US2377972A - Television transmitting system - Google Patents

Description

o. H. SCHADE 2,377,972

TELEVISION TRANSMITTING SYSTEM June 12, 1945.

Filed Aug. 29, 1942 /MHGE ASE/v 4 Our/w 75 DEFLE GENE/mm? I INVENTOR 0770 I?! Sci/40E.

ATTORNEY Patented June 12, 1945 TELEVISION TRANSMITTING SYSTEM Otto H. Schade, West Caldwell, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 29, 1942, Serial No. 456,601 11 Claims. -(o1. 178-72) This invention relates to an improvement in television transmitters, and, more particularly, to an improvement in transmitters utilizing low velocity cathode ray beam pick-up or camera tubes of the "Orthicon type, particularly where a separate collector electrode is used as a signal output terminal for the pick-up tube.

In order to intensify the output signal from a television pick-up or camera tube, it is desirable, in some instances, to use a secondary electron multiplier to produce the desired degree of ampliflcation, the multiplier usually being incorporated within the tube envelope so that the net output signal from the pick-up tube may have an appreciable intensity. Secondary electron multipliers, per se, have been applied to television pick-up or camera tubes of the Orthicon type, and an example of a tube employing the use of secondary electron multipliers is shown and described in the patent to Iams, No. 2,288,402, issued June 30, 1942.

Furthermore, in all television pick-up tubes it is desirable to use some form of scanning beam return blanking in order that no stored image charges will be removed or cancelled, and in order that no image signals may be generated during the interval in which the scanning cathode ray beam is returned to an initial scanning position. Generally, adequate blanking may be accomplished by controlling the current intensity of the scanning cathode ray beam by varying the intensity of the beam between zero and some finite value. In such cases the scanning cathode ray beam is either entirely or substantially entirely blocked or suppressed during the return time interval, the intensity of the cathode ray beam being permitted to return to it normal scanning intensity at the conclusion of the blanking interval.

In Orthicon transmitter equipment where an electron multiplier is used, the multiplier is generally located so that it collects and responds to the returned or reflected cathode ray beam, the returned beam having an intensity equivalent to the originally produced beam less the electrons which are removed from the beam in cancelling or neutralizing the charge on the mosaic or target electrode. The signal current fluctuation is therefore often a small percentage of the total return beam current. When an electron multiplier is so associated with an "Orthicon pick-up tube, return line blanking by modulation of the scanning cathode ray beam is not practical or desirable, since the very large variation in the electron beam intensity at the electron multiplier produces extreme variations in signal intensity which are undesirable and which affect the operation of the system.

It is, therefore, one purpose of the present invention to provide -a system whereby return beam blanking may be accomplished in an "Orthicon pick-up or camera tube without the necessity of resorting to a per cent or heavy current modulation of the scanning cathode ray beam.

Another purpose of the present invention resides in the provision of circuit means whereby the charge on the mosaic or target electrode will not be affected in any manner during the cathode ray beam return interval, the intensity of the cathode ray beam being maintained uniform at all times.

A further purpose of the present invention resides in the provision of means used in connection with an Orthicon television pick-up or camera tube whereby the scanning cathode ray beam will not be permitted to impinge upon the mosaic or target electrode during the beam return interval irrespective of the charge condi tion on the mosaic electrode.

A further purpose of the present invention resides in the provision of means in connection with a television transmitter whereby return beam blanking may be accomplished without producing undesired variations in the signal output intensity during the blanking interval, and whereby a correct black level signal is established during the blanking interval.

A still further purpose of the present invention resides in the provision of a circuit arrangement associated with a television pick-up tube of the Orthicon type whereby the target or mosaic electrode may be subjected to'a negative potential during the beam return interval whereby the cathode ray beam will not be'permitted to impinge upon the target electrode during such interval.

Various other purposes and advantages will become more apparent to those skilled in the art from the following detailed description, particularly when considered in connection with the drawing wherein like reference numerals represent like parts and wherein:

Figure 1 shows a conventional form of the tube and a preferred circuit for practicing the present invention; and

Figure 2 shows a portion of the tube of Figure 1 taken along section lines 2! in Figure 1 looking in the direction of the arrow.

