US2396865A - Signal-generating system - Google Patents

Description

A. v.' LOUGHREN 2,396,865

SIGNAL-GENERATING SYSTEM Filed March zo, 194s 2 sheets-sheet 1 Mafch 19, 1945.

March 19, 1946. A V, LOUGHREN- 2,396,865

SIGNAL-GENERATING SYSTEM Filed March 50, 1943 2 Sheets-Sheet 2 FIGB ' UMMILMWUUULVUUUMWWWWUU FIG-4- INVENTOR ARTHUR V. LOUGHREN Patenied Mar. 19, 1946 .UNITEDv STATES PATENT :o1-FICE- y SIGNAL-GENERATING SYSTEM v Arthur V. Longhi-en, Great Neck, N. Y.. assigner to Hazeltine Corporation, a corporationot Deb aware Application March 30, 1943, Serial No. 481,104

15 Claims.

This invention relates to signal-generating systems and is particularly directed to the reduction oi undesirable shading signals which are genery ally produced along with the desired signal components in those generating systems that utilize a camera tube as the signal generator. While the invention has a wide range of application, it is especially ysuited to television signal-generating systems, and in the description which follows, its apllication to suh ay system is described indeta In ay television signalvgenerating system oi the type under consideration, the camera tube may comprisea cathode-ray tube having a photosensitivemosaic target electrode, an electron gun for developing, accelerating and focusing anelectron beam on the target electrode, and means for scanning the target with the beam duringsuccessive scanning intervals. In such an arrangementan optical system establishes a charge distribution on the target electrode in accordance with an image to be translated so that during the scanning operation video-frequency signal components corresponding to the image are produced in the output circuit of the tube. It has been lfound, however, that undesired spurious signals, commonly referred to as' shading components; are v also produced duringthe scanning operation along' with the desired video-frequency signal components. If suchspuriQuS Signals ,arenas-remi wed. they' causellndesrblefshading in the reprbdug' ',fimage, n Y. p

In order l to`eliminate` `or effectively reduce 'the undesired shading "signals v studies 1 have been: made to determine their cause'.l These studies revealed that, during the scanning operation, secondary electrons are emitted from the target elec- Atrode and the return'if'rain-back of' these secn ondary electrons to the target produces a current which is one component of-the shadingsignal,

Since this phenomenon is produced by thescanning operation, the shading signal has compo- -nents which correspond infrequen'cy to the scan'- ning frequencies. It has been proposed, there,-

.Ifore, tominimize vthese shading components by .l inserting a suitable and artificial compensating signal in the video-frequency signal which `is derived from the cathode-ray signal-generating tube. It'has been found thatv such an articia-l signal having a saw-tooth, exponential, or paraj' bolic lwave form and having frequency compofnent's related to one 0r both 0f the Scanning tre:

quencies of the camera tube may, insome cases, -be effective tol reduce the shading signal to a limited extent. This method of compensation, how- (ci. iva-1.2) A' ever, has the disadvantage-that itfrequires elaborate and expensive equipment which is both difcult and inconvenient to operate successfully the loss of desired video-frequency components involves the use of a line raster or grid in the optical system, or an equivalent arrangement for 2l) causing the optical image on the targetelectrodeto be broken up or. for'causingjthe lines oi the l l target ,electrode tofhave alternately exposed and y unexposed elementary areas. vWhen the (electron beam .isf incident yupon an exposed elementary 25 area of the'gtarget electrode in such an arrangeexpQedelementar-y jarea, vthe output 3 0 sista solely. of shading components.-A

' nin'gftheesuccessivelyexposed elementary'areas the image isto be reproduced without impairment and furthe arrangement to-be operable, .the scanning 4spot must be ofy neness comparable, nottdthe'tele'v'ision image structure..but to the 45; structure er the une raster. Present techniques where this requirement can be readily met so `that such arrangements have limited'practical value.

It is anobject of the present invention, there-l fore, to provide an improved signalfgenerating system which'avoidsone or more of the above'- mentioned disadvantages of the prior art arrangements. It is another 55 ia signal-generating system having an improved ment, the`joutput signal consists of video-ffre-.xv quency-signalecomponents and shading compov nents'butwhenthe beam is incident upon anvjun-"z signalconv- Means are provided-@for deriving as e useful-signal rthe difev ference' between the signals obtained when scanthefvunexposed elementary v areas.Y While.

