US2250476A - System for phototransmission - Google Patents

Description

July 29, W41. 5, R, EVANS SYSTEM FOR PHOTOTRANSMISSION Filed June 14, 1938 Illlllfllllll "(llllllll Hill.

HORIZONTAL VERTICAL DEFLECT/O/V DEF-56770 I RAD/0 AMPUF'EQ TRANSMITTER -L-' I)?! Z l EfiT/CAL r- DEFLEC TOR Kl/VESCOPE P/C ruRE fiAo/o VOLUME REC AND AMP BACKGROUND 1 CONTROLS F szaw-pms/sravcc SCfif/F/V H0/?/Z0/VTAL DEFLECTOR IN-VENTOR Patented July 29, 1941 UNiTED STATES PATENT OFFICE" 2,250,476 p SYSTEM FOR PHQ'I'OTRANSMISSION Earl R. Evans, Jackson Heights, N, Y.

Application June 14, 1938, Serial No. 213,574 g Claims. (or 1786.8)

This invention relates to a method of and system for electrically transmitting pictures or images at a relatively slow rate as compared with the transmission of successive pictures in television systems whereby a comparatively narrow -1 frequency band is suitable for transmission and certain difficulties encountered at present in television systems are obviated.

The object of the invention is to simplify and improve phototransmission and reproduction systems and methods.

According to one feature of the invention apparatus is provided for quickly forming an electron image or electrical charge image to be transmitted and for then transmittingand-reproducing said picture at a relatively slow rate, for example during an interval of several seconds. In'this mannersuccessive snapshots of news events and the like may be transmitted at frequent intervals and reproduced for an observer for a suitable period with comparatively simple transmission and receiving apparatus; Since the speed of transmission may be only about one one-hundredth as great as that employed in television, an ordinary radio channel ofa width of ten kilocycles or less may be employed for the transmitting channel or ordinary wire lines may be used whereby the space or distance limitations of the high-frequency television channels are overcome and the radio transmitting and receiving apparatus is greatly simplified.

In accordance with the invention suitable storage mechanism for the picture at the transmitting station is employed whereby the picture may be transmitted at a desired rate after it has been formed upon a picture sensitive element such as the mosaic screenof an iconoscope. Likewise at the receiving station a transitory o-r evanescent image is formed and retained for a short time. The received picture is stored for asufficient time to be viewed by the observer, as for example, by forming the picture on an electro-luminescent screen of the retentive or slowpersistence type utilizing a phosphorescent ma- In the preferred form of the invention-an elec-- tronic image of the'picture to be transmitted is produced on the screen of an ic'onoscope similar in most respects to that described in a paper by V. K, Zworykin in the January, 1934, Proceedings :of the Institute of Radio Engineers.- In a television systememploying the iconoscope the picture carrier contains high-frequency com ponents which require a Wide frequencyb'and for transmission and necessitates the use of eX-' tremely high-frequency radiation in order to avoid interference with the broadcast range'of frequencies. The use of these high frequencies, however, involves great engineering problems and since the waves can be received only over relatively-short distances, a greatmany repeaters must be employed to cover more than a small area; In addition to the complexity and cost of this system", serious difficulties result from-in teraction between the separate transmitters and other phenomena. 4

In accordance with the invention, however, transmission is effected by the use of a narrow frequency band often kilocycles or less similar to that employed in the transmission of 'sound programs. Thus the transmission involves no more di-lficulty than -in=sound broadcast systems which are in general use, and the transmitting station can utilize one of the regular broadcast channels and broadcast the photo-transmission signals over a large area; The screen of the iconoscope is preferably arranged to be subjected to the picture to be transmitted for only a short time, for example a tenth of a second or less The screen may be subdivided into minuteareas as by ruling "dividing lines to separate the elements thereof in order that the charges applied to the'elementsof the screen will not be unduly diminished for twoor three seconds or untiltransmission-can be effected; The scanning beam of the iconoscope is operated to scan the picture which has been produced upon the screenin a desired period, for example two or three seconds. As this period is approximately one hundred times as great as the scanning speed of a television system the frequency band required for transmission is only a small fraction of that required for television. The time required for transmitting each picture obviously depends upon the size of the picture and fineness of detail, and upon the band Width of the transmitting channel but as indicated is ordinarily of the order of one hundred times as long a-sthe time required to transmit one frame in television.