In the drawing a. television pick-up tube of the "rthicon type is shown which includes an evacuated envelope having a target, preferably of the photo-sensitive mosaic type, at one end thereof. The tube also includes an electron source and an anode slightly offset from the axis of the tube and from the geometric center of the target at the opposite end of the envelope. The target, if of the mosaic type, is provided on its front surface with an extremely large number of individual photo-sensitive particles, the target being so positioned that it may be scanned by an electron beam originating at the electron source and being also positioned so'that it may have focused thereon an optical image of the object of which a picture is to be transmitted.

The electron source consists of a more or less conventional electron gun structure, the potential between the electron source and the target or mosaic electrode being so adjusted that the electron beam is projected at relatively low velocity in the region of the target electrode, and, in fact, the beam is directed toward the target with an impact velocity approaching zero. Electrons from the scanning cathode ray beam will then be absorbed by the mosaic electrode in accordance with the number of photo-electrons emitted there from, the number of emitted electrons being naturally a function of the intensity of the optical image projected thereon. Accordingly, those areas of the target that are more intensely illuminated acquire a more positive electrostatic charge with respect to the lesser intensely illuminated particles by reason of the release of photo-electrons, and these positive charges together with their distribution represent an electrostatic charge image of the optical image to be transmitted. When the scanning cathode ray beam is caused to traverse the mosaic or target electrode, areas of the surface of the mosaic electrode that are charged positively permit contact with electrons from the scanning beam which thereby neutralize or cancel the positive charges, the remaining electrons constituting the scanning cathode ray beam being repelled or reflected by the mosaic or target electrode. The electrons which are reflected by the target electrode (i. e.. the return beam), therefore, vary in intensity in accordance with the positive charge image which exists on the target electrode, since the electrons which are not absorbed by the target electrode are reflected bythe electrode and are ultimately projected against a collector electrode or a secondary electron emitter electrode.

Intermediate the electron source or the electron gun structure and the target electrode are positioned means to focus and deflect the cathode ray beam in order that the desired scanning operation may be accomplished. Television pickup tubes of the Orthicon type normally employ electromagnetic focusing and may use full electromagnetic deflection or a combination of electromagnetic and electrostatic deflection. In the tube shown by way of example in the drawing, the line deflection of the cathode ray beam is accomplished by electrostatic means, whereas the field deflection of the cathode ray beam is accomplished by electromagnetic means. Electromagnetic deflection may be employed for both line and field deflections if desired.

In addition to the focusing and deflecting means the television pick-up tube may include an electron multiplier for increasing the intensity of the controlled electrons that are returned by the mosaic or target electrode. Furthermore, certain shielding electrodes are used, and, in addition. it is generally desirable to use electrodes for lifting" the cathode ray beam in both its forward and return directions in order to separate the two paths.

Referring now to the drawing, the Orthicon television pick-u tubeas shown including an elongated evacuated envelope I. enclosing at one end a target or mosaic electrode 2 and at the opposite end an electrode assembly 3 adapted to develop and project electrons toward the mosaic electrode along the path 4, shown by dotted line. The mosaic electrode 2, which faces the electron gun structure 3, preferably comprises a substantially transparent sheet of insulation 5, such as mica or some similar ceramic material, having on its rear surface a, translucent or semi-transparent electrically conducting film 6. 0n the front surface of the mica sheet 5 is positioned a mosaic surface comprising an extremely great number of mutually insulated and separate photo-sensitiv particles 1. Various methods of making such a semi-transparent mosaic are well known in the art and examples of the method of constructing such mosaics are to be found in the method disclosed by the Essig Patents, No. 2,020,305, granted November 12, 1935, and No. 2,065,570, granted December 29, 1936. In place of the mosaic or target electrode, obviously other types of photoconducting or photo-voltaic targets may be used.

