US2476900A - Variable gain amplifier - Google Patents

Description

July 19, 1949. A. LE R. OLSON VARIABLE GAIN AMPLIFIER Filed April 30, 1945 WEI;

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M N 0 T Nw who mm R A momzom ATTORNEY Patented July 19, 1949 UNITED STATES PATENT 'O'l-FFI CE .2,47-;900

Alton Le Roy-folson Fort Wayne, -Ind., assignor, by mesneassignments, tOFfilllSWOlth Research Corporatioma corporation of Indiana Application Apr-i130, 1945,.Serial No. 591,096

8 Claims. (Cl. 179-471) This invention relates to signal amplifying apparatus and particularly'to videoamplifying apparatus for controlling and adjusting the gamma of video signals ma television system.

t is well recognized in the art that-thequality of a reproduced monochrome television image may be impaired through loss of detail which is present in the original subject by reason of its natural color. Another way in which the quality of a reproduced image may be impaired is by reason of the distortion and attenuation of the video signals in transmission and reception. Likewise it is well known that, by reason of the inherent characteristics of the apparatus employed to recreate a television image, the contrast ratio and average brightness of the reproduced image generally is lower than the corresponding characteristics of the subject. As a result, an image may be reproduced in sucha manner that each element thereof may be ex actly proportional in brightness ate-the corresponding element of thesubjectand yet the reproduced image may appear to have insulficient contrast to be of suitable entertainment value. Therefore, it generally is necessary in itelevision systems to modify the video signals :in selected portions of their amplitude rangefor the purpose of producing greater fidelity of .image reproduction. Such signal modification is commonly known as gamma control.

It is well known that in the past various expedients and devices have been employed for controlling the gamma of video signals whereby the contrast between the light and dark portions of the reproduced image is altered so as to provide for a more faithful production ofthe image. Many of the prior art devices, while accomplishing the desired gamma control have the disadvantage that in so doing the overall gain of the amplifier system is so afiected as to produce a corresponding change in the brightness level of the reproduced image.

It, therefore, is an object of the present invention to provide a novel arrangement of apparatus for controlling the gamma of 'video signals in a television system.

Another object of the invention is .to provide a video signal amplifier for a television system whereby to control the gamma of the video signals as desired without concomitantly afiecting substantially the overall gain of the amplifier.

In accordance with the instant invention, there is provided a signal amplifier which includes at least one vacuum tube having a cathode, an anode and a control grid. Facilities are provided with this vacuum tube, which'is employed :to produce a selectivemodification of the signals, for effecting cathode excitation thereof. These facilities include a source of biasingvoltage coupledrto the control grid having a polarity and magnitude appropriate to effect a portion of the operation of the tube in a non-linearregion of its characteristic. In order to excite or drive this tube and atthe same time to suitablyvary the inputcircuit biasing thereof, there .is an impedance device connected to the cathode in series with the spacedischarge path of Qthe-tube. The signals whichare to .be modified, for the'purpose of effecting igamma control in a television system for example, are impressed :upon the cathode-connected impedance device. The amplified and selectively modified signals arederived from the anode of the tube which may be coupled to-any desired utilization circuit.

More specifically, inaccordance with a feature of the illustrative embodiment of the invention included herein, there also is=provided an amplifying tube which includes a cathode, an anode and ;a control grid. This tube is arranged as a cathode follower by the-provision of animpedance device connected to its cathode in series with the space discharge path of the tube. Where the device is to be used as a gamma control for video signals in a television system the video signals are impressed upon the control grid of the amplifying tube and the amplified signals are derived from the cathode-connected impedance device. The cathode-connected impedance device of the amplifying tube is conductivelycoupled to the cathode-connected impedance device of thegamma. control tube insuch amanner that at least portions of the two impedance devices are connected in series.

In accordance with a further feature of .this invention, .the cathode -connected impedance .de- .vices, with which the tubes .are provided, are arranged to be variable so that there may be effected a variation in thedegreeof gamma control. For this .purpose the two impedance devices may be potentiometers and it has been found that particularly good results have been obtained by varying the potentiometers simultaneously and in correspondence with one another. This may be done by mechanically interconnecting the adjustable contacts of the potentiometers so that they may becontrolled suitably 'by the manipulation of a singleknob or like adjusting member.

For a better understanding of the invention, together with .other and further object's thereof,

reference is had to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. I

In the drawing:

Fig. 1 is a circuit diagram of that portionof a video signal amplifier which embodies the present invention; and y 1 Figs. 2 and 3 are curves illustrating the performance of the apparatus.