At "the receivingstation the picture may bereproduced upon any suitable means which will retain the elements of the picture long enough to be viewed, for example during the transmission period and for two or three seconds thereafter. A kinescope may be employed to reproduce the picture from the received signals, the kinescope being a conventional television receiving element described by Zworykin in the Proceedings of the Institute of Radio Engineers in December, 1933. However the kinescope in the present system preferably employs a phosphorescent screen or so-called slow-persistence screen in which the luminescence of the activated portions of the screen persists for the desired period. Screens of this character have been known and used in cathode ray tubes for various purposes, one example being disclosed in the patent to DuMont No. 2,000,014, dated May 1'7, 1935; and standard RCA cathode ray tubes designed for other uses are available with a slow-persistence screen having suitable characteristics.

Other objects and advantages of the invention will appear from the following description of the preferred embodiment shown on the accompanying drawing wherein Fig. 1 shows an arrangement according to the invention for transmitting pictures intermittently; and

Fig. 2 shows an arrangement for receiving and reproducing picture signals.

With reference to Fig. l, the reference numeral I designates the cathode ray tube for generating picture signals. This tube or iconoscope comprises a suitable photosensitive, mosaic screen 2 which may be made up of a great number of minute, photosensitive silver globules supported on a metallic base plate and each being insulated from the latter and also from each other either by their spacing or by ruling dividing lines between adjacent areas of the screen surface. A suitable electron gun 3 operates to develop a ray 1 of electrons and to direct the same at the screen structure. The tube is provided with a grounded anode. for example, in the form of a silver coating ID on the inside surface. The ray 1 is caused to scan the screen structure during intermittent periods and during the periods between the intermittent scanning periods the ray is returned to the initial starting point for the next succeeding scanning movement. During each scanning period. the ray scans the screen along a saw-tooth path. The control circuits and elements of the transmitting and receiving apparatus, except as hereinafter described, may be similar to those described 'in articles by Kell. et al., and Holmes, et al., in the Proceedings of the Institute of Radio Engineers for November, 1934.

According to the invention it is desired to expose the mosaic screen 2 momentarily in order to snap a picture of a scene in much the same manner as is done with the ordinary form of camera in taking a snapshot and to accomplish this result I provide a shutter 14 of the type which opens for any desirable period, such as TKth or ggth of a second. The light rays from the object I6 are focused through the shutter on the mosaic screen 2 by means of a suitable optical system represented by the lenses I8 and i9. In order to control the shutter I4 I provide an electrical control comprising a solenoid 20 for oneratin the shutter which is connected across the battery 22 and in series with the switch 23 and lamp 2 Lamp 24 is provided for the convenience of the operator so that he may know when picture transmission and reception is completed. The switch 23 may be controlled by a motor driven cam 30 which is driven from the motor 3! through the reduction gear train 32 and clutch 33. The gear train 32 drives the cam shaft at one revolution every six seconds, assuming that the successive pictures are sent at intervals of not less than six seconds. The clutch 33 is electrically controlled by means of the solenoid 35 which is connected across battery 38 and in series with the switches 42 and 43 respectively. With the arrangement shown, the solenoid 35 controls a latch engaging the movable element of the clutch and the clutch remains engaged until the cam shaft returns to its initial position. When the operator sees a scene which he desires to transmit, he closes the switch 42 to engage the clutch 33 to in turn drive the cam 30. As cam 30 rotates the switches 43 and 23 will be closed thus operating the shutter control 23. Switch 43 is provided in order to prevent the operator from stopping transmission of an individual picture. When he opens switch 42 the transmission does not cease until a complete picture has been transmitted and switch 43 opens the clutch circuit; or the clutch control elements may be constructed as shown so that the cam shaft always comes to rest in the position shown irrespective of the time of release of the armature of the magnet 35.

