US2476698A - Beam-velocity control system for cathode-ray tubes - Google Patents

Description

July 19,1949. R. G. CLAPP 2,476,698

BEAM-VELOCITY CONTROL SYSTEMl FOR GATHODE-RAY TUBES Filed Nov. 5, 1946 INVENTORQ ,WCHA/9p G. aA/J Patented July 19, 1949 UNITED!- TENT BEAMQVEIOCITYCONTROIJ SYSTEMFOR" CATHODE -RAY'TUBES Application November 5, 1946, SerialiN: 70751855 1X Claim.. (Cl. 1!78.2.5),v

The'- inventionherein-1 described?. and' claimed relatesy to cathode-ray tube'L systems andi' circuits; and; moref particularly., to. novel circuit'. means and? arrangements for-eiecting av [controll function overlv a cathodeerays beam:` or beam trace as iti moves-acrossthe-:face Ofabathodefray tube; The invention has importantnapplcation in connection. with.l cathoderray Oscilloscopes and in the picture reconstituting elements-of television receivers and; the.v likeln-A most applications. of.rthe: cathode-ray` oscilloscope to the observationoff voltage or. current wave-forms? itf is-.desrecb that: the cathode-ray beanrbe causedtamoveinnasgivendirection, e. g; from left; to right, at: constannvelocity, so that the:l horizontal axis off the. oscilloscope screen may:`v be utilized. as: a linear: timescales Against scale: theA amplitudes oi variable electrical quantities may` be displayed as`Y vertical devilations; in theytrace:

television; picture-reconstituting systems, employing cathode-rainv tubes; provided.` with the usual horizontal andverticah deflection; circuits; and. with.. means: for modulating the.v electron beam: intensity im accordance; with.. the video signal,4 itfisv particularly.'importantithatithezveloce ity: of. theV beam, asz, it V,movesffrom left; Ito right; helcl substantially.`V constant,. since. otherwise serieusedistortionseinA picture-* geometryfwill occur.

In cer-tainzv other: applicationsoii the, cathode,- ray: tube; and; particularly;Y in. oscilloscope work', it. preferred; that; the;Y horizontal: component of :beam velocity vary non-linearly with respect to time. E'or example;. obsenving; certain classes; of' variable;n quantities: itz may; beil con? veulent. to. display.: the` variable against: a; lega: rithmic-scalea-ontimbasea..

In accordance with thefpresentinvention novel means-z are. providedfonv precisely controlling: a coordinate component.. of velocita@A ofiy a cathode:- ray.: beam-z on beamstrace. The. intention .contenuplatesr not; only. suclr^.contmla=as.gwi1lz eiectpa', conistant; velocity: of: beaml` moyemenmlbut: also. such control: as willi eecte predetennnedn variable, beam velocities.:

Inisamobjectot tnespresenifinvention to pro;- vde ai: cathoderray;` tube; circuiiaarrangemeniz. by means of which the velocity of scan may be'zrea'dr predetermined and: precisely; controlled;

Itiisranothen obj ect .off the invention. to proyide ac.. proj eet-icm system; of; teleirisionv picture; recom stitntionn;whichdiebtifmmithe primary picture source; heretofore wasted:-r on unuseda utilized tof influence t or: cont-mii. the: reconstitutiom of; the televisiompicture.y

Itis another; objectiofi the: invention: to. pro;- vide afdelection-:fcontrol circuitiforrusefzwith.oathsv ode-rayxtubexsystemssbwmeans ofzwhichthezpro.- vision and-'maintenance of atimngaxis s ofi great precision. may.y be;v achieved;

It is still another obj ectgothefinventionfto:pro vide'. a deflection control; circuit.,.ft0r: use: with cathode-ray oscilloscopem-.bymeansfoiwhichfsucfeessive and equal time;z interi/.als may bem pre;- sented, alongy .the-.timing'laxisi bysuccessiyelyfvary.- ing space. intervals;

These and `otherf obj ects; off: inventiomA and the mannerk in .whiehiare attained;..will. ap;- pear` fromzthe.; following; detailed. descriptionl and thef accompanying: drawings-,.im Which-A Fig. 1i is a .diagrammatic-illustrationi ofA aspre# ferred embodiment ofi the= imlontion;

Fig. 2`v is an'villustrationaoff' :n.maslcfl usediir'rcon-y junction with-the:photo-electric'cell showninfFi'g-g 1; and

Fig. 3f is a'. fragmentary\ illustration;` showing a modication of aapartofthesystem illustrated in=Fig. 1.

