TELEVISION*

V. K. Zworykin

The new system of “cathode” receiving and transmitting television apparatus developed by V. K. Zworykin has opened up prospects of which mechanical television could not even dream. The best evidence of the advantages of this system is the fact that in America, as early as two years ago, mechanical television had been completely displaced by cathode television.

The merits of the new system and its significance have received due appraisal among us as well. At present a whole series of research laboratories in the Union are working on the problem of cathode television. We may already note a whole series of achievements, such as, for example, the development of the “kinescope” television receiver in the laboratory of the “Svetlana” plant.

Nevertheless, however great the importance of the problem of cathode television, information on this question in Russian is extremely scant. With the exception of Zworykin’s report, delivered by him at the Scientific-Technical Society of Electrical Engineers (Leningrad) during his stay in the USSR in the summer of 1933, published in a very small edition, and a short article in Radiofront, nothing has been printed on the subject up to now.

In view of this, the editors considered it advisable to place in the journal the translation printed below of Zworykin’s article from the January issue of the Journal of the Franklin Institute, devoted to the fundamentals of cathode television. This article, treating the question not only more broadly and more deeply than Zworykin did in his report, contains, in addition, a considerable number of illustrations, thanks to which the reader obtains a complete idea of the cathode-television system and of its merits.

The Editors

Brief summary. The present article gives an outline of a new system of television apparatus developed in the laboratories of the R. C. A. Victor Co.

The system is entirely electrical. It uses only electronic devices and has no moving mechanical parts whatsoever.

The transmission of the image is effected by means of a special electron tube called the iconoscope. This tube is a true electric eye and consists of a light-sensitive mosaic corresponding to the retina of the human eye and an electron beam playing the role of the optic nerve. The image of the object being transmitted is optically projected onto the mosaic and is converted by the iconoscope into a series of electrical impulses corresponding to the brightness of the individual points of the object.

The reproduction of the image is produced by means of another vacuum tube, the kinescope, which converts the electrical impulses back into fluctuations of the brightness of a screen, by bombarding a fluorescent screen with an electron beam of variable intensity.

The motion of the electron beam in both tubes is such that, thanks to

* Translation by N. S. Khlebnikov.

the image is divided into a number of parallel lines. These motions are synchronized, so that the instantaneous positions of both beams relative to the points of the picture are always the same. The synchronizing signals are transmitted simultaneously with the image transmission. In this way the receiver operates fully automatically.

The sensitivity of the iconoscope at the present time is approximately equal to the sensitivity of motion-picture film and makes it possible to transmit images successfully under natural illumination. The resolving power of the iconoscope is very great, greater than is necessary for obtaining a television image of the very highest quality.

The present article gives the theory of this new television system and describes its properties. Photographs are given of images obtained on the fluorescent screen of the receiver.

Introduction

The word “television” has recently become a familiar word and calls to mind for us the artificial reproduction of rapidly changing pictures. Since it is artificial,

Fig. 1. Image sharpness as a function of the number of lines for a picture with a large amount of detail.

we cannot expect an absolutely exact reproduction of the original, and therefore the word “television,” in order to characterize the degree of perfection of the reproduction, requires an additional adjective.

In making reproductions in printed publications, it is customary to characterize the clarity of a drawing by the number of dots falling per unit area. An ordinary drawing contains about 700 elements per \(1\ \mathrm{cm}^2\), or approximately 26 lines per \(1\ \mathrm{cm}\). For finer work a larger number of elements is used. In tele-

the necessity of using a single conducting channel compels one to decompose the image into elements transmitted sequentially. Owing to the fact that these elements are arranged in the form of a series of parallel lines, it has become customary to characterize an image by the number of lines per unit length. Thus, we speak of 30-, 60-, or 120-line television, which means that the entire reproduced image consists of 30, 60, or 120 parallel lines, the density of which varies along their length.

The more elements an image contains, the more difficult the problem of television becomes. It is therefore extremely important to establish,

Fig. 2. Image sharpness as a function of the number of lines for a picture with fewer details.

how far one should go in the direction of increasing the number of elements, or, in other words, what is the minimum number of lines at which the image may be considered satisfactory.

A considerable number of excellent works, both experimental and theoretical, have already been published on this question. The results of these works agree quite precisely with one another and indicate that increasing the number of lines at first leads to a strong increase in the sharpness of the image; then the rate of this increase slows down, and finally the sharpness reaches a certain limiting value, after which any further increase in the number of lines gives only a negligible increase in sharpness.

