US2321611A - Television - Google Patents

Description

June`15, 1943. J, J, MOYNIHAN TELEVISION Filed Feb. 12, 1942 2 Sheets-Sheet l HM Hwwvl BEBES: @Euhm Einw INVENTOR. JOHN J.MOYNIHAN BY fM/O VWM 1* ATTORNEYS June 15, 1943.

Image pickup apparatus Receiver memorytube J. J. MOYNIHAN TELEVISION Filed Feb. 12, 1942 scansion of Nth frame Syn hesizing Receiver equipment Conventional 2 Sheets-Sheet 2 Transmitted signal WMI-f Conventional N th Difference scanson of frame C lsjigtnal (N- th storage eween frage Nund (N-I) frame F1' 29 3o ,o l l r Optional i fmerv 28 l l v Remnantof View Sui-n slgrfol L difference N'h'fmm (N mh PUS D t signal less N remnant dG thon given frequency Conventional scansion of Nth frame (N'Ihh frame Storage `l2 A/ I3 I4 \I6 il l Sum signal N|)th plus N (remnant) Synt esizing transform r INVENTOR.

JOHN J.MOYNIHAN Mme 'A TTORNEYS Patented June 15, '1943 I TELEVISION Iohn J. Moynihan, Cleveland, Ohio, assignorof one-half to Joseph B. Brennan, Cleveland,

Application February 12, 1942, Serial No. 430,596

(Cl. P18-7.2)

9 Claims.

In transmitting a television signal, the system which has been brought closest to perfection employs a device for scanning the image to be transmitted and electrical circuits for modulating the transmitter in accordance with the light densities of the image scanned. Present-day methods make usev of the signal so scanned to reconstruct an image at the receiving station generally by means of a cathode ray viewing tube. In the present systems, the image ls scanned periodically generally at the rate of sixty frames per second or thereabouts. The reconstruction of the image at the receiving station is also accomplished periodically in synchronism and at the same rate with the scanning at the transmitter. The net result of this system of transmission is that individual frames are scanned as a unit and rebuilt into an image at the receiving station as a unit, approximately sixty times persecond. Since the number of scanned elements in each frame must necessarily be rather large for adequate definition, the frequency band required to transmit the signal corresponding to this large number of frame elements must also be very large.

Typical ngures for a current system of television transmission are 441 lines per frame, 30 frames per second, and 60 neldsper second where the difference between the number of frames per second and the number of elds per second is accounted for by interlaced scanning of the image. The resulting signal requires a band width of approximately two million cycles in order to transmit the signal adequately. f

The relatively high frequency of the side bands to be transmitted has so far been the major obstacle to the development of a commercial television system. For adequate transmission by radio frequency radiation, the side band irequen.- cies to be transmitted should be a small percentage of the frequency of the carrier of the transmitter. This means that when transmitting with a two million cycle side band, a carrier of considerably greater frequency must be employed. The characteristics of the transmission medium for these high frequency carriers are substantially dierent from those for frequencies now in use for sound broadcasting. In eiect,'

the range of the transmitting station is restricted to approximately the optical range of the antenna, that is, in the present state of the art, television signals of the high quality above described may be received only if the receiving antenna can actually see the transmitting antenna.

One of the commercial eiects of this situation is that'the audience capable ofJ receiving a television signal from a given station is severely restricted.

A second difilculty in the transmission of television signals under the present systems resides in the fact that high frequency signals cannot be transmitted with high delity over land lines of the conventional telephone type, and it is, therefore. necessary to provide expensive types of land line construction such as coaxial cables or to rely on radio relays for the network transmission of television signals or for the'transmission of a signal from a transmitter antenna at a substantial distance from the point of origin of the signal.l

With the above points in mind, it will be appreciated that a television system which could attain the definition and resolution of present systems as regards the received image, while not imposing a high frequency side band load on the system intervening between themodulator of the transmitter and the receiving equipment, would be of very great value. Such a system I have de? vised, and the following is a description of it.

The principal object of my invention or'fundamental principle of the system is to'scan a frame of the image to be transmitted, to compare the frame so scanned with a frame previously scanned, said comparison being point-by-point, and to transmit a signal which is a function of the difference between the present and a preceding image. f

To the accomplishment of the foregoingl and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may lbe employed.

