US2899600A

US2899600A - Wtoth of screen

Info

Publication number: US2899600A
Authority: US
Priority date: 1956-08-13
Publication date: 1959-08-11
Anticipated expiration: 1976-08-11
Also published as: GB869538A

Description

CATHODE RAY TUBE. SSTEMS UTILIZNG SECONDARY Aug. ll, 1959 J s BRYAN 2,899,600

ELECTRON EMISSION TO GENERATE INDEXING SIGNALS Filed Aug. 13, 1956 2 SheetsrSheet 1 Flj; l. [Pe/0@ HRT) TRAAZs/r r/ME IN V EN TOR. MM5-s J.- mem/v Aug. 1 1, 1959 J. s. BRYAN 2,899,500.

cm1-IODE RAY TUBE sYsTEMs UTILIZING SECONDARY l ELECTRON EMISSION TO GENERATE INDEXING SIGNALS Filed Aug. 15, 1956 2 Sheets-Sheet 2 GEA/Emme 47 E 1NVENTO& JHMEJ J. BRYAN /yrra/P/vey United States Patent O i CATHODE RAY TUBE SYSTEMS UTILIZING SEC- ONDARY ELECTRON EMISSION T GENERATE INDEXING SIGNALS James S. Bryan, Duncannon, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Penn- Sylvania Application August '13, 1956, Serial No. 603,622

9 Claims. (Cl. 315-12) This invention relates to systems employing cathode ray tubes having means responsive to electron beam impingement for generating a signal indicative of the rate of the beams traversal of the target area and of the position of the beam at any instant. More particularly, the invention relates to systems of this type which utilize secondary electron emission to produce the desired signal.

While the invention is applicable to any such system where the generated signal may serve a useful purpose, it is intended particularly for use in color television receivers employing a single cathode ray tube, wherein the proper color rendition is insured by means of a control or indexing signal generated by electron beam impingement on elements within the picture ytube which is adapted to generate the indexing signal by secondary electron emission. Such a receiver is shown, for example, in U.S. Patent No. 2,736,764 issued February 28, 1956 to F. l. Bingley. In one form of such a color television receiver, the screen of the picture tube may have vertical phosphor stripes thereon arranged in successive triplets, the stripes of each triplet being successively emissive of light of different colors in response to electron beam impingement during each horizontal line scan of the screen, The indexing signal-generating elements may be in the form of vertical indexing lines or stripes associated with the color triplets and formed of material having dilerent secondary electron emissivity than the adjacent portions of the screen. Examples of screen structures of this type are disclosed in a copending application of C. Bocciarelli, Serial No. 198,709 filed December l, 1950.

Referring more particularly to the structure and operation of such a cathode ray tube system, in practice the screen is preferably formed as a coating on the faceplate of the tube, and an electrode is provided, in the form of a coating on the wall of the tube, for collecting the -secondary electrons emitted by the screen. Through external connections to the 'screen and collector coatings, suitable voltages are applied to the coatings, the collector coating being at higher potential than the screen coating, and an indexing signal is derived by virtue of the secondary emission within the tube from the screen to the collector electrode. This signal may be utilized to insure proper color rendition, either `by control of the phase of the color video wave supplied to the cathode ray tube, as described in the copending application of M. E. Partin, Serial No. 382,384, tiled September 25, 1953, now U.S. Patent No. 2,759,042, issued August 14, 1956, or by control of the horizontal scanning, as described in the copending application of W. P. Boothroyd, Serial No. 219,093 iiled April 3, 1951. In any such system, the frequency and phase of the indexing signal should be determined solely by the rate of beam traversal of the screen area, as otherwise the indexing signal would not be truly indicative of said rate Aand the instantaneous beam position.

The principal object of the present invention is to solve a problem and overcome an objection in such systems which heretofore has caused color error.

This invention is predicated on the discovery of the cause of the color error and upon the further discovery of a simple and practical method `and means for eliminating the color error.

