US2298796A - Method and apparatus for picture transmission - Google Patents

Description

oct. 13, 1942. R D, KELL 2,298,799

METHOD ANDVAPPARATUS FOR PICTURE TRANSMISSION Suvenor f d, Gttorncg Fifa. 1.

R. D. KELL METHODAND APPARATUS FORPICTURE TRANSMISSION Filed Aug. 31, 1939 5 Sheets-Sheet 2 EtccEEt l Hlllu" mkum. GENS "Illu Oct. 13, 1942. R. D. KELL. 2,298,796

METHOD AND APPARATUS lFOR PICTURE TRANSMISSION Filed Aug. 3l, 1959 '5 Sheets-Sheet 3 faz: 6L'

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Patented Get. 13, 1942 METHOD AD APPARATUS FOR PICTURE TRANSMISSION Ray D. Kell, Haddonfield, N.J., assignor to Radio Corporation of America, a corporation of -Dela- Application August 31, 1939, Serial No. 292,925

(ci. 17a-7.2)

7 Claims.

My invention relates to the transmission of pictures and has for its object to provide an improved. method of and means for so transmitting pictures from motion picture film or the like that the picture may be reproduced at the receiver with the proper background or average overall illumination. According to one aspect of the invention, it provides a systemwherein the direct current component of the video signal may be reinserted at any point in the system in cases where this component has been lost as a result of passing the signal through alternating current, According,

amplifiers, transformers, or the like. to another aspect of the invention, it makes possible thetransmission of video signals including the direct current component through an alternating current amplifier or the likewithout any change in the location of their alternating cur- In applying a preferred embodiment of my invention to a television transmitter, there is employed a cathode ray picture transmitter tube of the type recently described by Iams and Rose. The main feature of this tube is that the electron beam velocity is so low at the point of impact at the mosaic .that the secondary emission ratio is less than unity. I have observed that when motion picture film istransmitted by projecting the picture frames upon the cathode ray tube mosaic A during the return lineperiods onlyand then vgsa'nning, the ,mosaic after the light has been cut off; the'sign'a'l level representative of black in the picture corresponds "exactly with the position of the A. C. axis of the signal. 'Ihe reason-for this is that the signal produced during a return line period (during which period a light image is projected upon the mosaic) is a result of electrons leaving the whole mosaic in proportion to the light and shade of the picture, while that produced during the useful scanning period is a result of the same number of electrons being replaced on the surface ofthe mosaic. Stated in another way, the integrated signal amplitude with respect to time during the image projection period is exactly equal to the integrated signal amplitude with respect to time during the useful signal period. e Y

Thus, it will be apparentvthat a signal of the above-described character may be transmitted through a system including alternating current amplifiers without any shift in the position of the A. C. axis. In other words, the D. C. component of the signal would not be lost.

However, because of the large amplitude of the signal pulse produced during the return line period, it is preferred to Aclip off the greater part ofit in an early amplier stage and to utilize the resulting signal later for reinserting the direct current component.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which .Figure 1 is a diagram showing one embodiment ofthe invention as applied to a television transmitter,

Figure 2 is a circuit diagram the transmitter shown in' Figure 1,

Figures 3, 4, 5 and 6;' are curves which are referred to in explaining'1V the invention,

Figure 7'is a diagramv showing the clipping action which occurs in the circuit shown in Figure 2,

Figure 8 is a curve illustrating the character of the video signal which is transmitted to the receiver, l

Figure 9 isa circuit diagram of a portion of a television transmitter illustrating another embodiment of my invention, and e Figures 10 and 11 are. diagrams whichare referred to in explaining the operation of the circuit of Figure 9.

Referring to Fig. 1, the cathode ray picture transmitting or pickup tube is shown at I. This tube, which is ofthe type wherein the electron vcontrol electrode 3 and accelerating electrodes 4 and 6. Suitable biasing and accelerating voltages are provided by the'batteries 'I and 8.