In said annexed drawings, Fig. l is a simplified schematic diagram of a transmitter constructed according to my invention: Fig. 2 is a simplified schematic diagram of a receiver constructed according to my invention; Fig. 3 is a block diagram of the signal trace through the system shown in Figs. 1 and 2.l

Referring now more specifically to the drawings and more especially to Fig. l which is a simplified schematic diagram of a, transmitter and receiver constructed according to my system, l is a scanning tube which may correspond to the commercial device known as an iconoscope; 2 is the 'lens for focusing an image of the subject to be transmitted upon the photo-sensitive plate 3;

4 is a cathode ray gun for scanning the photosensitive plate 3; 5 is a three-winding transformer, one winding of which, 6, is connected to the scanning tube I as shown. Another winding 'I is connected to the modulator 8 of the transmitter 9 by means of a low pass filter I0. The cut-off vpoint of the' low pass filter I0 should be equal to the maximum side-band frequency desired to be transmitted from transmitting antenna II.'

Connected to the signal train, after the low pass filter, is a special tube I2 which I have called a memory tube, and which comprises a photo-sensitive plate I3 in conjunction with a fluorescent screen I4, and so arranged either optically or by direct contact between the fluorescent screen I4 and the photosensitive plate I3. that for every portion of screen I4 there corresponds a portion of plate I3, the photo emission of which is proportional to the brightness of screen I4. Screen I4 is illuminated in the conventional way by means of an electron beam originating in the electron gun I5, and the intensity of which is controlled by a grid element or similar device in a manner well-known to the art, in response to signals originating-in winding 34 of transformer 3|.

In the tube shown, the electron beam from gun I5 is caused to scan screen I4, and the electron beam from a gun I1 is similarly caused to scan plate I3 in such fashion that the portion of plate I3 being swept over is very slightly ahead `in space of the portion of screen I4 being swept over. The defiecting plates or coils for scanning both the 'image tube and the memory tube are not shown in the drawings for simplification. The signal picked up by the memory tube from the fluorescent screen I4 is applied to a winding I8 on transformer 5, and also, through an intervening isolating coupling stage 35, to winding 33 on a transformer 3I The signal from the low-pass filter III is also applied to a winding 32 on transformer 3|, likewise through an intervening isolating stage. These isolating stages may most conveniently comprise an ordinary vacuum-tube amplifier stage, the purpose of which is to prevent feed-back of the signals in transformer 3| to either the analyzing transformer 5 or the modulator stage 8. A third winding 34 on transformer 3| is connected to the control element of the cathode ray gun I5, for modulating the screen I4.

The signal transmitted by transmitting antenna I I is picked up on receiving antenna I9 and is detected in the receiver and amplified in amplifier 2|, all in a manner well-known in the art. The amplified signal output is fed to a winding 22 of a three-winding transformer 23 as shown. Transformers 23 and 3I will be referred to hereafter as synthesizing transformers.

A second winding 24 on the synthesizing ,transformer applies the signal to a receiving memory tube 25 of substantially the same construction as memory tube I2 at the transmitter. The scanned signal from memory tube 25 is amplifled by amplifier 26 and applied to the third windngZI of the transformer 23. The amplified signal output of 26 is also applied to a conventional viewing tube 28.

In operation, an image being focused on plate 3 of tube I by means of lens 2 is scanned by cathode ray gun 4, and the resultant signal is aping that this is the initial scansion ofthe image (corresponding to the start of a program), no signal will be found on screen I4, and, therefore'. the gun I`I scanning plate I3 will apply no signal to winding I8 of transformer 5, or winding 33 of i transformer 3 I.

The windings 6 and I8 of transformer 5 are connected in phase opposition, so that equal signals on the windings will buck each other out. and no resultant signal will be induced in winding l of the transformer 5. However, under the conditions assumed, since no signal is applied on winding I8, a signal will be induced in winding 1. proportional tov the signal in winding 6.

