US2253312A - Television signal-reproducing system - Google Patents

Description

Aug. 19, 1941. J. c. wlLsoN TELEVISION SIGNAL-REPRODUJING SYSTEM Filed March 2. 1939 vvvvvv Patented ug. 19, 1941 OFFICE TELEVISION SIGNAL-REPRODUCING SYSTEM John O. Wilson, Bayside, N. Y., assigner to Hazeltine Corporation, a corporation of Delaware Application Merch z, 1939, serial No. 259,380

4 claims. (ci. 17a- 7.5)

This invention relates to television signalreproducing systems and especially to such systems by means of which images may be faithfully reproduced irrespective of certain operating conditions tending to impair the fidelity of reproduction.

In accordance with present television practice, atransmitted signal comprises a carrier wave, modulated during successive intervals or trace periods by Vvideo-frequency and unidirectional components representative of variations in illumination of successive incremental areas of an image being transmitted andof its average background. illumination, respectively. Between the trace periods, that is, during retrace intervals, the carrier wave has a predetermined amplitude level referred to as a blanking level and corresponding tol al given shade, usually black, and is modulated during a part of this retrace period by synchronizingsignal components which correspond toinitiations of successive lines and fields in the scanning of the image.

At the receiver, a beamis so deflected as to scan and illuminate a target or screen in a series of fields of parallel lines. The synchronizingsignal components of the received signal are separated from the other modulation-signal components and utilized to control the scanning apparatus of the receiver so as to synchronize its operation with that of similar apparatus utilized at the transmitter in developing the signal. The intensity of the beam. is controlled. by the light-modulation. components, thereby to reconstruct the image. l

Since the eye of an observer is responsive primarily to contrasts or ratios between illumination values rather than absolute value of any given shade of illumination of an object, such as black, it is important that the extreme values of illumination in the reproduced image corresponding to signal components representing white and black in the transmitted image have a constant ratio under al1 operating conditions. That'is, the contrast ratio of the reproduced image should be maintained constant for a given amplitude of a signal to be reproduced irrespective of variations in the signal level corresponding to such given shade value.

To this end it is attempted in conventional television receivingsystems -so to control the system that the modulation signal is stabilized with respect to a particular shade value or level, such as the blanking level. More particularily, it is attempted so to control the system that a given signal level or amplitude corresponding to black, or any predetermined shade value, is effectively held fixed with respect to the signalinput brightness characteristic of the reproducing device of the system, in order that signal components representing-any other given shade will at all times appear as the same shade in the reproduced image. However, due to various operating conditions, for example, variations in the average signal amplitude which are not compensated for, or to failure of the stabilizing circuits completely to fulfill their function, the signal level corresponding to black as applied to the reproducing device frequently is not maintained at a fixed value. Under these conditions, therefore, and in view of the fact that conventional television reproducing systems have either linear or logarithmic signal-input brightness characteristics, the contrast ratio tends to'vary, thereby impairing the fidelity of reproduction. In general, the system should be controlled so that the light gradations or shades from black to white, represented by the various light-modulation components, are all faithfullyrepresented in the reconstructed image, a given light gradation being represented by a constant contrast ratio under all operating conditions.

It is an object of the present invention, therefore, to provide an improved television signal' reproducing system whereby the contrast ratio for a given signal-input amplitude is maintained constant under all operating conditions.

In accordance with a feature of the present invention, there is provided a television signalreproducing system which comprises signalvaries in accordance with operating conditionsA in ther system, which variations tend to effect undesired variations of the contrast ratio of a reproduced signal of a given amplitude. There are further provided means included in .said

reproducing means forimparting to the repro- Y ducing means an exponential signal-input brightness characteristic over the operating range of said sytem eective to provide a substantially constant contrast ratio for a reproduced signal of a given amplitude irrespective of the black-level variations.

Also in accordance with a feature of vthe invention, there is provided, in a television signalreproducing system, a cathode-ray television signal-reproducing tube including an electron gun for developing, accelerating and focusing an elec tron beam, a fluorescent target in the path of pitch providing an exponential signal-inputbrightness characteristic for the tube.

For a better understanding of the present invention, together with other and 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 accompanying drawing, Fig. l is a circuit diagram, partially schematic, oi a television receiver including a signal-reproducing system embodying the present invention, while Figs. 2 and 3 are curves illustrating certain characteristics of the receiver of Fig. l.

