US2202605A - Television system - Google Patents

Description

May 28, 1940- F. scHRTER TELEVISION SYSTEM Filed Aug. '7, 1937 2 Sheets-Sheet 1 w QW INVENTR' F/Q/TZ SCHROTER BY f? ATTORNEY May 28,1940. F. scHRTER TELEVISION SYSTEM Filed Aug. f7, 19357 2 Sheets-Sheet 2 IN V E NTO R ,fR/rz SCH/zov-ER A AV J l-uw- ATTORNEY Patented May 28, 1940 UNITED vSTATES y 2,202,605 TELEVISION SYSTEM Fritz Schrter, Berlin, Germany, assigner to Telefunken Gesellschaft fr Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation oi Germany Application August 7, 1937, serial No. 157,818 l In Germany August 29, 1936 5 Claims. (Cl. 178-68) My invention relates broadly to the transmission of optical imagesand moreparticularly to a method of and apparatus for transmitting these images by television.

In the usual picture transmission method, more particularly in connection with television work, it is the absolute brightness or shading values of the constituent picture elements` that are transmitted to a remote point even in cases where they have not changed as compared with the values of the corresponding picture element in the :transmission of the preceding frame in moving picture transmission. The fact that this means anA unnecessary load on the transmission channel, both from the View point of the intensity of the signals to be transmitted as well as also, possibly, from the viewpoint of the required frequency band should be appreciated.

Therefore, according to the present invention the. brightness or shading or densityvvalues of the picture elements are transmitted only to the extent where, contrasted with the corresponding and respective picture elements of the preceding frame, they have experienced an actual change. But in accordance with the basic idea, namely, of transmitting only differences or diierent values, that is to say, to transmit to a distant point, as it were, only one picture which could be designated as a difference picture, 'rather than the entire picture, there are `conceivably quite a number of different embodiments.

Among the letter there is an embodiment vml which transmisison occurs at a velocity independent of the picture content and where, for instance, if the picture is dissected or scanned in straight parallel lines or strips, one -works with a constant line and frame frequency. Among.

the methods of transmitting` pictures at a velocity independent of the picture content may be regarded to fall also a spiral scanning method wherein the velocity of the scanning pencil is constant or wherein the velocity insidev the course or time of scanning each individual frame is subject to variation according to the same law. A transmission to a remote point of a difference picture which proceeds at a rate of a' velocity which, in the sensefas hereinbefore outlinedis independent o f the picture content, oiers the merit and advantage over the transmission method involving-always the absolute values of picture element brightness or shading as heretofore disclosed that the transmission channel, that is to say, for instance, an RF .carrier wave will be called upon to carry' smaller ,amplitudes `and that as a result interference of neighboring transmitters will be diminished.

But within the scope of the basic idea of the invention the diierential picture could be transmitted also at a rate of speed which is dependent uponV the comparative brightness values. This is feasible, for instancaby what is known in the art as velocity modulation. In this method, the scanning beam or pencil is so acted upon in reference to its yspeed that at the receiving end, the intensity of the pencil re-creating and delineating the image may stay constant and stable, while, on the contrary, the speedv of its migration or sweep over the area on which the incomingv image is to be re-createdwill be so much smaller the brighter any particular spot or element yof the picture is tobe re-created. If the differential picture is transmitted by this principle, then the scanning pencil or beam will have to sweep all those portions of the dilerence picture at the sending end at a high rate of speed at which no change has occurred, while the delineating speed will be slower in the case of such picture elements where the difference picture or frame has a finite brightness or shading value.

However, inasmuch as the major part of Ithe elements of a television image is'altered from one frame to the next (imagining, for example, a

rscene in which a limited number of persons appear on a very variegated and detailed `background) the transmission of the difference picture, for the same high deiinition, is accomplishable inside a briefer period of time than when using the method known in the earlier art in whichA again and again, the absolute shading values of the picture elements were handled andr transmitted rather than` the differential values. However, fundamentally speaking this means that the width of the requisite transmission channel is reduced without this being associated with a 4 loss of picture detail ashas heretofore been true.- This progress could be utilized in practice either in the manner that for the transmission of the video signals a channel of reduced width is used or else that the picture so sent is made of higher deiinition and richer in detail, in other words, by accommodating inside a given yframer area a greaternumber of picture elements without the necessity of enlarging the corresponding transmission channel. 4

My invention will best be understood by reference to the drawings in which Fig.1 is an embodiment of my invention in which the differential picture. is transmitted at a velocity independent of the picture content,

Fig. 2 is a receiving arrangement, and

Fig. 3 is a schematic arrangement.

