US2335983A - Television signal-translating system - Google Patents

Description

J. C. WILSON TELEVISION SIGNAL-TRANSLATING SYSTEM Filedv ug. 7, /1941 upm 5555@ GzmwE wuz...

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ATTORN EY MMI-F Dec. 7, 1943.

Or o a 0 o w.V QM SV ma n Patented Dec. 7, 1943 TELEVISION SSIglgTAL-TRANSLATING TEM John C. Wilson, Bayside, N. Y., assigner to Hazeltine Corporatioma corporation of Delaware Application August 7, 1941, Serial No. 405,753

(Cl. P18- 5.8)

v oi' the retina of the eye is above a critical fre- 13 Claims.

This invention relates to television signaltranslating systems and, particularly, to such systems of a type in which a time-modulated television signal, derived by scanning the televised object at a predetermined field-scanning frequency. is translated at one point of the system and from which there is derived, at a subsequent point of the system, another time-modulated television signal, the field-scanning frequency corresponding to which is different than that corresponding to the first time-modulated television signal.

As used throughout this specification and the appended claims, the term time-modulated television signal refers to a conventional television signal-representative of successive elements of a picture and comprising a single-valued electric signal which varies with time.

In conventional television systems it is custcmary to scan the object televised in two scanning directions and at frequencies which are suftlciently high to obtain satisfactory fidelity in the reproduced image. The band width of the frequency spectrum required for translating any television signal varies with the product of the scanning speeds utilized in developing the signal. Ordinarily, the minimum acceptable scanning speeds in a television system, neglecting the flicker phenomenon, are limited by the .speed of motion of the object being televised. However, in some cases, there is relatively little motion in the subject to be televised and the scanning speeds can be materially reduced While still obtaining satisfactory fidelity in the reproduced image. This results in a material saving due to the fact that it is necessary to provide signaltranslating stages adequate only to translate the signal of the reduced band width. Furthermore, the number of television programs which can be transmitted over the air in a given frequency range is dependent upon the average frequency band required for the individual programs and thus, if the average frequency band can be reduced, a larger number of programs can be transmitted within the given frequency range.

However, even though there is very little motion in the object being televised, there is a limit vto the minimum field-scanning frequency which can be used to translate and reproduce the image satisfactorily. This limitation is due to the socalled "flicker effect in image reproduction caused by the ability of the eye to discern' similar repeated impressions if the frequency of repetition is below a critical frequency. If the frequency, the visual sensation equals that given by the same quantity of luminous flux averaged over the total period. It is, therefore, possible to reduce or eliminate dicker effect in a reproduced image by increasing field-scanning or frame-scanning frequency above a critical value.

It is, therefore, desirable to provide a television system in which, in one part of the system, a television signal corresponding to a low field-scanning frequency is translated and in which. in another and subsequent part of the system, a related television signal is translated which has associated therewith a field-scanning frequency sufficiently high to eliminate flicker effects in the reproduced image.

It is an object of the present invention, therefore, to provide an improved television system in which one or more of the above-mentioned disadvantages of prior art television systems are eliminated.

translating the television signal is minimized and in which, in a subsequent portion of the system, there is derived a related television signal'which can be translated and reproduced without objectionable flicker.

In accordance with a feature of the present invention, a television signal-translating system comprises means for translating a time-modulated iirst television signal, derived by scanning a televised object at a predetermined field-scanning frequency, and energy-storage means responsive tothe translated signal for developing a space-modulated second television signal. The system also comprises means for commutating the energy-storage means at a finite frequencydifferent than the field-scanning frequency electrically tolderive a time-modulated third television signa Also in accordance with a feature of the invention, the 'method of translating a television signal comprises flrst translating a time-modulated television signal, derived by scanning a televised object at a predetermined field-scanning frequency, storing energy from the time-modulated rst television signal to develop a spacemodulated second televisi n signal, and extracting the stored energy at a nite frequency higher than the held-scanning frequency electrically to derive a time-modulated third television signal.

For a better understanding of theinvention,

quency of repetition of illumination of a part together with other and further objects thereof,

reference ls had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims..

