US2297524A - Color television system - Google Patents

Description

Sept. 29, 1942. E. I. ANDERSON COLOR TELEVISION SYSTEM Filed June 20, 1941 I INVENTOR Milk? IZen/pn ATTORNEY Patented Sept. 29, 1942 COLOR. TELEVISION SYSTEM Earl I. Anderson, New York, N. Y., casino: to Radio Corporation of America, a corporatlonpi Delaware Application June 20, 1941, Serial No. 398,899

8 Claims. (01. its-5.4)

This invention relates to a television transmitter and more particularly for the transmission of television pictures in color.

Ordinarily in the transmission of color images by television it is customary to transmit separate images or television frames in succession, the successive images or frames transmitted being representative of a particular color of the subject matter being transmitted. The transmitted signals are received at a television receiver and are then used to produce successive black and white images which are viewed through difierent color filters, the difierent filters used being rotated or placed in front of the television receiver in rapid succession in synchronism with the transmission of the separate images and in synchronism with the movement of corresponding filters in the television transmitter.

At the transmitter a single television transmitting tube is used and optical images of the subject matter to be transmitted are projected upon the light sensitive electrode or photo-sensitive mosaic in the transmitting tube through color filters so that each successive scanning cycle in the television transmitting tube will produce picture signals corresponding in intensity to the light values of a particular color in the subject matter transmitted.

The. light responsive electrode or the photosensitive mosaic which is used in the television transmitting tube does not have completely uniform response in so far as color is concerned or in so far as the wave length of the light projected thereon is concerned. Furthermore, the light transmission characteristics of the color filters used at the transmitter are not identical, and the artificial-light sources do not have the same output at all light frequencies.

Accordingly, in the absence of some correcting means, the signals representing one color are either too strong or too weak in comparison with the signals representing another color to permit the production of a natural and desirable balance of color in the composite images produced at the television receiver. In a three-color television system it is generally customary to use red, blue and green filters and in order to compensate for the response of the television transmitting tube, the differences in transmission characteristics of the filters and the artificial light output at these particular colors, separate amplifying channels have been used for each of the three colors together with electronic switching means for connecting the output of the television transmitting able in view of the obvious necessary complications in the circuits involved.

Normally in presently used television transmitting systems where sequential color images are transmitted in three colors, Wratten filters A, B and C5 are used. As stated above, these filters do not have the same percentage of light transmission-at their center frequencies and, accordingly, the overall intensity of light which is transmitted through the filters and projected upon the mosaic electrode is not uniform and not truly indicative of the light values of the various colors in the object to be transmitted. V Accordingly, this lack of identical transmission in the various filters results in an inaccurate proportioning of the color in the received image. Through the use of the present invention it is possible to compensate for this lack of uniformity in light transmission of the various filters so that a properly blended color image can be reproduced at the television receiver. a

In television transmitting studios the particular type of lighting used also affects the proper balance of the color at the receiver, as indicated above, since some lighting sources may have a preponderance of light in the red portion of the spectrum whereas other sources of light may have a preponderance of light in the blue or violet portion of the spectrum. Accordingly, an improper balance of color in the received image will result by reason of the particular light source used in the studio and through the use ofthe present invention it is possible to compensate for the non-linearity in the source of light used in the studio.-

It is a purpose of the present invention, therefore, to provide atelevision transmitting system wherein color images may be transmitted and wherein a single amplifying channel may be used without the necessity of associating therewith an electronic switching device.

tube to the one or another of the three amplify- Another purpose of the present invention reli ght responsive electrode in the television transmittingtube.

Anothenpurposeoi the present invention resides in the provision of means for compensating for the lack of uniformity in the percentage of light transmission of the various color filters used with the television transmitting camera.

Still another purpose of the present invention resides in the provision of a simple and convenient means whereby a lack of balance in color as a result of the particular source of illumination used in the studio may be fully compensated for.