In order to give a better idea of this relationship, Fig. 1 presents reproductions from one and the same picture, transmitted by 60, 120, 180, and 240 lines, respectively. These pictures were not transmitted with the aid of a television transmitter, but were

case, as in cinema, we have the transmission of many images each second, owing to which the missing details of one can be reproduced in others, while the eye perceives them all as belonging to one and the same image.

The practical difficulties in constructing a television system increase in proportion to the amount of material to be transmitted, and therefore grow with an increase in the number of elements of the picture. These difficulties consist not only in dividing the picture into elements. They are also due to the limited number

Fig. 4. Increase in image sharpness with an increase in the number of lines at different illuminations.

of electrical channels that can be used in transmission, the requirements imposed on electrical circuits, and also the light regime necessary for transmission and reception.

The following Table 1 illustrates the relationships between the number of elements of the image and the maximum frequency of light variation that may occur in transmitting the picture, assuming the ratio of its length to its width to lie between 3 and 4, at 24 repetitions per second.

The bandwidth of carrier frequencies necessary for transmitting images with a number of lines greater than 100 makes it impossible to use radio transmitters with the frequencies employed for sound broadcasting. The only solution to the problem is transmission at ultrahigh frequencies. Questions of a radio-engineering character, however, fall outside the scope of the present article. Nor will the history of the development of televisio-

were obtained by a special optical method, and therefore were not subject to distortions that may occur in various processes in the transmitter and receiver. They must therefore be regarded as television images of ideal quality. The series of pictures presented shows that, for a picture with a large number of details, we must consider 240 lines as a minimum. With a smaller number of details the number of lines may be somewhat smaller, as is easily seen from Fig. 2. Both examples given

Fig. 3. Sharpness of the image as a function of the number of lines for a black-and-white picture.

refer to images with halftones. Black-and-white pictures require approximately the same number of lines, as may be seen from Fig. 3.

The aforementioned relation between the number of lines and the sharpness of the image is also illustrated by the curve in Fig. 4*. This curve was obtained by calculation, proceeding from the resolving power of the human eye. It may be expressed by the equation:

\[ G = \rho \lg n, \]

where \(G\) is the sharpness of the image, \(n\) is the number of image elements, and \(\rho\) is a coefficient depending on the size of the image and the angle of vision. The three curves of Fig. 4 correspond to three different illuminations: 1, 10, and 100 lumens.

All the data presented refer to the transmission of images of stationary objects. Television images of moving objects have greater sharpness, owing to the fact that in this

* Ya. A. Riftin, Z. S. f. Techn. Phys., Vol. III, Nos. 2–3, 1933.

TABLE 1

It is enough to say only that practically all work up to the present time has been carried out with mechanical methods of decomposing the image into elements. In exactly the same way, mechanical methods have also been used for reconstructing the image in the receiver. Because of this, purely mechanical difficulties arose in the design of scanning devices, and difficulties connected with increasing the number of image elements, especially with respect to the light conditions.

The situation with light proved to be truly a stone wall against which all attempts to increase resolving power—which is necessary for obtaining a high-quality image—came up, and it made practically unattainable the true goal of television: the transmission of images under natural illumination.

To understand the cause of these difficulties, we must remember that the picture is transmitted point by point, so that in mechanical television the photoelement is under the action of light from each point of the picture only for a very short interval of time, equal to the duration of the exposure of an element. Suppose that, in order to obtain an image of good quality, 240 lines or 76 000 elements are necessary. With 24 repetitions per second this means that the transmission time of an element will be \(1/1824000\) of a second. The output of the photoelement is proportional to the intensity of the light and to the interval of time during which the light acts on the photoelement. Simple calculations show how negligible the output of the photoelement will be for the indicated number of picture elements. If we take an average camera with an aperture of \(1:4.5\), the total luminous flux falling on the plate when photographing a bright picture in the open air will be about 0.1 lumen. Replacing the plate by a disk of 76 000 elements and using a photoelement with a sensitivity of \(10\ \mu\mathrm{A}\) per lumen, we find that the current obtained from one picture element will be equal to:

\[ I_e=\frac{I\cdot 10^{-5}}{10\cdot 76000}=1.3\cdot 10^{-11}\ \mathrm{A}. \]

* 10% added for the loss of time for transmitting synchronizing signals.

The charge produced by this current during the passage of one element will be:

\[ Q = I_e t = \frac{1.3 \cdot 10^{-11}}{1824 \cdot 10^6} = 7 \cdot 10^{-17}\ \mathrm{C}. \]

Comparing this quantity with the charge of an electron \((e = 1.59 \cdot 10^{-19}\ \mathrm{C})\), we see that only 44 electrons correspond to one picture element. The amplification of such negligible impulses presents insurmountable difficulties in practice.