By means of a focusing coil 9, theelectron beam leaving the electron gun is focused to a small spot on a mosaic screen II. The screen II is of the well-known type commonly employed in iconoscopes with the exception that it is made translucent-I whereby the image of a frame of the motion picture film I2 to be transmitted may be projected on the photoelectric elements of the screen from the back side by means of a suitable optical system indicated at I3.

The electron beam is caused to scan the screen of a portion of vthe Radio provided for electrostatic shielding purposes. A

centrally apertured electron-collecting electrode I9 is provided to which the electrons of the beam not reaching the mosaic screen II are directed`v` and collected.

As explained in the above-mentioned Rose patent, the electrons of the electron beam are directed upon the mosaic screen II with a very low velocity, that is, a velocity approaching zero velocity at the point of impact therewith. The reason for employing this low velocity is that n the electrons of the beam should approach the point of impact on the mosaic with -a velocity such that the ratio of secondary electrons to primary electrons is less than unity.

In operation, elemental areas of the mosaic electrode acquire electrostatic potentials proportional to the intensity of light incident thereon. Particles of the mosaic which are more highly illuminated acquire the most positive electrostatic charge with respect to the unilluminated particles. The positive charges representing an electrostatic image of a picture to be transmitted are neutralized by the scanning beam electrons. When the electrons of the scanning beam are directed toward those particles of the target which are negative with respect to the cathode, they cannot reach those particles because of their low velocity. These electrons, since they lare prevented from impinging on the target are returned to an electron collecting electrode adiacent the electron gun. The scanning means which causes th'e electron beam to scan the mosaic electrode or target are so chosen as to prevent those electrons of the beam which do not reach the target from returning to the electron gun but rather causes them to be redirected along paths other than those `followed by those electrons in traversing the Adistance between 'the electron gun and the target.

A tube of the above type has a linear characteristic between light input and signal output. It is so adjusted that when the mosaic screen II is dark all of the electrons of the electron beam are repelled and returned to the collector electrode I9. Any light on the screen II causes some of the electrons of the beam to reach the screen for neutralizing the positive charges which the light has caused the photoelectric capacity ele- 55 ments to assume. Th'is neutralizing current is the picture current. Preferably, it is taken off the back or signal plate of the screen II and supplied to a picture amplifier 45 through an 6 output resistor 30.

Referring now more specifically to the circuit associated with the pickup tube, there is provided a suitable synchronizing signal and blanking impulse generator indicated at 26. This generator preferably supplies synchronizing and blanking signals similar to those described in Bedford Britishl Patent 448,065. It may comprise rotatable discs having apertures therein corresponding to the impulses desired as described ln-the said British patent, or it may comprise vacuum tube oscillators and shaping circuits as described in Smith Patent No'. 2,132,655.

ing circuit 28 which forces the desired saw-tooth current through the deecting coils I6.

Likewise, synchronizing impulses occurring at the horizontal deiiecting frequency are supplied over a conductor 29 to' a, suitable horizontal deflecting circuit 3I which impresses a saw-tooth deilecting voltage across the delecting plates I4,

The basic method of transmitting the motion picture film is that described in my Patent No. 2,166,214, issued July 18, 1939. Preferably, the system employs the so-called 2-3 intermittent mechanism described in Bedford Patent No. 2,082,093, which' makes possible the transmission of standard 24-frame-per-second iilm at a scanning rate of 60 frames per second.

The illm projector and associated apparatus comprises a film gate 32 through which the motion picture lrn i2 is drawn by means of an intermittent mechanism 33, preferably of the type described in Bedford Patent No. 2,082,093. This mechanism is driven by a synchronous motor 34 operated from the 60-cycle power line. The motor also drives a shutter disc 36 having an opening therein through which an image of a picture frame may be projected upon the cathode ray tube mosaic i I during the return line period, In the specic example illustrated, the motor 36 operates at 3600 R. P. M., and the shutter disc has only one shutter opening wh'ereby the shutter opens 60 times per second.

Obviously, the electron beam of the cathode t ray tube I must be deflected in the proper time relation to the projector and shutter operation.