The single figure of the drawing is a schematic diagram of a complete television signal-translating system in accordance with 'the invention including a television signal transmitter and a television signal receiver.

Referring now more particularly to the drawing, the television signal-translating system illustrated comprises a transmitter 1 including an antenna system 8, 9. 'I'he transmitter 1 is of conventional form except that the scanning frequencies of its image-analyzing device are considerably lower than customary, for example, its linescanning frequency may be approximately 2500 lines per second and its field-scanning frequency lines per second resulting in a signal of a band width of approximately 0.75 megacycle. The television signal-translating system also comprises a receiver of the superheterodyne type for receiving the television signal from. transmitter 1. This receiver comprises an antenna system I0, I I connected to a radio-frequency amplifier I2 to which are connected in cascade, in the order named, an oscillator-modulator I3, an intermediate-frequency amplifier I4, a detector I5, a video-frequency amplifier I 6, and an image-reproducing device I1. A line-frequency generator I8 and a field-frequency generator I9, each having an input circuit coupled to detector I5 through a synchronizing-signal separator 20, are coupled to line-scanning windings 2| and field-scanning windings 2|, respectively, which are associated with image-reproducing device I1. An automatic amplification control or A. V. C. bias is derived from detector I5 and applied to one or more of v the tubes of radio-frequency amplifier I2, oscillator-modulator I3, and intermediate-frequency amplifier I4, in order to maintain the signal input to detector I5 within a relatively narrow amplitude range for a wide range of received signal amplitudes. A sound-signal reproducer 6' is coupled to intermediate-frequency amplifier I4 in order to reproduce sound signals accompanying the received television program. 'I'he stages or units Ii-IS, inclusive, I8, I9, and may all be of conventional well-known construction, except that they are designed to pass a signal-modulation band of only approximately 0.75 megacycle rather than approximately 4 megacycles, as is customary, so that a detailed illustration and description thereof are deemed unnecessary herein.

Referring briefly, however, to the operation of the system described above, television signals radiated by antenna circuit l, 9 and intercepted by antenna circuit I 0, I I are selected and amplified in radio-frequency amplifier I2 and coupled to the oscillator-modulator I3 wherein they are converted into intermediate-frequency signals which, lin turn, are selected and amplified in intermediate-frequency amplifier I4 and delivered to the detector I5. The modulation components of the signal are derived by thedetector I5 and the video-frequency components thereof are supplied to the video-frequency amplifier I6 wherein they are amplified and from which they are supplied in the usual manner to a brilliancy-control electrode 22 of image-reproducing device I1. The intensity of a scanning ray of device I1 is thus modulated or controlled in accordance with the videofrequency voltages impressed upon the control electrode 22 in the usual manner. The synchronizing-component output of detector I5 is supplied through synchronizing-signal separator 20 to generators I8 and I9. Scanning waves are generated in the line-frequency and field-frequency scanning generators I8 and I9, respectively, which are controlled by synchronizing-voltage pulses from separator 20, and are applied to the scanning elements 2l, 2l' of image-reproducing device I1 to produce scanning fields, thereby to deflect the scanning ray thereof in two directions normal to each other, so as to trace a rectilinear scanning pattern within tube I1 in a manner to be hereinafter fully explained. The control voltage derived from detector I5 and applied to one or more of the tubes of stages I 2, I3, and I 4 serves to control the amplification therein to maintain the am-4 plitude of the signal input to detector I5 within relatively narrow limits for a Wide range of received signal amplitudes. Sound signals accompanying the received television program are reproduced in sound-signal reproducer B.

