US2248548A - Television transmitter - Google Patents

Description

y 1941- I K.SCHLESINGER 2,248,548

TELEVISION TRANSMITTER Filed Aug. 2, 1935 3 Sheets-Shet l 7/? ran for mag/w July 8, 1941. SCHLESINGER 2,248,548

TELEVISION TRANSMITTER Filed Aug. 2, 1935 3 Sheets-Sheet 2 July 8, 1941. K. scHLEsxNeER 2,248,548

I TELEVISION TRANSMITTER Filed Aug. 2, 1955 3 Sheets-Sheet 3 F 46 I I m; MM

. from the specification Patented July 8, 1941 2,248,548 TELEVISION TRANSMITTER Kurt Schlesinger, Berlin, Germany, assignor, by

mesne assignments, to Loewe Radio, Inc., a corporation of New York Application August 2, 1935, Serial No. 34,390 In Germany August 4, 1934 2 Claims.

The subject matter of the invention is a television transmission or a method of scanning an original image, whereby the simultaneous transmission of image signals and synchronisation signals takes place in particularly simple fashion and of small cost with the use of one and the same main amplifier. By way of explanation it is necessary to deal briefly with the existing state of the television art.

An object of the invention is an arrangement for producing synchronization impulses of constant peak potential.

Another object of the invention is an arrangement for producing said synchronization impulses by the scanning holes of the Nipkow disc.

Further objects of the invention will be seen in the accompanying drawings.

In the drawings,

Figure 1 shows diagrammatically an embodiment of a television film transmitter.

Figure 2 shows the course of the image potentials and synchronization potentials in the transmitter.

In Figure 3 there is illustrated a disc for scanning an image to be transmitted.

Figure 4 shows the size of the image projected on to the scanning disc in comparison with the spacing between the scanning holes.

Figure 5 shows embodiments of a television film transmitter according to the invention and Figures 6 and 7 show further forms of embodiments of the present invention.

In Fig. 1 an arc lamp I is used to intensely illuminate a film 3 by the use of a condenser or hollow reflectorl, and the film image is reproduced on to to the Nipkow disc 5 by means of one or two lenses 4 and 4'. If a photo-cell 6 with connected image amplifier l is placed behind the Nipkow disc, preferably with the interposition of a collecting lens 8, there is available at the output of the image current amplifier 1 the signalling potentials which correspond to the light intensity values of the original film 3. No synchronisation impulses are present at the output of amplifier I.

The transmission is assumed to be a so-called positive transmission with black synchronisation signals. A transmission of this kind is illustrated diagrammatically in Fig. 2. Although the invention described in the following is not limited to a method of transmission of this kind, the same will nevertheless be explained in conjunction with the diagram 2, as this method is becoming more and more popular. In Fig. 2 the time t has been entered as abscissa and the aerial current i of the transmitter as ordinate. As regards the transmission it is necessary that the light intensity signals increase the aerial current in the case of white and reduce it in the case of black, but even with extreme black the aerial current should not become zero. As a matter of'fact the entire current intensity interval between'a very white, 1. e., transparent point of the film' anda point of the film completely precluding the transmission of light must be situated be tween two defined limits, which are representedby the two broken horizontal lines 9 and IE! for white and black. Complete choking of the aerial current is reserved to the synchronisation impulses, of which in Fig. 2 the short-signal line impulses are represented by the curves II, and the image-change impulse of greater duration by the curve l2. The separation of the synchronisation signals from the'image signal takes place in the manner known per se by an amplitude filter in the receiver. The latter is accomplished in the most simple manner by the use of a glow lamp.

In order to efiect transmission of synchronizing signals in practice in accordance with Fig.2 there'has been provided a special'slot divisioning on the Nipkow disc of the transmitter (5' in Fig. l). A Nipkow disc of this kind, as shown in Fig. 3, is as follows:

On the outer periphery the apertures necessary for the scanning of the image are arranged'in a circle, it being a condition that the film 3 is continuously advanced during the rotation of the disc. there is employed, as well known, a disc having spirally disposed apertures. tion or image area I3 is selected of a size exactly corresponding with the spacing between two adjacent scanning apertures M. In order now to obtain the synchronisation signals, which were designated II and I2 in Fig. 2, the disc 5 in Fig.1