Still another purpose of the present invention resides in the provision of a simple and convenient system whereby the strength of the signals produced by the television transmitting tube for the differently colored images may be so altered as to produce the desired color balance at the television receiver without the necessity of using electronic switching means and without the necessity of using a plurality of separate amplifying channels.

A still further purpose of the present invention resides in the provision of means whereby the degree of compensation may be varied at will so that proper color balance may be maintained by a simple and convenient monitor means at the television transmitter.

Still another purpose of the present invention resides in the provision of means whereby the current intensity of the scanning cathode ray beam is controllable so that each image of the same color is scanned by a cathode ray beam of the same current intensity whereas images of difierent colors are scanned at diiferent beam intensities to thereby produce a variation and a control in the signal strength for each vertical scanning cycle.

Various other purposes and advantages of the present invention will become more apparent to those skilled in the art from the following specification, particularly when considered in conjunction with the drawing wherein:

Fig. 1 shows a schematic representation of a television transmitter circuit incorporating one application of the present invention.

Fig. 2 shows voltage variation curves which are present at various parts of the circuit and Fig. 3 shows the color disc which is provided with the various color filters.

Referring now to Fig. 1, there is shown schematically a television transmitter which includes a transmitting tube I0. This transmitting tube as shown is an Iconoscope but it is to be understood that an Orthicon or similar tele-' vision transmitting tube could as well be used. The Iconoscope or television transmitting tube l includes a cathode [2, a control electrode 14, a first accelerating anode I 6 and a second accelerating anode l8. When proper potentials are applied to these electrodes from a power supply unit or from a. potentiometer 20 connected across a power supply unit (not shown), a focused beam of electrons will be produced. The produced beam of electrons may be defiected in vertical and horizontal directions by means of the deflecting coils 22 and 24, respectively, which are energized by the vertical deflection generator 26 and the horizontal deflection generator 28. Normally, the horizontal deflection generator operates at a much higher frequency than does the vertical deflection generator since the former determines the number of lines scanned per second whereas the latter determines the number of complete fields scanned per second.

Also included in the television transmitting tube is a light responsive electrode 30 with which is associated a signal plate 3| which is connected to ground by way of load resistance 32. A collector electrode 34 is also included in the tube and is normally connected to'the second accelerating anode I8. The collector electrode operates to collect the photo electrons which are emitted from the mosaic electrode 30 when a light image is projected thereon.

The image to be transmitted is represented by the arrow 36 and through the use of a lens system 38 an optical image of the object 36 may be projected and brought to focus upon the mosaic electrode 30.

Interposed in the optical axis of the television transmitting tube is a color disc 40 (shown in detail in Fig. 3) which includes a plurality of segments each of which is provided with a color filter which will transmit a predetermined wave length of light. In a three-color television transmitting system the filters normally used are colored red, green, and blue and as the color disc is rotated by means of the motor 42, optical images of different colors are projected upon the mosaic electrode in accordance with the color of the filter interposed in the optical path at the particular instant. The motor 42 is shown connected to the vertical deflection generator since it is necessary that the motor be driven in synchronism with the vertical deflection of the cathode ray beam. In some instances, however, the. power for the motor may be derived from an entirely separate source'and impulses from the vertical deflection generator may be used'in synchronizing the motor or in controlling its speed.

Connected across a load resistance 32 of the television transmitting tube is an amplifier 44 the output of which is supplied to a modulator and transmitter 46 which supplies energy to the transmitter antenna 41.

The modulator and transmitter also receive energy from the vertical and horizontal deflection generators in order that synchronizing impulses of desired wave form may be transmitted along with the picture signals for use at the receiver to maintain the television receiver in proper synchronous operation with the television transmitter.

