US2255691A - Television signal-translating system - Google Patents

Description

ASept 9, 1941- J. c. wlLsoN TELEVISION SIGNAL-TRANSLATING SY'STEM Filed Jan. 14, 1939 nouns spun! INVENTOR JHN c.wu soN ATTORNEY Oil.

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tics of television 'apparatus as Patented Sept. 9, 1941 f John C. Wilson, Bayside, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application January 14, 1939,/Serial No. 250,914

`12 claims. (c1. 11s-1.3)

This invention relates to television systems and particularly to the video-frequency signal-translating portion thereof. The invention is especially concerned with controlling the characteristics of such systems relating to gradation and contrast of the illumination between the incremental areas of transmitted and reproduced images.

. It is frequently desirable in television systems..

to introduce predetermined distortion of videofrequency signals, vfor example, non-linear amplication, over certain portions of their amplitude ranges, in order to obtain apparenty optimum fidelity of reproduction of the images being transmitted and received. The desirability of such distortion is due to certain inherent characteriswell as to certain physiological phenomena of the observer. In' conventional vtelevision apparatus, for, example, certain of the component elements, such as aml pliers, cathode-ray tubes, and the like, have input-output or stimulus-response characteristics which deviate from linearity to such an extent as to result in appreciable distortion in the translated signal, unless compensated forA by the introduction of predetermined complementary dis,- tortion. Moreover, in some instances, if the 11- v lumination of the incremental areas of the rev produced image is proportional to the illumination'of the corresponding areas of the original image, that is, if the systemy has a linear over-all stimulus-response characteristic, the reproduced images may 'appear at or distorted to an observer. In such cases, therefore, it' is desirable to give the system. a predetermined non-linear over-all response characteristic.

In photography a film is said to have a gamma which deviates from. unity in accordance with differences in the relative detail or contrast for'the brighter detail or contrast for the brighter or darker portions of the scene represented thereby, compared to other portionsy thereof with respect 'to the corresponding contrasts of the original scene, This same concept is useful in television. Briefly, the gammaof any system may/ be definedy as the 'slope of the stimulus-response curve4 plotted on a logarithmic scale. Obviously, only where gamma is unity is there a linear relationship between the stimulus-and response over the entire response range of the system.

While the stimulus-response characteristic of alinearsystem on a linear scale is, of course, a

,straight line through the origin, in televisionsystems it is vconventional to plot the coordinates representing stimulus in terms ofthe minimum appreciable'stimulus and the resultant stimulusresponse curves generally obtained have power law distortions rendering them parabolic. That is, theyl have a deviation in their slopes with respect to their slopes near the origin, except for the one condition that the linearly responsive -system has a slope of unity. By transposing these curves to logarithmic scales, they all become linear, only the curve ,representing a linear relationship still having a slope of unity. 'I'he slopes of these latter curves, therefore, provide convenient referencesto the corresponding parabolic characteristic curves on linear scales and the slopes of such curves are referred to to describe the gamma of the system or device.

Whose characteristic they represent.

A television video-frequency signal-translating l system thus has a gamma4v greater or less than unitywhen its characteristic is such that it effects an expansion or contraction,respectively, of one portion of the amplitude range of the signal translated thereby relative to the portion of such amplituderange near its origin, and its gamma is unity only when its transmission characteristicI is linear over the entire signal amplitude range.

It isA highly desirable, therefore, that gamma control means be provided for television signaltranslating systems in order to compensate for various inherent non-linear response characteris-l tics of .such systems and the physiological phenomena referred to above.

Moreover, since the illumination of different successively reproduced images `may be substantially diierent so that the physiological effects requiring compensation vary accordingly, it isv .desirable that means be provided for automatically controlling the gamma of such a, system in accordance with the character of `the illumination of the image being transmittedi It is an object of the present invention, therefore, to provide an improved automatic. gamma control means 4for television signal-translating systems; that is, means-for effecting a predetermined distortion of a television video-frequency signal whereby improved contrast effects over a selected portion of the signal-amplitude range may be procured which distortion is automatically controlled in 'accordance with the illumination characteristics of the transmitted image.