This signal is applied to the input of a lowpass filter I0. Regardless of the frequencies comprising the signal in winding 1, some portion of these frequencies will pass through the low-pass filter IIJ and be applied alike to modulator 8 and to winding 32 of synthesizing transformer 3|. .The portion of the signal which passes through the low-pass filter .will not, in general, conform in frequency, amplitude, or phase with the signal applied on winding 1, but it will be a single-valued function of the signal applied on winding 1. The remnant of signal in winding 1 which is passed through low-pass filter I0 is used to modulate the transmitter 9, the output of which is radiated on 'antenna II.

- The remnant of signal which passes through low-pass filter I0 is alsoapplied to winding 32 of synthesizing transformer 3I. Inasmuch as vwinding 32 is connected in phase addition with winding 33, a signal will be induced in winding 34 which is equal to the sum of the signals in 32 and 33. At the instant under consideration, there is no signal on winding 33,-and therefore the signal in 34 will be equal to the signal in 32.

By 'a suitable choice of the cut-olf value of the low-pass'lter I0, the frequency of the station carrier required for emitting the signal may be substantially reduced over present practice.

The first frame of the image on plate 3 having been scanned, We proceed to a considerationI of the signal values involved in the second scansion.

Assuming that no motion has'taken place in the object being' scanned, and also assuming that the signal frequencies required for scanning the image on plate 3 are higher than those desired to be imposed on the modulator 8, a signal will be applied on winding 6 of transformer 5 identical with the signal from the previous frame. Now, however, the fiuorescent screen I4 of memory tube I2 has been activated by the signal due to the previous scansion, and since the Vtime delay characteristics of the screen I4 are such that the picture remains on the screen for a convenient length of time after the excitation of the screen, the cathode ray gun I1 will scan plate I3, and apply to winding I8 of analyzing transformer 5, and to winding 33 of synthesizing transformer 3I, a signal similar to, but not necessarily identical with ,the signal previously transmitted through the low-pass filter I0.

The reason that the signal so scanned will not be identical with the signal corresponding to the plied to winding 6 of the transformer 5, Assum- 75 We now. have a signal applied to Winding I of transformer 5 proportional to the signal derived from scanning the second frame of the image p n plate 3, and a signal applied to Winding I8 proportional to some function of the signal derived from scanning the first frame of the image on plate -3. The two windings being connected in phase opposition, a signal will be induced in winding 1 proportional to the diierence between the two signals, and this signal will be applied to low-pass filter Ill.

The second-frame remnant being applied to winding 32 with the first-frame remnant on winding 33, a signal will be induced in 34 proportional to the sum of the two remnants.

The process outlined above will continue 'until the picture built up on plate I4 of memory tube l2 corresponds to the image on plate 3 of scanning tube. l. When such correspondence occurs, no signal will be induced in winding i Vof' transformer 5, and consequently the transmitter will be unmodulated.

The reconstruction of the image at the receiving station occurs as follows:

It is assumed that Athe receiving station has been in operating condition from the start of the program transmission by the transmitter. The remnant of the first scansion, as outlined above,

having been used to modulate the transmitter,v

The signal applied to winding 22 will induce acorresponding signal in winding 25, which, when applied to the memory tubeii, will cause a iiuorescence of the screen 22 corresponding to the iiuoresc'ence occurring on the screen .tl of the transmitter memory tube l2. This uorescence may be used to excite the photo-sensitive plate 3@ which is scanned in a fashion identical with the scansion of plate i3 of the transmitting memory tube. l

The output of this scansion is fed through an amplifier .and applied to the controlled element of a viewing tube 2b. The output signal is also applied to a winding 2l on transformer 22.

Since the scansion of plate 3@ and also of plate i3 is effected just prior to the building up of the new picture on ythe uorescent screen, the signal in windings i8 and 2l will be proportional to the signals corresponding to the scanning of a picture one frame behind that being received at the instant.

Continuing with the discussion of the action of the receiving equipment, the signal corresponding `to the second remnant is applied to winding 22 at the same time, and in synchronism with a signal corresponding to the scansion of the first remnant is applied to winding 2l. The two windings being in phase addition, a signal will be induced in winding 22 corresponding to the sum of the signals in windings 22 and 2l. The signal in `winding 2Q being applied to memory tube 25 will cause screen 2i to be excited to an extent proportional to the sum of the two signals.

