US2086833A

US2086833A - Television

Info

Publication number: US2086833A
Application number: US572138A
Authority: US
Inventors: Walton George William
Original assignee: Individual
Current assignee: Individual
Priority date: 1930-11-13
Filing date: 1931-10-30
Publication date: 1937-07-13
Anticipated expiration: 1954-07-13
Also published as: DE641519C; NL40187C

Description

$111, 13, 192.7 w. WALTON 2,086,833

TELEVIS ION Filed Oct. 50, 1931 4 Sheets-Sheet 1 Fly/0 319;

July 13, 1937.- w WALTON 2,086,833

TELEVISION Filed Oct. 50, 1931 4 Sheets-Sheet 2 G. W. WALTON TELEVISION Filed Oct. 50, 1951 July 13, 1937."

4 Shets-She et 5 July 13, 1937.v 3. w. WALTON TELEVI S ION File'dpct'. so, 1951 4Sheets-Sheet 4 mmmnMA Ann/mm uuuuvv VUUUUUUUAV VUUU Patented July 13, 1937 UNITED STATES PATENT OFFICE TELEVISION George William Walton, London, England 12 Claims.

This invention relates to methods of synchronizing the scanning devices used in television and the like, and has for its object the provision of one or more control frequencies transmitted with the picture currents in such a manner that the picture received is not affected thereby and that the actual picture .transmission and reception may also be simplified.

Hitherto attempts have been made to transmit a control frequency separately from the picture currents either as a separate modulation of a separate intermediate carrier frequency, or in the intervals between the successive pictures or lines of the picture. These methods have proved to be complicated and not very successful. A further method has been to have a distinct band in the picture so that the picture currents are in distinct groups which are used for synchronizing. This method also is not satisfactory for some scanning time is lost and as the groups are not regular, details of the phase or the synchronization thereby causing great unsteadiness in the picture.

According to the present invention one or more sinusoidal control frequencies are transmitted with the picture currents, or as a modulation of the picture currents, which control frequencies are used at the receiver to maintain correct synchronism, phase and the like, the arrangement being, however, such that the actualpicture is thereby in no way disturbed even if the control frequencies are not electrically balanced out. This is accomplished by choosing such a control frequency that its effect in a received picture is balanced out in succeeding pictures by reason of the phase changes of the control frequency which appear in these succeeding pictures. Actually the control frequency itself need not change phase,.the change being only apparent, as it is caused through the relation of the picture frequency to the control frequency. According to another method use is made of optical filters which are so constructed that the scanning operation produces the control frequency as a modulation of the picture currents,

and a similar but opposite filter at the receiver removes the effect of such a frequency from the picture. This method is also useful in providing means for producing afrequency which can be used for transmission, since as it is a modulation of the picture currents, the resulting frequency, or frequencies, in the picture currents may be transmitted direct, for instance by wireless, be received and be applied without'rectification to control the intensity of light, which in combination with scanning will reproduce the picture, the filter removing any effect of the frequency. In this way, in wireless, the trans-- mission band is made narrow, thus allowing the transmission of a picture having greater detail,

picture disturb the It is to be understood that several different frequencies may in this way be applied to the picture currents for specific purposes: for instance, a frequency of half the picture frequency, for the automatic framing of the picture a frequency comparable with the line frequency for' the purpose of exact synchronization, and a high frequency for the transmission.

Optical stops or apertures may in some cases be used for producing a sinusoidal modulation of the picture currents, this being accomplished by the change of angle or position of a pencil of light passing through, or by the stop or aperture. This method is very'useful where oscillatory scanning is used and affords also a simple means of balancing out any uneven brilliancy in the picture caused by the oscillatory scanning.

The filters above mentioned may be very conveniently produced by photographic means, which besides being very precise, are cheap and can be produced in quantity by printing.

The invention will now be described with reference to the accompanying drawings, it being understood that the scope of the invention is not limited to the methods and means described and shown for they, are only given by way of example.

Figs. 1-6 are diagrams illustrating the prin-' ciple of the invention,

Fig. 7 is a circuit diagram showing one arrangement for separating picture signals from synchronizing signals,

Figs. 8, 9, 9a, 10, 10a, 11, 11m, 12, 12a, 13, 13a, and 14 to 19 illustrate diagrammatically various ways in which the invention may be performed,

Figs. 20 to 22 show diagrammatically various optical systems for use with the present inven tion,

Figs. 23 to 25 show various forms of optical stop' which may be used in the arrangements such as are shown in Figs. 20 to 22,

Fig. 26 is a circuit diagram showing a further arrangement for separating picture signals and synchronizing impulses,

Figs. 27 and 28 show two forms of filter for use with the invention, and Figs. 27a. and 28a show the distribution of shading on the filters of Figs. 27 and 28 respectively,

Figs. 29, 30, and 31 are wave-form diagrams showing the operation of the invention,

Fig.32 is a circuit diagram showing a further arrangement for separating electrical impulses, suitable for use with a cathode ray tube,

Figs. 33*and 34 are further diagrams illustrating a way of performing the invention,

Fig. 35 shows a filter disc,

Figs. 36 to 43 show the electrode system 0 Kerr cells suitable for use with the invention,