3 requires a second divisioning. This consists of a ring of slots l5, which are lighted by a special light source IS (in Fig. l), and the light transmitted through the slots are converted by means of a special photo-cell l1 and a special amplifier I8 into electrical synchronisation signalsof constant intensity. Unfortunately the output potentials of the image amplifier l and the syn chronisation amplifier l8 cannot merely be superposed, as in this case the constant amplitude of the synchronisation signals as set out in Fig. 2 would not be even approximately fulfilled. As a matter of fact there is required a special appara- With intermittent movement of the film" The film reproducdisc with special synchronisation slots as shown in Fig. 3 consists in the fact that the precision in the relative position of the slots l to the apertures I4 is never sufliciently good in practice, and that in consequence a perfectly linear perpendicular line of the image is always reproduced at the receiver with small curvatures. The

curvatures, to which the eye is very sensitive;

always occur when the precision in the position of the edge of a slot is less than the width of an image point. The particular line must then obviously commencesomewhat too early ortoo late, so that the impression, i. e., the intensity modulation, is displaced in position on the received image in the order of magnitude of an image point as compared with the correct value. It is obvious that the electrical portion of the system cannot compensate for errors'in divisioning'of this kind. Thus, for example, even the filtering out of the synchronisation frequency by highly selective apparatus, for example by tuning forks or by electrical tuned circuits, cannot assist'in straightening the perpendicular lines so that they may appear as if the image points have small dis placement errors. x

The new method is illustrated in Fig. 5. The film optical system remains the same as in Fig. 1, and comprises the arc lamp l with the reflector 2, the film 3, which is intended to run continuously, and preferably two lenses'4 and 4'. The film is assumed to be one of such brilliance that the light impinging on the Nipkow disc 5 com pletely disappears at the black points, and is transmitted at the white points practically without weakening. Now the idea according to the invention consists in projecting 'an additional uniform lighting over the field of the filmimage on the disc 5 by a special auxiliary light source 23. Whilst the film reproduction 24 as shown in Fig. 4, should be completely without light at the black points, the same, by means of the additional uniform auxiliary lighting, is made brighter in homogeneous fashion over its entire width with an adjustable light intensity. According to the invention, this additional lighting i projected over a width which is exactly the same in size as the width of the film image reproduction 24, i. e., auxiliary light and film light are caused exactly to register. The arouate portion of the disc for this lighting is,1however, shortened by an interval 25 amountingto approximately 10% of the spacing between two adjacent scanning apertures M. It will bequite obvious what occurs now in the photo-cell circuit B. It is assumed that the photo-cell currents are fully proportional to the light intensities behind the scanning aperture, and in particular theyshould reach their lowest value, or disappear, when the lighting entirely ceases. The primary light through the lenses 4 and 4' will disappear even at the blackest points of the film reproduction. The secondary light, however, by

way of the lamp 23, the slot 3!, the selecting lens 2| and the deflectin prism 26, will continue to prevail also at these points, and the photo-cell current will, therefore, not disappear during the entire image. The two light sources I and. 23 are cut off only in the black interval 25. It is, therefore, only in this interval that the photo-cell current becomes zero. If, as will be initially assumed, there is proportionality between intensity of light and aerial current throughout the entire transmitter, we obtain without further auxiliary apparatus of any kind an aerial current diagram according to Fig. 2 with complete constancy of the amplitudes of the synchronisation signals independently of the course of the image signals, and we can adjust by regulation of the light intensity of the lamp 23 by means of a series resistance 21 the amount by which the synchronising signals exceed the maximum image amplitudes. The brighter the lamp 23 is adjusted, the higher will be the peaks II and I2 as compared with the interval 9 and I0 in Fig. 2.

By means of the described arrangement in Figs. 4 and 5 there is obtained initially merely the short line-change impulse H. the long image-change or frame impulse l2 it is merely necessary in the case of a circular concentrically apertured disc to allow a rotary diaphragm 28 to rotate at 25 periods per second. the frame frequency, and to arrange this immediately in front of the disc 5. With an opaque arm 29 this diaphragm, operated by means of a small synchronous motor 30, intercepts the light from both light sources for the duration of approximately 5 lines, so that during this absence of light there is obtained in the desired fashion a disappearance of the photocell current 6 and, in the case of the assumed proportionality, also of the aerial current 20.

