US2242952A - Video-frequency signal-generating apparatus - Google Patents

Description

VIDEO-FREQUENCY SIGNAL-GENERATING APPARATUS Filed Dec. 29, 1938 TRANSMITTMG nEwcE GENERATOR 24 m ELD 0 FREQUENCY GENERATOR R OLF C. HERGENROTHER ATTORNEY INVENTO RU Patented May 20, 1941 UNITED: @sT-ATE VIDEO-FREQUENCY SIGNAL-GENERATING" J v APPARATU Rudolf .C. Hergenrother, Beechhurst, N. Y., as-- signor to Hazeltine Corporation, a corporation of Delaware Application December 29, 1938, Serial No. 248,245

10 Claims.

This invention relates to video-frequency signal-generating systems for television signal transmitters and, more particularly, to provisions for increasing the sensitivity and controlling the shading effect of cathode-ray signal-generating tubes utilized in such systems.

Video-frequency signal-generating systems commonly utilize a cathode-ray signal-generating tube including an electron gun in the tube for developing, focusing, and accelerating an electron beam on a photosensitive target electrode disposed in the path of the beam, means for scanning the target with the beam during successive scanning intervals, and a collector electrode disposed near the target for collecting secondary electrons emitted thereby. The amplitude of the output signal of such a tube is dependent upon the magnitude of the voltage existing between the collector electrode of the tube and each incremental element of the target electrode at the instant before the element is scanned. Thus, the potential of each element of the target electrode is negative just prior to being scanned and has its maximum positive value at the instant just after it is scanned. After scanning, the element relatively gradually assumes a negative potential which is a function both of the number of emitted secondary electrons which rain back upon the particular element and the illumination of the particular element. Therefore, an increase in the negative charge of the particular element during the interval when it is not being scanned increases the amplitude of the signal provided by the element upon scanning. It has, therefore, been proposed to subject the entire target to a flood of low-velocity electrons from an auxiliary source Within the tube to increase this negative charge.

One such proposal is to utilize an auxiliary electron gun within the tube which is effective to fiood the target with low-velocity electrons only during the retrace interval of the scanning cycle. While this may have the effect of slightly increasing the sensitivity of the tube, the target is subjected to the flood of electrons only during a minor fraction of the total interval between successive scannings of the particular element under consideration. Also the interruption of the flood of electrons to the target electrode at field-scanning frequency tends to introduce transient effects in the signal output of the tube.

It has also been proposed to make the collector electrode of a cathode-ray tube of the type under discussion photosensitive and to subject the collector electrode to illumination in order to provide a supply of low-velocity electrons which, during a portion of ,the' scanning cycle, may find their way to the target electrode and thus increase the sensitivity of the tube. However, the rain of secondary electrons back upon each element of the target electrode from the remaining elements is not, in general, uniform and, therefore, the several elements of the target electrode do not assume potentials accurately rep resentative of their respective degrees of illumination. This produces theeffect known as shading in the output signal of the tube. In arrangements utilizing the above-mentioned photosensitive collector electrode, there is no control of the distribution of electrons emitted from the collector electrode over the target electrode. This, in itself, produces additional undesirable shading effects which are not, in general, compensatory to those produced by secondary electrons emitted from the target electrode itself.

It is an object of the invention, therefore, to provide a video-frequency signal-generating system of the character described which is not subject to the above-mentioned disadvantages.

It is still another object of the invention to provide a video-frequency signal-generating system having an arrangement for compensating, as desired, the shading effect due to emitted secondary electrons from the target electrode of the cathode-ray signal-generating tube utilized.