:35 such lan-Iarrangement represents what is believed,

to'betheproper approach to shading component 'v correctilitfisgsubjecteto Acertain limitations.- 11twill-bev understood that theline raster must have l an appreciablm ilnei` structure than -l ,the struc-I l 40 ture of thetelevision image .to be translated if in electron optics have, not advanced to astage object ofthe invention to provide in the description of the erating system comprises, a cathode-ray signaly l generating tube, a photosensitive target electrode n the tube, means for establishing a charge distribution on the target electrode in accordance with a signal to be generated, a source of beam current in the tube, and means for scanning the Vtarget electrode during a scanning interval with the beam to generate desired signal components having frequencies that extend over a range de- For a better understanding oi' the present invention, together with other and further objects thereof, reference is had to the following description taken in connection withthe accompanying drawings, and its scope will be pointed out in the appended claims.

,In the accompanying drawings, Fig. 1 is a circuit diagram, partly schematic.. of a complete television signal-generating .and transmittingl system embodying the present. invention: Figs. 2 to 7, inclusive, comprise curves used as an aid generating system of Fig. 1; Fig. 8 is a circuit diagram of a modification of a portion of the system of Fig. 1; and Fig.

9 comprises curves used as anaid in describing the operation of the circuit arrangement of Fig. 8.

Referring now more particularly to Fig. 1 ot the drawings, the television signal-generating and transmitting system illustrated comprises a cathode-ray signal-generating tube I5 having a photosensitive mosaic target electrode i6, a

termined by the charge distribution on the target electrode and which also develops undesired shading components with the desired signal components. The system includes additional means for periodically and substantially decreasing the current of the beam from a predetermined value during the scanning interval at a frequency which is at least as high nal component generated, and frequency-responsive means coupled to the tube having a frequency characteristic related .to the frequency at which the beam current is decreased for deriving a signal which includes the desired signal components and which is substantially free of undesired shading components.

In accordance with a preferred embodiment of the invention, the beam current of-a cathode-ray type signal-generating tube is periodically interrupted during the scanning of the target electrode, the interruption frequency being high compared with the highest video-frequency signal component generated. During intervals when the electron beam isincident upon the target electrode, the signal produced in thetube includes both desired video-frequency signal components and undesired shading components. During intervals when the beam is interrupted, however, the signal produced comprises only undesired shading components. Stabilizing means coupled tothe output circuit of the tube derive a signal which effectively comprises the diii'erence between the signals produced when the beam is incident upon the target electrode and when the beam is interrupted. Consequently, `the derived signal includes the desired video-frequency signal y components andis substantially free of the undesired shading components.

In accordance with another embodiment of the invention, means including a band-pass lter are coupled to the output circuit of the cathode-ray signal-generating tube for deriving therefrom a signal which includes desired video -i'requency signal components and which is substantially free ofundesired shading components. In this arrangement, advantage is taken of the fact that the periodic interruption of the beam current at a rate which is high compared with thehighest video-frequency signal component generated .produces a carrier signal modulated only by the as the highest-frequency sig- 4 desired video-frequency Signal components. The r band-pass lter of this arrangement is Adeslilrneeis to select the carrier signal and one or bgth of its f sidebands of modulation.

.source of beam current including anlelectron gun structure as well as focusing and accelerating electrodes lto which suitable operating potentials are supplied from a source Il, beam-deiiecting elements I8, AI8 and an output circuit including a load impedance 20. An optical means I4 is provided for establishing a charge distribution on the target electrode in accordance with an image to be transmitted.

VConnected in cascade to the load impedance 2li of signal generator l5, in the order named, are a signal-translating unit 33 to be described in detail hereinafter, a combining amplifier 2|, a modulator 22 and an associated carrier-frequency oscillator 23, a power ampliiler 24, and a radiating antenna system 25, 26. Deilecting voltages of saw-tooth wave form are supplied to deflecting elements I8, I9 from outputcircuits oi' a. linescanning generator 21 and a field-scanning generator 28, respectively, to cause the beam of signal-generating tube l5 to scan the target electrode in a series of elds oi parallel lines. Blockout voltagesv are supplied from generator 28 through a blocking condenser 60 to a control electrode 38 of tube l5 to suppress the beam during retrace scanning intervals. Output circuits of generators 21, 28, 28 are coupled to combining amplifier 2l to supply thereto synchronizing signals and block-'out signals for transmissionwith the radiated carrier wave. These generators are synchronized by timing pulses from timing-impulse generator -30 which is preferably stabilized by means of a connection 3i to a suitable source of periodic voltage, for example,v to the powersupply circuit or to the synchronizing-voltage source of motion picture mechanism where such is employed. A high-frequency generator 22 of conventional design produces a control potential whicn isapplied tothe control electrode of tube i5 for a purpose to be described more fully hereinafter.