Referring to Fig. l, I0 denotes a strip of lm bearing frames II through I4. Anteriorly of the said lm strip are two devices I5 and I6, say, two Braun type or cathode ray tubes which for each of the film frames I2 vand I3 produce a spot designed to scan the two frames of the film in synchronism and in phase. Posteriorly of the film strip are mounted two photo-electric cells I1 and I8 which are provided with a common positive potential source of supply I9 and whose currents flow through two distinct resistances 2li,`

2I. These resistances 26 and 2l in the manner as shown in the drawing are united with the control grids of two tubes 22 and 23, the grid circuits thereof including also'the biasing voltage sources 24 and 25. The plate circuits of the two tubes contain the resistances 26 and 21 and distinct plate potential sources of supply 28, 29. Two cathode ray devices 36 and 3l have so-called storing surfaces 32 and 33, respectively are unbroken metal sheets (so-called mosaic elements). which latter are insulated from each other as well as from the joint co-operating coat so that in reference to this coat they represent tiny condensers.

j and 23, respectively. On the right-hand side of the storing surface 32 and 33, respectively, is a device known from the cathode ray picture scanning art and which, respectively, consistsy also of secondary emission anodes 44, 45, a pair'of deflector plates 46, 41, vanodes 48, 43, cathodes 50,

5I, and also control grids 52, 53, respectively.

r Connected with therdeflection plate pairs 36, 31

(which, however, could also be replaced by dev fiection coils) is a sawtooth wave generator 54 Y, which'furnishes deflection potentials for the two [pairs of plates producing deections in the two 'co-ordinate senses of the picture. "ing surfaces 32 and 33 and the anodes 44 and 45,

From the storleads are'brought respectivelyto amplifiers 55 and 56,l both of which feed an RF transmitter 51.

Also for the pairs of dei- lector plates 46, 41, which could yalso be replaced by deflector coils, there is provided va joint saw-tooth wave generator 58..

The control grids 52, 53 of the two storing dej vices are connected with a device 59 consisting, for example, of a so-called multivibratorA supplying positive and negative impulses of like duration and which furnishes the grids 52 and 53 alj 14 shown only schematically with the busbars 15;v 16 positioned perpendicularly in the picture proternately with a voltage designed to block out the cathode ray beam.

An arrangement of the kind shown in Fig. 1 f operates in the manner that upon the two'pho- :Vavtoelectrc cells I1 and I8 luminous uxesy corresponding to the absolute brightness or shading values of two congruous picture elements in the two frames I2 and I3 are made to impinge. In-

" asmuch as the drops of voltage across the re- Y' inthe two tubes 22 and 23, it will be'seen vthat the'potential of the control grid of tube 22 will be "shifted in the positivel sense only if the respective picture element of the lm frame I3 tube 23.

Vof supply 24 and 25 to suit this scheme is to l0 68 connection isestablished with two'switch-l ycorresponding to the' various picture points may lconsist, for instance, of small-sized.V cathode ray 'lamps I1 and 18 having'ajcontrol grid, with a condensor suchas` 19er 80, respectively,';being "interposed betweenthe control grid. and the..

cathode of each lamp.- It willbe understood that 70 happens to be darker than the corresponding picture point of the lm frame I2. In this case the potential of the control grid of the tube 23 shifts into the negative region seeing that the fall of potential across the-resistance 26 which furnishes 5 for the tube 23a negative grid voltage prevails over the fall of potential across the resistance 2| which represents a positive grid voltage for the The size of the biasing potential sources be chosen in such a way that, in the presence of the above assumptions, only'tube 22 will conduct current, while tube 23 is blocked. But if, conversely, the picture element under consideration. in lm frarneMI'Z is brighter than the respective 15 picture element in frame I2, then the drop of potential across resistance 2| willsurpass that across resistance 26 and tube 22 stays blocked, while rtube 23 will conduct current. Now. the cathode ray pencils issuing from cathodes 46 and 20 4I, by theaid of `control grids 42 and 43, re-