This time relation is insured by the use of a control circuit 31 which holds the impulse generator 26 in a fixed time relation to the 60-cycle power supply. Such' a control circuitis described in Bedford Patent No. 2,137,010.

Preferably, vertical and horizontal blanking impulses are supplied from the generator 26 through an amplifier and a coupling condenser to the control electrode 3 ofthe tube I, these impulses having the proper amplitude, polarity and timing to block the electron beam during 5 both the vertical return line time and during the synchronizing or driving impulses occurring at 4 the vertical deiiecting frequency are supplied over a conductor 21 to a suitable vertical deflecthorizontal return line time.

The video signal appearing across th'e output resistor 30 is of the character shown in Fig. 3 where the signal produced by the picture frame being fiashed or projected upon the mosaic is indicated at while the picture signal produced as the electron beam scans the mosaic is indicated at y. The horizontal blanking pulses cause th'e output signal to go to black periodically as indicated at z. It may be noted that, in order to simplify the drawing, ,only a small number of horizontal pulses 2 have been indicated. The

area of the signal a: above the A. C. axis is always the same as that of the signal y below the axis.

0 This is because, in the cathode ray tube used in the electron beam during the scanning period merely replaces the electrons that were released from the mosaic when the picture was projected upon it.

Thus, if the scene becomes darker, the amplitude of both signal portions :c and y decreases. The result is that the A. C. axis is always at the level representing black in the picture.

This is further illustrated in Figs. 4, 5 and 6. If the mosaic screen is dark, there is no signal, this condition being illustrated in Fig. 4. If the screen is uniformly illuminated to an intensity representing gray, the signal is as representedin Fig, 5. If the illumination is increased to the maximum illumination representing white, the

this system,

signal is as shown in Fig. 6. It will be seen that the A. C, axis has not shifted, but has remained at th'e level representing black in the picture.

From the foregoing, it will be evident that the cathode ray tube output may be transmitted to the cathode ray receiver tube through A. C. amplifiers without any loss of the direct current component of thepicture. This procedure, however, has several disadvantages, one being that the amplitude of the signal produced by the light flash on the mosaic is larger (about ten times that ofthe picture signal, for example) whereby amplifiers are quickly overloaded, and another being that there is no way of transmitting vertical synchronizing signals of the desired characteristics.

Accordingly, it is preferred to clip olf either all or a substantial portion of the impulse portion a: of the video signal at a suitable point either in the preamplifier 45 or in the video amplifier 50. -For example, as illustrated in Fig. 2, an early stage 31 of the video amplifier may be given a sufiicient negative bias to clip off the signal portions :c at the black level. This clipping action is shown in Fig. 7 where the characteristic of the tube is shown at 39. The video signal having the character illustrated in Fig. 3 4

is applied to the clipping tube with the pulses :c in the negative direction.l The pulses in the resulting signal appearing in the plate circuit of the clipping tube have a fixed amplitude at the black level and a varying height with respect to the alternating current axis. As will be described later, it may be preferred to rst clip o serted at any desired point orpoints in the system. Preferably, direct current transmission is employed for more efficient operation in which case the direct current is reinserted at or close to the modulation stage of the transmitter. In this case, as well as where A. C. transmission is employed, the D. C. component is'reinserted in the receiver at the cathode:,r ay tube to provide automatic background eontrol, since thevideo amplifiers in the receiver are usually of the A. C. type for best operation.-

Merely by way of example, there isv shown in Fig. 2 a D. C. reinserting stage for D. C. transmission. This particular stage comprisesl an amplier tube 4| to which the video signals are fed through a suitable .umber of amplifiers which have been omitted from the drawings as indicated by the. broken line and through a grid condenser 42, the signals being appliedlto the grid of the tube 4I with the synchronizing pulses of positive polarity. The grid leak resistor 43 has. such re sistance with respect to the capacity of the grid condenser 42 that, due to the synchronizing impulses driving the grid positive periodically, there is produced a grid leak biasing voltage that varies in' accordance with the height of the synchrof nizing pulses as measured from theV A.- C. axis.4

-. At the receiver, the D. C. may bereinserted after having been removed by the receiver video Aamplifier or amplifiers by means of the same l0` type of D. C. reinserting stage as that shown in Fig. 2 or by anyone of several other reinserting circuits such as the circuits shown in Holmes Patent 2,251,677, issued Aug. 5, 1941, led February 28, 1933, entitled Television systems, and assigned to the Radio Corporation of America.