The provision of a rotating diaphragm of this kind, which would not cause difficulties from a technical point of view, is rendered unnecessary by the use of a spiral aperture disc. When spiral apertures are used it is possible to arrange a black image angle such as 25 in Fig.4 also in a radial direction, i. e., the height of the lighted area may be made smaller than the height of the radial divisioning in the spiral disc. In this way it may be accomplished that the-five extreme apertures pass over a surface which its adjusted to be completely without light. In this case the short and the long impulses II and I2 in Fig. 2 will then be generated completely automatically. As well known, however, the arrangement with the circular concentrically apertured disc, particularly in the case of a large number of lines, is superior to the spiral disc.

Up to now it has been assumed that between the aerial current and the lighting of 6 there is a proportionality which is independent of the duration of the transmission of the signals, i. e.,

fication with condenser coupling true linearity cannot be accomplished without resorting to additional compensating means, and the synchronisation signals form under all circumstances a tangent to the zero lines of the aerial To producecurrent. If the subject matter in the film image persists at half-tone intensity for a comparatively lengthy period of time, the resistance-capacity coupled amplifier is unable to reproduce thev enduring lack of light, and its output potential induces merely weak potentials about the middle grey line. The difference in the photo-cell current upon the passage of the aperture I 4 into the dead angle 25 is, however, merely very small, as the film image light intensity at 24 was assumed itself merely to be very small, and the change in light intensity corresponds in substance, therefore, to the cessation of the auxiliary lighting over the surface 24. In consequence the synchronisation signals in this case do not reach the zero line of the aerial current. In a scene of this kind connected receivers necessarily fall. In exactly analogous but reverse fashion an error in synchronisation makes itself noticeable upon the transmission of a film scene taking place continuously in bright sunshine, since the resistance-capacity coupled amplifier is unable to reproduce the continuous presence of bright sunshine and records merely weak differences in intensity between more and less bright points in the original, does not dispose these small oscillations, as should be the case, about the maximum line 9 (Fig. 2) but instead they are disposed about the middle grey line. In this case, however, the jump in lighting when the scanning aperture leaves the image surface 24 and enters the dead angle 25 is particularly great, as the photo-cell 6 registers not only the omission'of the auxiliary light 23, but also in addition the disappearance of the reproducing light, which in the case of the stated scene is very bright. Owing to the incorrect mean value. of the amplifier output potential, which is not situated at 9 but between 9 and I (Fig. 2), the synchronisation signals fall this time below the aerial current minimum, assuming this to be possible at all.

If the aerial current is so adjusted that the same is reduced completely to zero by the impulses in the case of normal signalling and normal intensity of the scene, more strongly throttling signals are unable to diminish the aerial current still further. Proper reduction of the aerial current is possible, however, even with the use of a resistance-capacity coupled amplifier if the auxiliary lighting is increased in such fashion that even with a complete absence of image lighting the change in the amount of light projected on the photo-cell between 24 and 25 in Fig. 4 is so large that in each case the transmitting aerial current will with certainty disappear. There is accordingly included a saturation limit at a desired point in the transmission chain 7-20. Either the characteristic of the transmitter is so adjusted that the maximum fluctuation between black and white in the original film image does not completely make full use of the range of the transmitter (current value 9 and I!) (Fig. 2), and the intensity of the auxiliary lighting 23 amplified to such extent that cessation of the auxiliary lighting alone is sufiicient in each case to completely eliminate the aerial current, or a potential limitation of this kind is introduced within the low-frequency amplifier I. It is only required that the anode current of the final tube reach saturation, or that the grid bias of the final tube, by the application of grid current or by the use of a glow lamp particularly poor in capacity connected in parallel with the grid leak resistance of the final stage, with suitable bias, prevent an increase of the grid alternating potential in the final stage beyond a maximum surge value. It is then accomplished that greater values than this potential limit do not occur, 1. e., that all impulses reach the same absolute potential value, and by the increase in the auxiliary lighting as referred to above it is only necessary that this limit is regularly attained upon scanning with the auxiliary lighting alone, i. e., in the absence of film lighting.