spectively, willbe acted upon and modulatedin accordance with the differential valuesof they picture shading or brightness vvalues, in other words, in accordance withthe brightness of the differential picture in such a way that the elementary capacities of the storing surfaces "32 and 331 overk which the modulated cathode-'ray beamis swept by the aid of the pairs of deector plates 36 and 31 will acquire charges which will 3G correspond `to the positive orthenegative dif- Y ferential values, as the case may be.- `The cathode ray pencils issuing from the cathodes 5l) andA 5I, by the aid of the deiiection fields set up by the sawtooth wave generator" 58 are sequentially 35 swept over thefstoring ` surfaces 32 and 33. "By

' virtue of the device 59 the beam Scanning thestoring surface 33 will be blocked out while the storing surface 32 is swept, and conversely, during v the scanning'of the storing surface 33the cathodeg\f40 ray `beam sweeping surface '32 will be blocked out. While these scanning actions are kproceeding the RF 4transmitter 51 will be acted upon consecuv tively'by the 'positive and the negative brightness Dwhich busbars 64-,66 are kconnected which correspond't'o the various lines of the picture reecreatorlor projection surface 61, while through lead devices :69, 18 which are co-ordinated to the 55 verticalrows ofthe picture re-creating means on'picturefsurface 61.v Twocorresponding contacts of the said switches-69 and 10, say, contacts 1I and 12,'are each connectedwthrough a tube 14,',60

jection lsurface. 61, the outputs of thesaid .two tubes having a high internal vresistance.being interlc'onnected. 'I 'he picture r'e`creating means. .65

While mechanical switches 63,669 and 10 have rays switch devices of suitable design.

The arrangement shown in Fig. 2' operates in themanner that While the switch arm or wiper 63 is positioned on busbar 64, and while the cathode ray beam scans, in the storing device 30 of Fig. 1, for instance, the positive differential values and then causes the same to be transmitted, the switch 69 of Fig. 2, transiently connects in sequence all cathode ray lamps which are connected with the busbar 64. Incidentally, the condensers 19, 80 of those cathode ray lamps will be further charged for which the respective picture elements exhibit an increase in brightness compared with the preceding frame, which means that positive values of the differential picture result in a charge of the condensers at the cathode ray lamps. After a given contact element of rswitch 69 has been abandoned, the charge upcn the corresponding condenser is preserved so that thev luminescent screen of the cathode-ray lamp will be struck by a constant cathode ray current throughout the entire time interval until the recurrence of transmission of a possibly altered `differential value for the picture element in question. After the positive differential values have thus been transmitted, a motion of the switch 10 will initiate transmission of the negative differential values in a similar manner. Suppose that while the scanning beam in the storing device 3I of the transmitter sweeps the storing surface, and when the contact 12 of switch 10 is contacted, a negative differential value, for example, is transmitted so that through the amplifier 14 a discharge of the condenser 19 will be ybrought about and that the other potential at the control grid of the cathode ray lamp 11 is reduced. After the contact step 12 has been abandoned, for the full interval of ytime until the transmission of the next differential value, the luminescent screen of this cathode ray lamp will be impinged upon by a constant cathode ray beam. y

As already pointed out, particularly the switches l69 and 10, but optionally also the switch 63 could be of the cathode ray type it being recommendable in this connection to so dispose the contacts in the switch devices 69 and 10 that both switch pencils, by the agency of one and the same deflectoreld, may be moved in synchronism and in phase, such that the pencil in switch device 1U will'reach its contacts only after the pencil in switch device 69 has already swept its contacts.

' This condition, in fact, has been indicated also in the case of the mechanical switches in Fig. 2 in that the angular path of both wipers comprise the same angle a., and that in the switch 69 the contact steps are disposed on the first half of the stroke, while in switch 1I) they are arranged on the second half of the stroke. In a similar manner arrangements may be made so that also in the storing means 30, 3|, the storing surfaces 32 y and 33, respectively, will be scanned sequentially'.

are of a substantially lower frequency than the picture frequencies heretofore customary in teleswitch 63 will be fairly low.