Referring now more particularly to the portion of the system of the drawing embodying the present invention, there is provided in the cathoderay reproducing unit I1 a control grid 25 of the image-grid type which, per se, is known in the art, one such grid being described in the United States Letters Patent No. 2,280,191 to R. C. Hergenrother, granted Apri121, 1942. The image grid 25 of tube I1 constitutes an energy-storage means responsive to the signal translated by video-frequency amplifier I6, which is a timemodulated first television signal derived by scanning the televised object at transmitter 1 at a predetermined field-scanning frequency, and is utilized to developfromthe time-modulated signal a space-modulated second television signal. The image grid 25 consists of a. perforated or reticulated disc, one surface of which is made up of a thin dielectric charge film or plate 26 coated on a conductive sheet or backing plate 21 and capable of holding a surface distribution of electron charges corresponding to a television image which is formed thereon by facing it towards the source of the scanning beam modulated by electrode 22. Projections on the backing plate 21 of the image-grid structure are provided for blocking the direct paths through the grid for the highvelocity electrons of the scanning beam. 'I'he tube I1 is provided with an electron gun for developing a second television signal comprising a spacemodulated charge image on the image grid 25, this electron gun including a cathode 28, a control electrode 22, focusing and accelerating anodes 29, 30, and a collector electrode 3l.

Cathode-ray tube I1 also comprises an electron gun for commutating the energy-storage means or image grid 25 at a 'frequency higher than the field-scanning frequency of the signal transmitted by transmitter 1, electrically to derive a time-modulated third television signal. This electron gun includes a cathode 32, a control electrode 33,and an anode 34 for projecting a beam of electrons on image grid 25. In order to cause this last-named beam to scan' or commutate the target electrode 25 at a frequency higher than the eld frequency used for scanning the televised object at 'the transmitter, there are provided a line-scanning generator 50 and field-scanning generator 5I coupled respectively to line-scanning windings 52 and eldscanning windings 53. A timer 54 is provided for supplying synchronizing pulses to generators 50 and 5I. The structure of tube I1 is such that the beam from cathode 324 is incident on backing plate 21 and is eifective to develop lowvelocity secondary electrons in the vicinity of the point at which the beam is incident. Thel passage of these low-velocity electrons through the image grid 25 is controlled by the charge image on image grid 25, electrically to derive a timemodulated third television signal. The electrons ,comprising this time-modulated third television signal are directed to fluorescent screen 38 by means of focusing plates 39 in order to reproduce a visible image on the screen.

For most successful operation of a reproducing unit of the type under discussion, the charge image on the image grid 25 must be substantially discharged during each field-scanning cycle of the transmitter 1. The charge image on the image grid may be discharged between successive image scansions by any one of several arrangements, an arrangement hereinafter called a chasing-beam scanning apparatus being illustrated in the drawing. The chasing-beam scanning apparatus includes an electron gun comprising a cathode 4|) and focusing anodes 6l and 42, for directing a stream of relatively low-velocity biasing electrons upon surface 26 of image grid 25, and a chasing-beam scanning generator 44, having an input circuit coupled to synchronizing-signal separator and output circuits coupled to scanning windings 45 and 46, for causing the beam generated by the chasingbeam gun to scan the surface 26 of image grid in a way to be described more fully hereinafter. Suitable operating potentials are provided for the cathode-ray tube I1 in a manner which is well understood in the art. v

Before describing in detail the operation of the image-grid tube I1, it may be helpful rst to consider some of the operating characteristics of the particular elements of tube |1 and particularly the secondary electron-emission characteristic of the surface 26 of image grid 25. 'The secondary electron-emitting target of electron gun 22, 28, 29, 30 of cathode-ray tube I1 is effectively the insulating fihn 26 of image grid 25 which forms one plate of a condenser, the other plate of which is conducting plate 21 connected to the collector electrode 3| so that it is maintained at a proper operating potential. The grid 25 may take a variety of forms. For example, the grid may consist of a conductive surface on a dielectric or insulator, which surface is not continuous but is broken up into a number of minute elements to form a mosaic. For such an image grid any electricaln charge produced thereon by the primary electron beam from cathode 28 is localized at the region where the beam strikes the target and is not appreciably dissi- Alternapated over the surface of the target. tively, the target may consist simply of a dielectric sheet or lm. A primary electron beam incident on such a target releases secondary electrons from the outer layer of the target and behaves in much the same way as does, the conductive mosaic target described above; that is, any electrical charge produced by the incident primary electron beam remains localized where the beam strikes. The target of the mosaic type and the dielectric target are thus similar in electrical behavior ard the following discussion of the electrical charging properties of the dielectric target is valid for the target of mosaic type.

