US2302425A - Television apparatus - Google Patents

Description

Nov. 17,1942. F.'M. DE1ERHAKE 2,302,425

TELEVISION furtinkrus Filed Oct. 26. 1940 cumzzm' 4 0R VOLTAGE Fig.2.

on o VOLTAGE W i 39 I 1 I r lnyentorf y Franklin M.. l 'Je er-hake,

His ttorney.

which are attenuated or lost in through the amplifier,

, avoided. g

Amplifiers capable of amplliying low ire atent Nov. 17,1942

spires PATENT. orr ca 2,302,425 H 'rnrhvisrou'arraaa'rus Franklin M. Decrhake, Schenectady, N. Y., n-

signer to General Electric tion of New York Company, a corpora- Application. October 26, 19, Serial No. 868,021

6 Claims- (ci. iii-4.3)

My invention relates to television apparatus, and in mrticular to apparatus for reinserting, at the output a video amplifier, low frequency and set current components of video signals transmission than-black region.

In present-day television systems an image to be tratted is usually scanned in a series of lines, and electrical signals are developed which are representative of the brightness of -each elemental picture component of the image. These signals mint, in general, convey two kinds of information: first, the relative brightness oi'the picture elements, determined by the details 0! the image, and second, the average absolute ing to the absolute monly designated as the infra-black or blacker- Since the fixed level 0! the pedestal portions of the video signal corresponds to black in thepicture image, it provides a very convenient reierence level to which may be referred the instantaneous picture signal intensity, correspondments of image.

this level to the average value of the picture signal. As will be pointed out in greater detail in the brightness of the picture elements, determined 1 by the background illumination. Thus, the sigmust represent both instantaneous and steady state conditions and contain a wide range or frequency components down to and including following specification, ii a video amplifier is capable oi transmitting, without substantial distortion, the wave form oi the pedestal portions and allbut the lowest frequency components or the picture signal portions, a peak rectifier cirt current components. -All these components must be present in the final signals which are utilized in they receiving apparatus to reproduce the image, if distortion is to be c uencies and direct currents, as'well as higher frequencies extending over the wide range of frequencies present in television video signals, are

very costly and dimcult to build. Hence, it is.

common practice to use more economical resistance-capacitance coupled amplifiers in both television transmitting and receiving apparatus.

Since low lrequency and direct current components are attenuated or lost in transmission through such amplifiers, it is necessary to reinsert these components in the output to restore all components oi the video signals to their proper relative values. Accordingly, it is a main object of my invention to provide-improved cir-.

cuits for reinsertifig these components in the output of. a video amplifier. A preierred form of sentative oi decreases in illumination, and vice versa. Between successive trains of picture signals blanking pedestals are inserted which have an amplitude substantially corresponding to blaclr'in the picture image. These pedestals serve to blank out the retrace portions of the sc pattern in a manner familiar to the art. They also serve to establish a fixed reference level for the synchronizing pulses, which comprises impulses of greater amplitude superimposed on the pedestals and extending into the region comvideo signal is one in which increases in signal amplitude are reprecuit coupled to the amplifier output may be utiheed to develop signals substantially proportional to the non-transmitted low irequency and direct current components. These may then be mixed with the amplifier output and supplied to a utilizationcircuit. usually a final video amplifier or a modulator. The types of amplifier and modulator tubes which are now available for television service require a low impedance input circult for stable operation. However, the rectifier output must inherently be 0! high' impedance and the problem is presented of coupling it directly to the utilization circuit without causing circuit instability. Furthermore, the reinserted components must bear the proper amplitude relationships to the transmitted Si nal components.

It is therefore aiurther object of my invention to provide improved means for reinsorting low frequency and direct current components in the video amplifier output in the proper proportions and 101' directly coupling the reinsertion circuits to a utilization circuits: as

to insure stable operating characteristics.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with Iurther objects and advantages thereof, may best be understood by rei- I erence to the iollowing description taken in connection with the accompanying drawing, in-

which Fig. l diagrammatically represents portions 0! a television apparatus embodying-my invention, and Figs. 2a through 2d are representative oi the wave i'orms oi video-signals which will be referred to for a better understanding of the operation of my invention.

Reierring now to Fig. 1, video signals to be brightness oi the picture ele- The average background 11- himinationis proportional to the distance irom' amplified are'supplied from any suitable source, not shown, to the input of a video amplifier, il= lustrated as a pento-de it]. As shown, the signals are coupled to the control grid of the amplifier l by means of e. resistance-capacitance coupling network H, 52. A normal operating bias is impressed on the control grid by means of any suitable potential source, not shown, connected between the lower end of resistor ii and the quency response characteristics. 1

Whlleths video amplifier ill or 1 has been illustrated as a single stage, it will he observed that it may comprise a-plurallty oi stages each of which may be similar to the one just described.