Having particular reference now to Fig. 1 o the drawing, there is shown a source of video signals I which is coupled to the input circuit of an amplifying vacuum tube 2 arranged; as a cathode follower. The amplified video signals :representing picture white.

means of condenser '21 to the input terminals that the particular apparatus illustrated herein derived from the tube 2 are coupled to the input I circuit of a gamma control vacuum tube 3. The

amplified signals derived from the tube 3 are in turn coupled to a utilization circuit 4 which may include additional components of a video amplifier chain or the like. The amplifying tubev 2 is shown in the drawing as a pentode of the 6AC'7 type or its equivalent. The gamma control tube 3 also is illustrated as a pentode and may be a 6AB7 or its equivalent.

A potentiometer 5 which consists of a resistance element 6 and an adjustable contact I is connectedto the cathode of the amplifying tube 2.

A similar potentiometer 8 comprising a resistance element 9 and an adjustable contact I0 is connected to the cathode of the gamma control tube 3. Specifically, the cathode of the tube 2 is connected to one terminal of the resistance element 6 and also to the adjustable contact 1 of the potentiometer 5. The other terminalof the resistance element 6 is connected to the adjustable contact Ill of the potentiometer 8. .One terminal of the resistance element '9 of this latter potentiometer is connected to the cathode of the tube 3. The other terminal of this resistance element is connected through a resistor [2 to ground. It has been found desirable to mechanically link the adjustable contacts 1 and ID of the respective potentiometers 5 and 8, as ini dicated by the broken line so that they may be correspondingly and simultaneously adjusted by means of a single control knob I l.

The suppressor grid I3 of the tube 2 is connected to the cathode of this tube and likewise the suppressor grid [4 of the tube 3 is connected to its associated cathode. The screen grid 15 and the anode of the tube '2 are connected through a voltage dropping resistor I 6 to the positive terminal of a source of space. current such as a battery I1. The negative terminal of the battery is connected to ground. Also the anode of the tube 2 is maintained at ground potential for the relatively high frequency video signals by means of abypass condenser l8. The screen grid IQ of the tube 3 is connected through a current limitingresistor 20 to the positive terminal of the battery IT. The anode of the tube 3 is connected through a load resistor 2| to;the

positive terminal of thev battery l1. 7

The output terminals of the signal source I are coupled by means of a condenser 22 to the control grid 23 of the amplifying tube 2. A leak resistor 24 for the condenser 22 is connected from the grid side of the condensertoground. The control grid 25 of the gamma control tube 3 is connected to the positive terminal of a source of biasing potential-such as a battery 26. The

negative terminal of this battery is connected to ground. The anode of the tube 3 is coupled by operates most satisfactorily when a signal voltage control grid 23 of the tube 2.

As illustrative of the manner in which the apparatus embodying the instant invention operates to effect a substantial range of gamma control, also assume that the control knob II is so manipulated to place the adjustable contacts 1- and III of the potentiometers 5 and 8, respectively, in engagement with the extreme right hand terminals of the resistance elements '6 and 9. Then there is connected in series with the cathode of the tube 2 only the impedance of the resistor l2 which may have a value of approximately 120 ohms. The cathode circuit of the tube 3 includes, in'addition to the resistor l2, the resistance element 9 which may have a value of approximately 100 ohms. In this case the total voltage of the cathode of the tube 2 with respect to ground which varies under the control of the video, signals is impressed upon the cathode of the tube 3. In such a case, with the battery 26 connected in positive polarity as shown, to the control grid 25 of the tube 3, the apparatus has been found to operate, when this battery has a value of between 0.5 and 1 volt, in

such a manner that the signal voltages developed at the anode of the tube 2 vary with maximum linearity relative to the video signal. voltages impressed upon the control grid of the amplifier tube 2. 7

Referring to Fig. 2, the curve 28 illustrates the relationship between the light and shade values of the picture and the output circuit video signal voltages representative thereof. The gamma of these reproduced signals then is almost unity as shown by the substantial linear character of the curve 28. In this condition of maximum linear ity, the signal voltage in the output circuit of the tube 3 at the center of the register of picture light and shade values, which is the midpoint between picture white and picture black, issubstantially 60% of the signal voltage representativeof picture black. This middle register output video signal voltage is represented by the point 29 on the curve 28 and the black representative voltage is indicated by the point 3| on the curve. In the ideal case of perfect linearity between the input and output signals the middle register signal voltage in the output circuit would be exactly 5 0% of the signal voltage representative of picture black.