Neglecting for the moment the' functions of units I2 and 33, thev system Ajust described comprises the elements of a television signal-generating and transmitting'system of conventional design and, since the various elements I4 to 2i, inclusive, thereof may be of any well-known construction, a detailed description ot the general systemand its operation is unnecessary. Brie'ily, however, the image of a. scene to be transmitted is focused on., the target electrode i6 in signal-' generator tube Il and video-frequency signal components having amplitudes and frequencies aandoet that extend over a range determined .by the motion be at leasttwice as'nign as the highest charge; distribution on the target electrode and v'the' scanning frequencies are developed-across vload impedance 20 in the usual manner and are applied through unit 33 to vcox'nlziinin amplier synchronizing-signal and block-out signal components from generators 21, 28 and 23 areV also applied to amplifier 2| in which they are mixed with the video-frequency signal componentsand amplified. The amplied composite signal is supplied to modulator 22 where itis so impressed upon the carrier wave generated by oscillator 23 as to develop a modulated-carrier i signal. This signal is delivered to power amplier 24 for amplification therein and is thereafter impressed upon antenna system 25, 25 to be radiated. v

The signal appearing acrossvload impedance 20 of signal generator I when the generator is utilized in a conventional television signal-transmitting system of the type just described will be considered now more particularly. As is well un derstood, the photosensitive elements of target video-frequency 4signal componentv lgenerated and, in particular, it is desirable that the period of interruption be short compared with the average transit'time-of the returning secondary electrons.. The curve of Fig. 4 representsthe variations in beam currentduring thel scanning'interval in response to .this-control potential.A `It will be seen that the beam current is periodically 1 video-frequency signal components and undeelectrode i6 acquire charges dependent upon the varying values of light in the corresponding elemental areas of the image focused thereon by the optical system M. The curve of Fig. 2 is assumed to represent such a charge distribution for a portion of a single scanning line on the target electrode. In this gure, rzero value of charge represents the white level inthe image focused on the target and a charge value of -Ci vrepresents the black level in that image. The polarities of the ordinate axis have been assigned in a manner opposite to conventional practice in order to facilitate a comparison of the charge-distribution curve with curves, to be discussed hereinafter, representing the generated signal. When this line portion is scanned with ay cathode-ray beam, video-frequency signal components are produced in the output circuit of the generator. At the same time, however, secondary electrons emitted from the target electrode during the scanning operation rain back thereon and this electron movement develops the above-mentioned undesired shading components which also appear in the output circuit of the generator. Consequently, the signal produced across load impedance 20 'under the assumed condition may be as represented by the curve of Fig. 3. A comparivson of this curve with that of Fig. 2 shows that the undesired shading components are represented by the general slope of the curve of Fig. 3. If this slope is reduced to substantially zero value, the undesired shading components are removed and the resultant signal contains only the desired video-frequency signal components.

' This result is obtained, in accordance with the` decreased from a normal. predetermined value Ii to substantially zero value, indicated In.

When the'line portion o! target velectrode I6 having the charge distribution' represented by the curve of. Fig. 2 is scanned with the periodically vinterrupted beam, a signal is'developed across load impedance 2liA in the output circuit ofv .thegenerator as represented by the curve of Fig.

5. The general slope of this curve is again assuxned to be produced by the undesired shading components. Hence, it will be seen .that the instantaneous value e1' of signal voltage obtained when the beam .isenergized includes desired sired shading components, while the instantaneous value es of signal voltage obtainedduring intervals when the beam is interrupted consists only of vundesired shading components. Unit 33 is coupled to load impedance 20 for selecting, as a signal for transmission, the diierence between signal values er and e2 which recur in alternation during any scanning interval.

This unit comprises a videofrequency amplifier 38 having an input circuit coupled across load impedance 20 and an output circuit to which is connected a signal-stabilizing means-including Y a diode rectiiler 35, coupling condenser 36and' load resistor 31. Amplier 34 is selected -to have a .substantially flat frequency-response characteristic for a range of frequencies including. the desired' video-frequency componentsI shading l components, and interruption-frequency compresent invention, through the addition oli-units '32 and 33 to the generating system.