Referring now more particularly to Fig. l of the drawing, the system illustrated comprises a receiver of the superheterodyne type including an antenna system Ill, II connected to a radiofrequency amplifier I2, to which there are connected in cascade, in the order named. an oscillator-modulator I3, an intermediate-frequency amplifier Il, a detector I5, 'a video-frequency amplifier I6, and a signal-reproducing system I1, which, in this embodiment, comprises a cathode-ray signal-reproducing tube I8. The cathode-ray tube I8 comprises the usual envelope provided with an elongated neck containing an electron gun, constructed in accordance with the present invention as hereinafter more fully described, including a cathode IB having an external heater 20, an internal signal-input or con trol grid 2|, a screen 22, a first anode 23, and a second anode 24. 'I'he tube also includes the usual fluorescent screen 25 on theenlarged end of the tube opposite the electron gun. Linefrequency and field-frequency scanning coils 28 and 21 are disposed about the neck ofthe tube for deilecting the beam in two directions normal to each other. The output circuit of the videofrequency amplier I6 is connected to the control grid 2I by way of a leak resistor 29. Operating potentials for the several electrodes of the device I1 are supplied from a suitable source, as represented by the voltage-divider resistor 30.

A line-frequency generator SI and a'iieldfrequency generator 32 are coupled to the output circuit of the detector I for synchronization and are connected to the scanning coils 28 and 2l, respectively, of the cathode-ray tube in the conventional manner. Suitable means are included in the generators 3i and 32 for separating the synchronizing'pulses from the other modulation components and from each other. The stages or units Just described, excepting the cathode-ray tube, may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briey to the general operation of the system described above as a whole, television signals intercepted by the antenna circuit I0, Il are selected and amplied in the radio-frequency amplier I2 and supplied to the oscillator-modulator I3 wherein theyare converted to intermediate-frequency signals which, in turn, are selectively amplified in the intermediatefrequency amplier I4 and delivered to the detector I5. The modulation-signal components of the carrier wave are derived by the detector I5 and are supplied to the video-frequency amplier I6 wherein they are amplied and from which they are supplied to the control grid ill ci the cathode-ray tube Il.

With proper operating potentials supplied from the source d@ to the electrodes of the tube i8, an electron beam is emitted from the electron gun, its intensity being controlled by the grid 2i in accordance with the video-frequency voltages impressed thereon and in a manner hereinafter explained in detail, which is in accordance with the present invention.

Saw-tooth current waves are generated in the line-frequency and field-frequency generators 3l and 32, which are controlled by the detected synchronizing pulses, and these waves are applied to the scanning elements 26, 2 of the cathode-ray tube I8 to produce electric scanning elds, thereby to deflect the ray in twoidirections normal to each other so as to trace a rectilinear scanning pattern on the screen of the ltube and thereby to reconstruct the transmitted picture.

Referring now more particularly to the portion of the system of Fig. l in which the present invention is embodied, it will be appreciated that heretofore it was attempted to provide signalreproducing systems or tubes with input-signal brightness characteristics which were substantially linear or which closely approached linearity. This characteristic of most conventional cathode-ray tubes actually has been a three-halves power curve.` Furthermore, as has been stated, in the operation of television signalreproducing systems, it frequently occurs that the level of the signal representing' black, as applied to the reproducing means, is not maintained at a xed value even though some other amplitude level related to black, such as the level representing the peaks of the synchronizing components, does remain substantially fixed. In such arrangements, therefore, black-level variations have effected undesired variations of the contrast ratio of the reproduced signal. These conditions are illustrated by the curves of Fig. 2 wherein curve A represents the signal-input brightness characteristic of a reproducing system of the type employed in the prior art. The wave forms of an applied input signal are shown at V1 and V2 which represent the same signal under diierent operating conditions, V2 being identical with V1 except that it is stabilized at the level Stn, while the signal V1 is stabilized at the level Sti. in the wave forms V1 and Va, the portions S represent the synchronizing pulses which occur during the retrace periods of the 'wave while the portions V represent the video-frequency components, the levels indicated at b1 and b2 correspending to black and the levels indicated at w1 and ws corresponding to full white in the image being transmitted. Brightness values in the reproduced images corresponding to the levels b1, bz, wi, and wz are indicated at bin, h2o, w10, and w20, respectively. It will be seen that for the signal V1 stabilized at Str, the contrast ratio 101/1210 is of the order of 1p to l. However, for the signal V2 stabilized at St'z, the contrast ratio wao/b2o is substantially reduced, being of the order of 5 to 1. 'I'here is, therefore, a very appreciable distortion of the image reproduced from wave V: as it appears Ato an observer whose eyes are chieiiy responsive to the ratio of the limiting brightness values, as distinguished from absolute values, as explained above.