In considering in detail `the electrical properties of dielectric lm 26, it will first be assumed that the beam current from cathode 28 is unmodulated and unscanned. The current to the backing plate 21 is .zero after a steady beam current from cathode 28 has been supplied for an appreciable time and the current surge has died out so that an equilibrium condition has been reached. For equilibrium, the beam current must be equal to the collector-electrode current.` It is also assumed that the dielectric surface 26 was initially uncharged so that its surface potential was the same as that of the backing plate 21. The arrangement may be so proportioned that the primary electron beam striking the dielectric surface 26 has a secondary-emission ratio less than unity which simply means that fewer electrons are released as secondaries thanarrive as primaries. These secondary electrons go to the collector electrode 3| The portion of the surface of dielectric 26 upon which the electron beam is incident, therefore, becomes increasingly negatively charged and the potential rises negatively with respect to the potential of collector electrode 3| and the backing plate 21. This negative potential will rise until the dielectric surface 26 reaches the potential of the cathode 28 when the electron beam current will no longer be able to reach dielectric surface 26 and equilibrium will have been established. The potential of the dielectric surface under these conditions has a negative value relative to the collector electrode 3|.

If, on the other hand, the voltage of the primary electron beam from cathode 28 is such that the surface of dielectric- 26 loses more electrons as secondaries than it gains as primaries when it is at the potential of collector electrode 3| so that it has a secondary electron-emission ratio greater than unity, it becomes positively charged relative to the backing Aplate 21 and the collector electrode 3| Assuming, for example, that the dielectric 26 is initially uncharged and the primary beam from cathode 28 is turned on, the building up of a positive electrical charge on the surface of dielectric 26 has the effect of opposing the secondary electrons which leave the dielectric to go to the collector electrode 3|. This opposing field tends to suppress the secondary emission current to the collector electrode 3|.,

turning back to the dielectric surface 26 those electrons having an initial velocity of emission insucient to overcome the electrical field. As the charge on the dielectric becomes more positive, a'greater portion of the secondary emission current is thus suppressed until a point of equilibrium is reached. This occurs when the secondary emission current is suppressed to the extent that the part of the secondary emission current which reaches the collector electrode 3| is exactly equal to the primary beamA current from cathode 28. The value of the limiting positive potential of dielectric 26 depends upon the velocity distribution, that is, upon the voltage distribution of secondary electrons. These characteristics of an image-grid tube are explained in more detail in the above-mentioned patent and element of the present arrangement which are similar to those of the patent have identical reference numerals;

Considering'now the operation of tube I1 in the system of the invention, it is seen that the electron gun structure comprising cathode 28 and the image grid 25 is similar to that of a conventional reproducing tube except that the fluorescent screen of the conventional tube is replaced by the thin sheet of dielectric 26 attached to the conductive backing plate 21. It is seen that the control grid 22 of this electron gun structure is connected,l as in a. conventional reproducing tube, to a television receiver and its operation will be considered during the scanning of one `field of a television image starting with the to element in accordance with the modulated television signal. This results in a distribution of positive charge over the dielectric surface 26, that is, in the production of a'second television signal comprising a space-modulated charge image on the image grid 25 of the tube I1, which charge image is an electrical replica of the transmitted television image. This charge image remains on the dielectric surface 28 for an appreciable length of time. During this time the electron beam from cathode 32 is effective to commutate the energy-storage means formed by the elements of image grid 25 due to the fact that it is scanned over the surface of the backing plate 21 at a frequency higher than the frequency of scanning associated with the electron beam from cathode 28. The beam from cathode 32 is eifective to develop low-velocity electrons in the vicinity of the point at which the beam is incident on the energy-storage means 25. These lowvelocity electrons are developed by secondary electron emission fromthe plate 21. The projections on backing plate 21 comprise means for preventing the direct passage of high-velocity electrons from the cathode 32 through the image grid 25. The passage of the low-velocity electrons, which are developed at the backing plate 21, through the image grid 25 is controlled by the charge image upon dielectric 26. 'I'he electrons which pass through the grid are accelerated and focused on fluorescent screen 38 to produce a visible television image by focusing plates 38. 'I'he electron stream which passes through the image grid thus comprises a time-modulated third television signal.