Figs. 44, 45, and 4c illustrate diagrammatiure 2 shows the control frequency superimposedon a D. C. component. If 1 and 2 are superposed the result is as shown in Figure 3. If now in the next complete picture the phase of the current in Figure 2 is reversed, the detail of the picture shown in Figure 1 combined with the control frequency will be as shown in Figure 4. It will be seen that the curves according to Figures 3 and 4 will also correspond tothe light intensity at the receiver, 1. e. looking at a corresponding part of the reproduced pictures, the brightness indicated by Figure 3 will be seen in the one complete picture and that indicated by Figure 4 in a succeeding picture. As the phase of the control frequency is reversed in the succeeding picture, and as in television succeeding pictures are shown in rapid succession so that the human eye is unable to follow the change of picture, the control frequency cannot be seen, only an average illumination being apparent. However, as the detail indicated in Figure 1 does not change phase from Figure 2 to Figure 4 it remains positive and therefore it appears in the picture. If the control frequency according to Figure 2 modulates the picture current it varies the amplitude of the detail indicated in Figure 1, reproducing lt in one picture as shown in Figure 5 and in the next one as shown in'Figure 6. No control frequency is apparent in the reproduced picture as only the mean value of the impulses,

shown in Figures 5 and 6 is seen; the average value of the picture current obviously changes sinusoidally at the control frequency as shown in Figure 2. Where the control frequency is only added to the picture current as in Figures 3 and 4, the D. C. component shown in Figure 2, al-

though preferred, is not specifically required.

At the receiver the picture currents may be.

passed through the primary of a transformer, or an inductance, such as the coils of the synchronizing device. In the case of a control frequency used for synchronizing or phasing, since the picture frequencies are in general higher, they are to.a large extent choked back from the synchronizing or phasing coils. This selective action may be improved by the use of a condenser I offering low impedance to the picture frequencies but high impedance to the control frequencies. A preferred arrangement is shown in Figure '7 in which I are the input terminals receiving the total picture currents, 2 is an inductive impedance and 3 a condenser. Low frequencies pass more easily through the circuit containing the impedance 2 than through that containing the condenser 3, and the picture frequencies pass more easily through 3 than 2. A transformer l delivers the control frequency at the terminals Sf and the picture frequencies appear at the terminals 6. This arrangement does not provide complete separation, for this is not required specifically, but at the terminals 5 the percentage of low frequency is greatest, and at 6 least. Many arrangements are possible and are well known .in usual electrical practice, and there is no need actually to have any separation. This will be understood from Figures 3 and 4. The hump due tothe picture detail in the control frequency shown in Figure 3 will tend todisplace the phase of, synchronization towards the hu p, bill in AXB C wherein A is the number of complete pictures per second, and B and C any whole numbers, such that B is not a multiple of C. C gives the number of complete pictures over which the effect of the control frequency is zero. visable that C be as small as possible, viz. 2, in order that there be as little flicker as possible in the reproduced picture. a

When

the control frequency affords a means of securing completely automatic phasing or framing of the picture, being particularly convenient in os- .-cil1atory scanning. Such a frequency may also be used for synchronization, provided it is truly sinusoidal rnd the synchronizing device responds correctly to the variatidn, for it is, less susceptible to disturbing influences such as picture detail I and line frequencies, and will maintain synchronism for a longer period than is possible with higher frequencies.

The manner in which control frequencies are balanced out will be better understood from Figures 8-13.

Figures 8 and 9 show the appearance of a control frequency of /2 of the picture frequency in alternate complete pictures.

Figure 8' shows the appearance of a received picture, say during the positive half cycle ,of the control frequency, and Figure 9 shows the same during the negative half cycle,.i. e. the appear ance of two successive complete pictures. The shading of the pictures varies sinusoidally from top to bottom, a half cycle being shown at the side. As these pictures alternate in their sequence, and in television apparatus appear in such rapid succession that individual pictures cannot be detected, the picture of Figure 8 may be considered as superimposed on the picture of Figure 9, so that the shadings are added, with the result that the picture shown in Figure 10 is obtained. In Figure 10a it is shown how the addition of the positive and negative values produces a constant mean value. This cancellation takes place independently of the phase relation of the control frequency to the pictures, and is unaffected by, nor does it affect the picture details, which appear just as though the control frequency was not present.

Figures 11 to 13 illustrate the case when the control frequency is i The picture has four strips so that the frequency is comparable with the strip frequency. Figure 11 shows one picture and Figure 12 the next one.

'The curves in Figures 11a and 120. show the shade It is adconstant value shown as the dotted line. The curves shown in the strips show the shade value in the same strip, unidirectional scanning being presumed.

5 Similarly any control frequency, provided it bears a correct relation to the complete picture frequency, will cancel out in the picture as seen, and any number of frequencies may be superposed as described to the picture currents. Pro- 10 vided each has a correct relation to the picture frequency, none will appear in the picture.

Figures 14 to 1'7 show forms of filters according to this invention which permit the control frequencies to be produced and be balanced in the 1 picture received.