The electron gun structure 3 that is positioned at the opposite end of the envelope I includes a conventional heater, a cathode element ID from which electrons may be drawn, a control electrode or modulating electrode II, and an accelerating anode l2. The grid or modulating electrode H and the anode electrode I! are centrally perforated in order to permit the passage of the developed cathode ray beam. Sinc electron gun structures are generally well known in the art. a further description of the gun structure is not believed necessary and, likewise, the conventional showing is deemed ample for illustrative purposes.

In order to deflect the electrons and to maintain the electron beam in a properly focused condition between the electron source and the target electrode, there is provided a focusing coil l5 surrounding substantially the entire tube envelope I. The coil preferably extends beyond the limits defined by the electron gun structure and the target electrode 2. The focusing coil is so designed and wound that it produces an electromagnetic field strength of an intensity of between 50 and '75 gausses, the direction of the field extending longitudinally through the tube. An electromagnetic field of this strength has been found sufflcient to maintain the electron beam in a, properly focused condition. The effect of the electromagnetic field is to cause the electrons which are emitted by the cathode l0 and projected through the gun structure 3 to follow helical paths or trajectors of small amplitude so that the electrons constituting the beam can be brought into sharp focus in the plane of the target or mosaic electrode 2. Actually, in tubes of the construction shown in Figure 1, the cathode ray beam is not maintained in an actual focal condition throughout the entire beam path 4, since the beam, in fact, has a number of focal points along its path by reason of the helical trajectories followed by th electrons. There are, therefore, a series of parallel focal planes normal to the beam path and the parameters of the tube or the circuits associated therewith are so adjusted as to cause one of these focal planes to coincide with the plane of the target electrode 2.

The length of a single helix or the pitch of the helix may be varied by varying the strength of the magnetic field produced by the coil '5 or by varying the velocity of the cathode ray beam. Th path of the beam under the influence of the focusing coil I! may, however, for the purpose of explanation and for simplicity, be consideredas linear in the absence of any additional deflecting force and the beam may also be considered as in focus throughout its length.

For deflecting the cathode ray beam in one direction a pair of electrostatic deflecting plates 28 and 2| are provided, (if electrostatic line deflection is desired) these plates being positioned such that the cathode ray beam is permitted to pass therebetween. The deflecting electrodes or plates and 2| are not flat or planar as is the conventional practice in cathode ray tubes but are curved or flared, which has been conventionally shown in the drawing. It has been found, as a result of considerable research and by the construction and test of a large number of tubes, that when an electrostatic deflection force is used in combination with a uniform axial magnetic field, it is desirable to have the cathode ray beam enter and leave the deflecting force with only its original axial velocity and with no components of velocity transverse to the axis other than that resulting from the initial transverse emission velocity from the cathode. Accordingly, the deflecting plates 20 and 2! are curved in order that no abrupt electrostatic deflecting field will be produced at either the entering or the emerging portion of the field. A more complete description of the construction of such deflecting electrodes as well as a description of their action on the electron beam and the interaction on the electromagnetic field is to be found in the patent to Iams, et al., No. 2,213,175, issued on August 27, 1940. Inasmuch as the present invention is not concerned with the detailed construction of deflecting electrodes, and since it applies equally well to tubes using full magnetic scanning deflection,

a further description is not believed to be necessary.

When a potential difference is impressed upon the two deflecting electrodes, the cathode ray beam is caused to be deflected in a direction parallel to their faces so that the point at which the cathode ray beam emerges from the electrotatic field is a function of the intensity of the field and its direction.

For deflecting the cathode ray beam in a mutually perpendicular direction, electromagnetic deflecting coils 22 and 23 are provided. These coils are preferably positioned 'on the outside of the tube envelope and are located at one end of the tube near the target or mosaic electrode. These coils produce an electromagnetic field extending in a direction normal to the axis of the tube envelope, and when the cathode ray beam is projected through the electromagnetic field the beam is deflected in one direction or another .in accordance with the direction of the field and its intensity. After deflection by the electromagnetic coils 22 and 23, their electrons are then permitted to project in the vicinity of the mosaic or target electrode which, as stated above,

is preferably maintained at a potential approximately equal to the potential of the cathode N.