If we compare these conditions with those under which a photographic plate operates, we shall see that the latter are incomparably more favorable, since all its points are subjected to the action of light throughout the entire exposure. In indoor photography this time is measured in seconds; for scenes in the open air it is of the order of \(1/100\) sec., i.e., many thousands of times longer than the time of passage of one picture element. The human eye, which we regard as the ideal of sensitivity, operates under conditions just as favorable.

If a television transmitter could be constructed on the principle of the human eye, each point of the picture would act on the photosensitive surface during the interval of time allotted to the transmission of the entire picture. Then, under the conditions of our example, the output of the photocell due to a single picture element would prove to be 76,000 times greater than in transmission by the ordinary method. Since scanning of the picture is necessary in order to make it possible to use only a single conducting channel, to achieve the indicated aim it is necessary to have some device for accumulating charges in the interval between two successive transmissions of one and the same point.

The requirements imposed on the light regime, and the necessity of doing without mechanically moving parts, led to the development of an entirely electrical television system, in which special electron devices are used for transmission and reception.

Iconoscope

The transmitting device took the form of an artificial eye. It received the name “iconoscope,” formed from two Greek words and literally meaning “image observer.” A photograph of this instrument is shown in Fig. 5, a.

The iconoscope consists of two parts enclosed in a common evacuated glass bulb. One part is a photosensitive mosaic and consists of a metal plate covered with a large number of the smallest photosensitive points, insulated both from one another and from the plate. Each such point constitutes the cathode of a separate photocell; all the photocells have one common anode. The purpose of this mosaic is the same,

that of the retina of the human eye. It converts the light energy received from the image projected onto it into the energy of electric charges accumulating on it until they are removed one after another, thereby giving rise to electrical impulses by which the transmitter is modulated. The removal of charges from the mosaic elements is carried out by means of a moving electron beam, which plays the role of the nerve of this electric eye. In order to make the analogy between the eye and the iconoscope still more complete, let us note that this device possesses “electric memory,” since, given a good dielectric, the charges on the mosaic can be preserved for a considerable interval of time.

Fig. 5. General view of the iconoscope.

To clarify the principle of operation of the iconoscope, it is best to consider the circuit of a separate mosaic element, shown schematically in Fig. 6. Here \(P_c\) represents such an element, \(C\) is its capacitance with respect to the plate common to all the elements, which we shall call the “signal plate.” The whole circuit can be traced from the cathode \(P_c\) to \(C\), then to the resistance \(R\), the battery \(B\), and back—to the anode \(P_a\). When light from the projected image falls on the mosaic, each of its elements \(P_c\) emits electrons. Thus, under the action of light, the elementary capacitor \(C\) becomes charged. The magnitude of this charge is a function of the intensity of illumination. When the scanning electron beam falls on the positively charged element \(P_c C\), this element replenishes its supply of electrons and may be called discharged.

If we plot the curve of the change in the charge of the element \(P_c C\) as a function of time, as is done in Fig. 7, we shall see that, owing to the light energy received from the picture, the potential of the mosaic element will increase. The rate of this increase

\[ \frac{dV}{dt} \]

depends only on the brightness of the part of the picture projected

for the given element. In other words, at an unchanged illumination intensity the potential will increase linearly with time. This linearity will be maintained only until saturation of the capacitance \(C\) occurs. The magnitude of the latter is chosen, however, with the calculation that, at the given discharge frequency by the electron beam, saturation never occurs. Since the motion of the scanning beam over the mosaic takes place at a constant velocity, the time interval \(t=\frac{1}{N}\) elapsing between two successive discharges is also constant and, thus, the magnitude of the accumulated charge depends only on the brightness of the corresponding point of the image. Under the condition of constancy of the intensity of the scanning beam, the current pulse flowing through \(R\) when the charge is removed from

Fig. 6. Circuit of an iconoscope element:

Fig. 7. Change of the charge of an iconoscope element as a function of the illumination time.

an element of the mosaic and, consequently, the potential difference \(V\) arising at the ends of the resistance also prove to depend only on the brightness of the point of the image. This potential difference \(V\) is the output of each separate photoelement of the iconoscope mosaic, i.e. the elementary signal of the transmitted image.