. Unit 32 comprises means for periodically and I ysubstantially decreasing the current of the beam in vgenerator l5 `from a predetermined, value, or

the value which it would normally have during; the scanning interval, at a Afrequency which -is at least as high as the highest video-frequency signal component produced in the generator. In accordance with a preferred embodiment of the invention this unit'delivers to control grid 38- of generator l5 a control potential havinga substantially rectangular pulse waveform and of such value that the beam current of the -ger'i- -erator is periodically interrupted Aeluring the' scanning of each line of the target electrode; Also, it is preferred that the frequency of interl ponents so that the signal from generator l5 vmay be translated .therethrough without distortion. Further, in order to ystabilize this signal,

the values of condenser 36 and resistor 31 are so chosen as to give the signal-stabilizing means a time constant which is long'compared with the period of vinterruption of the beam current and short compared with the line-scanning period of the signal generator.

m considering the operation of unit sa, it win be seen that the composite signal produced across load 4impedance 23 of generator I5 which includes desired video-frequency signal components and undesired shading components is applied to the input terminals of amplifier 34 and, after amplification therein, appears at the output terminals with reversed polarity and Asuperimposed on the direct current Asignal component of the amplier asindicated` by the curve of Fig. 6, This amplified signal is peak-rectied in diode 35 and, due to the long time constant of condenser 36 and resistor 31, is effectively stabilized on its' positive peaks. That is, these peaks of the sig- -nal are aligned at' a fixed potential level, as indicated by the curve of Fig. '1, whereby the general slope of the curve which has been assumed to represent the `shading components is'reduced to a zero value. It will be clear upon a comparison of the curves of Figs. 2 and '1 that the signal thus applied to combining amplifier 2l for transv mission includes the'desired video-frequency sign al components and is substantially vfreeof the undesired shading components.

Since the time' constant ofthe stabilizing circuit is proportioned with respect to the period `at control potential of sinusoidal wave form. In ksuch `case also, the generating which the beam current is interrupted, unit 33 comprises frequency-responsive means coupled to tube IB having a frequency characteristic related to the frequency at which the beam current is decreased or interrupted for deriving a signal which induces the desired signal compo-r nents and which is substantially free Vof undesired shading components.

The described operation of unit 33 in removing the undesired shading components from the generated signal may be considered from a slightly diifer'ent view. The signal voltages developed by the camera tube under the inuence -of the scanning beam are inversely related to the values of incident light in the `image focused on the targetl electrode. In other words. small values of signal voltage are obtained during the scanning of those portions of the target electrode that are brilliantly illuminated, while large values' of signal voltage are obtained during the scanning of asados l tentials are supplied from sources indicated as +B and Se. The described amplifier arrangement may be substituted for video-frequencyl amplifier 34 in the television system of Fig. 1 for selecting the modulated carrier wave from the output circuit of the generator. After amplification, this wave is applied to diode 35 where its modulation components are derived and delivered tb combining amplifier 2l for transmission.

This modified form of the invention operates by virtue of the fact that a carrier wave modulated by the desired video-frequency signal components is produced ,across loadv impedance when the beam current of generator I5 is periodically interrupted at a carrier-frequency rate during the scanning interval. The carrier frequency of this modulated wave is equal to the frequency of beam current interruption so that the darker portions of .the target. Accordingly,

when the beamcurrent is interrupted, the sig'- nal obtained from the generator should represent an absolute level of picture brightness comparable to the white level. Therefore, the periodic interruption of the beam current .effectively produces recurrent signal components having a predetermined datum level corresponding to the white level of the image to be transmitted. Unit 33 in stabilizing the generated signal establishes the recurrent signal level which represents an absolute level of incident light at a xed potential and thus removes the undesired shading components.

In the'foregoing description of a preferred embodimentof the invention it was stated that the beam current control potential supplied by generator 32 should have a substantially rectangular pulse wave form. It is well known that vsignals of such wave form are rich in harmonics whereby a wide pass band is required to translateA them without distortion. 'I'he pass band requirement maybe reduced considerably -by utilizing a or other suitable system of Fig. 1 operates substantially as described to ldevelop a` television signal which includes the desired video-frequency components but which is substantially free of undesiredshading components.

In Fig. 8 there is disclosed a modification of the frequency-responsive means which is coupled to generator Isin accordance with the present invention for deriving a signal that includes comprising a transformer v4 5 double-tuned by condensers Il, included in its output circuit. Each i'llter section is tuned to the carrier frequency of a modulated carrier wave which, as

' will be described presently, is developed across the load impedance. 2l of signal generator I5.

