US2268523A - Method and apparatus for television communication - Google Patents

Description

Dec. 30, 1941.

METHOD AND APPARATUS F s. L. CLOTHIER EIAL 2,268,523

OR TELEVISION COMMUNICATION Filed March 15, 1938 3 Sheets-Sheet l mmm Dec. 30, 1941. s. 'CLOTHIER EI'AL METHOD AND APPARATUS FOR TELEVISION COMMUNICATION Filed March 15, 1938 3 Sheets-Sheet 2' mo, 1941. 5. L. CLQTHER Em 2 268,523

METHOD AND APPARATUS FOR TELEVISION COMMUNICATION Filed March 15, 1958 5 Sheets-Sheet 3 Patented Dec. 30, 1941 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR TELEVISION COMMUNICATION Stewart L. Clothier, East Orange, and Harold C. Hogencamp, Irvington, N. J.

Application March 15, 1938, Serial No. 195,939

7 1 Claim.

Our invention relates to improvements in methods and apparatus for television communicatiomand particularly to cathode-ray projection tubes for television reception and cathodetubes for television transmission.

In television communication employing at the receiving station a cathode-ray tube having a fluorescent screenwhich is scanned by a ray of electrons, it has been proposed to make the construction and thevarious electrical conditions such that the fluorescent image be sufficiently brilliant for projection onto a larger, externalscreen. Ithas been found, however, that on account of'the characteristics of the flourescent sc reen,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 eifects or characteristics 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 substantally greater than the frame or field 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 of 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 head 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 field period necessary to avoid fllicker. Furthermore, on account of the relatively high coefiicient of heat-conductivity of these screens, the 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 Patent No. 2,098,000, issued November 2, 1937, to

Philo T. Farnsworth et al., 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 electronray directed at such eleemntal 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 field period would have to be if there is to be no fllicker. Still another construction for such a screen is disclosed in Patent No. 2,097,994, issued November 2, 1937 to Harry S. Bamford, and comprises an incandescib-le 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 point of contact, the degree of incandescence at any one point being proportional to the intensity of 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 twenty-fouth of a second for good detail. Where interlaced scanning is employed, the field frequency may be as high as sixty, in which case the decay period must be no greater than one-sixtieth 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 ac- "count of the time lag, and at a frame frequency of twenty-four, or at the high fleild frequency of sixty, these screens, if useful at all, would only be so for a still picture. Furthermore, in the screen constructions proposed heretofore, selection of screen materials has necessarily been limited to those having a period of decay or a time lag sufi'iciently low for a given frame or field frequency to avoid excessive blurring, so that use of more likely materials, such as phosphorescent or incandenscible materials having a decay period many times the frame 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 invention 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 efliciency and better operating action.

Other objects and advantages will hereinafter appear.

For the purpose 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 22 in Fig. 3 is a view similar to Fig. 2, showing a modification; I

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, taken on the line 55 in Fig. 4;

Fig. 6 is an enlarged sectional view, taken on the line 66 in Fig. 5;

Fig. 7 is a'view similar to Fig. 4, showing a modification;

Fig. 8 is a view similar to modification;

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

Fig. 10 is an elevational view partly in section, the section being taken on the line l0l0 in Fig. 8; and

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

In Fig. l of the drawings, the reference nu- Fig, 1, showing a erates to deflect the ray I 2 in one dimension only, and in a plane parallel to the axis of ro meral l0 designates a cathode-ray tube provided with an electron gun ll of any suitable, conventional construction, for developing a ,ray l2 of electrons directed at the effective surface of the screen structure which is in the form of a drum I3, rotatably supported in the tube. For this purpose, the drum 13 is fixed on a spindle l4 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 tation of the drum l3. However, the ray I2 may be deflected electromagnetically, if desired, and a second set of deflecting plates or other deflection means may be employed for vertical positioning of the ray.

In operation, deflection of the ray l2 in the one dimension effects line scanning, and movement of the screen surface in the direction transverse to this dimension effects frame scanning. With picture signals applied to the control electrode or grid IQ 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 2| to a large external screen 23, the beam of light producing the large images being first reflected from the mirror drum 22, which effectively 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 twentyfour mirrors pass a given point at the periphery in one section. The diameter of the screen drum [3 in such case will be such that with this drum being 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 l2 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 l3 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 per second. Other figures and speeds can be 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 the product of the frame frequency and the height of one screen-drum image.

: 'Ihe operating action may be analyzed in the following manner. With both the screen-drum i3 and the mirror or frame-drum 22 stationary, the result on the large screen 23 will be only a single, Stationary, horizontal line. With the screen-drum l3 rotating and the mirror-drum stationary, the result is a series of images moving-vertically downward on thelarge 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 series will be immobilized to cause the moving images to stand still onthe 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 picture will be stretched out in height, but it will remain fixedon the projection screen.