In order to afford bi-dimensional scanning, the coils 22 and 28 are arranged such that their deflection force on the cathode ray beam is norill mal to the deflection force produced by the deflecting plates 28 and II and, as indicated in the drawing, the electrons of the beam in passing through the field produced by the coils 22 and 23 may follow a path such as indicated at 30.

Additional means are provided for affecting the path of the beam in this same area, these means being the "lifting" electrodes or plates and 25 (see Figure 2). The purpose of these plates is to lift the center of the scanning pattern in order that it will coincide with the geometric center of the target or mosaic surface 2. Since the field produced by the electrode plates 24 and 28 is in the same area as the fleld produced by the coils 22 and 28, the lifting or raising of the beam is imparted simultaneously with the deflection of the beam as produced by the coils 22 and 28. 'Preferably and most conveniently, the electrodes 2 and 2B are positioned on the inside surface of the tube envelope, as indicated in Figure 2 of the drawing, and, furthermore, intermediate electrodes 60 and I are provided, these being connected to a ground potential.

For providing an accelerating force for the developed cathode ray beam as produced by the gun structure 2, a source of potential H (such as a power supply system or a battery) is com nected between the anode i2 and the cathode I0. Naturally, the anode i2 is maintained positive with respect to the cathode. The negative end of the battery 84 is then connected to a frame 8 surrounding the mosaic surface of the target electrode and to the ring electrode II (to be later described) by conductor H and in this conduction is interposed a source of potential 15 and a potentiometer H6 in order that the average potential of the frame 8 and ring electrode Il may be controlled with respect to the potential of the cathode '8. Between ground and the conducting surface 8 is a resistance elegntent 11, the purpose of which will be described a er.

For deflecting the cathode ray beam in a horizontal direction, a horizontal deflection generator 18 is provided, the output from the generator being supplied to the deflecting plates 28 and 2|. This voltage variation is preferably of saw-tooth wave form, as indicated by the curve 19. For controlling the operation of horizontal deflection generator I8, a series of control impulses of line deflection frequency, such as indicated at 88, may be applied to the terminal 8|. Likewise, for energizing the vertical deflecting coils 22 and 23, a vertical deflection generator 82 is provided which supplies a current variation of saw-tooth wave form for magnetic deflection at field frequency. The wave form may be such as indicated by the curve 83. In order to con trol the operation of the vertical deflection generator 82, a voltage variation of field frequency, such as indicated at 85, may be applied to the input terminal 84 of the vertical deflection generator.

In the operation of the system as shown and described, a cathode ray beam of predetermined uniform intensity is developed by theelectron sun structure 3 and is projected along path 4, this path bein offset with respect to the geometric center of the'target or mosaic electrode 2. The beam is then projected between the horizontal deflection plates 28 and 2|, and when the potential of these plates is varied by avoltage variation of substantially saw-tooth wave form, the cathode ray beam will be caused to be deflected in a plane parallel to the plane of the deflecting plates and 2| by reason of the presence of the strong axial focusing fleld generated by the coil ll.

Shield electrodes are associated with the horizontal deflecting plates, these shields being 50 and SI. The shield 50 is provided with a circular aperture 52, whereas the shield BI is provided with an elongated aperture 53, the long dimension of the aperture 53 extending in the direction in which the cathode ray beam is deflected. Accordingly, the cathode ray beam is permitted to pass through the aperture 53. After the cathode ray beam has been deflected horizontally at line deflection rate, the beam is then sub jected to the influence of the vertical deflecting coils 22'and 23, and also to the influence of the lifting plates 24 and 25. The lifting plates impart a predetermined vertical deflection to the cathode ray beam in order that in its undeflected condition it will be directed against substantially the geometric center of the target electrode. Those electrons of the scanning cathode ray beam that are returned by the target electrode are also lifted and may traverse a path such as indicated at 3| in the drawing. The returned electrons are then deflected through elongated aperture 56 in the shield plate 54. Naturally, the shield plate 54 also includes another elongated aperture 55 through which the scanning beam is initially directed.