Another chance to carry the basic idea of the invention into practice, as already pointed out above, is that the scanning of the storing surfaces 32, 33 at the sending end is effected by what has been called as the velocity modulation method. For this purpose, the arrangement shown in Fig. 1 need tofbe altered but little, as illustrated in Fig. 3. The amplifiers 55, 56, which furnish the positive and the negative brightness or intensity values of the differential picture in a way as known from the velocity modulation method, act upon the sawtooth wave generators 58', 58; the potentials produced by them are applied, on the one hand, upon the deflector plate pairs 46, 41, while, on the other hand, they are fed to the transmitter 51. The scanning pencils, as previously stated, rapidly sweep over those portions of the storing surfaces 32, 33 where the f differential picture has no finite amplitude, whereas a slower scanning takes place only where the differential picture exhibits contrast of detail. In this manner a reduction in the requisite frequency band as above stated is realized, for

Avision 'work; hence, the speed of rotation of it will be understood that at all points along a line where the differential picture is of zero amplitude the scanning beam will slip over quickly so that no appreciable time for transmission is` required.

In case the velocity modulation method is employed at the sending end, the receiver apparatus maybe built fundamentally similarly as described by reference to Fig. 2. All that is required is that the switch arms or wipers of switch devices 69 and 16 must be caused to revolve at a rate of speed that will vary in accordance with the velocity modulation or scanning used at the sending end, rather than rotating the same at a constant rate of speed. Switch 63 which should in this case v in the switch 10.

As can be inferred from what precedes, re-

creator means have been assumed to exist at the f receiving end according to the present invention for the various picture elements which, after having been once excited, exhibit a constant after-effect.- In the exemplified embodiment as shown in Fig. 2 this is produced by that the condenser coats 19, 80, retain their charges set u-p or supplemented through the amplifiers 13, 14. In order to compensate for such gradual leaking away of these charges as may happen across the insulation resistance of these condensers, and in order to effect originally the adjustment of the picture, one may proceed in such a way that,

from time to time, the frame I2 of the film strip f is completely masked at the sending end, while over the picture field of frame I3 a shutter is made to travel which will expose, for instance, only a few lines of the latter picture eld. Thus the absolute values of the picture point brightness or shading of the latter picture section (which, because of the complete masking of the lm frame I2 are the same as the differential values) will be transrriitted and be recreated at the receiving end. Thereafter, for a comparatively long period of time, only the differential values in the brightness distribution of the picture can be transmitted, provided that there occursno complete change of scene in the picture to be transmitted to a distant point. While the major portion of the television picture is transmitted only in so far as the differential values compared with the preceding frame is concerned, it would also be feasible to transmit for a few picture lines always the absolute values of the picture element brightness and to change periodically the position of these picture lines. With this end in View, a few picture lines or strips inside the picture eld of frame i2 would have to be completely masked so that inside the picture field of frame i3 the absolute values rather than the differential values of the picture element brightness in the masked lines would be televised.

lvVhat I claim is:

l. A system for transmitting optical images by television comprising a transmitter for sequentially transmitting a differential value o-i simultaneously developed picture signals representing corresponding elemental areas on at least two complete picture representations, a receiver wherein each picturel point representation may be pre-set to an optical value representative of the optical value of the transmitted picture for one frame thereof, and means for sequentially changing the optical Values at the receiver in accordance with the transmitted differential representation. l

2. A television system comprising a transmitter, said transmitter including means for sequentially developing a differential representation of the optical values of each elemental area of at least fg two complete optical representations to be transrepresenting means to represent definite optical valuations, and means for sequentiallychanging said picture point representations in accordance with the transmitted diierential representation senting means at said predetermined value for a predeterminable length of time, andA means for sequentially changing the operating level of said picture point reproducing means in accordance with the transmitted differential representation.

4.' Apparatus in accordance With claim 2 wherein said receiver means comprises a plurality of light sources each representative of the picturev point of the optical image to be reproduced, a

vacuum tube means having each of said picture point reproducing means in the output circuit thereof, condenser means connected in the input circuit of each of said Vacuum tubes for maintaining tne tube at a predeterminable operating level, and commutating means adapted to eneri gize said condenser means and to control the charge on said condenser means in accordance with the transmitted diierential representations. Y

5. Method of television comprising the stepsy of simultaneously examining at least two framesof the image to be transmitted, developing a differ-l ential representation of each of the'corresponding optical points on both of said frames, transmit ting saiddeveloped representations, and contro-lling the optical image reproduction in accordance with the developed diiferential representations.

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