A self-biasing arrangement consisting of a resistor 5t, by-passed.. for high-frequency signals bycondenser 5l, is provided in accordance with conventional practice and suitable operating-porange from 0 to fr and include desired video-fre if the interruption frequency is at least twice as high as the highest video-frequency signal' component generatedfupper and lower sidebands of modulation components individually representing the desired video-frequency signal components are produced and may be readily selected from the output circuit of the generator through a bandpass filter arrangement of the type just described. The undesired shading components do not appear as modulation on the carrier wave since the shading signal has substantially the same value during intervals when the beam is effective and sucneeding intervals when the beam current is interrupted so long as the period of interruption is high compared with the average transit time of the returning secondary electrons.

'.An operating characteristic of the arrangement of Fig. 1, assuming the radio-frequency amplifier of Fig. 8 to have been susbtituted for the video- 'frequency amplier 34, is disclosedin Fig. 9 where broken-line curve A represents the signal components generated in tube I5 during a scanning interval if the beam current is not interrupted.v

'I'hese signal components extend over a frequency quency signal components and undesired shading components. The operating frequency fz of generator 32 is considered to be adjusted to a value which is greater than twice that of frequency f1 corresponding to the highest video-freist quency signal component generated. Accordingly. the modulated carrier wave available in the output circuit of the generator, as illustrated by curve B of Fig. 9, comprises a carrier-wave component having a carrier frequency fz, a lower sideband of modulation components extending over the'frequency range fa-h, and an upper sideband of modulation components extending over the frequency range .f2-f4. the sidebands of modu. lation individually representing the desired videofrequency components. Since the modulated carrier wave is spaced in the frequency spectrum from the range O-ji, it may be readily selected to the exclusion of shading components by adjusting the tuned circuits of the radio-frequency amplier to have a mean operating frequency f2 and a pass band corresponding to the frequency range fs-fl. If necessary, damping resistors may be added to the tuned circuits in well-known Y manner to obtain the required frequency range for the lter circuits. These resistors are indicated in dotted construction in Fig. 8 since they may be comprised in part or in whole of the inherent resistances of the tuned circuits. After amplication in the radio-frequency amplifier, the modulated wave is applied to diode 3'5 where its modulation components which represent the desired Vshading components. then beapplied to diode 'may be transmitted directly Vassesses components derived at theutilizingapparatus, all

-as aforedescribed. v f t K 7It.will alsobe' understood that generator 32 need not necessarily be operated at afixed frej video-frequency components are derived and supplied to combining amplier 2| for transmission in `a conventional manner.

Thus in the described'modiflcation of the invention, cooperation with the quency-responsive means coupled to tube l having a frequency characteristic related to the frequency at which the beam'current is decreased or interrupted for deriving a signal for transmisthe radio-frequency amplifier of Fig. s in diode 35 comprises fre-l sion which comprises the modulation components l of the modulated-carrier signal developed in the tube and which is substantially freeof undesired shading components.

In certain applications it may be desirable to transmit the carrier wave modulated with the I desired video-frequency components and to derive the modulation components by detection in the usual manner at the receivervorutilizing apparatus. In such instances a radio-frequency ampliilerof the type disclosed in Fig. 8 may be coupled to the load impedance of tube |5.` The pass-band characteristics ofthe amplier are to be chosen in the manner already described so that the amplifier comprises frequency-responsivemeans coupled to ,the tube having a frequency characteristic related tothe frequency at whichthe beam current is decreased or interrupted for deriving a signal which comprises the modulated carrier wave and which is .substantially free of undesired shading components. The modulated carrier wave, after amplication, may be applied directly to an antenna system 'or it may be. applied to a frequency changer and heterodyned toa frequency more suitable for radiation. In these applications the necessary synchronizing signals may be conveniently transmitted to the utilizing apparatus by way of a separate channel or carrier wave.

It will be understood that it is not necessary to select both sidebands of modulation components quency operation of I While there-havebee'n describedwhat are at skilled inthe art since the entire range of video-frequency signal components is included within either one. Therefore, the radio-frequencyamplifier may be adjusted, if desired, to select only one of the sidebands of modulation. It will'also be understood that the pass band of theamplifier need only be sufficiently wide to select those sideband com` ponents that represent the video-frequency-signal components which the associated apparatus is capable of utilizing. In other words. even when transmitting a single sideband of modulation components, it will not be necessary to transmit Aal1 the modulation components if., the receiving apparatus is able to utilize only a limited portion of the video-frequency -signal components'produced in-the generator 15. Accordingly the phrase "desired'signal components asemployed.

f :throughout this description and in the appended f claims is intended to mean the signal components desired for'u'tilization.