The scanning cathode ray beam is also subjected to the vertical deflecting fleld to cause the cathode ray beam to be deflected in a vertical direction over the surface of the mosaic electrode. This deflecting fleld deflects the electrons going to the target electrode and returning therefrom so that the returned electrons always pass through the elongated aperture 56.

In order to increase the intensity of the image signals, a secondary electron multiplier is provided including cold cathode emitter plates 32, 33 and 34, as well as an anode collector plate 35. These electrodes are maintained at progressively increasing positive potentials in order to accelerate the produced secondary electrons between the cathodes, and in order that the electrons may be finally collected by the anode 35. The intensified image signals appearing in the anode 35 are then applied to the control electrode of amplifier tube 43. A source of potential and a potentiometer 4| are Provided in order that appropriate potentials may be applied to the multiplier electrodes.

The impact surface of the secondary electron emitting electrodes 32, 33 and 34 are provided with a coating such that secondary electrons will be produced in a quantity many times in excess of the arriving impinging primary electrons. Naturally, it is difficult to provide a surface such that the secondary emission ratio remains absolutely constant throughout each elemental area of the surface, and in order that small variations in the secondary electron emissive ratio will not appreciably affect the image signal output as derived from amplifier tube 43, it is desirable that the electrons of the returned cathode ray beam impinge upon the cathode surface 32 in an out-of-focus condition. If the electrons are not in focus when they strike the emitting surface 32, then local variations in the secondary electron emission ratio will not aiTect the image signal intensity. In order to assure that the electrons of the cathode ray beam will be sharply in focus mar-1,072

in the plane of the target electrode 2 and the return electrons out-of-focus at the emitting surface 32, an electrode H is provided and positioned adjacent the target electrode. This ring electrode H may be in the form of a conducting coating on the inside surface of the tube. This electrode serves the additional purpose of providing a substantially uniform electrostatic fleld over the scanned area of the target. It is, therefore, possible to alter the parameters of the tube and its associated circuits such that the scanning cathode ray beam will be in proper focal posi-- tion at the mosaic surface of the target electrode, and it is also possible at the same time to have the returning cathode ray beam out-of-focus in the plane of the secondary emitter electrode 32.

With the tube and associated circuit elements as above described, it is possible to scan the mosaic or target electrode to produce image signals corresponding to the electrostatic charges on the mosaic electrode which in turn correspond to the light intensities of the optical image projected on the mosaic electrode. Although the cathode ray beam is deflected in the return or fly-back direction at a very rapid rate both horizontally and vertically, the return beam is, however, effective at least to a certain extent in cancelling part of the charges which exist on the mosaic electrode so that'the next line or the next field scanned does not produce the proper image signals, since a portion of the charge image has been removed by the immediately preceding return deflection stroke of the cathode ray beam. This loss of charge on the mosaic electrode during the return interval may become quite serious, particularly when operating at relatively high beam currents and at low illumination levels.

By means of the present invention, it is possible to produce return line blanking without the necessity of interrupting or suppressing the cathode ray beam, as stated above. If blanking were ascomplished by intermittent interruption of the scanning cathode ray beam, very high signal impulses would be produced during the interruption intervals, particularly in tubes where electron multipliers are incorporated. The very high signal impulse is produced because of the fact that the signal output from the secondary electron multiplier is a function of the change in the return beam intensity, and the modulation percentage of the returned beam by the picture signal as a result of the scanning action is very much lower than the modulation caused by completely interrupting the scanning beam. The change in beam intensity naturally during blanking intervals is whereas the change in beam intensity due to the absorption of electrons by the mosaic during the scanning intervals may only'be a matter of a few, percent. Scanning beam blanking is feasible, however, in tubes of the type described where the signal plate or conducting layer 6 and its associated circuit is used as an output electrode, since in that case only the electrosatic charge signal current from the mosaic is passed to the amplifier, and a complete beam current interruption does not produce any undesirable results.