Coupled across the output of the amplifier ii! is a rectifier l8, illustrated as a duo-diode having its cathode electrodes and anode electrodes respectively connected together to function as a single diode rectifier. The cathodes of the rectifier is are connected to the anode of the video amplifier ill through a capacitor 2d and the anodes are connected to the cathode oi the amplifier it through a bypass capacitor ill. The anodes are also connected to a movable tap-23 on a bias potentiometer ii. The potentiometer H is connected across a suitable source of potential, indicated by the terminals 22 and i8, which is oled to maintain the right-hand end of the potentiometer negative. The unidirectional potential of the anodesof the rectifier HQ with respect to ground may be adjusted by adjusting the movable tap of the shunt type for rectifying the'slgnals ap The peering in the output or" the amplifier id.

rectified signals appear across the load resistor 26. This circuit develops low frequency and unidirectional signals, proportional to those present in the video signals supplied to the input of amplii'ier ill, in 2. manner shortly to be described in greater deb-oil.

The resistor is also included in the grit ircuit of a triode amplifier This amplifier is connected in a cathode-coupled circuit, sometimes called a cathode-follower circuit. The anode'oi the amplmer 25 is connected directly to the positive terminal ll of the source of anode potential and the cathode thereof is connected to the grounded terminal ill of this source through resistors 26 and 27 and the potentiometer ii. The grid is connected directly to th upper ter mluul of the load resistor 26.

it willhe. observed that the resistors 26 and are included in both the grid and anode circuits of the amplifier 25, thus providing a large amount of cathode degeneration. When operated in this fashion the amplifier has a gain somewhat less than unity, which is practically constant for relatively lowfrequencles down to and including unidirectional components. The gain is always less than unity but it can be made very nearly equal to unity by using a high gain amplifier. Furthermore, this circuit permits stable operation with a relatively high value of resistance in its grid circuit and low resistance in its common cathode circuit.

The resistor 21 is connected across the termlnals of a suitable utilization circuit. As illus trated, it is connected between the grid and cathode of a tetrode 28 through the bias potentiometer 28. The tetrode 28 and associated circuits may constitute 21 final power amplifier for the video signal; or, in a. television transmitter, it may comprise a modulator stage. A portion of the video signal output of the amplifier as, appearing across the load circuit elements l4, l5 and i6, is also coupled to the grid of the amplifier 28 through the capacitor 29.

The operation of the circuit of Fig. i will best be understood in connection with .the wave forms of television signals illustrated in Fig. 2. Fig. 2a

video signal, including the picture signal portions 39 and also the pedestal and synchronizing portions 4| and 43, take place about some alternating current axis, such as is indicated by the dashed line 40. As previously mentioned, the pedestal portions ii of the signal assume a reference level corresponding to blacliin the pic ture image. This level is indicated by line 32. The synchronizing pulses extend. a defimte distance above this level into the infra black region. The direct current component of the video signal during the interval represented is proportional to the distance between the axes tit and ii.

A video signal produced by scanning the same image line, but with decreased background lllumi nation, is graphically represented in Fig. 2b. it will he observed thatthe wave form of the picture signal 39 is the same as in Fig. 2a, but the direct current component of the signal is less than belore, corresponding to a darker image. The ped-. estals ti and synchronizing pulses d3 lie on the some threshold level 42 as before.

Thus, for proper rendition of the absolute brightness values of all elements of a scanned image, the pedestal portions must establish components will necessarily be attenuated or lost in transmission through the capacitor 52. For example, the signals of Fig. 2a and Fig, 2?) will appear on the grid of amplifier to generally as represented in Figs. 2c and 2cZ,-respectlvely. 1f

' the capacitor i2 is large enough, the wave forms of the pedestals M, the synchronizing pulses 33 and the picture signal as will be preserved, but the signal will be referred to the arbitrary alternating current axis so such that the areas of the video signal above and below this axis are su'ostantlally equal. it will be seen that this axis has no relation to the background illumination age level so of the stout of the capacitor and resistor 24 is mode long relative to the interval between adjacent pedestals. Therefore, through detector action as fixed reference level 44 for the rectified signed is established Just slightly below the tips of the synchronizing pulses. Therecrifled potentials appearing on resistor 24 correspond to the distance from this level to the avervideo signals.- The slight difierence between this new reference level and the level of the blorimhg estsls is nearly constant since the height of the synchronizing pulses is is easily eliminated by a sim pie adjustment of other portions of the reinsercoustent, and this tiou circuits now to be described.