It may again be remarked that when using purely direct current coupled amplifiers, carrier frequency amplifiers and amplifiers with glow lamp coupling, i. e., with stationary proportionality of the photo-cell 6 up to the aerial 20 saturation conditions of this kind are unnecessary, andrelatively small auxiliary lighting 23 is sufficient, in addition to which there is also the advantage that the medium intensity of the scene can at least be reproduced in the aerial current. As regards the received image, however, this method of transmission results in no improvement so long as the receiver does also not operate without condensers and a stationary proportionality is provided between strength of field and light intensity in the receiver. So long as the television receiver operates with a condenser coupling in front of the receiving light relay, use of the stated method of transmission cannot offer advantages. In these cases useof the resistancecapacity coupled amplifier in conjunction with saturation effects and strong auxiliary lighting is of particular importance in practice, as the resistance-capacity coupled amplifier is very simple in assembly and use.

The invention, which has been described in the above in conjunction with a perforated disc transmitter, may also be employed, however, quite generally. For example, it is quite readily possible to make use of the same in transmitters where the subject matter transmitted originates in a studio rather than being derived from a film.

An arrangement for carrying-out the invention in practice in the case of a cinema transmitter is illustrated by way of example in Fig. 6, said arrangement containing in addition to the features set forth in the above a particularly advantageous arrangement. for producing the auxiliary lighting.

In the same:

I is the arc lamp, this .beingthe only source of light occurring in the system. By means of the reflector 2 or condenser the light is concentrated on to the film, the image to be transmitted on each occasion being represented by the section 3a. It'is assumed that the light intensity of the image field is uniform over the entire picture area 311, as may be expected in the kinematographic art.

Now the transmitter according to the invention is intended to operate with a circular concentrically apertured disc- 5, which accordingly has the property of scanning merely one single line in the entire height of the image 3a; The image-3a is reproduced sharply on the disc 5 by means of two lenses 4 and 4. The line 32 scanned by the disc 5 represents the only line in the entire image which is utilised optically, and all remaining lines of the film are not embraced by the scanning operation. Accordingly the total remaining light on the picture area 3a is useless for the television transmission. On the other hand, as shown in the above,a very considerable additional amount of light must be projected on to'the film. reproduction 24 on the Nipkow disc, and this additional lighting must be approximately equal in amount to the lighting intensity of snowy white points in the original. In order to solve this problem without the heavy expense of a second arc lamp, as merely an arc lamp could supply the requisite intensity of light, it is very natural in accordance with the invention to make use of the part of the light which is not utilised for the scanning of the image and traverses the picture 3a outside of the scanned line 32. For this purpose, in accordance with the invention, the scanned line 32 is not, as hitherto usual, disposed at the centre of the image, but the line of the original reproduced on the scanning disc is displaced in relation to the path of the bundle of rays 33 and located at 32. This may readily be accomplished in practice by lifting or lowering the entire projection apparatus together with the film 3 and the lenses 4 and 4', with respect to the position of elevation of the axis of the Nipkow disc. The reproduction of the film 24 is then lowered or lifted on the disc, and in this manner the edge of the reproduced image is practically made to register with the radius defined by the scanning apertures. Now in accordance with the invention, the following possibility is provided of rational utilisation of the remaining light:

' A deflecting prism 34, preferably one composed of durable glass (Ignal glass) projects that portion of the light which otherwise would impinge on the picture area 3a above the line 32 to be transmitted vertically to the path of the reproducing rays. The part of the film 3, which otherwise without a prism would be lighted, accordingly remains black. The rays impinge on a slot or mask 35, of which more particularly the width e requires to be reproduced sharply. Behind the slot or mask 35 there is located a second deflecting prism 35. One or two lenses 31 and 38 are provided, and then in turn two deflecting prisms 39 and 40. The last-mentioned prism is tilted to a somewhat greater extent, so that the auxiliary light, as required, covers the same area of the scanning disc 5 as the reproduction of the original picture area 3a. The intensity of the auxiliary light may be adjusted in very simple fashion by an iris shutter provided for example at 4|, whereby the height of the synchronizing impulse may be optically varied. If the maximum auxiliary light intensity obtainable in this way is not sufficient, an additional increase in the auxiliary light may be efiected with the assistance of a condenser lens furnished at 42 by concentration of the light coming from 34 on the slot or mask 35.