In the present invention, blanking is accomplished by applying momentarily short negative impulses to the mosaic or target electrode, the duration of the impulses corresponding to the time required to return the scanning cathode ray beam to its starting position. The current intensity of the cathode ray beam is in no wise altered, and these momentary negative impulses that are applied to the target or mosaic electrode are eflective to repel the electrons of the scanning beam from the surface of the mosaic, thereby preventing any discharge of the electrostatic charge condition that exists on the surface of the mosaic during the beam return interval. All of the electrons which constitute the cathode ray beam are, therefore, returned and are directed upon the first cathode of the secondary electron multiplier so that no signal impulses are produced in the output circuit. The beam return current, and hence the multiplier output current, accordingly have zero modulation during the blanking interval, corresponding with and producing thus a correct black level signal.

For producing the desired negative signal blanking impulses and to accomplish the blanking in accordance with the purpose of the present invention, the negative control impulses 80 and 88 of line and field frequency, respectively, are applied individually to the control electrodes 81 and 88 of tubes 90 and 92, respectively. Each of tubes 90 and 92 include a cathode, a control electrode and an anode. The negative impulses of line frequency, such as indicated by the curve 88, are applied to the control electrode 81 through coupling condenser 93, whereas the negative impulses of field frequency, such as indicated by the curve 85, are applied to the control electrode 88 by coupling condenser 94. The anodes of tubes 90 and 92 are maintained positive with respect to their associated cathodes by means of independent load impedances 95 and 96 and by means of a potential source (not shown, but indicated between the positive and negative terminals). When the negative control impulses of line frequency are applied to the control electrode of tube 81 the control impulses bias the tube to cutoff so that positive impulses of line frequency and of rectangular wave form are available at the anode of tube 90. The curve 91 represents the potential variations that may be available at the anode of this tube. These positive impulses of line frequency are then applied to the cathode of tube 92 by way of coupling condenser 98, the cathode of tube 92 being connected to ground by way of cathode resistance I00. Tube 92, therefore, has applied to its control electrode negative impulses of field scanning frequency and to its cathode positive impulses of line scanning frequency. The impulses of both series, therefore, tend to drive the tube to cut-ofi with the result that a composite series of positive impulses is available at the anode of tube 92 corresponding to both line and field scanning frequencies. The wave form shown at I02 represents the voltage variation available at the anode of tube 92.

The composite series of positive impulses representing both line and field frequency which are available from the anode of tube 92 are then applied to the control electrode of tube I94 by way of coupling condenser I06. This tube also includes a cathode, a control electrode and an anode, the latter element being maintained positive with respect to the cathode by means of a source of potential (not shown) and by means of load impedance I08. Corresponding negative impulses are, therefore, available at the anode of tube I04 and the wave form of these impulses may correspond to the curve shown at I I0. These negative control impulses are then applied to the conducting layer or signal plate 6 associated with the mosaic 2 by way of coupling condenser 2. These negative impulses are, in fact, the desired negative signal blanking impulses, and operate to momentarily drive the mosaic surface sufficiently negative (1. e.- below the potential of the cathode III of the gun structure 8) to prevent the scanning cathode ray beam from reaching the mosaic electrode during the return time interval. The negative blanking impulses, therefore, cause a deceleration of the cathode ray beam with the current intensity of the scanning cathode ray beam remaining unchanged. The resistance TI is included in the conductor between the signal plate or conducting surface 6 of the mosaic and ground in order that the signal plate may be maintained at a proper average potential and in order to isolate the blanking impulses.

Through the use of this system of blanking, the scanning cathode ray beam is permitted to reach all points of the mosaic in the normal manner during the scanning deflection interval, but is prevented from reaching the mosaic during the return time intervals. There are, consequently, no image signals developed during the return time intervals, since the full current intensity of the scanning cathode ray beam is returned without modulation, i. e. no disturbing impulses or signal components are produced and a correct black level may thus be derived from the multiplier or from a collector electrode when no multiplier is used. The image signal, as derived from a collector electrode or from the electron multiplier, has the opposite polarity to the signal that would be derived from the signal plate 8, and, in addition, it differs from the image signals from the signal plate in Orthicon pick-up tubes only by the steady direct current component which is equal to the beam current multiplied by the gain of the electron multiplier. Naturally the direct current component is not passed by the image signal amplifier 43 and the proper black level of the image signal is the zero modulation level obtained during blanking periods,

amplifier system by automatic black level setters or direct current inserters well known to those skilled in the art.