In the operation of the transmitter as so far described, the cathode ray beam which is generated in the television transmitting tube is deflected horizontally at a rapid rate and vertically at a relatively slow rate in order to scan the mosaic electrode 30.. .When an optical image of the object 36 is projected on the mosaic 30 through one of the color filters, for example red, a charge image is produced on the mosaic in proportion to the intensity of the light values projected thereon and in accordance with the response of the mosaic at the wave length of red light. The mosaic electrode is then scanned by the cathode ray beam in order to produce a series of picture signals corresponding to these light values and immediately following the vertical scanning cycle or the vertical deflection of the cathode ray beam, the blue filter is interposed in the light path so that the charge image which is produced on the mosaic electrode prior to the next scanning cycle will be in proportion to the light values of the image projected through the blue filter and in accordance with the response of the tube at that particular color. The mosaic electrode is again. scanned to produce picture signals representative of these light values. The same cycle is repeated with an optical image of the object 36 being projected through the green filter on to the mosaic electrode.

With the transmitting system as shown the color image produced at the television receiver would not be perfectly balanced and would not be entirely satisfactory since the response of the mosaic electrode of the television transmitting tube is not uniform for all colors and as explained above, various other factors affect the color balance. Accordingly, in the absence of some corrective networks or in the absence of some means whereby the intensity of the picture signals for the different color images may be controlled, a true and proper color balance cannot be reproduced at the receiver. By controlling a current intensity of the scanning cathode ray beam, it is-possible to produce a proper balance of color and one form of an apparatus which will accomplish this result will be explained presently. Furthermore, the produced picture may have some degree of haziness in view'of the response of the mosaic to infra-red light. In order to eliminate the effects of the infra-red light, an

infra-red filter 48 is preferably placed in the optical axis of the television transmitting tube in order to remove all light having a wave length greater than approximately 8,000 Angstrom units.

In order to compensate for the lack of linearity of color response and in order to produce the proper balance of color at the receiver, a systemhas been provided whereby the current intensity of the scanning cathode ray beam at the transmitter may be controlled. This control is produced by'applying diiferent voltages to the control electrode I4 of the television transmitting tube II]. It will benoticed that the control electrode I4 is connected to the potentiometer by means of a resistance 50 and the point of connection to the potentiometer is negative with respect to the cathode. The control potentials may be applied to the control electrode l4 along conductor 52 through the coupling condenser 54.

In order to supply the necessary voltage variations to the control electrode M, three impulse generators 56, 58 and 60 are provided. These impulse generators are controlled in operation by energy derived from the vertical deflection generator 28 in order that the impulses supplied by the generators 56, 58 and 60 may be properly synchronized and phased with respect to the vertical deflection generator 26. The impulse generator 56 supplies an impulse extending in a positive direction for each third field scanned and as indicated in the drawing this impulse generator might correspond to the repeated cyclic scanning of the red image fields. Likewise, impulse generators 58 and 80 produce impulses extending in a positive direction which correspond respectively to the green and blue image fields. The number of impulse generators used of course must correspond to the number of different colors used in color disc 40 and, naturally, if a two color system is used then oly two impulse generators are necessary.

At the output of the impulse generator 55 is a potentiometer 62 with which cooperates an adjustable contact; in order that the intensity of the impulse supplied to the control electrode l during the scanni ng of the red image field may be controlled. Likewise, similar potentiometers 64 and 66 are associated with the green impulse generator 58 and the blue impulse generator 60. The moveable contacts of the potentiometers are connected to the conductor 52 by way of isolating resistances 68, I0 and 12. By this connection the impulses supplied by the three impulse generators are applied to the control electrode I 4 of the transmitting tube in order to vary the current intensity of the scanning cathode ray beam. The intensity of the impulses supplied by any one of the three impulse generators may be individually controlled-by a manual manipulation of the potentiometers 62, 64 and 86.

In Fig. 2 are shown curves representing the impulses supplied by the three impulse generators as well as a curve indicating the composite wave form which is present on conductor 52. The curve 14,,for example, shows the wave form which is applied to the conductor 52 by the impulse generator 56 and these impulses persist for a length of time equivalent to the scanning time of one field. The impulses concur for each third frame and are synchronized with the scanning of the red image field. The curve 16 shows the wave form of the voltage supplied to conductor 52 by the impulse generator 58 and these impulses are of the length similar to the impulses shown at 14 but the amplitude of the impulses which constitute curve 16 may be greater or less than the amplitude of the impulses of curve 14. These impulses are synchronized with the scanning of the green image field. Likewise, curve 18 shows a similar voltage variation which is synchronized with the scanning of the green image field, the intensity of the impulses being still different from the intensity of the impulses of curves '4 and 76.

