circuit of which a substantially saw-tooth electrical wave appears at a given frequency, an impedance network connected in the output circuit of said tube and operating to effect a feed-back action to the input circuit of said tube of such character as to reshape, at said frequency, the voltage wave which would otherwise appear across said input circuit, the reshaped wave being such that it is effective to cause a substantially saw-tooth electrical wave to appear in said output circuit. and a second tube forming part of said deflecting circuit for causing said deflecting circuit to oscillate.

5. In a circuit for causing a substantially sawtooth current wave to pass through ray-deflecting coils, an electron tube at the output end of the deilection circuit, a relaxation oscillator circuit connected to the input circuit of said tube, and an impedance network connected in one of the circuits of said tube and operating to effect a feedback action to the input circuit of said tube of such character as to reshape the voltage wave which would otherwise appear across said input circuit, the reshaped wave being such that it is eective to cause a substantially saw-tooth current Wave to pass through the load circuit of said tube.

6. In the art of electronics wherein it is required that a substantially saw-tooth electrical wave be caused to appear in an electrical circuit, the method oi operation for such purpose which comprises developing electrical waves each at the same frequency and of generally similar shapes but of different curvature's, deriving from said waves another electrical wave of such shape that it is eective to cause a. substantially sawtooth wave to flow through said circuit, and utilizing such derived Wave to cause the required saw:

tooth wave to ilow through said circuit.

7. In a circuit for def'lecting an electron beam along a substantially saw-tooth path, an electron tube in the output circuit of which a substantially saw-tooth electrical wave appears, and an impedance network connected to the common gridcathode, plate-cathode circuit of said tube and operating to eect a feed-back action through said cathode to the input circuit of said tube of such character as to reshape the voltage wave which would otherwise appear across said input circuit, said impedance network comprising a resistance and an inductive shunt across at least a part of said resistance.

8. In a circuit for deflecting an electron beam along a substantially saw-tooth path, an electron tube in the output circuit of which a substantially saw-tooth electrical Wave appears, and an impedance network connected to the common gridcathode, plate-cathode circuit of said tube and operating to effect a feed-back action through said cathode to the input circuit of said tube of such character as to reshape the voltage wave which would otherwise appear across said input circuit, said impedance network comprising a resistor and a capacitive shunt across at least a part of said resistance.

9. In a circuit fordeflecting an electron beam along a substantially saw-tooth path, an electron tube in the output circuit of which a substantially saw-tooth electrical wave appears, and an impedance network connected to the common gridcathode, plate-cathode circuit of said tube and operating to eiect a feed-back action through said cathode to the input circuit of said tube of such character as to reshape the voltage wave which would otherwise appear across said input circuit, said impedance network comprising a resistor, an inductive shunt across at least'a part of said resistor, and a capacitive shunt across at least a part of said resistor.

10. In a circuit for deecting an electron beam along a substantially saw-tooth path, an electron tube in the output circuit of which a substantially saw-tooth electrical wave appears, and an impedance network connected to the common gridcathode, plate-cathode circuit of said tube and operating to effect a feed-back action through said cathode to the input circuit of said tube of such character as to reshape the voltage wave which would otherwise appear across said input circuit, said impedance network` comprising a resistor, an inductive shunt across a part of said resistor, and a capacitive shunt across another part of said resistor.

11. In combination, means for generating a voltage wave having a saw-tooth wave form, each saw-tooth consisting of a portion of gradually changing amplitude in one direction which occurs during a useful deiiecting period and a portion of rapidly changing amplitude in the opposite direction which occurs during a return line period, a wave-utilization network coupled to said wave generating means, and means for introduc ing distortion in said iirst portion of each sawtooth to compensate for distortion unavoidably caused in said network.

12. In combination, means for producing a saw-tooth voltage wave having a steeply sloping portion and a more gradually sloping portion and comprising sine wave frequency components covering'a wide frequency range, a wave-utilization network, means for coupling said wave producing means and said network, said network having the characteristic that it causes the more gradually sloping portion of a saw-tooth wave to bend over, and means for so accentuating the low frequency components of said voltage wave as compared with its higher frequency components that said bending over is compensated.

13. In combination, a vacuum tube having an input circuit and an inductive output circuit, means for generating a voltage Wave having a saw-tooth wave form and comprising sine wave frequency components covering a wide frequency range, means for impressing said wave upon said input circuit, and means for so preferentially acting upon the low frequency components of said wave as compared with its higher frequency components that an electrical wave of substantially perfect wave form appears in said output circuit.

14. In combination, means for producing a saw-tooth voltage wave having a steeply sloping portion and a more gradually sloping portion, a wave-utilization network, means for coupling said wave producing means and said network, said network having the characteristic. that it causes the more gradually sloping portion of a saw-tooth wave to bend over, and means for causing that portion of said saw-tooth voltage wave which corresponds to the bent over portion of the saw-tooth wave in said network to bend up before it is impressed upon said network at least enough to compensate for said bending over.

15. In combination, means for producing a voltage wave having a saw-tooth component which has a steeply sloping portion and a more gradually sloping portion, a wave-utilization network, means for coupling said wave producing means and said network, said network having the characteristic that it causes the more gradually sloping portion of a saw-tooth wave to bend over; and means for causing that portion of said saw-tooth component which corresponds to the bent over portion of the saw-tooth wave in said network to bend up before it is impressed upon said network at least enough to compensate for said bending over.

16. In combination, means for generating a voltage wave having a saw-tooth component. each saw-tooth of said component consisting of a portion of gradually changing amplitude in one direction which occurs during a useful deecting period and a portion of rapidly changing amplitude in the opposite direction which occurs during the return line period, said voltage wave having the undesired characteristic that said iirst portion contains an undesired distortion, a wave utilization device coupled to said wave generating means, and means for introducing distortion in said iirst portion to compensate for said undesired distortion.

17. In combination, means for generating a voltage wave having approximately a saw-tooth shape, said saw-tooth consisting of a portion of gradually changing amplitude in one direction which occurs during a useful deecting period and a portion of rapidly changing amplitude in the opposite direction which occurs during the return line period, said saw-tooth wave having the undesired characteristic that said rst portion contains an undesired curvature, a wave utilization device coupled to said wave generating means, and means for introducing a correcting curvature in said first portion to compensate for said undesired curvature.

18. In combination, means for generating a voltage wave having approximately a saw-tooth shape, said saw-tooth consisting of a portion of graduallychanging amplitude in one direction which occurs during a useful defiecting period and a portion of rapidly changing amplitude in the opposite direction which occurs during the return line period, said saw-tooth wave having the undesired characteristic that said flrst portion contains an undesired curvature, an output tube having an input circuit and an output circuit, a wave utilization device coupled to said output circuit, said input circuit being coupled to said wave generating means, and means for introducing a correcting curvature in said iirst portion at said input circuit to compensate for said undesired curvature.

19. In combination, a vacuum tube having an input circuit and an inductive output circuit, means for generating a voltage wave having a saw-tooth wave form, said saw-tooth having a gradually sloping portion of gradually changing amplitude and a portion of rapidly changing amplitude and comprising sine wave frequency components covering a wide frequency range, means for impressing said saw-tooth voltage wave upon said input circuit, said inductive output circuit having the characteristic that it causes the said gradually sloping portion of a saw-tooth wave to bend over, and means for causing that portion of said saw-tooth wave which corresponds to the bent over portion of the saw-tooth wave in said output circuit to bend up at said input circuit to compensate for said bending over. ALDA V. BEDFORD.

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