This Holmespatent claims broadly the feature of reinserting the D. C. component in accordance with the variation in height of the synchronizing impulses.

Referring more specifically to that portion of the circuit which combines the picture signals with the blanking and synchronizing impulses, it will be seen by reference to Fig. l that with a switch 56 closed the blanking impulses are applied to a stage of the video amplifier 50. Also, the synchronizing pulses are applied through an amplifier 51 to some stage in the video amplifier 50.- For example referring to Fig. 2, the picture synchronizing and blanking signals may be added at the same point in the circuit by applying them to the amplifier tubes 58, 59 and 6I, respectively, which have a common plate resistor 62.

The above-described apparatus may be 4operated with the switch 56 (Fig. 1) open whereby the synchronizing pulses are set directly on the picture signal at the black level. It is preferred, however, to have the pedestal or blanking portion of the final signal extend slightly beyond black as shown in Fig. 8 in order that the adjustment at thecathode ray tube of the receiver will not be critical for obtaining good blanking, it being understood that at the receiver tube the blanking portion of the signal drives the tube to beam cut-off.

Instead of clipping the signal pulses :c at the black level as described above, it is generally preferred to clip them somewhat above the 'black level as shown in Fig. 10. The reason this is preferred is that, unless special tubes are employed, the clipping should be done in an -early amplifier stage where the signal voltage is low, in order to avoid saturating ofi` any of an impulse as. On the other hand, vacuum trbe char-` acteristics are such that good clean clipping is difficult to obtain at low signal voltages.

It will be apparent that, when clipping is cone above the black level, there must be a second clipping step in order to bring the vertical pedestais to the same level (the black level) as that of the horizontal pedestals z. Since this second clipping step is necessary, it is immaterial that the first clipping of the signal is not as complete as' required in the final signal.

In Fig. 9 there is shown a video amplifier which Vmay be substituted for the .ideo amplifier 50 of Fig. 1 and Fig. 2. Like parts. in these figures are indicated by the same reference numerals. The first 'clipping of the signal may be done by the .tube 31 with less lbias on the control grid than that indicated in Fig. 7. As a result, the

pulses :c are clipped beyond the black level in the` black direction, as indicated in Fig. 10. `It may be preferred in some cases to perform this first clipping step in one stage of'the preamplifier 45,

since it should be done where the large pulses a: do not saturate the ampliiier. The important point is that at the rst clipping stage the A. C. axis of the signal should be located at adeilnite level such as the black level, as distinguished from some indefinite location which would result if portions of the pulses :v were saturated off before the iirst clipping.

After being amplified in one or more stages. the clipped signal is clipped the second time, as described above, by the tube 58 which has a variable bias applied thereto by means of a diode 66 and its associated circuit elements. The cathode of the diode 66 is connected to the control grid terminal of the tube 58, while the diode plate is connected to ground, preferably through a biasing battery 61. With this connection, negative pulses will cause a flow of diode current which charges the grid condenser 68. This charge leaks oir through a grid leak resistor 69. The relative values of elements 68 and 69 are such that the discharging circuit of condenser 68 has a long time constant as compared with the frequency of occurence of the pulses :c whereby the discharge of condenser 68 applies a bias to tube 58 which follows variations in the height of the clipped pulses :c as measured from the A. C. axis of the signal. 1l. It will be seen that the diode introduces a bias voltage which holds the peaks of the pulses :z: at a fixed level such that they are clipped oli at the desired black level. i

The output of the tube 58 as shown in Fig l1 is of the same character as that shown in Fig. '7, where the first and only clipping operation was at the black level. Therefore, it may have the blanking and synchronizing pulses added theret in the manner previously described to produce a nal signal like-that shown in Fig. 8.