It will thus be seen that the image formed on the uorescent screen 29 at the receiving station will gradually approach in lappearance the image formed on the fiuorescent screen it at the transmitter, provided that the rate of decay of the image on each fluorescent screen is the same. The image formed on the viewing tube 2B atv the receiver is merely an effective duplicate of the image formed on the fluorescent screen 29.

It will be observed that the values of signal in winding 22 of receiving synthesizing transformer 23 correspond to those in winding 32 of v transmitting synthesizing transformer 3|; the

' 29 and Il is assumed to be the same.)

It will be appreciated that by selecting the cut-oli point of the filter lli at an appropriate value, the radio frequency equipment-of the signal chain can be made to correspond more closely with normal sound transmission equipment than by attemptingto transmit the entire frequency band involved in the scansion of the image on plate 3. 'I'he portion of the equipment carrying I the high frequency band required for the delineation of a high fidelity television signal can be confined to equipment located at the source of the television signal and to equipment located at the point at whichthe signal is to be reproduced, all the intervening equipment being adapted to carry only low frequency signals.

It may be necessary to operate the memory tubes l2 and Z at a signal level such that a signal corresponding to white on the viewing tube corresponds to a relatively low signal on the memory tube. To express it in photographic terms, the gamma of the image on the memory tube may be very low even while the gamma of the image on the viewing tube is relatively high.

The reason for this condition is found in an analysis of the expansion of sine mi: in terms of sine .'r. Upon expanding sine nx in a power series in it is found that coeicients of the order of n occur in the expansion, and it maybe necessary to provide for a modulation level ,or gamma in the memory tube corresponding to these coemcients.

It will be appreciated that the above description is quite brief, and that several elements not shown in the drawings of Fig. l, nor discussed in the above description, are necessary for the operation of the device. l Some of these elements will now be listed, and it will be appreciated that the omission of these elements from the drawings and previous description is for the sake of brevity, and not with the understanding that the system will work without them. They constitute accessories well known to those skilled in the art of transmitting television signals.,

Deflecting plates or coils.

Saw tooth generators.

Time-delay networks for synchronizing the l signal scanned by the Amemory tube with that scanned by the image scanning tube in the transmitter and with the incoming signal in the receiver. f

. Telephone equipment for the transmission and reproduction of the sounds associated with the image.

Time-delay` networks for synchronising the sound at the receiver and the reconstructed image as viewed on the screen of the viewing tube.

. Amplifier stages as required to phase and po- Iarize the various signal currents properly.

G. Attenuation networks to vary or adjust the gain in the several circuits of the system..

Referring to item C above, it will be apparent thatl some arrangement must be made in both memory tubes by which `the scansion of the preceding frame can occur at a slight advance in space before the cathode ray gun scans the iluorescent screen in putting on the next. frame; in spite of this advance in space it is obvious that the circuits require that both scansions vbe applied simultaneously in time to the various transformers in the circuit. For this reason, it is necessary to include a delay network of some kind in the pick-up circuit of the memory tube in order to bring it into time synchronism with the picture build-up portion of the tube. Such a delay network should prefer ably have uniform delay over the rangev of frequencies employed and such networks are fairly well known in the art.

Referring to item E above, a study of the circuits, Fig. 1 and Fig. 2, reveals that the image created on the viewing tube screen is lagging the image on the pick-up tube screen to some extent. It will, therefore, be necessary to insert some type of delay network in the sound circuits associated with the television sending equipment in order to assure synchronism of sound and picture at the receiver. A

Referring to item F above, the circuits, Fig. 1 and Fig. 2, are extremely simplified, and in general, amplifiers are not shown except incidentally. It is quite possible that in the circuits shown here, the phase and polarity of the signal currents may be actually incongrucus, but such phase and polarity can be corrected as in conventional television equipment by the insertion of amplier stages which have the effect of reversing the polarity or displacing the phase 180. A conventional triode vacuum tube amplifier is such a circuit.

Referring to G above, the signal level in various partsof the circuits shown in Fig. 1 and Fig. 2 will vary widely. It will, therefore, be necessary to `insert attenuation networks to vary or adjust the gain in the several circuits of the system.

ing 22 of synchronizing transformer 23, is energized by a conventional scansion signal F of the n-lth frame from the receiver memory tube 25.