In experimental use of such systems, color error was observed in the reproduced image, and after considerable experimental effort to determine the cause of such error, it was found to be due to diierences of the transit times of secondary electrons emitted from different portions of the screen. More particularly, it was found that the time between primary electron impingement on the screen and arrival of secondary electrons at the collector electrode varied considerably over the screen area, and that this caused valiation in the phase of the indexing signal and thus produced color error. Upon careful investigation and study of the transit time differences in the cathode ray tubes that had been employed, it was found that during each line scan and during vertical scan the transit time varied according to a certain pattern.

An `attempt was made to obtain substantially constant transit time by arranging the coatings and adjusting the voltages, but this was found to be impractical because it required too close a voltage tolerance.

After further study and experimentation, the idea was conceived of arranging the coatings and adjusting the voltages in ysuch a manner as to cause the transit time variations to take the form of a simple geometric curve, such as a parabola, and introducing compensating nonlinearity into the horizontal deflection circuit to compensate for the transit time variations. This proved to be a practical solution of the problem.

The invention may be fully understood from the following detailed description with reference to the accompanying drawings, wherein Fig. 1 shows in sectional plan the structure of prior cathode ray tubes of the type here involved, and shows in block form the deflection circuits employed to effect horizontal and vertical deflection of the electron beam;

Figs. 2 and 3 are graphic illustrations of the variation in transit time of secondary electrons over the width and depth or height of the screen;

Fig. 4 is an illustration similar to Fig. 1, showing a tube structure and modified deflection means according -to the present invention; and

Figs. 5 and 6 lare graphic illustrations of the transit time characteristics of the tube shown in Fig. 4.

Referring -rst to Fig. 1, there is shown a cathode ray tube 10 having an associated deflection yoke 11 which is supplied with deection currents by the horizontal and vertical

scanning wave generators

12 and 13. The deflection yoke and the scanning wave generators are conventional and do notrequire detailed illustration or description.

The cathode ray tube 10 is typical of those employed prior to the present invention to produce an indexing signal as hereinbefore described. The tube envelope comprises the relatively thin walled bell portion 14 and the relatively thick faceplate 15 which has a peripheral flange 16 abutting and joined to the edge of the bell portion. The bell portion 14 and the flange 16 form a ilared wall portion extending between the neck of the tube and the front wall forming the face of the tube. On the inside surface of the bell portion 14, and preferably extending within the neck, there is provided a conductive coating 17 which serves as a collector electrode for secondary electrons emitted from the screen. The screen 'comprises a conductive coating 18 on the inside of the faceplate 15. This coating extends over the flange 16 and onto the bell portion 14. The screen is not illustrated in detail as this is unnecessary for the present purpose, but the indexing elements are represented in exaggerated form at 19. It will be understood, ofcourse, that this is not intended to be an accurate representation of the indexing elements, as this would be impossible on the scale to which Fig. 1 is drawn. The indexing elements are in the form of horizontally-spaced vertical stripes, Fig. 1 being a sectional plan view taken horizontally through the axis of the cathode ray tube.

External connection is made to the

coatings

17 and 18 by means of

metal buttons

20 and 21 extending through the wall of the bell portion 14. In some instances, insulation of the coatings from one another is provided merely by the space between them, while in other instances an insulating coating is provided in this space. For simplicity, Fig. 1 merely shows the space between the coatings as the insulating medium.

As shown in Fig. l,

conductors

22 and 23 are connected to the

buttons

20 and 21 for application of operating voltages from a source such as the battery 24, and for derivation of the indexing signal from the load impedance 25.

As previously mentioned, in operation of such a cathode ray tube, the indexing elements 19 are impinged by the electron beam during each line scan, and secondary electrons are emitted by the screen and are collected by the collector electrode 17. The indexing signal, which is intended to be indicative of the scanning rate and position of the electron beam, is produced by virtue of the fact that the indexing elements 19 have a secondary electron emissivity diierent from that of adjacent portions of the screen. The indexing signal is utilized to insure proper color rendition Ias previously indicated.