The scheme of operation of the iconoscope is in reality somewhat complicated by the circumstance that the scanning beam not only neutralizes the positive charge of the mosaic element, but also charges this element negatively. The potential that is actually established is determined by the velocity of the electrons of the beam and by the secondary emission from the photosensitive surface, arising as a consequence of its bombardment by electrons of considerable velocity. The magnitude of the equilibrium potential for an ordinary iconoscope in darkness is equal to \(-0.5\)—\(1.0\) V. Under the action of light, the mosaic elements acquire a positive charge, thereby lowering their normal negative potential, which is again restored by the scanning beam.

Another complication is due, by its origin, to the fact that

in addition to the discharge pulses from each individual element of the mosaic, there are pulses caused by the charging of all the elements of the mosaic under the action of the light falling on it. The charging current is constant for a stationary picture, but ceases to be so as soon as the image or some part of it begins to move over the mosaic. Changes in the charging current, however, are very slow and do not act on the amplifier if the latter is provided with a filter for eliminating frequencies below 24 cycles.

Fig. 8 gives an idea of the conditions on the surface of the mosaic. Here the dark part represents the electric charges accumulated by individual elements of the mosaic owing to the action of the light of the projected picture. Although in Fig. 8 the background of the picture has the same density at all points, in reality the corresponding charges at a given moment of time are not equal, but vary as shown on the left side of the figure. The charge reaches its greatest value at the moment immediately preceding the arrival of the beam at the given point. Immediately after the passage of the beam the charge is near equilibrium conditions and increases during the whole scanning period, attaining its greatest value at the moment before the arrival of the beam.

Fig. 8. Removal of charges from the surface of the mosaic by an electron beam.

By neutralizing a point of the mosaic, the electron beam almost instantaneously releases the entire store of energy accumulated there during \(1/24\) of a second. Therefore the magnitude of the elementary pulse in a system using the accumulation of charges will be as many times greater than in the ordinary one as the light from an element of the picture acts on the photocell for a longer time.

This is easy to show by calculating the magnitude of the elementary pulse in both cases.

The potential difference \(V_\mu\), arising across the resistance \(R\)

in the case of a mechanical transmitter, at the expense of one element of the picture, may be expressed as follows:

\[ V_m = RS \frac{\Phi}{n}, \tag{1} \]

where \(R\) is the magnitude of the resistance, \(S\) is the sensitivity of the photoelement (i.e., the photocurrent referred to a unit luminous flux), \(\Phi\) is the luminous flux corresponding to the entire image, and \(n\) is the number of image elements.

In view of the fact that the individual pulses must not interfere with one another, the circuit of the photoelement must satisfy a definite condition, namely that the product of its capacitance by its resistance must be no greater than the time of passage of a single element, i.e.,

\[ C_m R = \frac{1}{Nn}, \]

where \(C_m\) is the capacitance of the circuit and \(N\) is the number of repetitions of the picture per 1 sec. Hence we find the magnitude of the resistance:

\[ R = \frac{1}{NnC_m}. \]

Substituting the expression obtained in (1), we get:

\[ V_m = RS \frac{\Phi}{n}\,\frac{1}{NnC_m} = \frac{S\Phi}{Nn^2C_m}. \tag{2} \]

From this expression it follows, among other things, that in mechanical television the elementary pulses are weakened in proportion to the square of the number of elements.

To determine the elementary pulse of the iconoscope (under the same conditions), let us compute the magnitude of the charge corresponding to an element of the image. This charge is equal to:

\[ Q = S \frac{\Phi}{n} t, \tag{3} \]

where \(t\) is the time of action of the light on the element between two successive discharges, which, to a close approximation, may be taken as equal to \(\frac{1}{N}\).

The elementary pulse of the iconoscope is equal to:

\[ V_i = \frac{Q}{C_i}, \]

where \(C_i\) is the capacitance of the elementary circuit of the iconoscope.

Thus

\[ V_i = S \frac{\Phi}{n}\,\frac{1}{N}\,\frac{1}{C_i}. \tag{4} \]

The ratio between the elementary pulses of the iconoscope and of the mechanical transmitter will be equal to:

\[ \frac{V_i}{V_m} = \frac{\dfrac{S\Phi}{nNC_i}} {\dfrac{S\Phi}{n^2NC_m}} = n \frac{C_m}{C_i}. \tag{5} \]

Assuming that \(C_{\text{ж}}=C_i\), we have:

\[ \frac{V_i}{V_{\text{ж}}}=n. \]

Thus, in the example we are considering, the iconoscope pulse must be 76,000 times greater. This would be so if the efficiency of the device were equal to \(100\%\), which is practically impossible because of all sorts of losses. At present one has to be satisfied with \(10\%\) of the theoretical output.