Reference may= nowfbe'fhadi to Figi; 1i in which there is-illustratedi diagrammatically a;- cathoderay tube'system employing-:the principleslof the present invention. It1willbefassumed;A for-pur poses of description; thatlthe systemf illustrated constitutes ai portionM ofV the-picture-reconstituting apparatus of a television receiver, but it will be apparenty that the-apparatusdescribed -is equally adapted' for use in cathode-ray oscilloscopes, and the like; Tl'lesystem` illustrated includes" a numberu ofconventionalE devicesV4 conventionally arranged. Theseincludethefcathode-ray picture tube I, deflectione coil Vasserrrblml horizontaldis*- charge'tubescircuit 31A (preferably-oihigh'finternal impedance) constructedand" arrangedto develop a sav/tooth voltage wavee'e acrossythe capacitor C, a horizontaloutputpentode and: a coupling transformer 5' by meansY of; which" a' sawtooth current, wave is introduced: into: the'l horizontal deflectingcoil.'contained;withiniassembly 2: The vertical .d'eflecting,system,which may be conventional,l hasv ybeen ,omitted fromtliej drawing in the interests otsimplicity; TheA sphericalj mirror 6 andthe aspliericakcorrectingvlens 1i comprise. ele.- ments. of a Schmidtlsystem. ofltelevison picture projection adaptedtoprojectithe. primary image, appearing; on uor.esc ent.screen. 8,..onto a suit.- able viewing screen.` S.- positonedf. rearwardly of thecathodeeray.2 tuba- Il., Detailed iniormationlre.- lating to,-projlection-.systenisfoiithis tppeisnavail.-

ablei inthearticle` byfMamftlandEpsteim. appear.-

ing in the December 1944 issue of Electronica pp. 98-105. Y

In accordance with the present invention there are provided means, responsive to the movement of the beam spot as it crosses screen 8, for generating an alternating voltage whose frequency is proportional to the velocity of said spot. Cooperating means are provided, responsive to the frequency of said voltage, for generating a correcting signal which is utilized, in conjunction with the normal deecting current, to correct or control the velocity of said beam spot in a manner such as to maintain said frequency substantially constant. In the embodiment illustrated in Fig. 1, the foregoing means are comprised of the devices and circuits now to be described.

In the central aperture conventionally provided in the center of the spherical mirror 6 there is mounted a suitable lens 9 which is adapted to project the primary image or picture appearing on the screen 8 Vonto the sensitized face I0 of a photoelectric cell II. Lens 9 is preferably coated to prevent reection of incident light back onto the screen 8. The cell II may be, for example, a photovoltaic cell in which a voltage is generated in a layer of selenium during exposure to light, or it may comprise a vacuum-type photo-cell having a substantially rectangular, flat cathode. The light-sensitive face I of the photo-electric cell is provided with a vertically apertured mask I2, such as that illustrated in Fig. 2, comprising a plurality of alternately opaque and transparent areas. Accordingly, as the brilliant, rapidly moving spot travels horizontally across the screen 8 the photo-electric cell I I is excited intermittently and generates a voltage whose frequency, in megacycles per second, is equal to the number of transparentlines encountered by the moving spot per microsecond of spot travel. Since the flyback time is small compared to the horizontal line period this frequency Will approximate the horizontal line frequency multiplied by the number of transparent mask apertures traversed by the spot in one line scan. In a television system operating under the present standard of 525 lines, thirty frames per second, the frequency of the signal developed during the normal line period will be of the order of 3.15 megacycles per second for a system in which the mask I2 comprises 200 apertures.