In accordance with the present invention, a television signal-translating system comprises 'means for translating a television signal includf ing an input element and means for -`applying to' the input element the video-frequencyand background-illuminationl components of`the signal with predetermined amplitude values 'thereof corresponding substantially to` the black and white shade values of .the transmitted picture. 'I 'he system includes means for adjusting the gamma of the signal-translating means and means responsive to a predetermined illumination characteristic of the signal for controlling the adjusting means in accordance therewith.

For a better understanding of the invention, together with other a'nd further objects 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.

In the drawing, Fig. l is a schematic diagram of a complete television signal-receiving system including a video-frequency amplifier embodying the present invention, while Figs. 2 yand 3 are graphs illustrating certain operating characteristics of the system of Fig. l, to aid in the understanding of the invention.

' Referring now more particularly to Fig. 1, the system illustrated comprises a television receiver of the superheterodyne type including an antenna system I I, II connected to a radio-frequency ampliner I 2 to which there are connected in cascade, in the order named, an oscillatormodulator I3, an intermediate-frequency ampliner I4, a detector and A. V. C. supply Il, a videofrequency ampliner, I5, a second video-frequency amplifier I1 embodying the present invention, and an image-reproducing device Il. .A line-frequency scanning wave generator I3 and a neldfrequency scanning wave generator are also coupled to the output circuit oi the detector I5 and to the scanning elements of the signal-reproducing device. 'I'he stages I2-2II, inclusive, excepting the video-frequency ampliiler I1, may all be of conventional well-"known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly to the general operation of Vthe receiving system Just described, e television signal intcrceptedby the antenna I0, II is selected and amplified in the radio-frequency amplifier I 2 and supplied to the oscillator-modulator I3, wherein it is converted into an intermediatefrequency signal which, in'turn, is selectively ampliiled in the intermediate-frequency ampliner Il and delivered to the detector |5. The modulation components of the signal are-derived by the detector I5 and supplied to the video-frequency ampliier I6 wherein they are ampliiled and supplied to the further video-frequency ampliner I1 which translates the signal in accordance with the apparatus of the present invention as will be presently further described, and from which they areY applied to a scanning-beam control element of the image-reproducing device I8. The intensity of the scanning beam of the device Il is thus' modulated or controlled in accordance with the video-frequency voltage impressed upon its control elemcnt in the usual manner. The modulation signal is also applied to the generators I9 and 2l and the synchronizing components of the signal are utilized therein to synchronize the operations of these generators with the corresponding apparatus at the transmitter. Saw-tooth current or voltage scanning waves are generated by the generators I9 and 20 and these waves are applied to the scanning elements of the device I l to produce scanning fields, thereby to deiiect the scanning beam in two directions normal to each other so as to trace successive series of parallel lines or nelds on the target 0f the imageent invention, for the purpose o1' controlling the' contrast of the nne detailed structure of certain portions of the amplitude range of the illumina- -tion of the reproduced image relative to other portions thereof, the video-frequency amplifier I.1 is designed in accordance with the present inventio'n and comprises signal-translating means, for example, a multigrid vacuum-tube ampliiler 2|, preferably of the hexcde type as shown. A

-signal-input circuit is provided for the amplifier I1 comprising a voltage-divider resistor 22 included in the output circuit of the video-frequency ampliner I3'. The iirst grid or signalinput electrode ofthe tube 2l is coupled to the input circuit by means of a tap on the resistor 22, a vacuum-tube amplifier 23, and a diode reinserter 2l. A suitable coupling condenser 25 andleak resistor 25 are included in the input y circuit of the tube 23 and a load resistor 23a is included in its output circuit. A coupling condenser 21, leak resistor 28, and bias battery 29 are also included in the signal-input circuit of the tube 2l as shown, the leak resistor 28 serving as the load resistor of diode reinserter 24. The tube 2| thus comprises an input element or first grid and means for applying to the input element the video-frequency and background-illumination components with predetermined amplitude values thereof corresponding substantially to the black and white shade values of the transmitted picture.