The sum signal G, appearing across the winding 21 of transformer 23, is equal -to the signal F 'plus the transmitted signal E, and this sum sig? nal G is converted by the action of the receiver memory tube 25 and the viewing tube 28 into a visible excitation of the fluorescent screens in each of these tubes, and appears as the nth `frame of the sequence. A similar action to that shown occurs in the transmitter section comprising the synchronizing transformer 3| of the memory tube I2.

It will be apparent that the above description has been based on the assumption of an n-lth Referring now more specifically to Fig. 3 it is believed that the following, taken in conjunction with Fig. 3, will lead to a better understanding of the mode of operation of my invention. In Fig. 3, the physical components of the system are denoted by circles and given reference characters corresponding to the like parts of Figs. 1 and 2, while the signal passing between the respective physical components have beenv indicated by rectangular blocks to which letters have been assigned as described in the following: The analysis given below is assumed to take place during the scansion of the middle of the nth frame in a sequence of frames, where n is any number in the sequence. 'I'he only frames concerned in the analysis are the nth frame and the n-lth frame, lwhere n-l denotes the frame previous to the nth frame in the sequence, and Where at the start of transmission n-l may be zero.

The block indicated by the letter A is schematic of the signal generated by the iconoscope I and passed on to the winding 6 of the analyzing transformer 5. conventional signal due to the scansion of the image on the plate of the iconoscope.

At the time of the analysis, it is assumed that a frame, designated as the n-lth frame appears on the screen Il of the memory tube I2. The scansion of the screen develops a signal B of Fig. 3 which is characteristic of the conventional scansion of the n-lth frame, and which is applied -to the winding I8 of the analyzing transformer 5. 'I'he difference signal appearing across the winding 'I of the transformer 5 constitutes a signal functional of the difference between A and B, and is designated as the signal C of Fig. 3.

This signal comprises the frame already in existence on the memory tube screens I l and 28 respectively. By a process of induction, it is apparent that even when the n-lth frame is zero, that is, at the start of a transmission, the apparatus functions as described.

'I'hroughout the foregoing description. reference has been made to the addition of one image to another; the addition of signals of respective images or scansions, etc. It is to be understood, of course, that where the term "addition" is thus used in the specification and in the annexed claims, the algebraic addition is in each instance intended. Thus,for example', while two functions or components are thus added, the negative character of one such may actually amount to a subtraction. It is, therefore, believed proper to use the term addition" if it is interpreted as an algebraic addition andthe term is used with thisunderstanding in the specification.

'I'hroughout the foregoing specification and in the appended claims, the term "successive is used to denote different successive images or signals in the sequence of images and signals at both the transmitting and receiving stations. By the use of this term, I do not intend to be limited to two images or signals, for example which are immediately adjacent in the sequence. Thus for example, where the remnant of an imageframe at the transmitter is compared with a successive image frame, it is Within the contemplation of my invention that these two image frames thus compared may be separated by one or more image frames.

Throughout the foregoing description and in the claims, reference is also made to the image reconstructed at thereceiving station, and the fact that the same is substantially similar to the true image of the scene at the transmitting station. It will, of course, be obvious to those familiar with the art of television transmission that the reconstructed i-mage at the receiving station actually only approaches the scene at the transmitting station, since the` reconstructed scene will generally be subject to certain limitations as to degree of sharpness, degree of contrast. and the like.

It is obvious that other methods may be usedfor generating the difference signal which is a function ofthe optical changes in the scene at the transmitter. One such method is as follows:

A motion picture record is made of a scene to be transmitted. Successive frames of the motion picture record are scanned simultaneously by separate scanning devices, and the separate scanning signals thus derived combined in phase opposition. Such signals can be applied respectively, for example, to winding 6 and i8 of analyzing transformer 5, and a secondary signal proportional to the optical rdifference of successive frames of the motion picture record can be taken olf winding l. This signal, or some modification of it, may then be the one which is transmitted to the receiver station where the images of the scene at the transmitter station may be reconstructed in the same manner as outlined above.

Another way in which the difference signal may be derived is bythe insertion of a delay circuit between the original signal line and the winding it of transformer 5, provision being made for the application of the originally scanned signal to winding i8 through said delay circuit at a time exactly one frame later than its original scansion, and in phase opposition to the scansion of the successive frame.

