US2330682A - Method and apparatus for television communication - Google Patents

Description

455-605 AU 233 EX FIPBlOb xa 2,330,682

Sept. 28, 1943. s. L CLOTHIER El AL 2,330,682

. IETHOD AND APPARATUS FOR TELEVISION COMMUNICATION Original Filed March 15, 1938 3 Sheets-Sheet 1 .ATT NEY Sept. 28, 1943. s. L. CLOTHIER ETAL 2,330,682

IETHQD AND APPARATUS FOR TELEVISION COMMUNICATION Original Filed March 15, 1938 3 Sheets-Sheet 2 m I I I I I W sept- 23, 1943- s. L. CLOTHIER ETAL 2,330,682

ETHOD AND APPARATUS FOR TELEVISION COMMUNICATION Original Filed larch 15, 1938 3 Sheets-Sheet 5 Ham/daw n amp Patented Sept. 28, 1943 METHOD AND APPARATUS FOR TELEVISION COMMUNICATION Stewart 1. crooner, lunch, and Harold c. Hog- Iapicwood, N. 1.

Original application March 15, 1938, Serial No.

195,939, now Patent No. 2,288,523, dated December 80, 1941. Divided and this application December 30, 1941, Serial N0. 424,9

2 Claims. (Cl. 178-72) for television reception and cathode-ray tubes for television transmission.

In television communication employing at the receiving station a cathode-ray tube having a fluorescent screen which is scanned by a ray ofelectrons, it has been proposed to make the construction and the various electrical conditions such that the fluorescent image be sumciently brilliant for projection onto a larger, external screen. It has been found, however, that on account of the characteristics of the fluorescent screen, these tubes have definite limitations as to the brilliancy and sharpness of the image which can be produced for projection. For example, when operating at the relatively high intensities required, the fluorescent materials used heretofore no longer have a linear modulation characteristic. Furthermore, some screen materials which are purely fluorescent at moderate intensities become phosphorescent and even incandescent at the higher intensities of electron bombardment. These effects or characteristic result in pronounced blurring of the image, for the reason that at any elemental area of the screen, the

period of time elapsing during excitation of the fluorescent material and decay to the point of extinction or invisibility, is substantially greater than the frame or fleld scanning period which is necessary to avoid flicker.

Development work along this line has accordingly centered on a construction for the screen whereby a more brilliant image can be obtained. One such construction comprises a thin sheet oi metal which is maintained at red heat by current from an external source. In operation, as the ray of electrons is deflected to cause the screen to be scanned, any particular point is raised from red heat to higher values up to intense incandescence, depending upon the ray intensity. However, in these screens the period of decay is far in excess of that allowable if any particular point is to return from relatively intense incandescence to red heat within the frame or fleld period necessary to avoid flicker. Furthermore.

on account of the relatively high coefiicient of heat-conductivity of these screens, th heat of bright points flows in all directions to equalize the temperature, thereby causing adjacent points to become brighter than perhaps they should be, for good detail.

Another such construction is disclosed in Patout No. 2,098,000, issued November 2, 1937, to

Philo T. l 'arnsworth et a1., and comprises a screen having a surface of refractory textile material similar to that in a common gas mantle, and which is capable of being raised to incandescence by the impact of the electron ray deflected to scan this surface, the degree of incandescence to which any elemental area is raised being in proportion to the intensity of the electron ray di rected at such elemental area at the instant. Screens of this construction have a relatively low coefficient of heat-conductivity, so that during any frame or field period, a bright point of the image does not expand and include adjacent points. In these screens, however, return of any particular point from incandescence to red heat or lower, is largely by radiation, which fact again gives rise to blurring because of time lag. That is, the decay period is substantially greater than that which the frame or fleld period would have to be if there is to be no flicker.

Still another construction for such a screen is disclosed in Patent No. 2,097,994, issued November 2, 1937, to Harry B. Bamford, and comprises an lncandescible screen composed of a plurality of closely mounted helices of very fine tungsten wire having their axes parallel, and normal to the plane of the screen. In operation, the impact of the ray of electrons on the screen raises the elemental areas thereof to incandescence at the P t Of t the degree of incandescenceat any one point being proportional to the intensityof the electron ray directed at such point at the instant. In these screens, however, there is also the detrimental time lag, as in the others.