it should be noted that, while the time constout of cspscltor 2d and resistor 2d should be long enough to permit rectification of the peaks oi the synchronizing pulses, it should not be too long. Otherwise, variations in the intensity of the background illution will not be ialthfully reproduced. As at rough criterion the time constout should be short relative to the time inreuuired to scam one complete picture image. 1

v The potentiometer 2i periorms several important functions. in. the first piece, by reason of the potehtied drop across the portion of this potentiometer to the right of the top 2%, e small current how is meintoined through the diode i8 and resistor t8 This provides an initial bias on the diode is which insures linearity of operation even in the presence oi very weak signals. In the second piece, the potential drop across the resistor 2 bioses the grid of the amplifier 25, which in turn determines the minimum current flowing through resistors 26 and Ti. Itwill be observed that e. negative bies is impressed on the grid of amplifier til equal to the total drop across the potentiometer 2i mus the potential drop across resistor 2'21 due to this current flow. Therefore, adjustment of the top 28 on the potentiometer 2i simultaneously adjusts the bios poten tisls applied to the tubes to end 28.

Since the rectified potentials developed on the resistor 2d are substantially proportional to the components to be reinserted at the grid of ampliher so, it might be assumed that plifier 28 could be directly coupled to this resistor. However, in the usual case the resistor 2d will have 9. high resistance of severel'megohms. The in clusion of a high resistance of this order in the grid circuit oi amplifier 28 wouldnot be satisfacefllciency as the higher frequency components of the well-known amplifier 2! from the Il'ld of thecmpliner 28- in order to obtain better high frequency response.

That is, this resistance isolates the capacity between the cathode of amplifier and ground from the grid of amplifier 28 and thus reduces attenuation of high frequency currents transmitted through capacity 28 by this cathode to ground capacity. This resistance 26 may be relatively small compared to resistor 21 so that most of the amplifier output appears on the latter resistor.

For a. number of causes, the reinserted low frequency and direct current components developed on resistor 21 have amplitudes somewhat less than they would have bad if transmitted directly through the video amplifier H) with the same the video signal.- As previously mentioned, the detector is actually establishes a reference level for the rectified signal which differs very slightly net result is that a.

from the pedestal level. The cathode-coupled. amplifier 2! introduces a small amount of desmplification and 9. further loss occurs due to the drop through the decoupling resistor 28. The

loss in voltage of approximately ten per cent may be expected in the reinsertion circuits, in the ordinary case. I

The proper proportions of transmitted to reinserted components of the video signal are easily reestablished by connecting the coupling capacitor 28 to a. point of lower potential on the output circuit of amplifier l0 than the point to which the capacitor 20 is connected. Thus, the signal impressed on the rectifier i9 is' greater than the signal applied to the grid of the amplifier 28 through capacitor 2! by the drop across resistor 13 of Fig. 1. I! this drop is made exactly equal to the loss in the reinsertion circuits,

tory. In tubes of the t available for television work, such s connection would result in instability and tube runaway. The cathodecoupled amplifier 25 chars on efiective solution to this problem. As previously mentioned, such it circuit permits stable operation with a. high veins of resistance in its grid circuit and a. relatively low value of resistance in its output circult. It also permits the use of a. smell triode of a. type that is less susceptible to instability than the type of tube required for amplifier do, which will generally be e. power tube.

The resistor Ed is employed to decouple the unavoidable shunt capacity of the cathode of the perfect restoration of all video signal components at the inputto amplifier 28 is achieved. Obviously, it can also be made greater or smaller should over-compensation or under-compensation be desired.

The constants of the component circuit ele meats-of Fig. 1 are subject to considerable variation depending upon the particular case involved. Therefore, although the following values are given for a typical apparatus embodying myinvention, it is to be understood that they are only illustrative. These values have been found suitable in a. Ml-line, double-interlaced transmission system having 60 picture fields per second. The tubes i0, i9, 25 and 28 were times 1852,6H6, 6SF5 and 61.6, respectively. Other constants follow:

Resistor l3 ohms Resistor l4 do 1,000 Resistor l6 do 10 Capacitor 28 microfa.rad .01 Resistor 2| ohms 100,000 Resistor 24..-, -megohms l0 Resistor 28-.... ohms 8,000 Resistor 21 do 100,000 Capacitor 29 microfa.rad .01 Capacitor 30 do 1 Capacitor 3! do 1 Source li, i8 vo1ts (positive) 300 Source 22, i8 volts (negative) It will thus be seen that my invention provides .means for obtaining practically perfect restors.

tion of the low frequency and direct current components with a. minimum of distortion, even on weak signals, and insures stable operation.