During the return-line, or yback, period the beam spot is extinguished by the application of the usual blanking signal (details not shown) and in consequence no signal is generated during such period. If the blanking should not be complete during this period there would be developed in photo-electric cell I I a second alternating current voltage of much higher frequency, but the presence of this voltage would not affect the operation of the system, since the devices I4 and I5, now to be described, are substantially nonresponsive to this higher frequency.

llfhe alternating current voltage developed in the photo-electric cell II is applied, by way of conductor I3, to a frequency modulation discriminator or detector I4 which is preferably tuned to the frequency of the voltage developed by the photo-electric cell II when the spot velocity is that desired for normal operation of the system. Where the electron beam is intensity-modulated, as it is in conventional television receivers, the frequency modulation detector I4 is preferably of a type which is non-responsive to amplitude modulation, since, under the conditions mentioned, the-'voltage generated in the cell I0 is amplitude modulated in accordance with the intensity modulation of the cathode-ray beam. The 'requency modulation detector disclosed in the copending application of W. E. Bradley, Serial No. 576,057, filed February 3, 1945, and assigned to the Philco Corporation, is particularly advantageous in the present system. A conventional frequency detector may be employed, however, if, as illustrated, a suitable amplitude limiter l5 is interposed between the photo-electric cell and the detector.

The output of the frequency modulation detector I4 is a voltage (or current) whose magnitude and polarity is a function of the direction and extent of deviation of the spot velocity from its preassigned velocity. A low pass filter I6 is preferably provided in the output circuit of the frequency modulation detector I4 for the purpose of rejecting any high frequency signal that may be present in the output circuit of the detector I4. The constants of the frequency modulation detector I4-or of the frequency modulation detector I4 in combination with the low pass lter lil-are preferably such that the arrangement possesses high internal resistance and functions, accordingly, as a current generator, In this respect, the detector described in the above-identified copending application of W. E. Bradley is ideal.

The output circuit of low pass filter I6 is connected in shunt with the capacitor C across Which the sawtooth voltage ec, hereinbefore referred to, is developed. Thus connected the output current of the frequency modulation .detector I4 acts to Vary the charge contained in the capacitor C, and so tendsto control the Wave shape of the voltage ec. As indicated above, the output current of the frequency detector is proportional, in magnitude, to the deviation of the spotV velocity Vfrom the preassigned velocity. And, by proper choice of detector output circuit connections, the polarity of the detector output is such that if the spot moves too slowly, the capacitor C charges more rapidly, while, if the spot moves too rapidly, the capacitor C charges more slowly. In either case the tendency is to correct deviations from the preassigned spot velocity. In other Words, the effect of the feedback path comprising photocell I I and detector I4 is to so control the velocity of the beam spot as to effect the generation of a signal of constant frequency Yby the photo-electric cell III, which, in this case, means constant beam velocity. Y

Should the picture appearing on screen 8 of picture tube I contain black areas which are completely dark, there will, of course, be corresponding time intervals during which no moving spot of. light will traverse the photocell. Such completely dark areas are rare in practice however and the photocell will normally receive the illumination necessary to generate a voltage capable of driving the detector I4. However if the voltage generated in the photocell is insufficient to drive the detector it is to be noted that the presence of the feedback path cannot in any way interfere with the normal, uncompensated operation of the horizontal discharge tube circuits.

Attention is directed to the fact that, in projecting the primary image (i. e. the picture appearing on fluorescent screen 8) onto the photocell I I, no light is taken from the projection tube I which might otherwise be useful in the projection of the primary picture onto the viewing screen S. 111 previous projectionl systems of the type illustrated the light directed toward the central region of the spherical mirror 6 has been entirely wasted because such light is prevented from reaching the viewing screen by the intervening presence of the projection tube. In fact, in order to prevent deleterious reections of light from the aforesaid central region of the mirror 6 back onto the fluorescent screen of the projection tube, it has been the practice to blacken this central region, or, alternatively, to provide an aperture therein as illustrated in Fig. 1. It is apparent, therefore, that in accordance with one of the features of this invention, apparatus is provided by means of which light heretofore wasted is utilized to influence or control the reconstitution, or, more specifically, the geometry of the primary television image.