In order to avoid flicker, the frame frequency should be at least sixteen, and is preferably at least twenty-four. This means that in order to realize the desired operating action in screens of the character referred to, the decay period, whether it is one of radiation or one of conduction, must be no greater than one twen y-fourth of a second for good detail. Where interlaced scanning is employed, the fleld frequency may be as high as sixty, in which case the decay period must be no greater than one-slxtieth of a second if any degree of detail is to be obtained. Even at a frame frequency of sixteen. there is blurring in the prior screens referred to on account of the time lag, and at a frame frequency of twentyfour, or at the high fleld frequency of sixty, these screens, if useful at all, would only b so for a still picture. Furthermore, in the screen construction proposed heretofore. selection of screen materials has necessarily been limited to those having a period of decay or a time lag suiiiciently low for a given frame or field frequency to avoid materials having a decay period many times theframe or field frequency, has not been possible.

With the foregoing in mind, an object of our invention resides in the provision of an improved method and apparatus for television communication whereby it is possible, in a cathode-ray projection tube of the character referred to, to employ materials. such as phosphorescent or incandescible materials, having a time lag or decay period many times any desired frame or field period.

Another object of our mvention resides in the provision of an improved method and apparatus for television communication whereby there is available at the receiver a sharply defined light source, modulated in accordance with the light intensity of the corresponding element of a still or moving image to be reproduced, and which has a substantially higher intensity than it is possible to obtain with the prior methods and constructions.

. Another object of our invention resides in the provision of an improved construction of cathode-ray tube for television transmission which has advantages over those proposed heretofore in the way of greater efllciency and better operating action.

Other objects and advantages will hereinafter appear.

For the P rp se of illustrating our invention, an embodiment thereof is shown in the drawings, wherein Figure 1 is a simplified, diagrammatic view, partly in section, of a television receiving system constructed and operating in accordance with our invention;

Fig. 2 is an elevational view partly in section, the section being taken on the line 2-2 in Fig. 1;

Fig. 8 is a view similar to Fig. 2, showing a modification;

Fig. 4 is a simplified, diagrammatic view, partly in section, of a television transmitting system constructed and operating in accordance with our invention;

Fig. 5 is an enlarged, detail,- elevational view, takenonthe lineHinFig.4;

Fig. 6 is an enlarged sectional view, taien on the line 8-4 in Pig. 5;

Fig.7isaviewsimiiartoFig.4,showinga modification;

Fig.8isaviewsimilartoFig.1,showinga modification;

Fig. 9 is a detail, elevational view, taken on the line 0-! in Fig. 8;

Fig. 10 is an elevational view partly in section, the section being taken on the line ll-ll in Fig. 8; and

Fig. 11 is a schematic diagram illustrative of the operating action in Fig. 8.

' In Fig. 1 of the drawings, the reference numeral il designates a cathode-ray tube provided with an electron gun ll of any suitable, conventional construction, for developing a ray ii of electrons directed at the eflective surface of the screenstructurewhichisintheformofadmm i3, rotatably supported in the .tube. For this purpose, thedrum ll isfixed on a spindle ll having a bearing at each end thereof in the wall of the tube, as shown. If desired, jewels may be used for these bearings. The effective cylindrical surassaeaa able fluorescent, phosphorescent, or incandescible material.

The drum it is rotated in the clockwise direc- 7 tion and at a uniform rate by a motor It, the

' tation of the drum it. However, the ray I! may be deflected electromagneticaily, if desired, and a second set of deflecting plates 25 or other defiection means may be employed for vertical posiidoning of the ray.

In operation, deflection of the ray I: in the one dimension effects line scanning, and movement of the screen surface in the direction transverse to this dimension eilects frame scanning. With picture signals applied to the control electrode or grid is of the electron gun to modulate the ray intensity in accordance with the lights and shadows of the respective elemental areas of the image at the transmitter, a like and brilliant image is produced at 20 on the screen surface. This image or series of images of course moves with the screen surface. The moving images are projected by a lens system 21 to a large external screen 23, the beam of light producing the large images being first reflected from the mirror drum 22, which eflectively arrests the vertical motion of the large image.

Assuming that the frame frequency is to be twenty-four, the drum 22 can have twenty-four mirrors and be rotated at a constant rate of sixty revolutions per minute, so that twenty-four mirrors pass a given point at the periphery in one second. The diameter of the screen drum II in suchcasewillbesuchthat withthisdrumbeing rotated at a constant rate of sixty revolutions per minute, the screen surface may be composed of twenty-four frame areas which are scanned by the ray i2 and presented to the lens system 20 in succession and at the rate of twenty-four'per second. From this it will be seen that repeated scanning of any one elementary line of the screen structure takes place only after an elapsed period of one second, which is far in excess of the time lag or decay period which the screen material might have. In our improved method and apparatus, therefore, there is always ample time for the screen material to decay from intense brightness to extinction before it moves around to be scanned again. If interlaced scanning is to be used, and the field frequency is to be sixty, with each of the drums being rotated at a constant rate of sixty revolutions per minute as before, the mirror drum 22 could have sixty mirrors and the diameter of the screen drum It could be such that the screen surface could be composed of sixty frame areas which would be scanned by the ray l2 and presented to the lens system 20 in succession and at the rate of sixty persecond. Otherfiguresandspeedscanbe used to obtain these same or other frame or field speeds.