The third grid or control electrode of the tube 2| is similarly coupled to the input circuit by means of an adjustable tap on -the voltagedivider resistor 22, vacuum-tube amplifier 30, ,and diode reinserter 3| Aso that tube 2| is effectively a self-modulating signal-translating device. A coupling condenser 32 and leak resistor 33 are included in the input circuit of the tube 30 while a load resistor 30a is included in its output circuit. A coupling condenser 34, leak resistor 35 and bias battery 36 are also provided in the input c'ircuit of the third grid of the tube 2|, resistor 35 constituting the load resistor of reinserter diode 3|. A load resistor 31 is included in the anode circuit of the tube 2| to which the input circuit of the image-reproducing device I8 is coupled, for example, the upper end of the resistor 31 may be directly connected to the brilliancy-control electrode of a cathode-ray tube where such a tube constitutes the image-reproducing device. Operating potentials are applied .to the electrodes of the tubes 2|, 23, and 30 from and having a parallel load resistor 4I. 'I'he negaplied to-*the thirdl albanesi .tive terminal of the resistor Il is connected, by way. of a lter including series resistors `42 and .shunt condensers 43 and the leak resistor 33, to

.the control grid of the amplifier tube 30 as shown.

In the-operation of the system, the video-frel quency signal is supplied to the'ampliier I1 froxn the output circuit of the-amplifier I6 and appears across the voltage-divider resistor 22 with such polarity that positively increasing signal v voltage corresponds to increased illumination in the image represented thereby. The taps on resistor 22 are so adjusted that predetermined portions of the signal-input -voltage are applied to .the input circuits of the tubes 23 and 30, preferably in-a `ratio of approximately 1:3, The tubes 30`and 23 serve to amplify, and reverse the polarities of, the applied signal voltages, which are then applied by .way of the reinserter diodes 24v The diodes 24 and 3| function to derive from the signal input to the control electrodes` negative unidirectional-bias voltages equal to thepeak value ofJ the signal on the black side of the zero axis, which voltages appear across the resistorsl 28 and 35, respectively, andare thus applied to the i'lrst and third grids of the tube 2| to supplement their-initial xed biases. The resultant signals applied to the control grids, therefore, include the xed-bias voltages and the unidirectional background-illumination component as y well as thevideoor high-frequencypicture-signal components and they are so applied to their respective grids that a predetermined characteristic level thereof, preferably the level corresponding approximately to black, corresponds to zero signal voltage on each of'the control electrodes. Thestage thus comprises means for developingin the output 4thereof video-frequency and illumination components of the translated signal with' predetermined amplitude values thereof corresponding substantially to the black and white shade values of the transmitted picture.

The rectier 38 operates, as explained above, to develop a unidirectional-bias voltage responsive to a predetermined illumination characteristic of the signal, specifically, to develop a bias voltage proportional to the average value of the video-frequency signal, and this Avoltage is applied negatively to the control electrode of the tube 30. The amplication of the signal translated by thetube 30 is varied to control the gamma-adjusting means of stage l1 automatically in accordance therewith, thus to control the average amplitude of the signal applied to the third grid of the tube 2| in accordance with the average background-illumination component of the signal translated by the tube 2|.

The results obtained by the system of Fig.. 1 may. best be explained with referencev to the curves of Figs. 2 and 3. In Fig'. 2, the abscissae represent signal voltages applied to the rst or `signal-input grid of tube 2| and the ordinates represent anode current of the tube 2|, The curves g3 represent the mutual conductance characteristics of the tube for different voltages aptrlling grid, the values given to g3 being in terms of anyV arbitrary unit rather than voltage values.

These curves vintercept the zero axis approxior mutual conductance con- ,is represented at-V. 'I'he third-gridvoltage varies instantaneously in accordance with the signal Y yapplied ythe/reto, thereby instantaneously modi-y fying the'signal-transfer ratio, that is, the ratio of the output current to the input voltage, of the tube, In other* words, the signal applied to the third grid of tube is effective instantaneously to vary the mutual conductance of the rst grid with respect to the anode of tube 2| inl accordance with the instantaneous values of the signal.

vthereby to eect a predetermined distortion of the wave form of the translated signal. The eiective or dynamic mutual conductance characterlstic of the tube for a'video-frequency input signal of one predetermined average amplitude is as illustrated by curve A i. l