While I have shown a. particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and 1. contemplate by the appended claims cover any such-modifications asfall within'thefirue spirit:

and scope' of my invention, a

what I claim as new and des' Letters Patent of the United States, is:

i. In combination, a source of signal electromotive force having frequencies extending over a band including low frequency and unidirectional components varying with respect to a predetermined signal level, said electromotive force having recurrent portions of an amplitude substantially equal to said level, alternating current translating means for said composite signals, a utilization device having low impedance, and means to reproduce from the output of said translating means currents in said utilization device having said high frequency, low frequency,

ascends able fosamplification of said alternating electrc= and unidirectional components in desired relafor high frequency and low frequency compo-- nents, said path for low frequency components including a peak rectifier having a high impedance load, and a cathode follower amplifier connected between said high impedance load and said low impedance utilization device, the cathode of said cathode follower amplifier oscillating motive force but too*low. for direct=connection to said lcad resistance, a cathode follower am plifier connected between said load and said input impedance tos'upply low frequency and unidirectional components 2mm .said load to said amplifier for amplification, and a separate high frequency path between said source and said input impedance to supply high frequency components of the televised picture to said amplifier for amplification thereby.

.4. In combination with a source of composite signals extending over a band of frequencies which include low frequency and unidirectional components with respect to a predetermined signal level, said si nals having recurrent portions of an amplitude substantially equal to said level, means for translating said composite signals without said components, a diode rectifier coupled to the output of said translating means, means biasing said diode for operation on a sub-' stantially linear portion of its operating characteristic, means interconnected with said diode for efiecting peak rectification of the translated signals impressed on said diode, thereby to develop low frequency and unidirectional signals substantially proportional to said non-transmitted comabove ground and having impedance to ground high at the highest frequencies transmitted through said low frequency path and having low.

impedance for currents transmitted through said high frequency path, and an impedance between said cathode and said utilization device to reduce attenuation of currents transmitted through said high frequency path by said low cathode to ground impedance.

2. In combination, a source of electromotive force varying in accordance with a televised picture and its background brilliance and having only alternating current components varying with said background brilliance, a utilization device, means to supply to said utilization device high frequency current varying in accord with v said televised picture and low frequency and unidirectional currents varying in accord with said background brilliance,' said means comprisin separate paths between said source and said utilization device for said high frequency currents and for said low frequency and unidirectional currents, said low frequency path having a peak rectifier for said alternating electromotive force, a load resistor therefor, and a cathode follower amplifier connected between said load resistance and said utilization device to transmit to said utilization device said low frequency and backforce varying in accord with a televised image and'ithackground brilliance and having only alternating current components, a peak rectifier for said alternating electromotive force having a load resistance on which appears unidirectional electromotive force varying as the low frequency and background components of said televised picponents, a thermionic amplifier having a load impedance common to it input and output circuits, means coupling said low frequency and unidirectional signals to the input circuit of said amplifier, ,a utilization circuit adapted to translate substantially all components of said composite signals, means coupling said impedance to I said utilization circuit, and mean coupling the output of said translating means to said utilization circuit.

5. In television apparatus for translating video signals having components extending over a wide band of frequencies and including very low fre quency and unidirectional components with respect to a predetermined threshold level, said signals having portions which extend to said level at intervals, the combination comprising means for amplifying said signals without said low frequency and unidirectional components,

means for rectifying said amplified components comprising a diode detection circuit having a time constant which is long relative to the longest interval between said' portions, a thermionic amplifier having an output impedance common to its grid and anode circuits, means conductively coupling said detection circuit to said grid circuit, a video signal amplifier having its input circuit conductively coupled to said impedance,

whereby low frequency and unidirectional potentials substantially proportional to said nonamplified components are developed on said impedance and impressed upon said input circuit, means for impressing a corresponding proportion of said amplified components upon said input circuit, means for impressing a relatively low positive potential on said diode sumcient to maintain it biased far operation on a substantially linear portion of its characteristic, said low potential also being effective to determine the grid bias potentials on both said amplifiers, and means for varying said low potential to adjust all said bias hire, an amplifier having input impedance suitpotentials simultaneously.

6. In television apparatus for translating video signals having components extending over a wide band of frequencies and including very low frequency and unidirectional components with respect to a predetermined threshold level, said signals having portions which extend to said fier having its grid connected to its cathode through a grid circuit serially including said im- 10 pedance, a decoupling resistance and an output load resistance in the order named, an anode circuit Jor said amplifier serially including said resistances, means for impressing a fraction of said amplified components across said load resistance, and means conductively coupled to said load resistance and capable of translating substantially all components of said video signals.

FRANKLIN M. DEERHAE.

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