The diameter of the screen-drum I3 is not critical. However, its peripheral speed, the frame frequency and the image size on the screen-drum surface are directly related. For example, having the desired size of the images at the screendrum known, itis possible to fix the peripheral face of the drum may be composed of any suit- 76 speed at a value determined by the product of the frame frequency and the height of one screen-drum image.

The operating action may be analyzed in the following manner. with both the screen-drum It and the mirror or frame-drum f2 stationary. the result on the large screen 2! will be only a single, stationary. horizontal line. With the screen-drum I3 rotating and tie mirror-drum stationary, the result is a series of images moving vertically downward on the large screen 23. and which are visible only as a blur. However, with the mirror-drum rotating at the proper speed and in the right direction, the vertical movement of the image serie will be immobilized to cause the moving images to stand still on the screen 23 and the action of the subject to be reproduced.

In normal operation, if the screen-drum is run more slowly, the picture will remain on the projection screen, but it will be collapsed in height. If the screen-drum is rotated too fast. the picturewill be stretched out in height, but it will remain fixed on the projection screen.

The mirror drum 2! may be driven by a separate motor 24, or the same motor may be used for driving bot-h drums II and 12.

As shown in Fig. 3, in lieu of the magnetic flux coupling between the motor II and the spindle I4, one end of the latter may pass through the wall of the evacuated tube through a grease seal, at which point there is a conical section 26 which is held seated in the srease bearing by the atmospheric pressure. The use of such driving means is made possible by the slow speeds of rotation required.

From the foregoing it will be seen that in our improved method and apparatus, it is possible to use, for the screen, materials such as phosphorescent or incandescible materials which, although more desirable, could not be used in the prior methods and constructions on account of their relatively high time lag or decay period.

In our improved construction, furthermore.

the greatest possible efliciency is obtained because of the fact that the image is taken from the same side of the screen surface which is scanned by the ray I2. Also, on account of the arrangement and operating action, there is no keystone and no variation in focus of the ray on the screen surface.

In cases where it is desired to use certain types of interlaced scanning, it may be desirable to cause a small amount of intermittent deflection ofthe ray I2 in a direction at right angles to the line-scanning dimension. in which case a second set of electrostatic plates or electromagnetic deflection coils may be used for this deflectlon.

In the transmitting system shown in Fig. 4, except for the following the construction and action is the same as in Figs. 1 and 2. The drum Ito, corresponding to the drum ii in Fig. 1, is provided with a mosaic, photosensitive surface. A

' light image of the object 21, after reflection from the mirror-drum 22a, is proiected by the lens system 2 la onto the photosensitive surface of the drum l3a. .A collector electrode, which may be in the form of a metal plate as provided with a slot 29 as shown in Figs. and 6, is supported close to the drum surface at the region thereof bombarded by the electrons of the ray lie, the arrangement and size of the slot 2! being such that during normal operation the electrons of the ray can pass freely through the electrode 28 to the drum surface,

With the drum "a rotating in the counterclockwise direction at the desired uniform rate, and with the mirror-drum Ila rotating at the proper speed and in the right direction accordlog to the same principle in Fig. 1, there will be produced on the surface of the drum Its a succession of complete, electron images of the object 21, each comprised of photoelectric charges corresponding respectively to the lights and shadows at the corresponding elemental areas of the object 21. These photoelectric charges would, if desired, remain on the photoelectric surface for an appreciable time, which might be a period greater than that for one revolution of the drum Ila. There is therefore no loss in emciency while any part of the drum surface rotates from lightimage position to the position at the slot 2| of the collector electrode. As each elemental line at of the photoelectric surface of drum Ila moves across the slot fl, It is scanned by the ray I In which is being deflected at line-scanning frequency by the plates "a. Electrons of secondary emission, represented by the arrows 3 I, are thereby released. and in intensity corresponding to the respective photoelectric charges along the line 30 being scanned at the instant. On account of the close proximity of the electrode II to the drum surface, an appreciable percentage of the secondary emission 3| may be collected by this electrode and fed by the connection 32 to an amplifier and transmitter circuit, as shown. The current in the return circuit of the electrode 32 will consist of a certain direct-current component modulated by the video signals. The elsetrade 28 need be only slightly positive with respect to the adjacent photosensitive surface, in order to collect the desired secondary electrons. Since the electron path is relatively short, the

output impedance of the collector electrode 28 is low as compared to such impedance in the various constructions and methods proposed heretofore. This is advantageous where a wide band of frequencies is to be used, and results in a better signal-to-noise ratio.

