US2281893A - Television scanning apparatus - Google Patents

Description

May 5, 1942.

TELEVISION SCANNING Filed June '21,

J. VAN DER MARK APPARATUS 1939 3 Sheets-Sheet l INVENTOR JAN VAN DER MARK ATTORNEY y 1942- J. VAN DER MARK I 2,281,893

TELEVIS ION SCANNING APPARATUS Filed June 21, 1939 3 Sheets-Sheet y I, I I

INVENTORY JAN VAN 0? MARK ,uru-M/ ATTORNEY y 1942- J. VAN DER MARK 2,281,893

I TELEVISION SCANNING APPARATUS Filed June 21, 1939 5 Sheets-Sheet s INVENTOR JAN VAN DER MARK ATTORNEY Patented May 5,1942

UNITED STATES PATENT ,TOFFICE algnor, by meme assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June 21, 1939, Serial No. 280,283 In the Netherlands June 23, 1938 4 Claims. ('01. lie-1.2)

also means for obtaining a scanning beam, usually a beam of cathode rays, which is caused to travel over the screen by means of deflector coils or plates in such manner that the various points of the screen are successively scanned. Thus one obtains in an output circuit a current, the ampli- I tude of which is varied as a function of the brightness of the picture points to be scanned.

It is found, however, that a picture scanned by such an electron scanner does not exactly correspond to the original picture after reproduction and, that particularly, shade formations occur which were not present in the original picture. One reason for this phenomenon lies in the fact that during scanning of the points of the screen secondary electrons and photo-electrons are dislodged which may return to other points of the screen and produce at these points potential variations. Owing to this, the potential at those points will no longer correspond to the brightness of the picture point there projected.

To avoid this undesirable phenomenon, according to this invention, means are provided,

between the stationary or moving picture to be transmitted and the radiation-sensitive screen of the electron-image scanner, which ensure that at least part of the picture points adjacent each other in the direction of line scanning is projected in an exposed manner and another part is projected in an unexposed manner onto the screen such that exposed and unexposed parts of adjacent picture points adjoin each other. In addition, means are provided for deriving a current from the output currents obtained after exploration of the screen, the amplitude of this current being substantially dependent upon the diiierence between the successive amplitudes of the output currents brought about by scanning of successive exposed and non-exposed portions of the screen.

The invention will be more clearly understood with reference to the accompanying drawings in which:

Fig. 1 shows one embodiment thereof; Figures 2 through 6 are explanatory curves; Fig. '7 shows a lens raster construction. The form of construction shown in Fig; 1 comprises an electron-image scanner .I together with a photoelectric screen 2 on which the picture I to be transmitted is projected by means of an optical-system constituted by an objective 4, a field lens 5 and a second objective 1. Between the picture 3 to be transmitted and the screen 2, there is positioned behind the field lens 5 a line raster 6, whose lines are normal to the direction in which the lines or the picture proj'ectedbnto the screen 2 arescanned.

Due to the presence of the raster 6, it is ensuredthat at least part of the points of the picture 3 adjacent each other in the direction of line'scanning is projected in an exposed manner and part is projected in an unexposed manner onto the screen such that exposed and unexposed parts of adjacent picture points adjoin each other. opaque lines of the line raster 6, if a picture has 'to be scanned according to 405-1ine definition and the ratio of the length of a picture line to the height of the image is 4/3, while only is of the length of the picture lines is scanned (the remaining 1 6 part is reserved for synchronization purposes) is in this case a 4 9 -405--486 These opaque lines are separated by transparent lines of equal number and equal width.

The screen 2 of the electron scanner l on which the picture 3 to be transmitted is thus projected is constituted by a number of insulated photoelectric particles 8 applied on an insulating underlayer which is carried by a conductive area 10, to which is connected the output circuit of the electron image scanner.

a voltage is set up through a resistance It connected to the conductive area iii, the variation of this voltage as a function of time for the duration of a scanning line being schematically represented in Fig. 2. This figure shows the voltage e .01 the point P relatively to the point Q earthed,

for example, and this voltage is negative.

Due to the scanning electron beam striking successively exposed and unexposed parts of the screen 2, the output current of the electron image scanner and consequently the voltage By way of example, if the number ofv through the resistance I3 is periodically interodically interrupted is double the highest picture current frequency determined by the numberof picture points to be scanned per second.