Nowassume thatit is desired tocompress to a maximum degree the signal voltages in the region of picture black- The control knob ll of Fig. 1

.each'have'valuesof ohms. Thesignal voltage V which is developed in the cathode follower output circuitof the'amplifier tube 2 with'the described adjustment of the potentiometers is the voltage drop produced through the entire series arrangement of the resistance elements 6 and 9 and-the resistor l2-which is a total of 320 ohms.

The portion of this signal voltagewhich is impressed upon the input circuit of the gamma control tube 3 is slightly greater than two-thirds of the total. It has been found that under these conditions the signals reproduced in the output circuit of the tube'3 correspond in a substantially linear manner to the signals impressed upon the input circuit of the amplifier tube 2 throughout a considerable portion of the range of voltages corresponding to picture white and light greys. Asthe input signal voltage increases positively in magnitude to represent darker portions of the picture, the tube 3 functions to reproduce these signals-with increasing modification or deviation from asubstantial linearity of response.

.Fig. '3 shows the performance characteristic of the'amplifier under these conditions, the curve 32 graphically depicts the relations between the light andshade values of the picture and the output circuit video signal voltage. With the apparatus described and adjusted as indicated in this instance, it has beenfound that, at the middle register of picture light and shade values, the signal .voltage, indicated at point 33 on the curve 32 as reproduced in the output circuit of the gamma control tube 3, is approximately 90% of the signal voltage representative of picture black. Picture black video signal voltage is indicated by the point 34 on the curve. In other words, a signal voltage which varies from a minimum amplitude representative of picture, white to 90% of its maximum value is derived to represent the light values of the picture from white to a grey shade appearing substantially in the middle of the register between picture white and picture black.

The remaining of the developed video signal voltage represents all of the dark picture shades between the middle register and picture black. It is seen, therefore, that the signal voltages representing the darker shades of the picture are compressed into approximately 10% of the maximum video signal voltage variation. For a lesser degree of signal voltage compression in the black region, the control knob ll may be manipulated, if desired, to-efiect engagement of the adjustable contacts land in of the respective potentiometers 5 and 8 with intermediate points on the corresponding resistance elements 5 and 9.

The curves 28 and 32 of Figs. 2 and 3 respectively are drawn to the same scale and thus indicate graphically one of the important attributes of this invention. It is to be noted that the point 34 of curve 32 indicates a video signal voltage representative of picture black which is only slightly less in magnitude than the corresponding video signal voltage indicated by the point 3! of curve 28. Inasmuch as these curves indicate the same video signal voltage to represent picture white, it is evident that the range of video signals from black to White is not materially difierent regardless of whether the gamma is substantially unity or materially deviates from unity. In other words, the overall gain of the amplifier system is not affected appreciably. Consequently, the brightness level of an image reproduced from video signals subjected to gamma control in accordance with the instant invention remains substantially constant throughout the entire range of gamma control.

The apparatus embodying the present invention operates in the manner described to effect gamma control by reason of the suitable biasin of :the control grid :25 of the tube :3 .to. efiectv operation of this tube in-thelewer portion of its characteristic inthe region .adjacentto the point of space currentcutofi where it. departs materially from linearity. With .this gridbiased, as described, to approximately 0.5 to 1 volts positive relative to ground, the operation of the tube is such:that.for signal voltages impressed upon the cathode circuit thereof of relativelysmall positive magnitudes'there is effected a substantially linear changeinthe space. current inthe tube. However, ,as. the signal voltage impressed upon the cathodecircuit of the tube 3 is increased in positive magnitude, the space current decreases and by reason'of the described biasing of this tube theoperationoccurs in the non-linear region of the characteristic immediately below thelinear portion thereof. It is seen,; therefore, that as the impressed signal voltage increases further in positive magnitude, there is effected: less of a decrease in the space, current. conducted by the tube.

It. is considered to be Within'the purview of this invention that, instead of compressingthe signal voltages representative ofthe darkerpicture elements, the apparatus alternatively -may be employed to. equally good advantageto compress the signal voltages representative of the lighter picture areas. In such a case it is'merely necessary to arrange that the'polarityof the signal voltage variation in the cathode circuit of the tube 3 be opposite to thatdescribed. Specifically, the impressed voltage shc-uldincrease in-positivemagnitude to represent lighter picture areas. Inasmuch as the gamma control tube 3 is arranged to produce the maximum modification of those of the signals impressed upon itscathode circuit which are of the greatermagnitudes, it is seen that insu-ch a case a compression of the signal voltages representative of the lighter picture areas will be produced in the output circuit of this tube.