In the two examples cited above, the motion picture film and the delay circuit partake of the nature of the memory tube i2 in the system outlined previously. It is obvious that any method, by which Ia signal, proportional to the scansion signal of a preceding frame and in phase opposition with the scansion signal of a succeeding f frame can be derived, may be used instead of the memory tube.

A further method of deriving the difference or secondary signal would be to use two motion picture film records, one a positive, and one a negative, developed to a suitable and preferably approximately the same gamma. If the two prints are then placed back-to-back with correspending frames in register, no signal will be transmitted by the scansion of the two transparencies because the portions corresponding to highlights in the positive will be dark in the negative, and vice versa.

If, however, a displacement of the two films is made by one frame, then the scansion of the two transparencies simifltaneously will result in a signal, if there has been motion in the interval between two successive frames, said signal having an amplitude corresponding to white in the portions along ,the edges of the moving object on one side, and a signal amplitude corresponding to black in the portions along the edges of the moving object on the opposite side of its direction of motion.-

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I, therefore, particularly point out and distinctly claim as my invention-z l. The method of generating television signals which includes the steps of scanning a succession of image frames, comparing the signal due to the scansion f each frame with the signal due to the scansion of a preceding frame, and utilizing the signal differences between successive frames for transmission as the television signal.

2. In the transmission of signals having componentsrepeated periodically, the steps of comparing the amplitudesv of the components in successive periods and transmitting a signal which isa function of the differences thereof.

3. The method of generating television signals which includes the steps of scanning a succession of image frames, extracting a remnant of the signal due to such scansion, storing said signal remnant at the transmitter, and transmitting said remnant, and transmitting a signal proportional to the differences between an image-frame scansion and the corresponding remnant-signal scansion.

. 4. In the art of television transmission, the steps of scanning a succession of image frames, comparing the signal due to the scansion of each frame with the signal due-to the scansion of a preceding frame, transmitting a function of said signal differences to the receiver, constructing an image at the receiver by means of an accumulation of such transmitted signals, and then sequentially varying such constructed image in accordance with successively received signals whereby the constructed image is made to approximate each instantaneous image frame at the transmitter by being sequentially modied to the extent by which such instantaneous image frame at the transmitter diers from, a preceding image frame at the transmitter.

5. Zin the art of television transmission'of a scene,(the steps of scanning a succession of image frames of such scene, transmitting a succession of signals which are respectively functions of the differences between the signalsderived from said scansion of a succession of image frames, storing the signals thus transmitted to cumulatively construct said'scene at the receiver, and then varying said constructed scene only to the extent necessary that it may reect the change between successive image frames at the transmitter as denoted by the signals transmitted to the receiver.

6. In the art of ielevisiontransmission, the steps of scanning a succession of image trames, forming on a uorescent screen an image which is a remnant of the said scansion, transmitting a signal which is a function of the difference between said remnant and said scansion, and accumulating a succession of such transmitted functions on a fluorescent screen on which will beaccuinulated a function of the entire scene at the transmitting station, and which will be varied by the transmitted image to the extent necessary to keep this function of the constructed image at the receiver in conformity with a function of the entire image at the transmitter, scanning the function of the entire image as thus constructed at the receiver, and therefrom constructing in a viewing tube a true image of the scene at the transmitting station.

7. 'I'he method of varying the image at thereceiving station of a television system which comprises adding to an established image at the receiver the optical difference between said image and a successor image.

8. In' the art of television transmission, the steps of producing a succession of substantially complete images of the scene to be transmitted, generating a succession of primary signals which are respectively a function of each such substantially complete images, generating a succession of secondary signals which are respectively functions of a difference between successive primary signals and transmitting to the receiving station such secondary signals.

9. In the art of television transmission, the steps of producing a succession of substantially complete images'of the scene to be transmitted. generating a succession ofprimary signals which are respectively a function of each such substantially complete images, generating a succession of secondary signals which are respectively functions of a diierence between successive primary signals, transmitting to the receiving stations such secondary signals, combining a succession of such secondary signals at the receiving station to reconstruct a succession of signals which are substantially like the said succession of primary signals at the transmitting station, 'and from such reconstructed signals producing a succession of.images substantially like the succession of images at the transmitter station.

JOHN J. MOYNIHAN.

Images