While no D. C. reinserting stage has been shown in Fig. 9, it will be understood that such a stage may be employed as previously described.

From the foregoing, it will be apparent that various modifications may be made in my invention without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations be imposed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

1. A picture-transmitting system comprising in combination a cathode ray transmitter tube of the type in which there is a mosaic screen hav- Aing elemental capacity elements which are electron-emissive and in which an electron beam is directed toward said screen, the electrons at the point of impact on said screen having a velocity such that the ratio of secondary electrons emitted from said screen to primary or beam electrons is less than unity, means for projecting images of a picture or scene upon said screen intermittently at a certain rate whereby periodically recurring signal pulses of a certain polarity appear in the output circuit of said tube in synchronism with said projection, means for scaning said screen with the electron beam at the end of each projection whereby lpicture signals of opposite polarity to said certain polarity appear in said output circuit, the resulting video signal in said output circuit having the characterstic that the alternating current axis is always at the level representing black in the picture, clipping means having a fixed bias for clipping oi said signal pulses at said black level whereby pedestals are produced, means for producing periodically recur- This action is shown in Fig.

Iing said synchronizing pulses to said clipped video signal that they extend beyond said pedestals in the direction of black.

2. A picture-transmitting system comprising in combination a cathode ray transmitter tube of the type in which there is a mosaicscreen having elemental capacity elements which are electron-emissive and in which an electron beam is directed toward said screen, the electrons at the point of impact on said screen having a velocity such that the ratio of secondary electrons to primary electrons is less than unity, means for projecting images of a picture or scene upon said screen intermittently at a certain rate ,whereby periodically recurring signal pulses of a certain polarity appear in the output circuit of said tube in synchronism with said projection, means for scanning said screen with the electron beam at the end of each projection whereby picture signals of opposite polarity to said certain polarity appear in said output circuit, the resulting video signal in said output circuit having the characteristic that the alternating current axis is always at the level representing black in the picture, means for clipping oi said signal pulses at a fixed level beyond black, means for next clipping off said signal pulses at said black level whereby pedestals are produced, means for producing periodically recurring synchronizing pulses that occur simultaneously with said pedestals, and means for so adding said synchronizing pulses to said clipped video signal that they extend beyond said pedestals in the direction of black.

3. A picture-transmitting system comprising, in combination, a cathode ray transmitter tube of the type in which there is a mosaic screen having elemental capacity elements which are electron-emissive and inwhch an electron beam is directed toward said screen, the electrons at the point of impact on said screen approaching zero velocity, means comprising an intermittent mechanism and shutter for projecting images of a picture or scene upon said screen intermittently at a certain rate whereby signal pulses of a certain polarity appear in the output circuit of said tube in synchronism with said projection, means for scanning said screen with the electron beam at the end of each projection whereby picture signals of opposite polarity to said certain polarity appear in said output circuit, the resulting video signal in said output circuit having the characteristic that the alternating current axis is always at the level representing black in the picture, means for clipping off said signal pulses at said black level whereby pedestals are pro-y duced, means for producing periodically recurring synchronizing pulses that occur simultaneously with said pedestals, and means for so adding said synchronizing pulses to said clipped video signal that they extend beyond said pedestals in the direction of black.

4. A picture-transmitting system comprising in combination a cathode ray transmitter tube of the type in which there is a mosaic screen having elemental capacity elements which are electron-emissive and in which an electron beam is directed toward said screen, the electrons at the point of impact on said screen approaching zero velocity, means comprising an intermittent mechanism and shutter for projecting images of a picture or scene upon said screen intermittently at a certain rate whereby signal pulses of a certain polarity appear in the output circuit of said y tube in synchronism with said projection, means for 'scanning said screen with the electron beam signals of opposite polarity to said certain polarity appear in said output circuit, the resulting video signal in said output circuit having the characteristic .that the alternating current axis is-always at the level representing' black in the picture, means for clipping off said signal pulses ata iixed level beyond black before. any substantial' distortion of the video signal has been produced, means' for lnext clipping off said signal pulses. yat said black level whereby pedestals extending to said black level are produced,means for producing periodically recurring synchronizing pulses that occurl simultaneously with said pedestals, and

` means for so adding said synchronizing pulses to the video signal after said second clipping that' theyextend beyond -said pedestals'in the direction of black.