Furthenuit maybe seen that generator '3 2 may l be operated at avfrequency comparable to that of the highest video-frequency signal component to 'l be generated. Inf such a case, however, it is necessaryl to adjust the tuned circuits of the radiofrequency amplifier to select theuppersideband #fof modulation components in order to obtain a signal which is substantially free of undesired The selected signal may components are derived and supplied tofcomloining amplier 2l for transmission,

and its modulation 35 where its modulation' or the signal quency since theinterruption frequency may vary over a considerable range without adversely affecting the operation of the system. ,Accordingly, the phrase .means for periodically and substantiahy decreasing the ycurrent lof :the beam and similar expressions used'in the foregoingr de. lscription and 'in the appended' vclaims is intended to include both fixed-frequency and variable-frethis generator.

present` considered tofbe the preferredemb'odiy.ments of this invention,

that' various changesandmodiflcations :may be made ,threinwithout departing from the invention, and it is, therefore', aimed in the appended claims to cover all such changes and modifications as fall within the true spirit andscope of theinvention. l' i Whatis claimed is:

erating ,systemy comprising, a

l. A signal-ge' cathode-ray sig al-generating tube, a photosensitivejtarget electrodel in -said tube', means for' establishing a charge distribution on said target electrode in accordance with a signal Ate be gen- `erated, a source of be'am current in said tube,

`means for scannings'aid target electrode duringk a scanning interval with said beam to generateA desired signal components having frequencies that extend over a range determined by the charge distribution on said target electrode andA which also develops undesired'shading components with said desired components, means for periodically and substantially decreasing thevcurrent of saidjbeam from a predetermined value during said scanning interval at a frequency which is at least as high as the highest-frequency signal component generated, and frequency-responsive means coupled to said tube having a frequency characteristic related to the frequency at which said beam lcurrent is decreased for deriving a signal which' includes said desired signal components and .which is sub- `tantia1ly`free of said undesired shading com-.

ponents. j

2. A signal-generating system comprising, a

' cathode -ray signal-generating tube, a photosensitive target electrode in said tubetmeans for establishing a charge distribution on said target electrode in accordance` with a signal to b e generated,

'a source of beam current in said tube, means for scanning said target electrode during a scanning interval with said -bcam to generate desired signal components having frequencies that extendl over a range determined by the charge distribution on said target -electrode and Whichalso 'develops undesired shading components with said y -I y desired components, .means `for periodically andsubstantially decreasing thecurrent of said beam from a predetermined value during said scanning said beam current is decreased for deriving a siginterval at a fixed` frequency which is at least as high as the vhighest-frequency signal'com--' and frequency-responsive v ponent generated, means coupled to said tube having a frequency characteristic related to the frequency at which nal which includes said desired signal components and which is substantially free of said undesired shading components.

l3; A signal-generating ysystem comprising, a

.cathode-.ray signal-generating tube, a photosensitive'target electrode in said tube, means for establishing a charge distribution on said target 4elecit will be obvious to those fated', a for scanning said target electrode during a scantrode in accordance with a signal to be generated, a source of beam current in said tube, means for scanning said target electrode during a scanning interval with said beam to generate desired signal components having frequencies that extend over a range determined by the charge distribu- `tion on said target electrode and which also develops undesired shading components with saiddesired components, means for periodically and substantially decreasing the current of said beam from a predetermined value during said scanning interval at a frequency which is atleast twice as high as the highest-frequency signal component generated, and frequency-responsive means coupled tosaid tube having a frequency characteristic related to the frequency at which said beam current is decreased for deriving a signalV which includes -said desired signal components and which is substantially free of said undesired shading components.

4. A signal-generating system comprising, a cathode-ray signal-generating tube, a, photosensitive target electrode in said tube, means for establishing a charge distribution on said target electrode in accordance with a signal to be genersource of beam current in said tube, means ning interval with said beam to generate desired signal componentshaving frequencies that extend over a range determined bythe charge distribution on said target electrode and which also develops undesired shading components with said desired components, means for periodically interrupting the current of said beam during said scanning interval at a frequency which is at least as high as the highest-frequency signal component generated, and frequency-responsive means coupled to said tube having a frequency characteristic related to the frequency at which said beam current is interrupted for-deriving a signal -which includes said desired signal components and which is substantially free of said undesired shading components.

