US2232190A - Television transmitter - Google Patents

Description

Feb. 18, 1941. A. W. lvANcE TELEVISION TRANSMITTER Filed March 30, 1957 Z'Sheets-Sheet Hrthur W Vance Fei). 18, 1941. A W VANCE 2,232,190

TELEVISION TRANSMITTER Filed March 30, 19557 '2 Sheets-Sheet 2 #www Mingus/wwe SIG/ML INPUT P l, 05 'FII/Ew' Srwentor Hrt-lfu/r W Vance Gttomeg ing of the received picture.

Patented Feb. 1.8, 1941 UNITED STATES TELEVISION TRANSMITTER Arthur W. Vance, Haddoneld, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 30, 1937, Serial No. 133,773

6 Claims.

My invention relates to television transmitters, and particularly to transmitters of the type employing a cathode ray tube for converting light images into picture signals for transmission to a receiver.

A preferred form of cathode ray transmitter tube is the type having a mosaic of photo-sensitive globules upon which a light image is projected whereby an electric charge is stored upon each globule. These charges are released as picture signals by scanning the mosaic with an electron beam.

An objectionable characteristic of this transmitter tube is that spurious signals are mixed with the picture signals as they appear in the output circuit of the tube, due to what is referred to as the black spot effect. Unless compensation or correction of some kind is provided, thespurious signals result in uneven shad- The black spot effect may be greatly reduced by mixing shading signals with the picture signals, as described and claimed in Patent No. 2,166,712, issued July 18 1939, in the name of A. V. Bedford, and; assigned to the Radio Corporation of America. However, the Bedford method of reducing the unelven shading is not entirely satisfactory, partly because it requires a trained operator for manually adjusting the shading controls.

It is accordingly an object of my invention to provide an improved means-for and method of reducing or eliminating the so-called black spot eifect or similar phenomenon.

A further object of my invention is to provide an improved means for and method of transmitting pictures from motion picture lm.

A still further object of my invention is to provide an improved amplifier for reversing the polarity of electrical signals.

In a preferred embodiment of my invention, I print a moving picture film with certain of the picture frames positive and the others negative, Preferably, alternatepicture frames are positive and the other picture frames negative. Images of these picture frames are projected upon the cathode ray tube mosaic in the usual way. In order to transmit all picture frames to the receiver with the same polarity, means are provided for reversing the polarity of a picture signal amplifier each time the picture frames change from positive to negative and vice versa.

The action of this system in eliminating the black spot is as follows:

Since the major part of the black spot or uneven shading is determined by the characteristics of the mosaic itself, the signal representing the black spot will remain substantially unchanged for successive 'picture frames.- In the preferred system, the black spot signal will be reversed at the transmitter for each new picture frame whereby the black spot" signal at the receiver and the corresponding uneven shading on the fluorescent screen of the cathode ray receiver tube is similarly reversed. Thus positive 1 and negative black spots appear alternately at 10 the receiver, and since they alternate at a rate above persistence of vision, they are balanced out in the eye of the observer.

The invention will be better understood fr om i the following description, taken in connection with the accompanying drawings, in which Figure 1 is avschematic and block diagram of a televisionV transmitter embodying my invention,

Figure 2 is a circuit diagram of the reversing amplifier employed in the system shown in Fig. 1,

Figure 3 is a View of the shutter disc shown in Fig. 1, and

Figures 4 and 5 are views of moving picture nlm showing how it may be printed for transmission in accordance with my invention.

Referring to Fig. 1, a television transmitter system is illustrated which includes a cathode ray transmitter tube l of the type described in an article by Dr. V. K. Zworykin which appears in the January, 1934, issue of The Journal of The Franklin Institute. This tube comprises an evacuated envelope having an electron gun 2, a focusing and accelerating anode 3, and a mosaic 4 of light sensitive elements thereon. Deflecting means, such as deflecting coils 6 and l, are pro- 35 vided for deflecting the electron beam horizontally and vertically, respectively, in order to scan the mosaic 4 with the electron beam.