In view of the foregoing it is, therefore, possible to produce return line blanking in an Orthicon pick-up tube of the type where a collector electrode or electron multiplier is used without interruption of the scanning cathode ray beam and without producing any undesired results such as the production of excessively strong peak impulses in the image signal series.

Although the particular Orthicon pick-up tube is shown and described in some detail, it is to be understood that various other designs of similar pick-up tubes may as well be used, and the present invention can be applied to any one of a number of similarly designed pick-up or camera tubes. An example of various different television pick-up tubes of the 0rthicon" or low velocity scanning type are shown and described in the U. S. patent to Iams, No. 2,213,177, issued on August 27, 1940. Furthermore, although an electron multiplier tube is shown as associated with the Orthicon television pick-up in the drawing, it is to be understood that the electron multiplier may be omitted and a single collector electrode positioned, for example, in the same location as the first electron multiplier plate 82, could be used for collecting the returned electrons to thereby produce an image signal output.

Various other alterations and modifications may be made in the present invention without departing from the spirit and scope thereof, and it is desired that any and all such modifications be considered within the purview of the present invention, except as limited by the hereinafter appended claims.

Having now described my invention, what I claim is;

1. A television transmitter including a lightresponsive pick-up tube having a target electrode, means for generating a cathode ray beam in said tube for scanning said target electrode at a low velocityto produce image signals,- and means to apply potential impulses of a predetermined polarity to said target electrode during predetermined short intervals, the intensity of the applied impulses being sufllcient to prevent the scanning cathode ray beam from reaching the target electrode during those intervals.

2. A television transmitter including a lightresponsive pick-up tube having a target electrode, means for generating a cathode ray beam in said tube for scanning said target electrode at a low velocity to produce image signals, ,and means to apply negative potential impulses of short duration to said target electrode to prevent the scanning cathode ray beam from reaching the target electrode.

3. A television transmitter including a lightresponsive image pick-up tube having a target electrode, means to generate a cathode ray beam in said tube and to direct the generated beam toward said target at a low velocity, means to deflect the generated cathode ray beam to scan the target electrode according to a predetermined pattern, and means to apply potential impulses of a predetermined polarity to said target electrode to prevent the scanning cathode ray beam from reaching the target electrode, the frequency of the applied impulses corresponding to at least one of the deflection frequencies of the scanning cathode ray beam.

4. A television transmitting system including a television pick-up tube having a target electrode, means for generating a cathode ray beam in said pick-up tube, means for deflecting the generated beam in one direction at one predetermined rate, means for deflecting the generated beam in another direction at another predetermined rate, the two deflections causing the cathode ray beam to scan the surface of the target electrode accord- .ing to a, predetermined pattern, and means for applying relatively short impulses to said target electrode to thereby prevent the cathode ray beam from reaching the target electrode during the application of the impulses, whereby the target electrode may be unaffected by the scanning cathode ray beam during a predetermined portion of each deflection cycle.

5. A television transmitting system including a television pick-up tube having a light-responsive target electrode, means for generating a continuous cathode ray beam in said pick-up tube. means for deflecting the generated cathode ray beam in mutually perpendicular directions to scan the surface of the target electrode according to a predetermined pattern, and means for momentarily decelerating the scanning cathode ray beam to thereby prevent the cathode ray beam from reaching the target electrode whereby the target electrode may be unaffected by the scanning cathode ray beam during moments of deceleration.