As hereinbefore set forth, in experimental use of cathode ray tubes such as that shown in Fig. 1, color error was observed in the reproduced image, and it was eventually discovered that the cause of such error was variation in transit time of secondary electrons emitted from different portions of the screen. From an investigation and study of the transit time variations in such cathode ray tubes, it was determined that over the width and depth or height of the screen, the transit time of the secondary electrons varied generally according to the

curves

26 and 27 in Figs. 2 and 3. Thus during each horizontal line scan, the transit time of secondary electrons varied generally according to curve 26 of Fig. 2, and during each vertical scan the transit time of secondary electrons also varied generally according tocurve 27 of Fig. 3. When an attempt was made to achieve substantially constant transit time by arranging the coatings and adjusting the voltages, this was found to be impractical because it required an extremely close voltage tolerance and it did not give suiciently constant transit time at the beginning and at the end of the scan.

Attempts to solve the problem led to conception of the idea of arranging the coatings and adjusting the voltages so as to cause the transit time variation to be a simple geometric curve, and introducing compensating non-linearity into the horizontal deflection circuit to compensate for the transit time variation. It was envisioned that if lthe transit time variations were caused to assume the form of a simple geometric curve, the adverse effects of the transit time variations might be nulliiied by providing a compensatory shape of the horizontal scanning waveform. The compensating eifect, of course, would result from the fact that the rate of scanning aifects the frequency and phase of the indexing signal.

Referring now to Fig. 4, there is shown a cathode ray tube 28 constructed according to one embodiment of the present invention. Associated with the cathode ray tube is the yoke 29 which is supplied with deflection currents by the horizontal and vertical

scanning wave generators

30 and 31. As in the case of Fig. 1, the cathode ray tube 29 comprises the bell portion 32 and the faceplate 33, on which the coatings are provided.

In this instance, the collector coating 34 on the inner surface of the bell portion 32 extends substantially to the edge of that portion, and the screen coating 35 on the inner surface of the faceplate 33 extends only over the faceplate proper and does not extend over the inner surface of the peripheral ilange 36. It was found that this arrangement of the coatings and suitable adjustment of the voltages (e.g. 27 kv. on the screen and 30 kv. on the collector) caused the transit time variations to take the form of simple geometric curves, such as the

curves

37 and 38 in Figs. 5 and 6.

In the cathode ray tube of Fig. 4, metal buttons 39 and 40 are provided on the bell portion 32 of the tube, and

conductors

41 and 42 are connected to the buttons to supply operating voltages from source 43 and to derive the indexing signal from across the load impedance 44, as before, in response to electron beam impingement of the indexing elements 5S. In this instance, the coating 35 is extended at 45 to engage button 40, and the coating 34 is recessed at 46 to preserve the required spacing between the coatings.

Further, in accordance with this invention, the horizonal sawtooth scanning waveform is caused to assume a non-linear form, such as shown at 47, so as to compensate for the transit time variation shown in Fig. 5. Short transit time advances the phase of the indexing signal, while long transit time delays or retards the phase of the indexing signal. Therefore, for proper compensation, the sweep should be slow during short transit time and fast during long transit time. The scanning waveform 47 is of non-linear sawtooth form such that its slope is small at the start, large at the middle and small at the end of each line scan. Thus the sweep is first slow, then fast, and then slow, to compensate for the transit time variation represented by curve 37 in Fig. 5.

In order to compensate for the vertical transit time variation, as represented by curve 38 in Fig. 6, the horizontal scanning waveform is also varied during each vertical scan according to curve 38. Thus at the beginning and end of each vertical scan the horizontal sweep is slower than at the middle of the scan.

The modification of the horizontal scanning waveform for the purpose of this invention may be accomplished in various ways, as will be apparent to those skilled in the art. In the arrangement of Fig. 4, a resistor 48 and a capacitor 49 are series connected in the output of generator 30 and given values (e.g. 20 ohms and 0.3 microfarad) such that the horizontal scanning waveform is modified to compensate for the transit time variations represented by curve 37. A resistor 50 and a capacitor 51 are series connected in the output of generator 31 and are given values (e.g. 20 ohms and 120 microfarads) such that a correcting voltage is derived at point 52 according to curve 38. This voltage is applied through coupling capacitor 53 to the control grid of the damper tube 54 so as to cause variation of the horizontal scanning waveform during each vertical scan, as described above.

While one embodiment of the invention has been illustrated and described, the invention is not limited thereto but contemplates such modifications and other embodiments as may occur to those skilled in the art.