5. A television signal-generating system comprising, a cathode-ray signal-generating tube. a photosen'sitive target electrode in said tube, means for establishing a charge distribution on said target electrode in accordance with an image to -be generated, a source of beam current in said tube, means for scanning said target electrode during a scanning interval with said beam to generate detrode during a scanning interval with said beam ,u

to generate desired signal components having frequencies that extend over a range determined by the charge distribution on said target electrode and which also develops undesired shading components with said desired components, means for applying to said control electrode a control potential for periodically and substantially decreasing the current of said beam from a predetermined value during said scanning interval at a frequency which is at leastas high as the highest-frequency signal component generated, and frequency-responsive means coupled to said tube having a frequency characteristic related to the frequency at which said beam current is a signal which includes said desired signal components and which is substantially free of said undesired shading components.

7. VA signal-generating system comprising, a

cathode-ray signal-generating tube having a control electrode, a photosensitive target electrode in said tube, means for establishing a charge distribution on said target electrode in accordance with a signal to be generated, a source of beam current in said tube, means for scanning said target electrode during a scanning interval with said beam to generate desired signal components having frequencies that extend over a range determined by the charge condition of said target electrode and which also develops undesired shading components with said desired components, means for applying to said control electrode a control potential having a rectangular-pulse wave form for sired video-frequency signal components having frequencies that extend over a range determined by the charge distribution on said target electrode and which also develops undesired shadingA components with said desired-components, means for periodically and lsubstantially decreasing the current of said beam from a predetermined value during said scanning interval at a frequency which is at leasthas high as the highest videofrequency signal component generated, and frequency-responsive means coupled to said tube having a frequency characteristic related to the frequency at which said beam current is decreased for deriving a signal which includes said desired video-frequency signal components and which is substantially free of said undesired shading components. y

6. A signal-generating system comprising, a cathode-ray signal-generating tube having a control electrode, a. photosensitive target electrode in said tube, means for establishing a charge distribution on said target electrode in accordance with a. signal to be generated, a source of beam current in said tube, means for. scanning said target elecperiodically and substantially decreasing the current of said beam from a predetermined value during said scanning interval at a frequency which is at least as high as the highest-frequency signal componentl generated, and frequency-responsive means coupled to said tube having a frequency characteristic related to the frequency at which said beam current is decreased for deriving a signal which includes said desired signal components and which is substantially free of said undesired shading components. i

8. A signal-generating system comprising, a cathode-ray signal-generating tube having a control electrode, a photosensitive target electrode in said tube, means for establishing a charge distribution on said target electrode in accordance with a signal to be generated, a source of beam current in said tube, means for scanning said target electrode during a scanning interval with said beam to generate desired signal components having frequencies that extend over a range determined by the charge distribution on said target electrode and which also develops undesired shading components with said desired components, means including a high-frequencyl generator for applying to said control electrode a control potential for periodically and substantially decreasing the current of said beam from a predetermined value during said scanning interval at a frequency which is at least as high as the highest-frequency signal component generated, and frequency-responsive means coupled to said tube having a frequency characteristic related to the frequency. at which said beam current is decreased for deriving a signal which includes said desired signal coinponents and which is substantially free of said undesired shading components.

.9. A signal-generating system comprising,4 a cathode-ray signal-generating tube, a photosensitlve target electrodeI in said tube, means for establishing a charge distribution on said target electrode in accordance with a signal to be generdecreased for deriving y ning interval with said beam ated, a source of beam current in said tube, means for scanning said'target electrode during a scani to generate desired signal components having tribution on said target electrode and which also develops undesired shading components with said desired components, means for periodicallyand substantially interrupting the current of said beam during said scanning interval at a frequency which is at least as high as the highest-frequency signal component generated, and peak-rectifying means coupled to said tube having a time-constant which is long with respect to theperiod of' said interruption frequency for deriving a signal which includes said desired signal components and which is substantially free of said undesired Shad-- ing components.

10. A signal-generating system comprising, a

cathode-ray signal-generating tube, a photosensitive target electrode in saidl tube, means for establishing a charge distribution on said target electrode in accordance with a signal to be generated, a source of beam current in said tube, means substantially interrupting the currentof saidk beam during said scanning interval at a frequency which is at least as high as the highest-frequency signal component generated to produce recurrent signal components having a predetermined datum level, and means coupled to said tube having a frequency characteristic related to the frequency at which said beam 'is interrupted for stabilizing said last-named signal components at a fixed signal level to derive a signal which includes said frequencies that extend over'a range determined by the charge disdesired signal components and which is substanv tially free of said undesired shading components.