Reference may now be had to Fig. 3, in which there is illustrated a modification of that portion of the system of Fig. 1 which comprises the cathode ray tube and the elements associated immediately therewith. In Fig. 3 those elements which correspond identically to elements in Fig. 1 are identied with corresponding reference characters. The arrangement of Fig. 3 is particularly adapted for use with direct-viewing television receivers and with direct-viewing cathode ray Oscilloscopes where the projection apparatus 6-1 of Fig. 1 is not present. While, in the illustration, deection of the cathode ray beam is accomplished magnetically through the agency of deflecting currents supplied to yoke 2 by the usual horizontal and vertical deiiection circuits (the latter not shown), it will be apparent that a conventional electrostatic deflection arrangement may be substituted if desired.

In the embodiment of Fig. 3, there is provided, in place of the photo-electric cell arrangement of Fig. 1, a conducting grid I8 engaging either the inner or outer surface of face I9 of cathode ray tube 20. The number and arrangement of individual conductors 2| comprising the grid is controlled by the same factors which control the number and spacing of the apertures in the mask I2 of Fig. 2. Grid I8 is conveniently applied to the face of tube by a suitable evaporation process, employing a stencil or mask to control the character or configuration of the grid. The metal (e. g. aluminum) comprising the grid may, if desired, be evaporated onto face 'I9 in such a thin layer as to be substantially invisible to the eye. The individual grid wires 2I are connected together by means of a horizontal conducting element 22 which, in turn, is connected to the input circuit of frequency modulation detector III .by way of conductor I3.

Where the grid I'8 is positioned on the outer surface of tube-face I9, the grid derives an electrical impulse from the cathode ray beam, through capacitive coupling therewith, whenever said beam passes directly beneath a grid conductor. Accordingly there will appear on the common conductor 22 an alternating voltage Whose frequency is a function of the beam velocity and of the spacing of the individual grid conductors 2l, This voltage is applied to'detector I4 by way of conductor I3, and the control of beam velocity is then accomplished as described hereinbefore with reference to Fig. 1.

If grid I8 be positioned on the inner surface of tube-face I9 (e. g. between the glass and the phosphor), the grid derives an electrical impulse from cathode ray beam, largely through conductive coupling therewith, whenever the beam impinges on a portion of the phosphor overlying one of the grid conductors.

In some instances it may be desirable to provide a time base or axis in which equal divisions or units of time are displayed as unequal distances along the axis. A logarithmic time base, for example, is frequently convenient in radar A scope presentations and in plan position indicators. Such a logarithmic time base may easily be provided by means of a grid I8 whose individual grid elements ZI are logarithmically spaced along the common conductor 22 as illustrated in Fig. 3.

In general, the shape of the deflecting 4current Wave ec (Fig. 1) should roughly approximate that necessary to provide the type of deflection desired, but the precise shape of the wave is not important since the circuits of the present invention can be relied on to compensate for very substantial irregularities in the wave-form supplied by the horizontal discharge tube circuits.

It will be apparent that, through the utilization of the principles of the present invention, deflection wave shapes of virtually any desired form can be attained .by the provision of suitably designed masks or grid. In this Way special time bases may be provided which otherwise might be very difficult of attainment.

In the foregoing the instant invention has been described with particular reference t0 certain specific, illustrated embodiments; it will be apparent to those skilled in the art, however, that various alternative arrangements may be employed within the scope of the invention defined in the appended claim.

I claim:

In a television receiver, a cathode-ray picture tube having means for forming a cathode-ray beam, means for deflecting said beam horizontally at an approximately constant velocity, means including a photo-electric cell and a grating positioned externally of said picture tube for generating an alternating voltage whose frequency is a direct function of the horizontal component of beam velocity, amplitude-insensible frequency-responsive means for generating a control current whose magnitude and polarity are substantially direct functions of the deviation of said frequency from a preassigned reference frequency, and means responsive to said control current and operative upon said deflecting means to minimize said deviations.

RICHARD G. CLAPP.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 1,745,528 Clark Feb. 4, 1930 1,976,400 Ilberg Oct. 9, 1934 2,090,801 Bowman-Manifold Aug. 24, 1937 2,171,936 Kucher Sept. 5, 1939 2,243,600 Hulst, Jr. May 27, 1941 2,251,525 Rosenthal Aug. 5, 1941 2,385,563 Beers Sept. 25, 1945 2,415,059 Zworykin Jan. 28, 1947 FOREIGN PATENTS Number Country Date 505,197 Great Britain May 5, 1939 687,728 Germany Jan. 11, 1940

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