The curve shows the composite wave form which is present in the conductor 52 and which is applied to the control electrode I4. This composite wave form is, of course, the aggregate of the Wave forms l4, l6 and 18. The impulses marked R, G, and B of the wave form 80 correspond respectively to the successive impulses ap plied to the conductor 52 by the impulse generators 56, 58 and 60 and the relative intensities of these impulses may be readily appreciated byobserving their amplitude as indicated in the curve 80. Between these impulses is an interval corresponding to the vertical return time of the scanning cathode ray beam and during this interval the control electrode of the transmitting tube is driven negatively to beyond cut-01f. T his eliminates the beam entirely during the return interval so that no picture signals are generated at that time. This interval may constitute from '7 to 10 percent of the entire vertical deflection cycle. The length of time occupied by the various impulses which constitute the composite wave form 80 therefore correspond to the length of time occupied by the actual vertical scanning operation.

It may be seen, therefore, that by adjusting the moveable contacts along the potentiometers 62,

64 and 66, the current intensity of the scanning cat ode ray beam may be controlled and may be causedto vary for each successive vertical scanning operation however, the beam intensity for each scanning'bperation of each particular color image may remain the same. By varying the beam intensity, it is possible to compensate for the lack of uniformity of response of the transmitting tube, for the lack of identical light transmission of the color filters, and for the lack of uniformity of power output at .various color frequencies lin the artificial light source so that a properly color balanced reproduction may be produced at the receiver. The changing of beam currentfor any one color does not afiect the setting for the other colors and, accordingly, each is manually controllable to the exclusion of the other so that the 'color balance may be, varied at will.

The impulse generators 55, :58 and 60 although shown diagrammatically may be of any desired form and well known counter circuits or similar circuits may be included in the impulse generators in order that they will respond only to each third impulse from the vertical deflection generator 26. This response to each third impulse produces the impulse for controlling the beam intensity.

Although a coupling condenser 54 is shown and described above, this condenser may be dispensed with and a direct current'connection provided instead but if this is done it is then preferable to operate the cathode I2 at or near ground potential with the second anode l8 at a high positive potential with respect to ground. When the transmitting tube is so operated voltage difierences between the control electrode [4 and the outputs of the deflection generators do not interfere. From the foregoing it may be seen that a decided improvement has been made in television transmitters where sequential color transmission is used and the necessary circuits have been materially simplified since a single amplifying channel may be used without the necessity of providing electronic switch means as has heretofore been necessary. Through the-use of the present invention, proper and adequate color balance can be accomplished by merely adiusting the potentiometers 62, 64 and 66 to thereby regulate the current intensity of the scanning cathode ray beam during the scanning of the separate red, green and blue image fields. This adjustment can very conveniently be done by the monitor in charge of the television transmission.

Various 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 allsuch modifications be considered within the purview of the present invention except as limited by the hereinafter appended claims.

I claim: I

1. A television transmitting system including a transmitting tube having a light responsive electrode, an optical system for projecting a light image upon the light responsive electrode along an optical axis, a plurality of differently colored light filters, means fpr successively interposing the light filters in the optical axis, means in said transmitting tube for producing a scanning beam of electrons, and means for altering, within predetermined operation limits, the current intensity of the beam of electrons in synchronism with the successive interposition of the light filters in the optical axis.

' beam of electrons for scanning said electrode, and

means for cyclically altering the current intensity of the beam of electrons, within predetermined operation limits, in synchronism with the successive interposition of the light filters in the optical axis.