The form of embodiment set forth in Fig. 6 having one are lamp has the additional advantage as compared with the stated arrangements having two arc lamps in that the ratio between the original lighting and the auxiliary lighting is always maintained even upon fluctuations of I, and by means of an automatic regulating means for the common photo-amplifier which is usually provided in any case an error or variation of this kind on the amplifier side may be fully compensated.

The method of auxiliary lighting may be employed in simple fashion for automatically regulating the degree of amplification according to the lighting intensity. For this purpose a part of the by-passed light, is directed upon a white plate 43 positioned near and inclined with respect to the slot 35, and is reflected against a control photo-cell 44. Across the load resistance 46 of the photo-cell 44 there is then developed a potential which is negative with respect to earth in proportion to the intensity of the arc lamp Therefore, if 41 is a tube in the train of the main amplifier behind the disc 5, this regulating potential may be conducted in the form of a bias to the control grid of the tube 41 by way of the line 48 in order to attain the desired effect and the maximum output potential is maintained constant independently of the lighting intensity, the sensitivity of the amplifier being increased upon a decrease in the illumination by the are I, and vice versa.

Further, in accordance with the invention, the amplification is so adjusted that (1) Only upon scanning such points of the original which are as clear as'glass on the one hand, and points completely without light on the other hand, at which the auxiliary lighting disappears, i. e., in the dead angle of the image, are the limits of the modulation characteristic of the transmitter reached.

(2) The aerial feed current is adjusted to that particular stationary value, which upon passage hrough a black-and-white scale in the original corresponds with the grey value of this scale. This aerial feed current is relatively high and amounts to approximately 60% of the maximum aerial current.

Another optical system for carrying out the method is shown in Fig. '7. This arrangement has the advantage that the same lenses which are employed for reproducing the film also perform simultaneously re-reproduction of the light ing slot. There is again provided a light source l with a condenser reflector 2, and by means of a prism 34 a large part of the light, which does not fall on to the scanned line 32, is deflected out of the main ray. As compared with Fig. 6, however, the by-passed light and the primary light are united before reaching the first lens 4. If the light proceeding from I has already been directed along parallel paths by the hollow refiector 2, only a portion of the surface of the lens 4 need be employed for reproducing the scanned line 32, via, the part situated below the broken line 49. It is, therefore-possible without loss of light to unite the by-passed light 50 with the primary light 5| in the lens 4 and to project and focus the image of the slot or mask on the disc 5 provided the spacing 52 has been correctly adjusted. The slot or mask 35 is so adjusted with respect to its spacing 52 and the prism so rotated that the reflected image of the slot 35 is situated above the line 32. As well known, this is the case when the optical length 52 is equal to the length 52', and the base of the prism bisects the angle formed between the two. By means of a diaphragm 4| and a condenser lens 42 an improvement and quantitative apportioning of the compound lighting may be obtained.

I claim:

1. A television transmission arrangement comprising a Nipkow disc containing a row of scanning holes, a photoelectric cell, a single source of light, means including said source of light for projecting an image of the object to be transmitted onto said Nipkow disc, and means including said source of light for projecting an additional light bundle upon the image projection onto said Nipkow disc, said additional lighting exactly covering the image field, the length of said image field being smaller than the spacing between said scanning holes, and means for producing an image current in said photoelectric cell by scanning said image field, an electronic amplifier for amplifying said image currents, a light responsive element, said light responsive element being positioned to receive light from the single source in proportion to the intensity of the source to produce a voltage proportional to the light intensity, and means to control the effectiveness of the electronic amplifier by the produced voltage whereby the strength of the amplified image currents will be substantially unailected by changes in the intensity of the source of light.

2. A television transmission arrangement comprising a Nipkow disc containing a row of scanning holes, a photoelectric cell, a single source of light, means including said source of light for projecting an image of the object to be transmitted onto said Nipkow disc, and means including said source of light for projecting an additional light bundle upon the image projection onto said Nipkow disc, said additional lighting exactly covering the image field, the length of said image field being smaller than the spacing between said scanning holes, and means for producing an image current in said photoelectric cell by scanning said image field, an electronic amplifier for amplifying said image currents, means responsive to the light from the source for producing a potential substantially proportional to the intensity of the single source of light, and means for controlling the image current amplifier by the produced potential whereby the strength of the amplified image current will be substantially unaffected by changes in the intensity of the single light source. I

KURT SCHLESINGER.

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