Where filters are employed in the transmitter, receiver or both, the control frequency must be a multiple of the picture frequency unless the filter is movable. In Figure 14 the frequency is four 20 times the picture frequency, the picture being scanned in four horizontal strips. If the picture had any other number of strips the control frequency per second would be that number multiplied by the number of pictures per second. In

25 Figure 14 is shown a filter having a sinusodial variation of shade from left to right, but no variation from top to bottom; the scanning of the strips is effected horizontally. Obviously when the filter is suitably placed, preferably in the focal 30 plane of the image at the transmitter, the picture currents are modulated by a control frequency produced by the filter. -A filter as shown in Figure 15, which is a negative of Figure 14, is

used at the receiver so that the effect 'of the control frequency is balanced out of the'picture. The filter method of producing a control frequency is very simple and yet very effective. Figures 16 and 17 show the transmitter and receiver filter for the same frequency as in Figures 14 and 15, but the phase is displaced through 90. The filters as shown should preferably be of double width so as to cover two cycles instead of one and thus enable the receiver filter to be ad- 45 shown in Figure 18.

- It will be seen that such a filter may be the same, irrespective of the number of strips in the picture. The frequency will always be the picture frequency multiplied by the number of strips. The filter may also be used with the variation at right angles to the strips to produce a control frequency equal to the picture frequency, which frame the picture. In this case, the control device shouldbe polarized so that there will be only one position in which the picture will frame.

Moreover, two such filters may be used at right angles, either separate or combined in one filter, one variation being parallel to the direction of scanning and the other one at right angles thereto, whereby two control frequencies are produced, one for automatic framing and the other or synchronizing.

In Figures 14-17 the filter has one cycle 'per (35 picture width, but it is possible to arrange that when a picture of a definte number of strips is to be transmitted any number of complete cycles in the whole picture is obtained. In the latter case the filter should only be used with pictures hav- 7 ing the number of strips for which the filter is made. Such a filter is shownin Figure 19, having four strips and cycles per complete picture,

directional.

For oscillatory scanning filters must be more a,oee,aaa

carefully made and used, the general rule being justed to balance out exactly. Such a filter is i may be used automatically to adjust the phase or the scanning being again assumed to be unithat there, must be a whole number, or a whole number and a half, of complete cycles per swing, and if the scann ng during a swing is not at constant speed, then the filter must be so made that the frequency produced is constant. The end of a swing should be always at a maximum or minimum shade value, otherwise there is a phase reversal of the control frequency produced with alternate swings in scanning.

- may be effected by applying the same frequencies to control the intensity of light falling on. a photographic surface placed preferably in a position where the image is formed or seen with the apparatus when receiving or transmitting actual pictures. The photographic surface is exposed for a sufficient length of time to attain the required density. When this photograph is formed it may be used as a master print, from which any number of copies can be taken by contact printing, or any other process.

Stops and apertures may be used to produce control frequencies, and to remove them from a received picture wherever a beam of light has a traversing or angular movement. An arrangement is shown in Figure 20 illustrating the arrangement at a television transmitter using a Nipkow disc.

The Nipkow disc I has formed on it the image to be transmitted. A section of the image throws light through the scanning aperture 5 forming a divergent pencil of light, which the condenser lens 9 focusses on to a photo cell Ill, in such a way that as 5 is moved to the position II, there is no movement of the spot of light at the focus of 9. An aperture I2, the size of which depends on the amount of variation required, is suitably placed between 9 and Iii. As shown, only half of the light from the aperture at 5 passes through I2, and as 5 moves towards II, the amount of light passing through I2 in},

creases to a maximum when 5 is midway between,

the positions 5 and II, and with further mo 7 ment towards II, it decreases until at again half the maximum value. With a suitable aperture at I2 and a suitable shapejof holes in swings the reflected spot of light moves between I the points I6 and II. In the extreme positions of swing the light passing through the aperture I5 is reduced, and is a maximum midway between I6 and I1. The result is the same as that obtained with the filter according to Figure 15.

In Figure 22 light from the source I3 is reflected by the oscillating mirror I4, the reflected spot moving between the points I6 and II. The.

stop I8 is of such a size andsopositioned that the light is maximum at the points I6 and I1, and minimum midway -between-- them at .the,

point I9. The effect is therefore. the same :as

in the case of the filter according to Figure 14.

It will be seen that a side displacement of an aperture or stop will give the same effect as the filters according to-Figure 16 or 17. tiple apertures or stops may be used to produce higher frequencies provided that the pencil of light at the aperture or stop is of suitable size. -The effect is always similar to that of the filters above described.

It will also be seen from Figure 20 that two control frequencies may be produced by an aperture or stop. This will be described with reference to Figures 23 to 25.

In Figure 23, the aperture 20 has a suitable form dependent on the type of scanning. The

extreme frame of the area over which the pencil of light moves in the plane of the aperture is indicated by the points 2|, 22, 23. Suppose the strips of the picture to be parallel to 2l-22 it will be seen that the movement of the pencil of light in that direction will produce a control frequency equal to the strip frequency as described with reference to Figure 20. As there is also a slower secondary scanning in the direction 2322 another control frequency is produced equal to the frequency of complete pictures. The first frequency may be used for synchronization, and the second one for the automatic phase adjustment.