l1. A television signal-generating system comprising, a cathode-ray signal-generating tube, a photosensitive target electrode in said tube, means for establishing a charge distribution on said target .electrode in accordance with an image tob generated, a source of beam current in said tube, means for scanning said target electrode during a scanning interval with said beam to generate desired video-frequency signal components having frequencies that extend over a range deter- `mined by the charge distribution on said target electrode and which also develops undesired shadrupting the current of said beam during said l scanning interval at a frequency which is at least as high as the highest video-frequency signal component generated to produce recurrent signal components having a datum level correspondingfrequency signal component y a carrier signal modulated by said desired signal components, and frequency-responsive means t yteristic related v tion components.

electrode in accordance with' a signal to be generated, a source of beam current in said tube, means for scanning said target electrode during a scanning interval with said beam Ato generate desired signal components having frequencies that extend over a range dition ofsaid target electrode and which also develops undesired shading components with said desired components, means for periodically and substantially decreasing the current of said beam during said scanning which is at least twice'as high as the highestgenerated to develop coupled to said tube having a frequency charactothe frequency at which said beam current is decreased for deriving a signal which comprises said modulated carrier signal and which is substantially free of said undesired shading components. f

13. A signal-generating system comprising, a

cathode-ray signal-generating tube, afphotosensitive target electrode in said tube, means for establishing a charge electrode in accordance with a signal to be generated, a source of beam current inl said tube, means for scanning. said target electrode during a scanning interval with said beam to generate desired signal components having frequencies that extend over a range charge distribution on said target electrode and which also develops undesired shading compo- `nents with said desired components, means forA periodically' and substantially decreasing the current of said beam during said scanning interval at a frequency which is highest-frequency signal component generated to develop r. modulated carrier signal having a carrier-frequency component corresponding to the frequency at which said beam current is decreased and having upper and lower sidebands of modulation components individuallyV representing said desired signal components, and frequencyresponsive means coupled to said tube having a frequency characteristic related tothe frequency atwhich said beam current vis decreased for de substantially free of said undesired shading components and which comriving a signal which is prises at least one of said sidebands of modula- 14. A signal-generating system comprising, a

cathode-ray signal-generating tube, a photosenf sitive target electrode in said tube, ing components with said desired components,

means for periodically and substantially inter-A means for establishing a charge distribution on said target electrode in accordance with a signal to be trans-r mitted, a source means for scanning a scanning interval of beam current in saidtube, said target electrode during with said beam to generate charge distribution on saidtarg'et electrode and to the white level of said image," and frequencyselective means coupled to said' tube having a' frequency characteristic related tothe frequency at which said beam lcurrent is decreased forstabilizing said last-named signal components at a fixed level to derive -a signal which includes said vcathode-ray signal-generating tube, a photosensitive target electrode in s aid tube, means for desired videofrequency signal components and which is substantially free of said undesired shadperiodicallyand sub which also develops undesired shading components with said desired components. Ymeans for tantially ldecreasing the during said scanning interthe `highest-frequency signal component gener-Jg ated todevelop a modulated carrier signal having 1 a carrier-frequency component correspondingtov I 'the frequency at which said beam cinrent` is decreased and having upper and lower sidebands /o modulation components individually repre-l h s nting said desired signal components, and establishing a charge distribution ln-said target 5 nl eans 'including a band-pass nltercoupled to determined by the charge coninterval at a frequency'. fl

distribution on said target' determined by the f at least as high as the 8 Y 2,396,835 Y said tube having a frequency characteristic rethe scanning frequency and which also develops lated to the frequency at which said beam curundesired shading components with said desired rent is decreased for deriving a signal which is components, means for periodically and substansubstantially free of said undesired shading comtially decreasing the currentV of said beam durponents and which comprises at least one of said 5 ing said scanning interval at a frequency which sidebands of modulation components. is atleast as high as the highest-frequency sig- -15. A signal-generating system comprising, a nal component generated to develop a carrier cathode-ray signal-generating tube, a photosensignal modulated by said desired signal compositive target electrode in said tube, means for nents, and frequency-responsive means coupled establishing a charge distribution on said target l0 to said tube having a frequency characteristic reeletrode in accordance with a signal to be translated to the frequency at which said beam curmitted, a source of beam current in said tube, rent is decreased for deriving a signal for transmeans for scanning said target electrode during mission which comprises4 modulation components a scanning interval with said beam to generate of said modulated carrier signal corresponding to desired signal components having frequencies )5 said desired components and which is substanthat extend over a range determined by the tially free of said undesired shading components. charge distribution on said target electrode and c ARTHUR V. LOUGHREN.

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