6. A television transmitting system including a television pick-up tube having a light-responsive target electrode, means for generating a cathode ray beam in said tube and for directing said beam toward said target at a low velocity,

directing an optical image upon said target electrode to produce an electrostatic charge image of a predetermined polarity, means to deflect the generated cathode ray beam in horizontal and vertical directions at different predetermined rates in order to scan .the target electrode to remove the electrostatic charge image, a collector electrode positioned in said tube to receive electrons of the scanning cathode ray beam not utilized in removing the electrostatic charge. and means for intermittently applying potential impulses of a polarity opposite to that of the electrostatic charge image and of relatively short duration to said target electrode to prevent the cathode ray beam from reaching the target electrode during the application of the impulses.

7. A television transmitting system including a television pick-up tube having a light-responsive target electrode, means for continuously generating'a cathode ray beam in said tube and for directing said beam toward said target at a low velocity, means for directing an optical image upon said target electrode to produce an electrostatic charge image, means to deflect the generated cathode ray beam in mutually perpendicular directions at different predetermined rates in order to scan the target electrode to cancel the electrostatic charge image, a collector electrode positioned in said tube to receive electrons of the scanning cathode ray beam not utilized in cancelling the electrostatic charge, and means for intermittently applying a deceleration force of relatively short duration to the continuously generated cathode ray beam to prevent the cathode ray beam from reaching the target electrode during the application of the deceleration force.

8. A television transmitting system including a television pick-up tube having a light-responsive target electrode, means for generating a cathode ray beam in said tube and for directing said beam toward said target at a low velocity, means for focusing an optical image upon said target electrode to produce a positive electrostatic charge image, means to deflect the generated cathode ray beam in horizontal and vertical directions at diifereht predetermined frequencies in order to scan the target electrode to cancel the produced electrostatic charge image, a collector electrode positioned in said tube to receive electrons of the scanning cathode ray beam not utilized in cancelling the electrostatic charge, and means for intermittently applying potential impulses of negative polarity and of relatively short duration to said target electrode to prevent the scanning cathode ray beam from reaching the target electrode during the application of the impulses.

9. A television transmitter including a television pick-up tube having a light-responsive target electrode, means to continuously generate a cathode ray beam in said pick-up tube and to direct the generated beam toward said target electrode at a low velocity, means to deflect the cathode ray beam in mutually perpendicular directions to scan the target electrode, means to project an optical image on the light-responsive target electrode to produce an electrostatic charge image thereon by photo-electron emission, a collector electrode positioned in said tube to receive electrons of the generated cathode ray beam not utilized for cancelling the electrostatic charge image, and means to apply a deceleration force to the continuously generated cathode ray beam during the beam return interval in both scanmeans for ning directions to prevent the cathode ray beam toward said target electrode at a low velocity,

means to produce an electrostatic charge image on the target electrode, means to repeatedly deflect the cathode ray beam in mutually perpendicular directions at diflerent deflection frequencies in order to scan the target electrode to cancel the produced electrostatic charge image, a collector electrode in said tube positioned to receive electrons of the cathode ray beam not utilized in cancelling the electrostatic charge image. and means to apply a deceleration force of short duration to the continuously generated cathode ray beam during its return deflection in both directions to thereby prevent the cathode ray beam from reaching the target electrode during the return deflection intervals whereby blanking may be accomplished.

11. A television transmitter including a television pick-up tube having a target electrode, means to generate a cathode ray beam in said pick-up tube and to direct said beam toward said target electrode at a low velocity, means to produce an electrostatic charge image on the target electrode, means to repeatedly deflect the cathode ray beam in both horizontal and vertical directions at diflferent deflection frequencies in order to scan the target electrode to remove the produced electrostatic charge image, a collector electrode in said tube positioned to receive electrons oi the cathode ray beam not utilized in v removing -the electrostatic charge image, and

means to apply negative impulses of short duration to a conducting surface adjacent the target electrode during the return deflection of the cathode ray beam in both horizontal and vertical directions to thereby prevent the cathode ray beam from reaching the target electrode during the return deflection intervals whereby both horizontal and vertical blanking may be accomplished.

O'I'l'O H. SCI-IADE.

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