Figure 24 illustrates an arrangement similar to that shown in Figure 23, exceptthat a stop 24 is used instead of an aperture, the frequencies produced having a phase displacement of 180 relatively to those produced according to Figure 23. The arrangement may therefore be used in a receiver when an arrangement as shown inFigure 23 is used in the transmitter, or vice versa, in order to eliminate the control frequencies from the received picture.

Figure 25 shows an arrangement of a stop 25 and

apertures

26, 21, 28, 29, 30, 3|, which likewise produces two frequencies, one equal to the picture frequency and the other one double the strip frequency. The cross section of the pencil of light should be approximately the same as that of the apertures. Obviously such arrangements are possible which enable other control frequencies to be obtained, exactly as is possible with filters.

The methods hereinbefore described may be applied to all systems of scanning and assembling. How this can be effected will be readily apparent, as regards mechanical systems. In the case of cathode ray scanning the application is not so apparent, and such arrangements will now be more clearly explained.

As the control frequencies are mixed with the picture currents, and the cathode ray has practically no inertia, picture impulses will completely upset the action of the control frequencies in the receiver. For this reason it is necessary to impart to the cathode ray some artificial inertia,

or produce some equivalent effect so far as the movements of the ray are concerned, so that impulses of short duration, such as from picture details, will not seriously disturb the posi:- tion of the cathode ray. This is most conveniently ensured by the use of electrical means, such as inductances and condensers, though intermediary electro-mechanical means may be used.

A typical arrangement is shown in Figure 26. wherein the'whole of the received currents are applied to the terminals 32. The actual picture detail freqencies are takenfrom terminals 34,

Mul-

through a condenser 33 which offers little impedance to those frequencies, but a great impedance to the control frequencies required for scanning. The

inductances

35 and 36 offer a high impedance to the picture detail currents, and a low impedance to the control frequencies, so that the latter pass through the primary of the trans former 31. The secondary of the transformer 31 supplies only very weak picture detail currents, but the control frequencies are strong. The higher control frequency is taken to the deflecting plates 38 of the cathode ray tube through a condenser 39 which offers little impedance to them, but high impedance to the low control frequency. The inductance 40 offers a high impedance to the higher control frequency and a low impedance to the low control frequency, so that the latter passes through the primary of the transformer 4 I. The secondary of the transformer 4' I .supplies a strong low control frequency, a weak high control frequency, and very weak picture detail frequencies. The two latter frequencies are further reduced in strength by a by pass condenser 42, offering a high impedance tothe low control frequency, which is applied strongly to the deflection plates 38' of the cathode ray tube. A by pass condenser 44 may be connected across the secondary of the transformer 31, which condenser offers a low impedance to picture detail frequencies but a high impedance to the other frequencies. An inductance 43 is connected in the circuit of the deflecting plates 38 to prevent picture detail frequencies from being applied at high strength to the said plates. This arrangement may be modified in many respects and use may be made of numerous other similar arrangements well known in practice.

, The effect of these arrangements is that strips of the picture are practically undisturbed by detail frequencies, and only. slightly by the higher control frequency, which may be an advantage for avoiding or reducing as much as possible dark bands between strips. The positions of the picture details in a strip are only very slightly disturbed by the strength of those details, and in any case the extent of any possible displacement is such that the eye cannot detect it in the picture. With electro-mechanical methods, the control frequencies instead of being supplied direct to the deflecting plates 38 and 38' are supplied to synchronous rotating orv oscillating devices, which generate or control currents producing control frequencies independent of the currents scanning method. The filter decreases in shade value from 44 to 45 in accordance to the curve 46 which has a constant slope from 41 to 48. In scanning in the direction from to 45, the picture currents are modulated as shown in Figure 29, and such a form of current variation in the receiver when applied to the deflecting plates will cause the spot of cathode rays to move slowly at a constant speed from one side to the other, and then at the end of the movement to jump quickly back again, the process being then repeated.

Figure 27 will give only one scanning, but two quency of Figure 31. One part of these changes filters as shown in'Figure 2'larranged at right angles to one another will give the double scanning. .The two filters may be combined in one, such as is shown in Figure 28, wherein the shade value decreases continuously at a constant rate from 44 to 48 diagonally. Actually Figure .28 shows the same filter as Figure 27,- but the shading has been rotated so that maximum variation of shade is along the diagonal. Suppose the filter according to Figure 28 to be scanned rapidly from 44 to 45, and the secondary scanning to be from 44 to 4|, then the filter will be scanned in strips, the number of which will depend on the number of scannings from 44 to during one scanning from 44 to 41. The resulting modulation of I the picture current will be as shown in Figure 30, if there are four scannings from 44 to 45 during the scanning from 44 to 47.

The

points

49, 50 and Bishow the end of one complete scanning and the beginning of the next, and the

points

49, 52, 53, 54 and 50 the beginnings of horizontal scannings and the ends of those preceding them.

Obviously there is a combination of two variations as shown in Figure 31, viz. one of high frequency marked by the

points

49, 52, 53, 54, 50, 55, 56, 51, 5t, and the other one of low frequency from 49 to 50 and from 50 to 5!, the former being four times greater than the latter.