To produce the desired current flow through the denectmg coils s and 1 for deflecting the 40 electron beam,` an electric discharge tube impulse generator is provided. This generator comprises a main oscillator 8, from which are derived electrical impulses of the desired frequency for horizontal deflection and also electrical impulses of the desired frequency for vertical deiiection. The main oscillator 8 may be a blocking' oscillator of the type described in U. S. Patent 1,999,378, a multi-vibrator, or the like. 50

'Ihe voltage impulses produced by the oscillator 8 are impressed upon a chain of frequency dividers 9, l I and I2, each of which may be a blocking oscillator operating the same as the main oscillator 'but at a lower frequency. Each oscil- 55 upon the mosaic 4.

lator is locked in step with the oscillator of next highest frequency.

The frequency of the main oscillator output is divided by seven in each of three frequency dividers, as indicated on the drawings, to produce voltage impulses occuring at the rate of 60 per second, this being the desired vertical deflecting or frame frequency. The GO-cycle impulses are supplied to the input circuit of a saw-tooth Wave generator or deflecting 'circuit E3, which converts them into voltage impulses of the necessary shape for causing a flow of current having a saw-tooth wave shape, through the deilecting coils 1 The horizontal deecting impulses are obtained by impressing a portion of the output energy of the main oscillator 8 upon the input circuit of a frequency divider I4 which divides the frequency of the main oscillator output by two to produce 10,290 impulses per second.' The output of the frequency divider |4 is impressed upon a saw-tooth wave generator or deflecting circuit i6 which has its output connected across the horizontal deflecting coils 6 for causing a flow of saw-tooth current therethrough.

It will be noted that the vertical deflecting and horizontal deecting frequencies are such as` to produce a 343 line picture, the scanning being interlaced. Although my invention is described in connection with an interlaced scanning system, it should be understood that it is of general application, and that its usefulness is not restricted to this'system.

Motion picture film having thereon the pictures to be transmitted is indicated generally at il. The picture'frames are moved intermittently into position in a film gate I8 by means of an intermittentmechanism I9 of conventional construction. The intermittent mechanism is driven by means of a synchronous motor 2| at the proper speedto project 6|! picture frames Del" second A shutter disc 22 is also driven by the synchronous motor, this disc being rotated at 60 rotations per second. As shown in Fig. 3, the shutter disc has a singleopening 23 therein, whereby4 the shutter periodically cuts off the light projected toward the mosaic. The timing of the shutter disc 22 is so related to that of the intermittent mechanism and to that of the deflecting means that a picture frame is projected upon the mosaic 4 and then scanned by` thefcathode ray after the shutterA disc has cut off the projected light and during,v the time the film is being moved into position for projection of the next picture frame. This method of transmitting pictures from motion picture film is described and claimed in, Patent No.f 2,166,214,

issued July 18,1939, in the name of R. D. Kell,V

and assigned to the Radiol Corporation` of America. v

In order to keep the scanning of the mosaic in the correct time relation to the projection of pictures thereon, a suitable control circuit, indicated at 24, is provided for maintaining the oscillator` locked in with the 60-cycle power line to which the motor is connected. One -satisfactory type 0f control circuit for thispurpose is described and claimed in Patent No. 2,137,010, issued November l5, 1938, in the name of ALV. Bedford,-, and assigned` to the Radio Corporation of ,l adds GOA-cycle impulses which are suppliedto' the 'y control circuit from frequency divider |2 through a wave shaping amplifier 21.

It will be apparent from the foregoing description that the control circuit 24 and its associated circuits cause the mosaic 4 to be scanned synchronously with the projection of the picture frames thereon. The exact picture projection and scanning operation is not described in detail as it is, in itself, no part of the present invention.

The picture. signals appear across a resistor 28 connected between the mosaic 4 and the second anode 3, and are impressed upon a picture signal amplifier 29 which amplies them and feeds them through a reversing amplifier 3| to a radio transmitter 3 2.

In order to maintain the scanning at the" receiver synchronized. with the scanning at the transmitter, theV horizontal synchronizing impulses are impressed through a wave shaping amplifier 33 upon a suitable amplifier stage in the transmitter 32. Likewise, the vertical deflecting impulses are impressed through a wave shaping amplifier 34 upon a suitable amplifier stage transmitter 32. Preferably, the. horizontal and vertical synchronizing impulses are impressed upon such an amplifier stage in the transmitter 32 so that they appear at the receiver with a polarity opposite to the polarity ofthe picture signals in thev whitedirection, whereby the synchronizing signals` and picture signals may be moreeasily separated. It will be 'understood that, in accordance with usual practice, the horizontal synchronizing impulsesarey transmittedv at the endv of each scanning linewhile the, vertical synchronizing impulses are transmitteduat the end of each picture frame whereby the picture signals and the synchronizing signals: are not transmitted simultaneously.