` 5. ,A picture-transmitting Asystem comprising,

l' in combination, a cathode raytransmitter tube of the type in which there is a mosaic screen hav- `ing elementalcapacity elements-which are elec` tron-emissive andin which an electron beam is i directed towardsaidscreen, the electrons at the point of ,impact on said.screeniapproaching zero velocity, means comprising an intermittent mechanisnil and shutter for projectingimagesofa picthe point'of impact on said screen approaching zero velocity, means comprising an intermittent mechanism and shutter for projecting images oi a picture or scene upon said screen intermittently at a certain rate whereby signal pulses of a cerltain polarity appear in the output circuit of said tube in synchronism with said projection, means for scanning said screen with the electron beam at 'the end of each projection whereby picture signals of opposite polarity to'said certain polarity appear in said 4output circuit, the resulting video signal in said output circuit having the characteristic that the alternating current axis is al- Ways at the level representing black inthe picture, means for clippingl oir said signal pulses at a iixed level beyond black before any substantial distortion of the video signal has been produced by ampliner saturation or the like, means for amplifying said clipped signal, means for next clipping oir said signal pulses at said black llevel whereby `pedestals extending to said black level are produced, meansfor producing periodically recurring blanking pulses that occur simultaneously `with said pedestals, means for also producing periodically recurring synchronizing pulses that fccur simultaneously with said pedestals, and means fcrso-adding said blanking and syn- 'chronizing pulses tothevideo signal after said second clipping that said blanking and synchronizing lpulses extend beyond said black level in the directionof black with` the synchronizing pulses set on top of. said blanking pulses.

polarity appear in the output circuit of said tubey in synchronism with said projection,means for scanning said screen with the electron beam at' the end of each projection whereby picture signais of opposite polarity to said certain polarity appear in said foutputcircuit, the resulting video signal in said output circuit havingthe characteristic that the alternating current axis is 'always at the level representing black in the picture, means for clipping oil said signal pulses at'v a xed level beyond black before any substantial distortion of the video signallias been produced by amplifier saturation or the like, means for next clipping off said signal pulses at said black level whereby pedestals ,extending to said black level are produced, means for also producing periodically recurring blanking and synchronizing pulses that occur simultaneously with said pedestals, and means for so adding said blanking and synchronizing pulses to the, video signal after. said second r clipping that said blankingand synchronizing pulses extend beyond said' black level in the direction of black with the synchronizing pulses set.

on top of said blanking pulses.

6. A picture-transmitting system comprising, in combination, a cathode ray transmitter tube of the type in whichhthereis a mosaic screen having elemental capacity elements which are n electron-emiss'ive and in which an electron'beam is directed toward said screen, the electronsat 7. The methodv of transmitting pictures in a picture-transmitting `system of the type including a cathode ray transmitter tube having a mosaic screen comprising elemental capacity elements which are electron-emissive and having an electron beam directed toward said screen, the electrons of said beam at the point of impact on said screen havinga .velocity such that the ratio of secondary electrons to primary electrons is less than unity, said method comprising projecting images of a picture or scene upon said screen intermittently at a certain 'rate whereby periodically recurring signal pulses ofa certain polarity appear in the output circuit ofsaid tube in synchronism with said projection,4 scanning said screen with said electron beam 'at the end of each projection whereby picture signals of oppositev polarity to said certain polarity appear in said4 output circuit, the .resulting video signal in said output circuit having the characteristic that the alternating current axis `is always at the level representing black in the picture, clipping off said A signal pulses at said black level whereby pedestais are produced,producing periodically recurring synchronizing' impulses that occur simultaneously with said pedestals, and so adding said synchronizing impulses to said clippedvideo signal that they extend beyond said pedestals in the direction of black.v j I RAY. D. KELL. i

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