If the modulation according to Figure 30 or the two separate variations according to Figure 31,

are present in the received picture, they can be eliminated by using a filter similar to that shown in Figure 28 but reversed, that is to say, with the shading decreasing from 48 to 44.

The combined control variations according to Figure 30 cannot beusefuily applied direct to the cathode ray tube, and so the components shown in Figure 31 must be separated before they are applied to the respective deflecting plates in the tube. A convenient way of doing this is shown in Figure 32. The picture currents modulated by the control currents are supplied to the input terminals 5a, the impulses being applied to the grid of the thermionic valve 68, through a resistance 59. Therefore the currents appear in full strength in the .anode resistance ti. The grid of the valve 62 is in parallel with the valve but is fed through an inductance or resistance 63 which offers a high impedance to picture detail changes, a condenser 64 assisting in by-passing the picture detail changes, so that the latter are considerably reduced at the grid of the valve 62. The valve taoperates in a similar way, except that the inductance or resistance 6E5 has a high impedance, and the condenser M a low impedance to picture detail changes and to the higher frequency changes shown in Figure 31,; consequently the low frequency changes are greatest at the grid of the-valve 55. The resistances 68 and 69 allow a current to flow through I53 and respectively so that the latter resistances are operative. The low frequency change in Figure 31 is greatest in the anode resistance Iii of valve 65. The tapping on 10 allows part of the voltage to be passed through the resistance ii and be applied to the grids of the valves 66 and B2. The phase of the currents so applied to the valves 60 and 62 is opposite to the phase they receive from the terminals 58 and consequently with correct adjustments the low frequency changes of Figure 31 will balance out from the currents applied to the valves 60 and 62. Therefore the changes of current through the anode resistance 12 ofthe valve 62 correspond to the high'freflows through the resistance 18 to the grids of the valves and BI, and because of the opposite phase it balances out that frequency from the anode currents of those valves. As the-low and.

high frequency changes of Figure 31 are balanced out in the valve 60, the current changes in the anode resistance correspond only to the picture detail changes, and a part flows through the resistance 14 to the grids of the

valves

82 and 66 balancing out those changes in the currents of these valves. The final result is that the valve 60 passes only picture detail changes, the valve 62 only the high frequency control changes, the valve 85 only the low frequency control changes, and these may be taken independently from one another from the terminals 15, 16 and 1'! respectively. Ii the terminals 16 and 11. are connected to a cathode ray receiver, unidirectional scanning is obtained in correct synchronism and phase with the transmitter. The arrangement of valves in parallel with interlocked negative reaction as It is also useful as a means of eliminating control frequencies from the received picture. The separation, though not absolutely complete, may be made very good with only a small percentage of the undesired frequencies appearing, if adjustments are correctly made. Many modifications are possible, but the characteristic of all of them is the negative reaction of circuits on other circuits in order to eliminate the frequencies of those circuits from the one on which they react; one circuit reacts on all other circuits or on those which require complete elimination of the frequency of that circuit.

Another method of cancelling control frequencies froma received picture consists in arranging one strip of a picture to compensate the next. In this case no filter is required at the receiver but the size of the scanning aperture or light spot is such that it covers two or more strips. Generally if more than two strips are covered the detail is impaired. The method will be explained with reference to Figures 33 and 34. Figure 33 shows a received picture of eight strips without compensation-and Figure 34 shows how one strip compensates half of each of two adjacent strips. The control frequency shown is half the strip frequency. It will be seen that the of scanning being it, 33, is. As til is negative relatively to it and it and lies over them, it compensates the latter strips to give an even shade.

The relation of the control frequency to' the line frequency for use with this method is A B+C, wherein A is the line frequency, B any number not a multiple of C, and C the number of lines of the picture in which compensation is complete. The scanning aperture or spot must be increased C times in the direction of the width of the strip. In Figures 33 and 34 6:2 and 3:1. Scanning by an aperture twice the width of a strip of the picture is known from the previous invention described in British Patent No. 218,766, but the application for the purpose of eliminating control frequencies from a received picture is new v Theiinvention as above described has been accomplished with stationary filters, apertures and .stops, and by what may be termed static arrangements. There are however many possible methods using moving filters and the like. Generally these will vary with the different systems of scanning, although the principles are the same. For instance in the case of a Nipkow disc a filter may be attached to the disc, the shade of the filter changing with the angle, so that one or more complete cycles are produced per revolution. The highest number of cycles should preferably not be more than a quarter of the number of holes required to scan one complete picture, otherwise'the frequency produced will deviate considerably from the sinusoidal form. Figure 35 shows such an arrangement, giving one cycle per-revolution, which may therefore be used for the automatic adjustment of the phase. A similar filter used in the receiver, displaced through 180", eliminates the frequency from the picture. Naturally, the filter need not be continuous, allthat is required being that the shade covering the apertures varies around the disc as shown in Figure 35.

An alternative method which produces the same result'consists in making the apertures of varying area, preferably by varying the size in' the direction of scanning, so that the light passed by the respective apertures shall be the same as would be obtained in Figure35. The receiver would of course be modified in the opposite sense, i. e. the hole in the receiver disc having the maximum area would correspond to the one with minimum areain the transmitter, and the holes in the two discs would have the same relative position in the spiral.