As previously described, and asvshown in Figs. 4 and'5, in the preferred embodiment of my invention the motion picture film is printed with alternate'picture framesvpositive and the remaining' picture A'frames negative. It will be apparent that 'during the transmission ofV ar` lm of this clfia'racter,V the picture signals appearing. across the'output resistor 2 8 of the transmitter tube, which 'represent a, cmpl'ete picturefrarne, will alternately be positive land negative inrpolarity with respect to each other, correspondingto the reversal of the picture1 frames. I n order to prevent'a corresponding reversal of the picture frames at the television receiver, the `reversing amplifier 3| is included in the voutput channel of the transmitter tube. f

As Worte describedin getan later. the polarity of the reversingarnpli-iieris reversed bysupplying to it a rectangular voltagey wavehaving a fundamental frequency of v30 cycles Iper second,

as indicated at 36. Since it is,A essential that vthis reversing Voltagewave occur the correct time relation to the scanning Aof the mosaic, it is derived from the 60-cyc'le frequency divider |2 which suppliesI the Yverticaldeflecting circuit.

at '36, by passing it through A.suitable clipping tubes, indicatedat 4|, as is` well known inv-the art. The rectangular impulses are` supplied through a transformer 42 to the reversing amplifier 3l, the primary and secondary, of the transformer being loaded'by resistors to'avoid the production of transients inthe transformer. 5 A suitable reversing amplifier Vis shown in detail in Fig. 2. 'Ihis amplifier comprises two amplifier channels connected in parallel, one of these channels including the amplifier tubes` 46 and 41, and the other channel including the amplifier tube 48. The tube 46, which is provided for the purpose of making the outputs of the two channels of opposite polarity, preferably is ofthe screen grid type having a cathode 49, a control grid 5|, a screen grid 52, anda plate 53. The tube 41, which isy blocked periodically to render the channel ineffective to pass signals,

also preferably is of the screen grid type having' a cathode 54, a control grid' 56, a screen grid 51, and a plate 58.'

The plate and grid circuits of the tubes 46 and 41, respectively, areresistance coupled through a plate resistor 59, a coupling condenser 6|, and a grid resistor 62. The resistor 59 is provided with a short-circuiting connection 63 and a variable tap 64, whereby the gain of the amplifier tube 46 may be adjusted to unity.

The output circuit of amplier tube 41 includes a plate resistor 66 through which voltage is supplied to'the plate 58 from a battery 61 or other voltage source. This battery 61 also supplies voltage to the plate 53 of tube 46 through a conductor 68 and the plate resistor 59.

The second channel includes only the one amplifier tube 48 in the specific circuit shown, this tube preferably being of the screen grid type having a cathode 69, a control grid 1l, a

i screen grid 12, and a plate 13.

The input circuits of tubes 46 and 48 are connected in parallel, the control grids 5| and 1I 40 being connected to each other through a conductor 14 and each control being suitably biased by voltage supplied through a grid resistor 16 by a biasing source 11.

The output circuits of tubes 41 and 48 are also 45 connected in parallel, the plate 13 of tube 48 being connected to the plate 58 of tube 41 through a conductor 18, whereby picture signals which are impressed upon the input circuits of tubes 46 and 48 appear across the resistor 66 50 either as the output of the tube 41 or the tube 48, depending upon which one of the tubes is'unblocked at the moment.

The tubes 41 `and 48 are blocked alternately by applying the rectangular voltage previously 55 mentioned to their screen grids 51 and 12. This is accomplished by connecting one end of the secondary of transformer 42 to theV screen grid 51,

and by connecting the other end of the secondary to the screen grid 12 while the mid- 50 point of the secondary is effectively connected to A. C. ground through the conductors 19 and 8l and the battery 61. The screen grids of the tubes 45, 41 and 48 are maintained as a positive direct currentl potential by the lower sec- 65 tion of the battery 61.

The rectangular voltage wave 35 which is supplied to the transformer 42 is of suflicient amplitude to block the tubes 41 and 48 alternately. Thus, one amplifier channel is inopera- 70 tive during the period the other channel is operative and, since the amplifier channels have outputs of opposite polarity, the picture signals supplied to the amplifier input are reversed in polarity each time a tube is blocked.