The motor 31 also controls the vertical motion of the cathode ray 1 by means of a cam 34 which closes the switch 35 to impress a starting impulse from battery 31 on the vertical deflecting circuit so labeled in the drawing. The cam 34 is arranged as shown in the drawing to control the vertical deflecting circuit. When the mosaic screen 2 is exposed and the electron image is created, switch 35 is closed to start the sawtooth wave generator and thereby start the scanning of the electron image, the scanning period being approximately three seconds. At the end of the three second period, switch 35 is opened for three seconds by cam 34 so that the vertical deflecting circuit causes the vertical scanning to cease while the picture is being viewed at the receiver. This deflecting circuit may be of the conventional type comprising a saw-tooth oscillator of the type wherein a condenser is alternately charged by a battery and discharged when a discharge tube is tripped to create a low resistance path therethrough. The starting i-mpulse generated when switch 35 is closed will bias the grid of the discharge tube in the saw-tooth oscillator circuit to such a potential that the tube is blocked and allows the potential across the condenser to rise. Thus the condenser is charged slowly for three or more seconds and discharged rapidly when switch 35 is opened to generate a saw-tooth voltage which may be impressed on an amplifier which may convert the saw-tooth voltage wave to a saw-tooth current wave and impress it on the vertical deflecting coils 40. The effect of the saw-tooth current is to cause the vertical scanning movement of the cathode ray 1 and the rapid return of the same to its initial position after the vertical scanning movement is completed.

In order to provide horizontal deflection of the cathode ray 1 a self-oscillating saw-tooth wave enerator so labeled in the drawing is provided for the purpose of impressing saw-tooth.

current through the horizontal deflecting coils M. This horizontal deflection circuit may comprise a condenser alternately charged to a potential which will be of suificient amplitudeto break down a path through a gas discharge tube of the conventional type the output of which will be a voltage wave of a saw-tooth formation since the condenser is slowly charged and rapidly discharged. The saw-tooth'voltage in turn impressed across an amplifier the output ofwhich is a Saw-tooth cur-rent.- Thiscurrent produces horizontal deflection at the required rate or 11 lines in three seconds, where n is the number of lin-esin the picture'and thr'ee seconds is the transmitting time. 1 ii g The picture Signals from the mosaic screen 2 and the horizontal'and vertical deflecting voltages are all impressed'across the amplifier shown in the drawing whereby the output of this amplifier will be a voltage Wave comprised of vertical and horizontal synchronizing signals interspersed with picture signals. This may be fed to the input of a conventional radio transmitter and broadcast in the usual manner.

Fig. 2 shows a television receiver comprising a conventional picture radio receiver and radio frequency amplifier. The output of the radio frequency amplifier is connected to the vertical deflecting circuit, the picture signal amplifier and the horizontal deflection circuit. The deflecting circuits may be of the conventional type wherein filters are used for selecting the proper synchronizing impulses and saw-tooth wave generators are controlled thereby. The output currents of the deflecting circuits are connected in the usual manner to the deflecting coils of a slow persistence picture reproducing cathode ray tube and the picture signal amplifier is connected to the control electrodes of the same tub-e whereby the picture signal is reproduced on a fluorescent or phosphorescent screen. In accordance with this invention the fluorescent or phosphorescent screen must be of the kind which has a considerable lag for instance 3 to 10 seconds so that when the picture is reproduced on the screen it may be viewed for a period of 3 to 5 seconds before the following picture is received. The brightness and duration of the reproduced picture is also dependent upon the intensity of the cathode ray beam and may be regulated by varying the same as in the ordinary cathode ray o'scillograph.