The filters may be separate from the scanning member and may move at a speed different from that of the latter so that the control frequency produced is dependent on the scanning and the speed of the filter. When the scanning is unidirectional it is advisable to move the filter in the same way, and with oscillatory scanning the filter should also oscillate,',otherwise irregular frequencies may be produced.

The filters need not move in-the scanning directions, but may be arranged to move at an angle thereto, so that one filter can produce two control frequencies at the same time by adiust-. ing the angle of movement in correct relation to the two scanning directions.

Moreover, the points of even shade value in the filter may be at an angle to the direction of scanning, i. e. the filter may be made such as shown in Figure 15 and be slightly displaced angularly when it will produce the effect of the filter shown in Figure 11. This applies equally tostationary as well as to moving filters.

Apertures and stops, which as has been explained above are the equivalents of filters, may also be moved in a similar way.

The control frequencies at the transmitter may be produced by ordinary electrical methods, for instance by separate generators and the like, the scanning being suitably synchronized therewith, or the generators coupled to the scanning arrangement. For instance, a generator may be provided on the shaft of a Nipkow disc. The modulation of the photoelectric current may also be effected by known electrical .means, such as thermionic valves.

In the receiver use may be made instead of the filters, apertures and the like of generators coufrequencies for the neutralization of the control frequencies in the picture currents, or their elimination from the picture.

Another way, of eliminating unwanted frequencies in a received picture consists in the modulation of light, for instance by such devices, as Kerr cells, the modulation being such as to balance out the unwanted frequencies. In light control devices using polarized light, additional poiarizers and analyzers need not be used; for instance more than one Kerr cell or when using the Faraday rotation of polarized light more than one coil may be used in tandem or side by side between the crossed Nichol prisms, one giving the normal light modulation required for the picture, and the other or others a modulation at the unwanted frequencies, but with a light variation which balances out the unwanted frequencies from the picture.

In the case of the Kerr cell, the cell may be provided with additional electrodes for this purpose. There may be one or a pair of added electrodes for each unwanted frequency, although this is not necessary, since all such frequencies may be applied to one, or one pair of added electrodes. Figures 36 to 43 show a number of different arrangements of such Kerr cells,only the electrodes being shown.

In all the arrangements, light passes through in the'direction of the arrows, and in Figures 36 and 37 it may pass in any direction in the plane of the arrows. In the arrangements shown in Figures 36 and 42, the unwanted frequencies are applied, say, to the pair of electrodes 8| and the picture frequencies to the pair 82. In the arrangements according to Figures 37 and 40, 83 is acommon electrode for both frequencies, the unwanted frequencies being applied to one of the other electrodes 88 and the picture impulses to the electrode 88. In Figure 38, the unwanted frequencies may be applied to the

electrodes

88 and 81 and the picture impulses to the electrode 88 as one pole and to the

electrodes

88 and 81 as the other pole, a resistance, inductance, or two condensers in series being connected across 88 and 81, and the impulses supplied to 88 being applied to a middle point of the'shunt across 86 and 81. In Figure 38 the two kinds of currents may of course be applied in the opposite way to the electrodes.

In Figure 39, the

electrodes

88 and 88, 88 and 82, 82 and 8| and 8| and 88 have shunts across them as mentioned in connection with Figure 38. The one kindof impulses are applied to the midpoints of

theshunts

88 and 88, and 8| and 82 and the other kind to the midpoints of the

shunts

88 and 82 and 88 and 8|. The arrangement according to Figure 30 maybe used also'without shunts, one kind of impulses 'being applied to 88 and 82 and the other to 88 and 8|.

In Figure 41 the connections may be as in Fig-. ure 39 although the effect is somewhat different. In Figures 38, 39, and 40 the plane of polarization lies at' an angle of with respect to the planes of the gaps, and in Figure 41 to the plane of 88 and 82. .This angle is generally the best for 9. Kerr cell. In Figures 36, 42 and 43 one kind of impulses is applied to the electrodes 8| and the other kind to the electrodes 82.

When using modulated neon lamps or similar devices, an additional electrode may be provided, to which the unwanted frequencies are applied,

, transmission, similar methods maybe used, the

picture impulses being modulated by, or mixed with, one or more control frequencies, and the frequencies being balanced out fromthe received picture by one or more of the methods hereinbefore described. In the methods in which the cancellation takes place in two or more com,- plete pictures, two or more pictures must be superimposed in order that the cancellation be effected. The other methods are preferable in picture telegraphy.

Where pictures are transmitted as modulations of a carrier frequency, improvements are secured by using this invention, since there is no need to rectify, the received modulated frequency being suitably applied to vary directly the intensity of a source of light and a compensating method being used to balance out the carrier frequency. Except when electrical compensation is used, the relation of the carrier frequency to the picture frequency must be as above described in connection with the various methods.