It will be apparent'to those skilled in the art that the polarity or phase of the signal voltages impressed upon'the cathode circuit of the tube 3, as they are related to the black and white limits of the picture, may be produced in many ways other than that illustrated herein. For example, under the assumed conditions where the video signals are impressed upon the amplifier tube 2 from the source I in such polarity that the black picture areas are represented by maximum positive voltages, the amplified signal voltages, if derived from the anode of this tube for impression upon the cathode circuit of the gamma control tube 3, would be in such polarity that maximum positive voltage at the cathode of the gamma control tube would represent picture white. Of course, with the present apparatus the same result may be achieved by simply reversing the polarity of the video signals impressed upon the input circuit of the tube 2 from the signal source I'.

It also obviously is not essential that the specifically illustrated conductive coupling between the amplifying tube 2 and the gamma control tube 3 be employed. It is contemplated that, in some systems, capacitive or inductive coupling may be employed successfully within the scope of this invention providing signal voltages of sufficient amplitude are available. Of course, where the sigrial voltages are derived from the anode circuit of the amplifier tube 2, capacitive or inductive coupling is preferable in order to isolate the cathode circuitofthe tube3 fromthe relatively high voltage source ILz: i c c I 1 Whileapparatus, which has been successfully operated in accordance with the present invention, has employed a unitary ccontrol of the potentiometers 5 and 8 to advantage, it is apparent that this feature is not an absolute essential'for the successful practicing of the invention. Either one or both of the adjustable contacts I and illl may be manipulated independently, if desired.-

While there has been described What, at present, is' considered the preferred embodiment of the'invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: V

1. In apparatus for effecting gamma control 015 video signals, a vacuum tube having'a cathode,

an anode and a control grid, a source of fixed biasing voltage connected to said control grid to effect a portion of the operation of said vacuum tube in the non-linear region of its characteristic, means coupled to said anode to supply space current to said tube, a resistance device connected to said cathode in series With the space discharge path'of said tube, said resistance device having a degenerative efiect upon the operation of said tube by reason of the traversal thereof by the space current of saidtube, whereby the control grid-to-cathode voltage is varied in magnitude inversely to variations in magnitude of the space current, means for impressing a Video signal in positive polarity upon said resistance device to control the magnitude of the space current in said tube, a utilization circuit coupled to the anode of said tube, and means for adjusting the point on said resistance device upon which said video signals are impressed to vary the proportion of said video signal effective to produce operation ,of said vacuum tube in the non-linear region of its characteristic. 1 I a a 2. A signal amplifier comprising, first and second vacuum tubes each having an anode, a cathode and a control grid, a variable resistance device connected to each of said cathodes in series with, the space discharge paths of the respective tubes, said resistance devices also being connected ance devices, a source of signals coupled to'the control grid of said first tube, a source of fixed biasing voltage of positive polarity connected to the control grid of said second tube and having a magnitude suitable to .efiect operation of said second tube in the non-linear region of its characteristic adjacent to the point of space current cutoff, a source of energy connected to the anodes of said tubes toisupply space current thereto, and a utilization circuit coupledto the anode of said second tube. c

3. A video signal amplifier'com'prising, first and second vacuum tubeseachhaving an anode, a cathode and a control grid, a source of space current having its positive terminal coupled to said anodes and its negative terminal, connected to ground, a series arrangement of two resistance devices, one terminal of said series arrangement being coupled to ground and the otherterminal being connectedto the cathode of said first vacuum tube, a connection from the junction point between saidresistance devices to the cathode of said second vacuum tube, a source of video signals coupled in positive polarity to the control grid of said first tube, a source of fixed biasing voltage connected between round and the control grid of said second tube and having a polarity and magnitude suitable to eifect operation of said second tube in the non-linear region of its characteristic adjacent to the point of space current cutofi in response to signals of relatively great positive amplitude, a utilization circuit coupled to the anode of said second tube, and meansifor 'efiecting' corresponding simultaneous'variation of said resistance devices to control the input circuit'biasing of said second vacuumtubes.

: ALTON LEROY OLSON. 1

REFERENCES CITED The following referenlces areof record in the file of this patent: c V

UNITED STATES PATENTS Mitchell Sept. 7,1943

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