When the scanning electron beam strikes'an exposed element of the screen 2, the amplitude in of the voltage through the resistance 53 is determined firstly by the brightness of the part of a picture point projected onto this element and,

secondly, by the variation of potential which this a element has obtained due to the secondary electrons and photo-electrons being dislodged during scanning from otherelements of the screen, which electrons have returned to the screen and have in part gained access to the screen element in view.

When the scanning electron beam strikesthe next non-exposed element of the screen i. the amplitude as of the voltage through the resistance 13 is exclusively determined by the variation of potential due to the secondary electrons and.

photo-electrons fallen on this unexposed element, since the number of secondary electrons striking adjacent exposed and unexposed elements of the screen 2 is approximately the same. The difierence between the amplitudes a1 and a2 is merely dependent upon the brightness of the point of the picture 3 to be transmitted which corresponds to the exposed and unexposed element of the screen 2. l

Consequently, a voltage must be derived from the voltage through the resistance it, the amplitude of this voltage being determined by the difference between the successive amplitudes a1 and or across the resistance l3 to a rectifier, for example a diode 11, through an amplifier i4 transmitting all frequency components of the said voltage, and through a condenser i5, which rectifier may be supplied with a bias, a leakage resistance It being provided between anode and cathode of the diode. The voltage ea set up in the output circuit of the amplifier H, i. e. the voltage of the point R relatively to the point S earthed, for example, is represented in Fig. 3. This voltage corresponds to the voltage of the point P relatively to Q, the amplification brought about by the amplifier it not being considered. The voltage of the point R relatively toS is positive because the voltage variations amplified by the amplifier H are superimposed on a constant D. C. voltage, viz. the D. C. anode voltage of the last amplifying tube of the amplifier M. The polarity of the voltage an is such that during the occurrence every time of the amplitude oz in the voltage e through the resist This may be effected by supplying the voltage that the voltage through the condenser cannot follow the rapid decrease of the voltage ea to the voltageea which comes after the impulse I1, but

can follow the slow variation of the amplitudes of ,the impulses I1, 12, I3 etc. the condenser voltage ance I3, the anode of the diode I1 is more positive than during the occurrence of the amplitude at in the voltage e When the anode of the diode I1 is supplied with a. positive voltage with respect to the cath ode, a current flows in the diode charging the condenser l5. If the diode is dimensioned sumciently large so that this is capable of supplying a high current at low anode voltages, the condenser line.

cc, 1. e. the voltage of the point T relatively to R will have a variation as indicated in Fig. This voltage as therefore is varied only in accordance with the slow variation of the impulses I1, 12, 13 etc.

The voltage en set up across the diode i'i is the diiierence between the output voltage ea of the amplifier it and the voltage as through the condenserv i5. This voltage an is represented in Fig. 5 and, as will be clear without further etiplanation, this voltage varies with thedifierence between the amplitudes ea and ea etc. up to and including ca and 6's Consequently, this voltage so does not contain anymore the component giving rise to shade formations.

From the voltage en may be obtained, by means of rectification, a current or voltage the amplitude of which is varied in accordance with the enveloping curve or in accordance with the average curve indicated in dotted line in Fig. 5.

With the usual electron-image scanners, the scanning electron beam is generally suppressed after completion of 'the scanning of each line or each image. Consequently, the voltage through the resistance 13 is reduced to nought at the end of each line or each image, as shown in Fig. 2. Accordingly, the output voltage of the amplifier it takes the most positive value at the end of each line or each image (see Fig. 3) with the result that the condenser is charged to a maxi mum voltage at the end of each line or each image, as represented in Fig. 4.

When the next line to be scanned begins at a level corresponding to this maximum voltage of the condenser l 5, as indicated in Fig. 4, no trouble is experienced. In most cases, however, the starting level of a line will not correspond to the maximum voltage which is the case when components giving rise to shade formations occur already inscanning the first picture points of the In this case, the condenser voltage cannot suddenly decrease to the value corresponding. to the starting level with the result that during reproduction a new disturbance arises in the image reproduced. I

According to the invention, this is obviated by preventing the condenser from being charged further after completion of the scanning of each line or each image. This may be ensured by blockingthe rectifier at the same time as the scanning electron beam is suppressed. In'this' case, the condenser I5 is discharged between the scanning of two successive lines or images to a voltage ec which is lower than the starting level of the next line or the next image rectifier having a control electrode. which elec-- trode is supplied with such a voltage during the time between the scanning of two successive lines or images that this voltage blocks the rectifier in the manner known per se and prevents the con denser it from being charged. Preferably. for this purpose the synchronization impulses are supplied to the said electrode.