It has been stated above that the control frequencies are sinusoidal, but this is not essential. The requirements are that the control frequencies should be continuous and regular as regards periodicity, wave form, phase and amplitude,

since these are the characteristics which make it possible to cancel the-m out from the received picture, i. e. to separate'fromthe picture the currents which are wholly irregular. Even when frequencies occur in the picture currents which are equal to the control frequencies, they will not affect the cancellation, unless they are absolutely as constant and continuous as the said control frequencies. When filter compensation is. used, such frequencies in the pictures may be as constant and continuous as the control frequencies without the received picture being affected. Aperture and stop compensation is of course equivalent to filter compensation When compensation takes place in succeeding pictures, frequencies equal to control frequencies cannot occur in ordinary practice, for if they could, movement of details in a scene must be such as to produce a change of phase of the corresponding picture impulse in the succeeding picture, which means such a rapid movement in the scene that it could not be seen by the eye, or such a structure of the scene combined with a precise movement, as does not occur in an ordinary scene, except very rarely. The same remarks apply to methods where compensation occurs automatically in succeeding strips of the picture. 4

In electrical methods of compensation of the control frequencies, equal. and constant frequencies produced by picture details are not disturbed, provided the amount of compensation is maintained constant with a fixed value corresponding to the control frequencies only.

Generally true sinusoidal control frequencies are most satisfactory, more particularly with unidirectional. scanning, for departures from sinus form obviously produce a number of sinusoidal frequencies, which are more likely to disturb the received picture. Filter compensation is preferable where the wave form is not sinusoidal.

With oscillatory scanning, however, a wave.

corresponding to the form of a full wave rectiends. This occurs almost wholly in the receiver and may be removed by thefilters or apertures above described. The effect is shown in Figure 44, where the ordinate Y,represents intensity oi illumination and the abscissa X, time, 93-94 being the period of one swing. It is seen that the curve has the form of a full wave rectified sinusoidal frequency. Let us assume that the picture currents in the transmitter are modulated, or have added to them, a frequency such as shown by the dotted curve. If the effect of the frequency applied to the picture currents in the received picture is equal to the variation due to the oscillatory scanning in the receiver, and shown by the full line, then the picture appears to be evenly illuminated, since the applied frequency is cancelled out of the picture. by the scanning in the receiver. Nevertheless that frequency may be used in the receiver in order to synchronize and adjust the phase of the oscillatory scanning. Naturally, a control frequency of the form shown may very easily be produced in the transmitter, by using a filter of the form shown in Figure 1.5. Further, the filter may be such that the control frequency is perfectly cancelled out of the received picture, since it can be produced photographicallyby the oscillatory scanning. In Figure 44, the period 95-93 represents a swing of the scanning device in one direction and the period tit-Edits swing in the opposite direction; therefore the position oftfi in the picture is the same as that of 9d, and a stationary detail in the picture which is scanned equally in two successive swings will produce the hump 96 in the dotted curve in one swing, and a corresponding hump 911 in the next swing. As the control frequency shown by the dotted curve is used,to drive or control the oscillations of the scanning device, 98 will tend to advance the one swing, and 971 will likewise tend to retard the next swing, so that over two swings the disturbance is zero, and in any case so rapid that the eye cannot detect it in the received picture; if the inertia, or its equivalent, of the oscillatory scanning device is adequate no disturbance will occur.

Another method of applying control frequencies to the picture currents in such a manner that they shall not appear in thereceived picture consists in adding to the picture currents, or modu at ng them with, a changing frequency of constant amplitude, the period of change being the control frequency, and sinusoidal. This will be better understood from Figure 45, wherein the curve 98 represents a sinusoidal variation and the curve 99 a rectified sinusoidal frequency. The Y axis gives the frequency and the X axis represents time. The curves show the change of frequency with time, the actual frequency being better seen in Figure 46 for the curve 98. As shown in Figure 46 the amplitude is constant, although it may also be modulated. In the receiver the frequencies of Figure 46 may be cancelled out of the picture by an appropriate method, which depends largely on the production of frequencies in the transmitter. For instance,

el mination of frequencies.'

ill)

The received currents may be applied direct to the synchronizing device, if the latter consists of a non-polarized'magnetic, electrostatic, or electrodynamic device of the dynamometer type; otherwise rectification is necessary. The frequencies are passed through a circuit having two branches, the one containing an inductance and the other one a condenser, as shown in Figure 4'1. This arrangement produces an amplitude modulation of the frequencies of opposite phases in the two branches. I00 are the output terminals and I0! are rectifiers. If no rectifiers are used, the primary of the transformer I02 is replaced by the coils or the like of the synchronizing apparatus. The invention may be used with, equal advantage in picture telegraphy and be readily applied to the various systems. When a cylinder is employed on which the picture is attached or received, and use is made of filters, the latter may also be of cylindrical form, or made" flexible so that they can be bent to therequired curvature.

It is an advantage that the filters be in contact with the picture, for which purpose use is preferably made of atransparent cylinder such as glass, with the filter permanently provided thereon, the picture in' the transmitter, and thephotographic surface in the. receiver being placed inside or outside the cylinder according to the general construction of the apparatus, so that it is in contact with the filter. As will be under-' stood, the filter will remain in the correct position with respect to the phase adjustment and the synchronizing devices;

The methods preferred for picture telegraphy are those in'which the compensation is effected by means of multiple electrode Kerr cells, or in which it takes place automatically in two strips of the picture, using the double size aperture or spot of light. Two control frequencies may be used, one corresponding to the number of revolutions of the cylinder, or to half. thereof, and the other one which is higher and is used to control the speed of the driving motor.