-The mirror drum 22 may be driven by a separate motor 24, or the same motor may be used for driving both drums l3 and 22.

Q As shown in Fig. 3, in lieu of the magnetic flux coupling between the motor l5 and the spindle l4, one end of the latter may pass through the wall of the evacuated tube through a grease CJI seal,-at which point there is a conical section 26 which is held seated in the grease 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 efficiency 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 l2. 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 defiection of the ray l2 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 deflection.

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 I3a, coresponding to the drum I3 in Fig. l, is provided with a mosaic, photosensitive surface.

A light image of the object 21, after reflection from the mirror-drum 22a, is projected by the lens system Zia onto the photosensitive surface of the drum l3a. A collector electrode, which may be in the form of a metal plate 28 provided with a slot 29 as shown in Figs. 5 and 6, is supported close to the drum surface at the region thereof bombarded by the electrons of the ray l2a, the arrangement and size of the slot 29 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 l3a rotating in the counterclockwise direction at the desired uniform rate, and with the mirror-drum 22a rotating atthe proper speed and in the right direction according to the same principle in Fig. 1, there will be produced on the surface of the drum 13a a succession, of complete, electron images of the object' 21, each comprised of photoelectric charges corresponding respectively to the lights and shad-;

ows 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 onev revolution of the drum 13a. while any part of the drum-surface rotates from light-image position to the position at the slot 29 of the collector electrode. As each elemental line 30 of the photoelectric surface of drum l3a moves across the slot 29, it is scanned by the ray I2a which is being deflected at line-scanning frequency by the plates l8a. Electrons of secondary emission, represented byv the arrows 3|, 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 28 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 elece trode 32 will consist of a certain direct-current component modulated by the video signals. The electrode 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 28, 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 elecitrode 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 effects are thus eliminated.

In the transmitting system shown in Fig. '7, the object is in the form of a moving picture film 34 moved at a uniform rate in front of a mask 35 provided with a slit aperture 36. The rate of linear movement of the film may be the same as the peripheral speed of the drum l3a provided the lens 2!!) 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 36, an image of this slit is projected onto the mosaic, photoelectric surface of the drum l3a,, producing photoelectric charges thereon in accordance with the light and shade conditions along the respective linear element of the film. The operat- There is therefore no loss in efficiency ing' action, otherwise, is the same as in Figs. 4, 5 and 6.

In Figs. 4 and 7, the ray may be positioned with respect to the slot 29 by the plates 25a.

In the receiving system shown in Fig. 8, there is no mirror-drum as in Fig. 1, anda diflerence in the construction shown in Fig. 3 resides in the fact that the screen-drum I312 is rotated intermittentlyby the motor l5b through a Geneva gear mechanism 31 of any suitable, conventional construction. Also, in Fig. 8 a shutter 38 of a conventional construction is used, and is disposed between the lens system Zlb and the external screen 23b. Both sets of deflecting plates are used in Fig. 8, so that the ray l2b is 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 [3b remains stationary, a complete picture is-scanned onto one frame area of its surface. At the end of one complete picture-scamthe 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 l3b 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 232). 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 B2) 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 31 could be such as to cause the drum l3b to remain stationary'for about A of a second, and to be moved to its new position during the next ti foil a second, making a total time cycle of of a second. Such operating action is illustrated in Fig. 11.

It will be understood that the shutter 38 may be driven, through suitable gearing, by the motor [517.

It is of course preferable to have the screen drum constructed with a minimum of weight and low moment of inertia to facilitate its intermittent motion without excessive driving power re- I quirements.

In Fig. 7 the intermittent drive shown inFlg. 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 Fig. 8.

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

We claim as our invention:

In a television communication system, a cathode-ray device including an evacuated envelope; screen structure in the form of a substantially cylindrical drum disposed in said evacuated envelope, coated on its peripheral surface with a screen material which is luminous under the impact of an electron beam and supported for rotation about a given axis; means comprising a cathode-ray gun structure within said envelope for developing a beam of electrons and for focussing said electron beam to an elemental area on the coated cylindrical surface of said drum;

means for deflecting said electron beam to cause.

scanning of said drum surface; means for modulating the intensity of said beam in accordance with electrical impulses representative of received television picture signals; means for rotating said cylindrical drum; and means including a transparent window in said envelope for viewing luminous images produced on said cylindrical screen structure.

STEWART L. CLOTHIER.

HAROLD C. HOGENCAMP.

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