In the operation of signal-reproducing device I1 for transmitting successive pictures or fields it is necessary to provide some biasing arrange-l ment for bringing the surface of dielectric 26 to a reference potential between successive picture scansions by the electron beam from cathode 28. If this is not done, the charge over the entire dielectric surface approaches a maximum constant value and the image disappears. This biasing may be effected either by electrical leakage of the charge through the dielectric or by bombarding the dielectric 26 with electrons of suiliciently low velocity that the secondary electronemission ratio is considerably less than unity, causing the charge on a dielectric to fall approximately to the cathode potential of the bombarding beam. This cathode potential may have any convenient value. For this purpose the electron gun structure 40, 4I, 42 is provided for supplying electrons which are sharply focused to a scanning beam and scanned over the surface of dielectric 26. This beam is scanned in the same manner and with the same scanning frequencies as the beam from cathode 28 is scanned, but the phase of the saw-tooth iield-scanningwaves is retarded by nearly a full cycle or advanced-by a relatively small angle so that the beam from cathode I8 follows or chases the signal beam from to have a gap of appreciable width, for ex.

ample, 20 lines, between the image-forming beam from cathode 28 and the biasing beam from cath-v ode 40. This phase relation can be obtained by advancing the phase oi' the field-scanning wave for the bias beam developed by the chasing-beam scanning generator M by the equivalent of 20 lines relative to the held-scanning wave developed by the generator I9.

In summary, therefore, it is seen that the television signal-translating system described comprises ya transmitter 1 and the stages I2-I6, inclusive, of a receiver for translating a time-modulated first television signal, derived in the transmitter 1 by scanning the televised object at a predetermined held-scanning frequency. 'Ihe image grid 25 of image-grid tube I1 comprises a capacitive energy-storage means, responsive to this translated time-modulated television signal, for developing a second television signal comprising a space-modulated charge image on the dielectric 26. Ihe electron gun structure comprising cathode 32 and deilecting windings 52, 53 comprises a means for commutating the image grid 25 at -a finite frequency higher than the frequency of iield scanning at the transmitter 1, while the backing plate 21 comprises a means for developing low-velocity electrons in the vicinity of the point at which the beam from cathode 32 is incident on the image grid 25, electrically to derive a time-modulated third television signal by the passage of the low-velocity electrons through the image grid.

It will be understood that the present invention is not limited to an arrangement in which this last-mentioned time-modulated television signal is directly reproduced as by the action of screen 38, but also includes arrangements in which an electrical signal is derived from the time-modulated electron stream passing through target 25 and further translated in the system.

It is seen that, in the arrangement of the invention, the field-scanning frequency associated with the reproduced image is unrelated to, and is preferably materially higher than, that associated with the television signal translated by transmitter 1 and for this reason the flicker effects can be substantially eliminated, as described above. It is also seen that the band width of the signal translated through the air from antenna system 8, 8`to antenna system l 0, I I is materially less than would ordinarily be required to reproduce a television image in a conventional system with the same flicker properties as the image reproduced by uorescent screen 38.

It will also be understood that various known modifications of an image-grid tube, some of which are described in detail in the above-mentioned copending application, can also be incorporated in an arrangement in accordance with the present invention.

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 modiflcations may be made therein without departing from the invention and, it is, therefore, aimed in the appended claims to coverall such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: 1. A television signal-translating system comto said translated signal for developing a Spacemodulated second television signal, and means for commutating said last-named means at a finite frequency higher than said field-scanning frequency electrically to derive a time-modulated third television signal.

2. A television signal received comprising, means for translating a time-modulated rst television signal derived by scanning a televised object at a predetermined field-scanning frequency, energy-storage means responsive to said translated signal for developing a space-modulated secondtelevision signal, and means for commutating said last-named means at a finite frequency higher than said field-scanning frequency electrically to derive a time-modulated third television signal.

3. Av television signal-translating system comprising, means for translating a time-modulated first television signal derived by scanning the televised object at a predetermined field-scanning frequency, capacitive energy-storage means, means responsive to said translated signal for developing in said energy-storage means a spacemodulated second television signal, and means for commutating said energy-storage means at a finite frequency higher than said field-scanning frequency electrically to derive a time-modulated third television signal.