One important feature of my reversing amplifier is that ythe rectangular voltage impulses which are supplied through the transformer 42 do not produce a disturbance across the output resistor 66 of the amplifier, since theydo not change the current flow therethrough. The reason for this is that when the screen grid of tube 41 goes negative to block the tube 41, the screen grid of the tube 43 goes positive, whereby the plate current of one tube is increased the same .amount the .plate current of the other tube is decreased. It will be apparent that tubes 41 and 48 should be operated along the straight portion of their characteristic by the varying screen grid voltage, if a perfect balancing out of the reversing impulses -is to be obtained. Also, the tubes should have similar characteristics and the tube 46 should be adjusted for unity gain.

A moving picture film may be prepared for transmission in accordance with my improved method by printing a single film, as shown in Figs. 4 and 5,' for example, from two separate films, one positive and one negative. 'I'he printing may be done by any suitable optical method.

It should be understood that it is within the scope of my invention` to project positive and negative pictures alternately upon the cathode ray tube mosaic from two separate films, one positive and the other negative, instead of from a single specially printed film.

In the system shown in Fig. 1, the film isv run through the projector at the rate of 60 frames per second. Obviously, if conventional practice is followed, the pictures will bev taken by a camera which takes pictures at this same speed, 60 frames per second. In this case, the film will be printed as shown in Fig. 4. In order to permit the use of standard lm having picture frames taken at the standard rate of 24 per second, the standard film may be printed as shown in Fig. 5, alternate frames from the standard film being printed twice and the other frames being printed three times. Such a lm, run at 60 individual frames `(several of them identical) per second, is running at the rate of 24 original picture frames per second.

Although I have described a system in Which the mosaic is scanned at the power line frequency, it will be understood that the film may be projected upon the mosaic at the rate of 24 frames per second, and the mosaic scanned at a corresponding rate, say 48 times per second, with interlaced scanning. In this case, the lm preferably is printed as shown in Fig.v 4.

I claim as my invention:

1. The method of transmitting pictures from motion picture lm which comprises the steps of printing alternate picture frames positive and the other picture frames negative, converting said picture frames successively into picture signals and reversing the polarity of said picture signals each time the picture frame which is being converted into picture signals changes from positive to negative or vice Versa.

2. The method of transmitting pictures from motion picture film which comprises the steps of printing certain picture frames positive and printing other picture frames interspersed with said positive frames negative, converting successive positive and negative picture frames into picture signals, and reversing the polarity of said picture signals each time the picture frame from which said picture signals are being derived is changed from positive to negative or vice versa.

3. In a television system having a cathode ray tube with a photo-sensitivescreen, the method of transmitting pictures from motion picture lm Which comprises the steps of printing alternate picture frames positive and the remaining picture frames negative on a single film, projecting said picture frames successively upon saidr screen to produce an electron image whereby picture signals are produced as said electron image is scanned by a cathode ray, and reversing the polarity of said picture signals each time the picture projected upon said screenis changed from positive to negative or vice versa.

4. In a television system having` a cathode ray tube with a photo-sensitive screen, the method of transmitting pictures from motion picture film Which comprises printing positive and negative picture frames in interspersed relation onY a single nlm, projecting said frames upon said screen to produce an electron image, scanning said irnage with the cathode ray to convert said pictures into picture signals, and amplifying and reversing the polarity of said picture signals each time the projected picture frame on said screen. changes from positive to negative or vice versa whereby the polarity of spurious signals but not of picture signals is reversed.

5. In'. a picture transmitter, a cathode ray transmitter tube of the type having a mosaic of capacity elements, means for projecting alternately upon said mosaic positive and negative picture images, means for scanning said mosaic With the cathode ray for converting said picture images into picture signals, an amplier connected to amplify said picture signals, and means for reversing theY polarity of said amplifier each timev the picture image projected upon said mosaic changes from positive to negative or vice versa.

6; In a picture transmitter, a cathode ray transmitter tube of the type having a mosaic of capacity elements, means for projecting alternately upon said mosaic positive and negative picture images at a rate greater than persistence of vision, means for scanning said mosaic With said cathode ray in synchronism With the projection of said picture images whereby picture signals are produced, an amplier connected to amplify said picture signals, and means for reversing the polarity of said amplifier in synchronism andin phase with said scanning.

ARTHUR WL VANCE.

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