To compensate for these undesired variations in the contrast ratio, therefore, the signal-reproducing device I8 is designed in accordance with the present invention. To this end, the cathode able thermionic emssive materiaL ness levels.

I9 of the electron gun of the tube I8, in the preferred embodiment illustrated in Fig. 1, comprises a sleeve of very small' diameter centrally located in the end of the neck \of the tube envelope. The cathode is preferably constructed/of nickel and its inner surface is coated with suit- The lamentary cathode heater 2u preferably comprises a helix of tungsten wire disposed externally around the cathode. For the purpose of controlling the intensity o'f the electron beam developed by. the gun, the control electrode 2l is disposed within the cathode sleeve a small distance from its inner surface. This electrode comprises a helical lgrid havinga nonuniform pitch, preferably a pitch which progressively increases thro'ughout its length, or at least a portion thereof, as shown in the drawing.

While there has been described what is at present considered to be the preferred embodiment 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 modifications as fall within the true spirit and scope of the invention.

What is claimed is: j

1. A television signal-reproducing system comprising, signal-reproducing means, means for developing and applying to said reproducing means The electron gun structure constructed as described above imparts. to the tube I8 an exponential signal-input brightness characteristic such as illustrated by curve B of Figsl 3 )and represented by the equation: Y

v =ee where =the 'illumination of the tube Atarget and e=the signal-input voltage.

That i's, the electron gun includes means for imparting to the signalfreproducing tubean exponential signal-input brightness characteristic overthe operating range of the system eiiective to provide a substantially constant contrast ratio for a reproduced signal of given amplitude irrespective of black-level variations thereof. This lis4 clearly illustrated in Fig, 3 ,where'-the. wave forms Viiand- V2. arev thesame as those shown in Fig. 2 and similar'designations are employed for the varioussignal components and bright- It is seen that, due vto the exponential signal-input brightness characteristic of curve B, the contrast ratiowzo/bzofor the signal V2 is substantially thesame as the contrast ratio 'uno/lno for the signal V1,irrespective.of the fact that the black level b2 of the signal Vzis considerably greater than. the black level b1 of the signal Vi.

' It will be appreciated that, while a particular means has been shownas the preferred embodiment of the invention for imparting the desired signal-input brightness characteristic to the signal-reproducing system for 'compensating for the black-level signal variations, various other suitable means may be provided in accordance with the present invention for accomplishing this purpose. For example, instead of utilizing the novel type of cathode-ray signal-reproducing tube described above, the desired characteristic may be imparted to the signal-reproducing means by utilizing a conventional type of cathode-ray tube having` an approximately linear characteristic and including in the reproducing means an ampliner tube having the required exponential characteristic and having a direct-current coupling to the cathode-ray tube.

a signal having an amplitude level related to black which is substantially fixed and having an amplitude level corresponding to black which varies in accordance with operating conditions in said system, such variations tending to Ieffect undesired variations of the contrast ratio of a reproduced signal of given amplitude, and means included in said reproducing means for imparting to said reproducing system an exponential signal-input brightness characteristic over the operating range of said system effective to provide a substantially constant contrast ratio for said reproduced signal irrespective of said blackf level variations. y

2. A television signal-reproducing system comprising an input circuit adapted to have a signal applied thereto having an amplitude' level re' lated to black which is substantially fixed and having an amplitude level corresponding to black which varies in accordance with operating conditions in said system and a cathode-ray signalreproducing tube coupledto saidinput circuit,

said black-level variations tending to effect undesired variations of the contrast ratio of a signal as reproduced by said tube, said tube cornprising an electron gun having a control electrode to impart to said tube an exponential signal-input brightnesscharacteristic over `the operating range of said system effective to provide a substantially constant contrast ratio for said reproduced signal irrespective of said black-level variations.

3. In a television signal-reproducing system, a

cathode-ray television signal-reproducing tube including an electron gun for developing, accelerating, and focusing .an4 electron beam, a flu()- rescent target in-the path of said beam, andal i helical signal-input electrode included in said gun and having nonuniform pitch providing anl exponential signal-input brightness characteristic for said tube.

4. In a television signal-reproducing system, a cathode-ray television signal-reproducing tube including an electron gun for developing, accelerating. and focusing an electron beam, av fluorescent target in the path of said' beam, and a helical signal-input electrode included in said gun having a progressively increasing pitch throughout at least a portion of its extent providing an exponential signal-input brightness characteristic for said tube.

JOHN C. WILSON.

Images