3. A television transmitting system comprising a television transmitting tube having a light responsive electrode, a lens system for projecting an optical image of an object to be transmitted upon the light responsive electrode, a plurality of color filters, means for moving the color filters so that they may be sequentially and successively interposed in the optical axis, means for generating a scanning beam of electrons in said transmitting tube, and means including a plurality oi impulse generators for varying the current intensity of the scanning beam of electrons, within predetermined operation limits, in synchronism with the sequential movement of the color filters.

4. A television transmitting system comprising a television transmitting tube having a light responsive electrode, a lens system for projecting an optical image of an object to be transmitted upon the light responsive electrode, a plurality of color filters, means for moving the color filters so that they may be sequentially and successively placed in the optical axis, means for generating a focused cathode ray beam in said transmitting tube for scanning said electrode, and means including a plurality of impulse generators corresponding to the filter colors forvarying the current intensity of the scanning cathode ray beam, within predetermined operation limits, in synchronism and in phase with the sequential movement of the color filters.

5. A television transmitting system for transmitting color images comprising a television transmitting tube having a light responsive electrode, a lens system for projecting an optical image of the object to be transmitted upon the light responsive electrode along an optical axis, a color disc having a plurality of color filters, means for rotating the color disc in the optical axis of the television transmitting tube in order that the color filters may be sequentially and successively placed in the optical axis, means for generating a scanning beam of electrons in said transmitting tube, a plurality of impulse generators corresponding in number to the different colors in the color disc, means for independently varying the intensity of the voltage variations produced by the impulse generators, and means for applying the voltage variations to the transmitting tube to control the current intensity of a scanning beam of electrons in synchronism with the rotation of the color disc.

6. A television transmitting system for transmitting color images comprising a television transmitting tube having a light responsive electrode, a lens system for projecting an optical image of the object to be transmitted upon the light responsive electrode along an optical axis, a color disc having a plurality of color filters, means for rotating the color disc in the optical axis of the television transmitting tube in order that the color filters may be sequentially and successively interposed in the optical axis, means for generating a focused cathode ray beam in said transmitting tube for scanning said light responsive electrode to produce picture signals, .a plurality of impulse generators corresponding in number to the different colors in the color disc, means for independently varying the intensity of the voltage variations produced by each of the impulse generators, and means for applying all of the voltage variations to the transmitting tube 7. A color transmitting system including a I transmitting tube having a light responsive electrode, means in said tube for generating a focused beam of electrons, means for deflecting the'loeamv in horizontal and vertical directions at difierent rates in order to produce picture signals, a lens system for projecting an optical image on the light responsive electrode, a color disc having a plurality of color filters, means for rotating the color disc so that a different filter is interposed in the optical axis for each vertical deflection cycle of the beam of electrons, a plurality of impulse generators corresponding in number to the different colors in the color disc, means including said impulse generators for producing'impulses of controllable amplitude in synchronism with the rotation of the color filters, a beam current intensity control electrode in said transmitting tube, and means for applying the impulses produced by the impulse generators to said control electrode whereby the current intensity of the scanning beam of electrons may be controlled in accordance with the color of the light filter interposed in the optical axis.

8. A color transmitting system including a transmitting tube having a light responsive electrode, means in said tube for generating a focused cathode ray beam, means for deflecting the beam in horizontal and vertical directions at different rates in order to cyclically scan the light responsive electrode to produce picture signals,

a lens system for projecting an optical image on the light responsive electrode, a color disc having a plurality of color filters, means for rotating the color disc so that a difierent filter is interposed in the optical axis for each successive vertical deflection cycle of the cathode ray beam, a plurality of impulse generators corresponding in number to the different colors in the color disc, means including said impulse generators for producing impulses of controllable intensity in synchronism with the rotation of the color filters, a beam current intensity control electrode insaid transmitting tube, and means for applying the impulses produced by the impulse generators to said control electrode whereby the current intensity of the scanning cathode ray beam may be varied in accordance with the particular color of the light filter interposed in the optical axis 1 during any particular scanning cycle.

. EARL I. ANDERSON.

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