After any line 30 passes beyond the collector electrode 2|, there may still be some electron charges remaining on the mosaic surface. For the purpose of removing such remaining charges. an erasing electrode 33 is employed. This electrode is supported close to the mosaic surface, and may be. for example. at about one hundred volts positive with respect to such surface. The mosaic surface, therefore, proceeds from the erasing electrode 33 to the re-exposure position in an uncharged and uniformly light-sensitive condition. Shading efiecm are thus eliminated.

In the transmittingsystem shown in Fig. 7, the object is inthe form of a moving picture film it moved at a uniform rate in front of a mask I! provided with a slit aperture 88. The rate of linear movement of the film may be the same as the peripheral speed of the drum Isa provided the lens III: is arranged to give one to one image size, and this rate is determined by the desired frame frequency, as will be well understood. In operation, as the film moves at constant speed past the slit aperture 3|. an image of this slit is projected onto the mosaic, photoelectric surface of the drum Ila, producing photoelectric charge thereon in accordance with the light and shade conditions along the respective linear element of the him. The operating MtiOl, otherwise, is the same as in Figs. 4, 5

In Figs. 4 and I, the ray may be positioned with respect to the slot 29 by the plates Ila.

In the receiving system shown in Fig. 8, there is no mirror-drum as in Fig. I, and a difference in the construction shown in Fig. 8 resides in the fact that the screen-drum ilb is rotated intermittently by the motor lib through a Geneva gear mechanism 31 of any suitable, conventional construction. Also, in Fig. 8 a shutter SI of a conventional construction is used, and is disposed between the lens system lib and the external screen 23b. Both sets of deflecting platesareusedinl ig.8,sothattherayflbis deflected simultaneously at the line-scanning frequency and at the frame frequency to scan a frame area of the drum surface.

In operation, while the screen-drum lib remains stationary, a complete picture is scanned onto one frame area of its surface. At the end of, one complete picture-scan, the screen-drum is rapidly moved, in the direction indicated by the arrow, a selected distance such that the scanned picture is in the field of the projection lens. During this movement of the screen-drum, the shutter 38 covers the projection surface of the lens. At the end of the intermittent movement, the drum lab is again stationary, and the shutter will have reached a position to permit passage of the beam to produce an enlarged image of the picture on the external, viewing screen 2312. This image is projected for a period during which a second picture is being scanned onto a different and succeeding frame area of the stationary screen-drum.

If desirable, to eliminate flicker, the shutter 38 may be provided with a second blade, as shown in Fig. 9, and operated as in a motion picture projector, to quickly cover and uncover the picture while it remains motionless.

when the second picture has been completed, the drum llb is again moved and the above described action is repeated.

If 24 pictures are to be produced each second, the design of the Geneva gear mechanism ll could be such as to cause the drum llb to remain stationary for about 95 of a second, and to be moved to its new position during the next M of a second, making a total time cycle of ,5 of a second. Such operating action is illustrated in Fig. ll.

a,sso,sse

It will be understood that the shutter 38 may llizbdriven, through suitable gearing, by the motor It is of course preferable to have the screen drum constructed with a minimum of weight and low movement of inertia to facilitate its intermittent, motion without excessive driving power requirements.

In Fig. 7 the intermittent drive shown in Fig. 10 might be used, in which case a standard motion picture projector, with intermittent film movement and associated shutter, would be employed, and the two intermittent movements would be synchronized so that the drum and film would be stationary during the-same periods. Likewise in Fig. 4 the mirror-drum could be omitted, and an intermittent and shutter employed as in Fi 8.

It will be understood that various modifications, within the conception of those skilled in the art, are possible without departing from the spirit of our invention or the scope of the claims.

This application is a division of our prior application, Serial No. 195,939, filed March 15, 1938, now Patent No. 2,268,523, December 30, 1941.

We claim as our invention:

1. In a cathode-ray device for television transmission, a tube, screen structure in the form of a drum rotatably supported in said tube and having a photosensitive operating surface, means for developing a ray of electrons directed at said surface, means for deflecting said ray, an electrode supported in proximity to said surface at the region thereof bombarded by the electrons of said ray, and a connection from said electrode to a point exterior of said tube.

2. In a cathode-ray device for television transmission, a tube, photosensitive screen structure in the form of a drum rotatably supported in said tube, means for developing a ray of electrons directed at the eflective surface of said screen structuremeans for deflecting said ray. means for imparting rotary movement to said drum to cause said surface to be scanned by said ray, and means including a mirror drum for projecting a series of images of the view for transmission on said moving screen surface.

STEWART L. CLOTHIER. HAROLD C. HOGENCAMP.

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