It is to be understood that theinvention is not limited to the'examples and details hereinbefore described, asthey may be modified according to requirements; without departing from the spirit of the invention.

What I claim is:--

1. A method of television, picture -telegraphy and the like, consisting in adding uninterrupted control frequenciesto the picture currents, transmitting them simultaneously and at the same time as the said .picture currents, using the said control frequencies in the receiver for synchronization, phase adjustment or framing and producing in the receiver a counter effect equal to that of the control frequencies and thereby canceiling the said control frequencies from the received picture as seen by the human eye.

2. A method as claimed in claim 1, wherein the control frequencies are added to the picture currents simply by superposition. I

3. A method as claimed in claim 1 wherein the control frequencies are added to the picture currents in the form of an amplitude modulation of the latter.

4. A method as claimed in claim 1, wherein the control frequencies are added to the picture currents in the form of a frequency modulation of a carrier frequency.

5. Amethod of television, picture telegraphy, consisting in adding an uninterrupted control frequency to the picture currents, transmitting it at the same time as the said picture currents,

using the said control frequency in the receiver for synchronization and the said control frequency being a fractional multiple of the frequency of the picture scanning so that there is a change of phase of the effects of the control frequency relative to the picture in a succession of picture scannings, which produces in the receiver an opposition control frequency effects in a group of successive picture scannings so that the effect of the control frequency, due to the persistence of vision, is zero in that group as seen bythe human eye.

6. A method of television, picture telegraphy, consisting in adding an uninterrupted control frequency to the picture currents, transmitting it at the the same time as the said picture currents, using the said control frequency in the receiver for synchronization phase adjustment the said control frequency being a fractional multiple of the strip frequency of scanning the picture so that there is a phase displacement of control frequency effects relative to the picture in the direction of scanning in a succession of strip scannings which are caused to, oppose in a group of successive strip scannings 'by scanning at the receiver with afractional overlap of strips, so'that the eifect of thecontrol frequency, due to persistence of vision, is zero in that group as seen by the human eye;

7. An apparatus for television and picture telegraphy, comprising in combination with a transmitter for the simultaneous transmission of picture currents and control frequencies, a receiver having shaded optical means interposed in the path of light forming the picture in the receiver for the purpose of cancelling out from the picture as seen the eflects of the control frequencies.

ST A method according to claim 1, consisting in using electrical ,means for suppressing some of the control frequencies from the picture screens in the receiver byselective electrical circuits and the application of opposing frequencies equal to the frequencies to-be suppressed.

9. In an apparatus for television and picture telegraphy, in which a number of control frequencies are added to picture currents and are transmitted continuously and simultaneously with the said picture currents and in which the said control frequencies are used in the receiver for synchronization and phase adjustment 1 a scanning device andalight control device which together produce the picture, the said light device having in'addition to the normal electrodes and control members to which the received picture currents are applied, other control members to which the control frequencies obtained by partial filtering from the received picturecurrents are applied to produce a variation of light opposite and substantially equal to the variation caused by thecon'trol frequencies added to the picture currentsapplied to thenormal electrodes and control members of the light control device for the purposeof cancelling the effect of the control frequencies in the picture.

10. Television and picture telegraphy receiving apparatus adapted for the reception of combined picture and control electrical oscillations and in which a counter balancing effect is produced for the purpose of removing the unwanted effects of the control oscillations comprising s,ose,sss creased intensity and said picture oscillation is i of reduced intensity compared with said combined oscillations, an electro-optical control device having a plurality of pairs of controlling members, means for applying potential difleren'ces corresponding to said combined oscillations across one of said pairs of controlling members and means for applying potential differences corresponding to said corrective oscillation across another of saidpairs of controlling members.

11. Television and picture telegraphy receiving apparatus adapted for the reception of combined picture and control electrical oscillations comprising means for producing from said combined electrical oscillations, a corrective electrical oscillation in which said control oscillation is of increased intensity and said picture oscilla-l tion is of reduced intensity compared with said combined oscillation, means for producing light varying in intensity in accordance with said combined oscillation and light varying in intensity in accordance with said corrective oscillation and means for combining the two light variations in such a manner that variations due to said control oscillation substantially cancel one another.

12. In apparatus for television and picture telegraphy-in which a number of control frequencies are added to picture currents and are transmitted continuously and simultaneously with the said picture currents and in which said control frequencies are used in the receiver for synchronization and phase adjustment. the combination with a scanning device of an alternator and means coupling the scanning device to the alternator to ensure synchronous working the frequency of the alternator being equal to the received control frequency and means for applying the output of the said alternator to a circuit containing the received picture signals-in opposed balanced relation to the control frequency contained in the said picture signals, for the purpose of eliminating the effects of the con- 20 trol frequency from the reproduced picture.

GEORGE WILLIAM WALTON.