Another method of removing the component of the voltage through a resistance II, which gives rise to shade formations, is based on the recognition of the fact that the frequency spectrum of the voltage set upthrough the resistance It comprises two frequency ranges, as indicated in Fig. 6. Firstly, a frequency range 3-!) con-- sisting of two side bands located symmetrically with respect to the frequency 1'' with which the output currents of the electron-image scanner, due to the presence of the line raster I, are periodically interrupted and on which the picture currents are :modulated, and secondly a frequency range O-A comprising the picture currents and the disturbing component giving rise to shade formation. In case of a really rectangular course of the impulses from which the voltage through the resistance It is built up, this voltage comprises further frequency ranges consisting of two side. bands of equal width as DC and CD which are located symmetrically with respect to the higher harmonics of the fre- 30 quency I". These side bands of higher order are not necessary for a proper working and may therefore be suppressed in the amplifier with the method described. In this case, however, the impulses in Figs. 3 and 5 are no longer rectangular, but sinusoidal.

Whentheline raster. has been chosen inthe above-mentioned manner the frequency 1" is double the highest picture current frequency so that the frequency ranges O-A and 8-D do not overlap each other. a

By conducting, according to the invention, the voltage set up through the resistance It across a selective device such, for example as a bandpass filter or a high-pass filter which transmits only frequencies higher than the highest picture current frequency. i. e. the highest frequency required for reproducing a picture with a certain definition, consequently the frequency range 3-0. the disturbing component giving rise to shade formation is suppressed. The picture currents transmitted by the filter and modulated gig:

iggi

a portion of the amount of light intended. for projection becomes lost. This loss may be avoided by utilizing a lens raster known per se instead of a line raster. One form of construction comprising such a lens raster is represented in Fig. '7. In this construction, a line of the picture to be transmitted is represented by the arrow It. By means of a lens IS, an image of this line is projected onto the radiation-sensitive screen I! of an electron-image scanner. Between the lens Ii and the screen It is interposed a lens raster 1i constituted by a number of cylindrical lenses mounted next to one another, which number is equal to the number of lines when use is made of a line raster whose axes are perpendicular to the plane of the drawing.- image of the lens I! is projected onto the screen 20 by each of the cylindrical lenses in such manner that the image of the line it projected on the screenll is constituteialternately by exposed and unexposed parts of approximately equal width.

what is claimed:.

1. In a transmission system for transmitting representations of an optical image, apparatus for correcting for spurious signal and black spot efiects comprising photo-sensitive target means onto which said optical image is electron beam scanning means for said photo-sensitive target means at recurring intervals, means interposed between said photo-sensitive target means and said optical image to interrupt the impression of discrete sections of the image onto said photo-sensitive means. means for sectionally comparing the valueof the signals developed from the portions of the target means onto which said image falls with signals developed from the target means from I the sections thereof which lack the discrete interrupted sections of the image to be transmitted, said latter means including electrical energy storage means for storing energy representative of the light values on said scanned sections of the target, a leakage path for said electrical energy stonage means, rectifying .means connected substantially in parallel with said leakage path, means for blocking the scanning action of the target scanning means during one direction of its motion, and means for blocking said rectifying means substantially simultaneously with the blocking of said scanning means.

2. Apparatus in accordance with claim 1, wherein said rectifying means comprises a diode.

3. Apparatus in accordance with claim 1, wherein said rectifying means comprises a thermionic uni-directional conductor having anode. cathode and at least one control electrode, and wherein the impulses for blocking the action of said rectifying means are'impressed onto the control electrode-cathode circuit of said thermionic uni-directional conductor.

4. Apparatus in accordance with claim 1, wherein said photo-sensitive target is positionedwithina'cathoderaytubeincludingmeans 'fordeveloping and'accelerating anelectronbeam withinthe envelope of the tube. and wherein the means for interrupting discrete seotiom of the optical image impressed onto the photo-sensitive-mea'ns is positioned within said envelope.

between the photo-sensitive side of said screen JliNvsnsssMARK.

Images