According to this invention it is desired to create quickly or practically instantaneously an electron image on the mosaic screen of the transmitting tube I and subsequently scan the electron image by means of the cathode ray I at a slow rate, for instance during a period of 2 secends or more, depending upon the band width of the transmission channel available and the number of picture elements to be transmitted or reproduced. The purpose of this slow scanning rate is to reduce the frequency of the picture current variations and thereby minimize the difi'lculties of long distance transmission. To start transmission the operator of the transmitting camera will close the switch 42 to start the earns 33 and 34. Cam 33 controls the shutter l4 through switch 23 so that the switch 23 energizes the shutter to momentarily expose the mosaic screen to the light reflected from the object I6. At the same time that the operator closed his starting switch the cam 34 closed the switch 35 to allow the saw-tooth generator to start the generation of a saw-tooth voltage whereby the vertical deflecting circuit starts the scanning ray across and down the mosaic screen. The horizontal deflecting circuit is allowed to continuously oscillate-so that the cathode ray beam constantly' 'scans themosaic screen ina horizontal 'directio'nf Owing to the nature of this systemit-is not necessary to start the vertical and horizontal deflection at thesame instant since it isimmaterial if a portion of a line is omitted from the picture. During idle periods wh-en no' picture'is transmitted, the ray maybe allowed to oscillate across the top of screen. The earn 30'is arranged to rotate once in, say, six seconds so 'that if continuous operation is desirable the shutter I4 will be opened every six seconds; the scanning period for each picture frame may be 3' s-econds'an'd. the viewing period may also'be' 3 seconds. If the operator desires to photograph a particular scene which will occur within a period of 7 o'r'8 seconds he maystop transmission by opening the switch, the result of which is to continue the previous frame of the picture until the cam 30 opens switch 43 to thereby deenergize coil 35 and disengage the clutch 33. When the desired action or scene occurs the operator may start operation of the picture transmitter by again closing the switch 42. Cam 34 is arranged to close the switch 35 at the same time that the shutter 20 is operated and for a period of 3 seconds so that the picture will be scanned vertically and horizontally for 3 seconds, after which, the cathode ray will oscillate horizontally at the top of the mosaic screen for a period of 3 seconds, namely the viewing period.

The observers at the receiving stations will be shown views of the game or other event which is being photographed at six-second intervals, 3 seconds of which is the scanning period and the remaining 3 seconds of which is the viewin period during which the picture remains on the fluorescent or phosphorescent screen of the picture reproducing tube.

The particular timing described is by way of example only and may be varied to furnish any desired viewing and scanning periods.

It may be desirable to enlarge the reproduced picture, in which case a cathode ray projecting tube may be substituted for the ordinary reproducing tube. Such a device and the manner of operation is described in the Proceedings of the Institute of Radio Engineers, vol. 2-5, of 1937, pa es 937 to 976.

Further modifications and advantages within the scope of the appended claims will be obvious to those skilled in the art.

I claim:

1. In. a photo-transmission system, means for storing and transmitting pictures or images in succession at intervals of the order of a few seconds. means for varying the start of successive cycles of said storing and transmitting means to vary the spacing of successive pictures and picture-storage receiving means for reproducing and temporarily maintaining each of the successive pictures or images for a period of at least a few seconds.

2. In a photo-transmission system, means for storing and transmitting pictures or images automatically at intervals of the order of a few seconds, manually operable means for controlling the start of successive cycles whereby selection of the pictures to be transmitted is permitted and picture-storage receiving means for reproducing and maintaining the pictures or images long enough to be viewed by an observer.

3. In a photo-transmission system, means for storing successive pictures or images, timing means for controlling the transmission of the pictures or images at intervals of the order oi. a few seconds and receiving means comprising a cathode ray tube provided with a slow-persistence screen for reproducing the pictures or images long enough to be viewed by an observer.

'4. In a picture transmission system, a mosaic screen, a shuttercfor exposing said screen to form an electron image, means for generating a beam of electrons, means for deflecting said'beam of electrons in both horizontal and vertical directions, switch means for controlling said vertical deflecting means, a second switch means for controlling said shutter, cams for operating said switches, driving means for rotating said cams, a clutch through which said cams are driven, and means for controlling scanning of the mosaic screen whereby successive pictures forming separate and distinct impressions are visibly reproduced.

5. In a. picture transmission system, a mosaic screen, a shutter for exposing said screen to form an electron image, means for generating a beam of electrons, means for deflecting said beam of electrons in both horizontal and vertical directions, switch means for controlling said vertical deflecting means, a, second switch means for controlling said shutter, cam means for operating said switch means, driving means for rotating said cam means, a clutch through which said cam means are driven, and means for controlling the shutter and the scanning means whereby successive pictures are visibly reproduced.

EARL R. EVANS.

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