Since the amplification of the tube 30, and hence the ,average amplitude of the signal applied tothe third grid of tube 2|, is controlled in accordance with the bias voltage supplied from the rectiiier 38, the eiective mutual conductance characteristic of the tube 2| automatically is varied in accordance with this average illumination characteristic of the signal. vThus, the

curves An, .A3, and A4, which is the same as 9s, illustrate the-effective mutual conductance characteristics of tube 2| for signal inputs to the third grid of progressively decreasing average amplitudes. -As explained above, this decrease is effected by successively increasing negatively bias voltage supplied by the rectier 38 resulting from receivedV signals of ,progressively decreasing average amplitudes, that is, representing images of decreasing average illumination. the variation in the bias voltage, and hence the transition from one of the curves A1-A4 to another, is, of course, continuous rather than in discrete steps, as illustrated.

It will be seen from curves AiAi, inclusive, that predetermined distortions of the wave forni of the translated signal are effected by thetube 2|. In other words, when the system has a characteristic such as is illustrated by curve A1,' since the slope of this curve is greater over the upper portion thereof than in the region of itsorigin or over the lower portion thereof and since the upper portion of the curve corresponds to the Iportions of the amplitude range of the applied signal representing the darker areas of the image, the darker portions of the reproduced image -are expanded. Hence, the contrast between the .relatively dark incremental areas of the repro- .duced image is increased relative to the contrast 'contrast between the darker elemental areas thereof. successively lesser distortions are effect'ed for received signals of successively lesser ,average amplitude values as shown by the curves Aa, A3, and A4, the latter being substantially linear, which is the desired relationship for optimum reproduction of the relatively lighter image.

The curves Bi-B4 of Fig. 3 represent curves A1-A4 of Fig. 2 plotted on logarithmic scales so that the parabolic curves of Fig. 2 are changed to the linear curves of Fig. 3, which latter indicate more clearly the gamma of the system which is measured by rthe slopes of the curves B1-B4 which, in the instances shown, are 2,-1.6, 1.3, and

Ordinarily,

1.0,I respectively. The gamma of the system is thus varied over the range of. 1-2 in accordance with the average value of the signal translated by the system. In other words, the contrast between the darker incremental areas of the re- 5 nal with predetermined amplitude values .thereproduced image is progressively increased relative to the contraest between the lighter areas of the image for progressively darker images. There is thus provided an arrangement whereby the gamma of the system is automatically controlled in accordance with the illumination characteristics of the images which the signals translated by the system represent, to the end that an apparent optimum delity of reproduction of the images being transmitted and, received is procured. d

With the input signal to the amplier Il stabilized on the peaks of the synchronizing pulses as shown, these pulses will share in the gamma changes or distortions edected by the system of the invention. In some systems, therefore, it may be preferable to stabilize the signal at the black level and remove the synchronizing pulsesl before they are applied to the amplifier l1.

While there have been described what are at present considered to be the preferred embodiments of this 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 it is, therefore, aimed in the appended claims to cover all such changes and modiilcations as fall within the true spirit and scope of the invention.

What is claimed is:

l. A television signal-translating system comprising means for translating a television signalY including an input element and means for applying to said le ent the video-frequency and ba-ckground-illurnitrrila` tion components of said signal with predetermined amplitude values thereof corresponding substantially to the black and white shade values of the transmitted pictures, means for adjusting the gamma of said translating means, and means responsive to a predetermined illumination characteristic of said signal for controlling said adjusting means in accordance therewith.

2. A television signal-translating system comprising means for translating a video-frequency signal including an input element and means for applying to said element the video-frequency and background-illumination components of said signal with predetermined amplitude values thereof corresponding substantially to the black and white shade values of the transmitted pictures, means for adjusting the gamma of said translating means, means for`developing a controlling effect proportional to a predetermined illumina tion characteristic of said signal, and means for 4utilizing said. controlling effect to. control said adjusting means.