In accordance with the invention, a video-frequency signal-generating system comprises a v cathode-ray signal-generating tube including an electron gun for developing, focusing, and accelerating an electron beam on a photo-sensitive target electrode disposed in the path of the beam and means for scanning the target with the beam during successive scanning intervals, together with a collector electrode adjacent the target adapted to be biased to a given potential. An auxiliary source of electrons is providedin the tube together with means for flooding the target with electrons from the auxiliary source during the scanning intervals. Optically-responsive means are provided for imparting to the flooding electrons any desired predetermined distribution to bias negatively successive elements of the target with respect to the collector electrode at the instant before each particular element is scanned. It will be understood that the bias referred to increases gradually and is maximum at the instant just before each particular element is scanned. In this manner, the sensitivity of the tube is increased and the effect of the flooding means upon the shading characteristic of the tube may be predetermined as desired.

In accordance with the preferrred embodiment of the invention, the auxiliary soin-ce of electrons comprises an auxiliary photo-sensitive cathode together with means for flooding the target electrode of the signabgeneratlng tube with an electron image of said photosensitive cathode.

For a better understanding of the invention, together with other and further objects thereof, reference is had to thefollowing description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. V

The single figure of the drawing is a circuit diagram, partly schematic, of a television transmitting system including a video-frequency sigpal-generating system} embodying the present invention.

Referring now more particularly to the drawing, the television transmitting system illustrated includes a conventional camera or projector I I and a cathode-ray signal-generating tube II. This tube comprises an envelope, including two cylindrical end portions or n'ecks I 2 and I3, and an enlarged cylindrical body portion l4. In the end portion I2 there is disposed a conventional main electron gun comprising a cathode [5, a control grid li, a focusing first anode l1, and a second anode or collector electrode 48, arranged in the order named. I The collector electrode 48 preferably comprises a,metallic film or coating on the inner surface of the tube, as shown. A photosensitive target I! is disposed in the body portion l4 of the envelope and may be constructed in the conventional manner including a thin sheet of dielectric, such as mica, having a metallic backing coating or signal plate and a photo sensitive mosaic front surface of suitable substance such as a silver-caesium-oxygen mixture or compound, thereby providing, in efiect, a multitude of minute condensers having the signal plate as a common electrode.

Suitable operating potentials are supplied to the above-mentioned electrodes of the tube II from a source of direct current 20 having its positive terminal grounded. The output circuit of tube ll includes a load resistor 2| connected at one end to target electrode I9 and at its other end to the positive terminal of source 20. There are also provided lineand field- frequency scanning generators 23 and 24 having their output tion of the corresponding elements of the image. Electrons are generated, accelerated, and focused into a beam by the electron gun including elements ii to 11, inclusive, and 48, the beam being focused on target electrode l9 and causing secondary electron emissionby the mosaic elements. Currents of saw-tooth wave form, generated by generators 23 and 24, are passed through the scanning coils 25 and 26, respectively, thereby producing magnetic fields which deflect the beam in two directions normal to each other causing it to scan frames or series of parallel lines on the target in the usual manner, thereby developing in the output circuit of the tube, including the output electrode IS, a videofrequency voltage which is proportional to the variation in the light and shade values in the image of the scene to be transmitted. This voltage is utilized in the transmitter apparatus 21 to modulate a carrier wave which is broadcast by antenna 28 in the usual manner.

Coming now to the portion of the system involving the present invention, there is provided I in the projection l3 of the envelope ll an auxcircuits connected to deflecting coils 25 and 26,

respectively, of the tube II, and synchronized from suitable timing apparatus, not shown. It will be understood that the system also includes suitable pedestal or blockout generators, synchronizing-signal generators, and other conventional apparatus not related to this invention. The output resistor II is connected to conventional television transmitting apparatus indicated generally at 21 and terminating in an antenna 28-28.