4. A television signal-translating system comprising, means for translating a time-modulated first television signal derived by scanning the; televised object at a predetermined fieldscanning frequency, energy-storage means responsive to said translated signal for developing a space-modulated second television signal, means for commutating said last-named means at a nite frequency higherthan said fieldscanning frequency electrically to derive a timemodulated third television signal, and means for reproducing from said third television signal a visible image of said object.

5. A television signal-translating system comprising, means for translating a time-modulated first television signal derived by scanning the televised object at a predetermined fieldscanning frequency, an image-grid tube, means for applying said translated signal to Said imagegrid tube to develop a second television signal comprising a space-modulated charge image on the image grid of said tube, and means for commutating the image grid of said tube at a. nte frequency higher than said field-scanning frequency electrically to derive a` time-modulated third television signal.

6. A television signal-translating system comprising, means for translating a time-modulated first television signal derived by scanning the televised object at a predetermined fieldscanning frequency, an image-grid tube, means for applying said translated signal to said imagegrid tube to develop a second television signal comprising a space-modulated charge image on the image grid of said tube, and means for scanning the imagegrid of said tube with an electron beam at a finite frequency higher than said field-scanning frequency electrically to derive a time-modulated third television signal.

7. A television signal-translating system comprising, means for translating a time-modulated first television signal derived b y scanning the televised object at a predetermined fieldscanning frequency, an image-grid tube, means y tube to a predetermined reference potential after Y each scanning by said first-mentioned beam, and means for commutating said image grid at a nite frequency higher than said field-scanning frequency electrically to derive a time-modulated third television signal.

8. A television signal-translating system comprising, means for translating a time-modulated first television signal derived by scanning the televisedobject at a predetermined fieldscanning frequency, an image-grid tube, means responsive to said translated signal for developing a second television signal comprising a spacemodulated charge image on the image grid of said tube, means for scanning said image grid with an electron beam at a finite frequency higher than said field-scanning frequency, and means responsive to said beam for developing low-velocity electrons in the vicinity of the point at which said beam is incident on said image grid electrically to derive a. time-modulated third television signal by the passage of said low-velocity electrons through said image grid.

9. A television signal-translating system comprising, means for translating a time-modulated first television signal derived by scanning the televised object at a predetermined field-scanning frequency, an image-grid tube, means responsive to said translated signal for developing a second television signal comprising a spacemodulated charge image on the image grid of said tube, means for scanning said image grid with an electron beam at a finite frequency higher than said field-scanning frequency, and means responsive to said beam for developing secondary electrons in the vicinity of the point at which said beam is incident on said image grid electrically to derive a time-modulated third television signal by the passage of said secondary electrons through said image grid.

10. A television signal-translating system comprising, means for translating a time-modulated first television signal derived by scanning the televised object at a predetermined field-scanning frequency, an image-grid tube, means responsive to said translated signal for developing a second television signal comprising a spacemodulated charge image on the image grid of said tube, means for scanning said image grid with an electron beam at a finite frequency higher than said field-scanning frequency, means responsive to said beam for developing secondary electrons in the vicinity of the point at which said beam is incident on said image grid electrically to derive a time-modulated third television signal by the passage of said secondary electrons `first television signal derived by scanning the televised object at a predetermined field-scanning frequency, an image-grid tube, means responsive to said translated signal for developing a second television signal comprising a spacemodulated charge image on the image grid of said tube, means for scanning said image grid with an electron beam at a finite frequency higher than said field-scanning frequency, means responsive to said beam for developing low-velocity electrons in the vicinity of the point at which said beam is incident on said image grid electrically to derive a time-modulated third television signal by the passage of said low-velocity electrons through said image grid, and means for preventing the direct passage of electrons from said beam through said image grid.

12. The method of translating a television signal which comprises. translating a time-modulated nrst television signal derived by scanning the televised object at a predetermined eldscanning frequency, storing energy from saidymodulated second, television signal, and means for commutating said last-named means at a finite frequency different than said field-scanning frequency electrically to derive a time-modulated third television signali JOHN C. WILSON.

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