3. A television signal-translating system comprising means for translating a television signal including an input element andV means for applying to said element the video-frequency and background-illumination components of said signal with predetermined amplitude values thereof corresponding substantially to the black and white shade values of the transmitted' pictures, means for adjusting the gamma of said translating means, and means responsive to the average value 4of the illumination components of of corresponding substantially to the black and white shade values oi the transmitted pictures, means for adjusting the gamma of said translating means, rectifying means for developing a .control-bias voltage prUPOrtlonal to the average value of the illumination components of said sigl nal, and means for utilizing said bias voltage to Cal Cil

said signal for controlling saidadjusting means in accordance therewith. f 4. A television, signal-translating system comcontrol said adiusting means.' l

5. A television signal-translating system comprising means for translating a video-frequency signs! with s predetermined distortion of its wave form including an input element and means for applying to said element the video-frequency and background-illumination components of said signal with predetermined amplitude values thereof 4corresponding substantially to the black and white shade values of the transmitted pictures, means for adjusting said translating means to vary said predeterminedA distortion, yand means responsive to a predetermined illumination characteristic of 'said signal for controlling said adjusting means in accordance therewith.

`6. A television vsignal-translating system comprising signal-translating means including a selfmodulating signal-translating device, means for applying to said device a television signal including video-frequency and unidirectional background-llumination components with predetermined input amplitude values corresponding substantially to black and white shade values of the transmitted pictures, thereby to effect a predetermined distortion of the wave form of the translated. signal in accordance with the instantaneous 'values thereof, means for adjusting said translating means to vary said predetermined distortion, and means responsive to a predetermined illumination characteristic of said signal for controlling said adjusting means in accordance therewth.

7. A television signal-translating system comprising signal-translating means, means for applying to said translating meansfor translation thereby a television signal including video-fre- 'quency and unidirectional background-illumination components with predetermined input amplitude values corresponding substantially to black and white shade values of the transmitted pictures, means for separately applying said signal to said translating means instantaneously to modify the signal-transfer ratio thereof in accordance with the instantaneous values of said signal, thereby to eiect a predetermined distortion-of the wave form of the translated signal, and means for controlling the amplitude-of said separately applied signal in accordance with a predetermined illumination characteristic of said signal, to adjust said distortion in accordance with said characteristic.

8. A television signal-translating system comprising a vacuum tube having a cathode, an output electrode. and a plurality of signal-input electIOdeS, meanslfor applying to a first of said input electrodes a television signal including videofrequency and unidirectional background-illumination components for translation by said tube, means for applying said signal to a second of said input electrodes instantaneously to vary the mutual conductance of said rst of said input electrodes with respect to said output electrode in accordance with the instantaneous values of tures, and lsimultaneously'adjusting the gamma said signal, thereby to effect a predetermined disy tortion of the wave form oi the translated signal, and means for controlling the amplitude of said i' signal applied to said second of said electrodes in accordance with a predeterminedl illumination characteristic of said signal, automatically to adjust said distortion in accordance with saidcharf acteristic.

9. A television signal-translating 'system comprising a vacuum tube having va cathode, an output electrode, and a plurality' oi control electrodes, means for applying to a rst one of said control electrodes a television signal including video-frequency and unidirectional background- .illumination components for translation by said tube, means including signal-repeating means for applying said signal to another of said control' electrodes instantaneously to vary the mutual conductance of said one of saidc'ontrol electrodes with respect to said output electrode in accordance with said signal, and means for controlling the repeating ratio of said signal-repeating means in accordance with the average predetermined' `illumination characteristic of said signal, auto-l matically to effect an adjustment of said predetermined distortion in accordance with said characteristic. I

10. The method of operating a television signal-translating system which comprises translatingthrough said system an input television signal with predetermined amplitude values thereof corresponding substantially to the black and white shade lvalues oi the transmitted picflating through said system an input television signal with predetermined amplitude values thereof corresponding substantially to the black and white shade values of the transmitted pictures, developing a controlling eiect proportional to a predetermined illumination characteristic of said signal, and utilizing said controlling effect to adjust the gamma of said system in accordance with said characteristic. i v

12'. The method oi operating a television signal-translating system which comprises translating through said system an input television signal including videofrequency and unidirectional background-illumination components with predetermined amplitude values thereof corresponding substantially to the black and white shade values of the transmitted pictures, sepa-A rately utilizing said signal instantaneouslygto modify the signal-transfer ratio of said system in accordance with said signal, thereby to eiiect a predetermined distortion of the wave form of the J OHN C. WILSON.

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