The apparatus just described, in general, represents a conventional television transmitting system. The construction and operation of such apparatus being well known in the art, a detailed description thereof is deemed unnecessary herein. In brief, however, in the operation ,of the apparatus described, an image of the scene to be transmitted is projected by camera III on the mosaicsurface of target electrode l9 eiiecting photoelectric emission by the various mosaic elements thereof in accordance with the illuminailiary electron source for continuously flooding the photosensitive target electrode I! with electrons. The auxiliary source of electrons com prises optically-responsive means such as a photosensitive cathode 40 which may be made to produce the electrons of the auxiliary source when subjected to light rays as represented by the beam 43. In order to accelerate the electrons so produced and focus them on target electrode l9, there is provided a cylindrical electron lens including acylindrical electrode 4| connected to, and at the same potential as, photosensitive cathode 40, and cylindrical electrode 48', electrically connected to collector electrode or second anode 48 by means of a conductive band 49 on the inner surface of the envelope of tube H. Suitable potentials are provided for the auxiliary electron source from a voltage-divider resistor 33, which is coupled across battery 34 having it positive terminal grounded. Thus, the photocathode 40 is biased slightly negative with respect to the collector electrode 48' and the potential divider 33 comprises means for adjusting the potential of the cathode over a range of potentials slightly negative with respect to that of the collector electrode for adjusting the sensitivity of the tube. It is essential that the operating potentials of the auxiliary source be so adjusted that the velocity of the electrons flooding the target I9 is so low that the secondary electron emission ratio of the target is less than unity. An electron velocity of less than 10 volts is suflicient in a tube having a mosaic with a caesium-silver-oxygen coating.

- In order to explain the effect of the auxiliary I electron source upon the output signal of the system shown, attention is again called to the fact that, in a video-frequency signal-generating system utilizing a cathode-ray Si nal-generating tube of the type described, the potential of any particular element of the target is a maximum positive at the instant just after it is scanned, while the negative potential of the element at the instant before scanning determines the amplitude of the signal produced when the element is scanned. It is for the purpose of increasing this negative potential that the auxiliary electron source is provided. It will be seen that the target electrode [3 is continuously flooded by low velocity electrons from the auxiliary source which are considerably greater in numbcr than any secondary electrons emitted thereby. thus providing the necessary increase in negative potential of elements of the target l9 to increase the signal output amplitude as the target is scanned.

Inasmuch as the rain of secondary electrons emitted from target electrode 19 back upon itself is a function of the illumination of the image projected upon target electrode is by the lens system I0, undesired shading effects may be produced, which shading effects vary in accordance with the scenes being transmitted. Therefore, the distribution of illumination of the opticallyresponsive means or photocathode 40 may be varied in any manner desired to impart to the :beam of flooding electrons any desired predetermined distribution to bias negatively successive elements of the target with respect to the collector electrode at the instant before each particular element is scanned, thereby to compensate at least partially for such shading effects. Inasmuch as an electron image of the cathode 40 is projected upon target electrode IS, the sensitivity of various elements of target electrode I9 is changed in varying degrees by the auxiliary electron source, thus rendering possible a compensating effect for the normal shading in a cathode-ray signal-generating tube.

While there has been described what is at present considered 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 this 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 this invention.

What is claimed is:

1. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive target electrode in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said tube being subject to undesired shading components, an auxiliary source of electrons in said tube, means for flooding said target with electrons from said auxiliary source during said scanning intervals, and optically responsive means for imparting to said flooding electrons any desired predetermined distribution at least partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the instant before each particular element is scanned, thereby to increase the sensitivity of said tube and to predetermine the effect of said flooding means on the shading components of the developed signal.

2. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive. mosaic target electrode in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said tube being subject to undesired shading components, an auxiliary source of electrons in said tube, means for continuously flooding said target with electrons from said auxiliary source, and optically responsive means for imparting to said flooding electrons any desired predetermined distribution at least partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the instant before each particular element is scanned, thereby to increase the sensitivity of said tube and to predetermine the effect of said flooding means on the shading components of the developed signal.

3. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive mosaic target in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said tube being subject to undesirable shading components, an auxiliary source of electrons in said tube having a cathode slightly negative with respect to said collector electrode, means for flooding said target with electrons from said auxiliary source during said scanning intervals. and optically responsive means for imparting to said flooding electrons any desired predetermined distribution at least partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the instant before each particular element is scanned, thereby to increase the sensitivity of said tube and to predetermine the effect of said flooding means on the shading components of the developed signal.

4. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive mosaic target in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said tube being subject to undesired shading components, an auxiliary source of electrons in said tube comprising a cathode, means for flooding said target with electrons from said auxiliary source during said scanning intervals, and optically responsive means for imparting to said flooding electrons any desired predetermined distribution at least .partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the instant before each particular element is'scanned, thereby to increase the sensitivity of said tube and to predetermine the effect of said flooding means on the shading components of the developed signal, and means for adjusting the potential of said cathode over a range of potentials slightly negative with respect to that of said collector electrode for adjusting the sensitivity of said tube.

5. A video-frequency signal-generating apparatus comprising, a. cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive mosaic and secondary emissive target electrode in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said tube being subject to undesired the potential of said collector electrode, means for flooding said target with electrons from said auxiliary source during said scanning intervals, and optically responsive means for imparting to said flooding electrons any desired predetermined distribution at least partially complementary to said shading components and such velocity that the secondary emission of said target electrode due to said electrons is less than unity to bias negatively successive elements of said target with respect to said collector electrode at the instant each particular element is scanned, thereby to increase the sensitivity of said tube and to predetermine the efi'ect of said flooding means on the shading components of the developed signal.

6. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive mosaic and secondary emissive target electrode in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said' tube being subject to undesired-shading components, an auxiliary electron source in said tube, means for accelerating electrons directly from said auxiliary source to said target to flood said target with electrons from said auxiliary source during said scanning intervals, and optically responsive means for imparting to said flooding electronsany desired predetermined distribution at least partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the instant each particular element is scanned, thereby to increase the sensitivity of said tube and to determine the efiect of said flooding means on the shading components of the developed signal.

I 7. A video-frequency signal-generating apparatus comprising, a. cathode-ray signal-generating tube, an electron gun in said tube for developing anelectron beam, a photosensitive mosaic and secondary emissive target electrode in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted 'to be biased to a given potential, the signal developed by said tube being subject to imdesired shading components, an auxiliary electron gun in said tube, means for flooding said target with electrons from said auxiliary gun during said scanning intervals, and optically responsive means for imparting to said flooding electrons any desired predetermined distribution a at least partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the instant each particular element is a scanned, thereby to increase the sensitivity of said tube and to predetermine the effect of said flooding means on the shading components of th developed signal. i

8. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for a developing an electron beam, a photosensitive to undesired shading components, an auxiliary source of electrons in said tube, means for accelerating electrons from said auxiliary source to said target with a velocity of less than 10 volts to flood said target with electrons from said auxiliary source during said scanning intervals,

and optically responsive means for imparting to said flooding electrons any desiredipredetermined distribution at least partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the instant each particular element is scanned, thereby to increase the sensitivity of said tube andto predetermine the eifect of said flooding means on the shading components of the developed signal.

9. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive mosaic target electrode in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said tube being subject to undesired shading components, an

auxiliary source of electrons in said tube comprising a photosensitive cathode, and means for flooding said target with electrons from said auxiliary source during said scanning intervals, and means for imparting to said flooding electrons various nonuniform predetermined distribution at least partially complementary to said shading components to bias negatively successive elements of said target with respect to said collector electrode at the'instant each particular element is scanned, thereby to increase the sensitivity of said tube and to predetermine the efiect of said flooding means on the shading components of the developed signal.

10. A video-frequency signal-generating apparatus comprising, a cathode-ray signal-generating tube, an electron gun in said tube for developing an electron beam, a photosensitive mosaic target in the path of said beam, means for scanning said target with said beam during successive scanning intervals, a collector electrode in said tube adapted to be biased to a given potential, the signal developed by said tube being subject to undesired shading components, an auxiliary source of electrons comprising a photosensitive cathode in said tube, and an electron lens for producing an electron image of said photosensitive cathode on said target electrode during said scanning intervals to bias negatively successive elements of said target with respect to said collector electrode at the instant each particular element is scanned, thereby to increase the sensitivity of said tube and to predetermine the effect of said flooding means on the shading components of the developed signal.

I RUDOLF C. HERGENROTHER.

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