I claim:

1. In a cathode ray tube system, a cathode ray tube having a Wall forming the face of the tube and having a neck and a ared wall portion extending between the neck and said wall, a screen coating on the inner face of said wall terminating substantially at the junction of said wall with the ared wall portion, said coating including indexing elements having a secondary electron emissivity different from that of adjacent portions of the screen, a collector coating on the inner surfaces of said neck and said flared portion for collecting secondary electrons emitted by the screen, said collector coating terminating short of said junction, means for effecting line and field scanning within said tube, means for applying operating voltages to said coatings and for deriving an indexing 4signal by virtue of the secondary electron flow from the screen to said collector coating, the transit time of the secondary electrons varying over the width and height of the screen according to simple geometric curves, whereby the phase of Ithe indexing signal tends to vary according -to the transit time variations, and means for modifying the line scanning Waveform so as to compensate for the transit time variations of said secondary electrons and thus prevent objectionable phase variation of said indexing signal.

2. In a cathode ray tube system, a cathode ray tube having an envelope comprising a bell portion and a faceplate, the latter having a peripheral flange through which fit is joined to said bell portion, a screen coating on the inner surface of said faceplate terminating substantially at the junction of the faceplate proper and the peripheral flange, said coating including indexing elements having a secondary electron emissivity different from that of -adjacent portions of the screen, a collector coating on the inner surface of said bell portion for collecting secondary electrons emitted by the screen, said collector coating extending substantially to the edge of said bell portion, means for effecting line and iield scanning within said tube, means for applying operating voltages to said coatings and for deriving an indexing signal by virtue of the secondary electron ow from the screen to said collector coating, the transit time of the secondary electrons varying over the width and height of the screen according to simple geometric curves, whereby the phase of the indexing signal tends to vary according to the transit time variations, and

means for modifying the line scanning waveform so as to compensate for the transit time variations of said secondary electrons and thus prevent objectionable phase variation of said indexing signal.

3. In a cathode ray tube system, a cathode ray tube having la wall forming the face of the tube and having a neck and a flared wall portion extending between the neck and said wall, a screen coating on the innerface of said wall terminating substantially at the junction of said Wall with the flared wall portion, said coating including indexing elements having a secondary electron emissivity different from that of adjacent portions of the screen, a collector coating on the inner surfaces of said neck and said flared wall portion for collecting secondary electrons emitted by the screen, said collector coating terminating short of said junction, horizontal and vertical deection circuits for effecting line and eld scanning within said tube, means for applying operating voltages to said coatings and for deriving an indexing signal by virtue of the secondary electron ow from the screen to said collector coating, the transit time of the secondary electrons varying over the width and height of the screen according to simple geometric curves, whereby the phase of the indexing signal tends to vary according to the transit time variations, means in the horizontal deflection circuit for modifying the line scanning Waveform according to the variation of transit time of secondary electrons over the width of the screen, means iin the vertical deection circuit for producing a voltage according to the variation of transit time of secondary electrons over the height of the screen, and means for applying said last-mentioned voltage to the horizontal deection circuit to modify the line scanning waveform accordingly, the modifications of the line scanning waveform serving to compensate for the said transit time variations, thus preventing objectionable phase variations of said indexing signal.

4. In a cathode ray tube system, a cathode ray tube having an envelope comprising a bell portion and a faceplate, the latter having a peripheral ange through which it is joined to said bell portion, a screen coating on the inner surface of said faceplate terminating substantially at the junction of the faceplate proper and the peripheral flange, said coating including indexing elements having a secondary electron emissivity different from that of adjacent portions of the screen, a collector coating on the inner surface of said bell portion for collecting secondary electrons emitted by the screen, said collector coating extending substantially to the edge of said bell portion, horizontal and vertical deflection circuits for effecting line and field scanning within said tube, means for applying operating voltages to said coatings and for deriving an indexing signal by virtue of the secondary electron flow from the screen to said collector coating, the transit time of the secondary electrons varying over the width and height of the screen according to simple geometric curves, whereby the phase of the indexing signal tends to vary according to the transit time variations, means in the horizontal deflection circuit for modifying the line scanning waveform according to the variation of transit time of secondary electrons over the width of the screen, means in the vertical deflection circuit for producing a voltage according to the variation of transit time of secondary electrons over the height of the screen, and means for applying said last-mentioned voltage to the horizontal dellection circuit to modify the line scanning waveform accordingly, the modifications of the line scanning waveform serving to compensate for the said transit time variations, thus preventing objectionable phase variations of said indexing signal.

5. A cathode ray tube system according to claim 4, wherein said screen coating has an extension onto said bell portion, said collector coating is recessed in the vicinity of said extension to preserve spacing between the coatings, a metal button is provided on said bell portion at said extension for electrical connection to said screen coating, and another metal button is provided on said bell portion for electrical connection to said collector coating.

6. A cathode ray tube system according to claim 4, wherein said indexing elements are in the form of spaced stripes extending transversely to the direction of line scanning.

7. In a cathode ray tube system, a cathode ray tube having a wall forming the face of the tube and having a flared wall portion joining said wall at the periphery thereof, a screen coating on the inner face of said wall terminating substantially at the junction of said wall with the flared wall portion, said coating including indexing elements having a secondary electron emissivity diierent from that of adjacent portions of the screen, a collector coating on the inner surface of said liared portion for collecting secondary electrons emitted by the screen, said collector coating terminating short of said junction, means for electing line and field scanning within said tube, means for applying operating voltages to said coatings and for deriving an indexing signal by virtue of the secondary electron flow from the screen to said collector coating, the transit time of the secondary electrons varying over the width and height of the screen according to predetermined curves, whereby the phase of the indexing signal tends to vary according to the transit time variations, and means for modifying the line scanning waveform so as to compensate for the transit time variations of said secondary electrons and thus prevent objectionable phase variation of said indexing signal.

8. In a cathode ray tube system, a cathode ray tube having a wall forming the face of the tube and having a liared wall portion joining said wall at the periphery thereof, a screen coating on the inner face of said wall terminating substantially at the junction of said wall with the flared wall portion, said coating including indexing elements having a secondary electron emissivity different from that of adjacent portions of the screen, a collector coating on the inner surface of said dared portion for collecting secondary electrons emitted by the screen, said collector coating terminating short of said junction, horizontal and vertical deflection circuits for effecting line and field scanning within said tube, means for applying operating voltages to said coatings and for deriving an indexing signal by virtue of the secondary electron flow from the screen to said collector coating, the transit time of the secondary electrons varying over the width and height of the screen according to predetermined curves, whereby the phase of the indexing signal tends to vary according to the transit time Variations, means in the horizontal deection circuit for modifying the line scanning waveform according to the variation of transit time of secondary electrons over the width of the screen, means in the vertical deflection circuit for producing a voltage according to the variation of transit time of secondary electrons over the height of the screen, and means for applying said last mentioned voltage to the horizontal deliection circuit to modify the line scanning waveform accordingly, the modifications of the line scanning waveform serving to compensate for the said transit time variations, thus preventing objectionable phase variations of said indexing signal.

9. In a cathode ray tube system including horizontal and vertical dellection circuits and a cathode ray tube of the type in which an indexing signal is produced, during line and field scanning of the cathode ray tube screen, by ow of secondary electrons from the screen `to a collector, and in which the transit time of the secondary electrons varies over the width and height of the screen and tends to cause variation of the phase of the indexing signal, the combination comprising means for causing the transit time of the secondary electrons to vary over the width and height of the screen according to predetermined curves, means in the horizontal deilection circuit for modifying the line scanning waveform according to the variation of transit time of secondary electrons over the width of the screen, means in the vertical deflection circuit for producing a voltage according to the variation of transit time of secondary electrons over the height of the screen, and means for applying said voltage to the horizontal deection circuit to modify the line scanning waveform accordingly, the modifications of the line scanning waveform serving to compensate for said transit time variations, thus preventing objectionable phase variation of said indexing signal.

References Cited in the tile of this patent UNITED STATES PATENTS 2,591,842 Llewellyn Apr. 8, 1952 2,613,273 Kalfaian Oct. 7, 1952 2,736,764 Bingley Feb. 28, 1956