A schematic representation of all the electrical circuits of the iconoscope is given in Fig. 9. As we see, here the parts of the photoelements (\(P\) in Fig. 6) are completely separated from one another. The cathodes of the photoelements are light-sensitive globules located on the surface of the signal plate and insulated from it. The anode,

Fig. 9. Complete circuit of the iconoscope.

common to all the photoelements, is the silvered part of the inner surface of the bulb. The capacitance \(C\) of each individual element with respect to the signal plate is determined by the thickness and dielectric constant of the insulating layer between them. The removal of the positive charge from the elements of the mosaic is carried out by means of a beam of electrons produced by an electron gun situated opposite the center of the mosaic at an angle of \(30^\circ\) to the normal. The mosaic and the gun are enclosed in a common, carefully evacuated glass bulb. The purpose of placing the electron gun at an angle is to make it possible to project the image onto the mosaic.

The resolving power of the iconoscope is determined by the number and dimensions of the mosaic elements, and also by the cross-section of the scanning electron beam. In practice, however, the number of mosaic elements is many times greater than the number of picture elements, the quality of which is thus entirely determined by the cross-section of the scanning beam. This is shown schematically in Fig. 10. From the basic assumptions made in the analysis of the ideal contour of an individual

element of the mosaic (Fig. 6), we find the conditions which the iconoscope mosaic must satisfy. These conditions amount to requiring that the photoelements have the same size, the same sensitivity, and the same capacitance with respect to the signal plate. The fact that the cross-section of the scanning beam is much larger than the dimensions of the individual photoelements modifies and simplifies these conditions. It is sufficient that the sensitivity and capacitance of a mosaic surface element, of a size equal to the area of the electron spot, be unchanged over the entire surface of the mosaic. This permits considerable deviations in the sizes of individual photoelements.

The question of the photoelectric and capacitive uniformity of the surface, the achievement of which would seem to be very difficult, is in fact solved quite simply. It is known that such a widespread material as mica can be obtained in the form of sheets of practically identical thickness at all points. Mica, therefore, is an ideal insulating layer for the mosaic. As for the mosaic itself, it can be obtained in several ways, the simplest of which consists in evaporating an alkali metal onto mica in vacuum. When the film obtained by evaporation is very thin, it is not continuous, but consists of miniature nests of metal, distributed quite uniformly over the surface of the substrate and separated from one another by non-conducting gaps. Another possible method is to obtain the mosaic from a continuous metallic film with the aid of a dividing machine.

Fig. 10. Relative sizes of the mosaic elements and the electron spot.

Initially the mosaic was obtained by the above-mentioned deposition of a thin film of an alkali metal, but the development of photoelement manufacturing technology compelled the use of another method, and at present the mosaic consists of a very large number of the finest silver beads, made photosensitive by special treatment with cesium.

Since the charges with which one has to deal are very small, losses due to the conductivity of the dielectric must be reduced to a minimum. Mica of good quality fully satisfies this requirement. Besides mica, other dielectrics may also be used. Thus, for example, thin layers of glass enamel (liquid glass) have proved quite suitable. The thickness of the insulating layer is made as small as possible.

The sensitivity of the mosaic has the same order of magnitude as the sensitivity of vacuum oxygen–cesium photoelements,

are identical, as are their spectral characteristics. The distribution of sensitivity over the spectrum for the mosaic is shown in Fig. 11. The sharp fall of the curve toward the short wavelengths is due to the absorption of light in the glass. The true distribution of sensitivity over the spectrum is represented by the dotted line.

The electron projector, which generates the scanning beam, is an extremely important part of the iconoscope. In view of the fact that the resolving power of this instrument is determined by the area of the electron spot sliding over the mosaic, the projector must be constructed in such a way as to give a spot of dimensions exactly corresponding to the number of elements for which the iconoscope is designed. In the example given above, of an image with 76,000 elements and with a mosaic plate about 100 mm high, the distance between two successive lines is about

Fig. 11. Distribution of the sensitivity over the spectrum of the iconoscope mosaic.

0.4 mm, and the diameter of the spot must be equal to half this value. Thus the construction of the electron projector proves to be a very serious problem.

The electron projector of the iconoscope is extremely similar to the analogous device of the receiving tube of cathode television—the kinescope, which has already been described more than once.* The arrangement of the projector is explained by Fig. 12. It consists of a cathode with indirect heating \(C\), the emitting surface of which is located on the bottom of the cylinder forming the cathode. This emitting surface is placed in front of the aperture \(D\) of the control electrode \(G\). The anode \(A\) is a long cylinder with three diaphragms arranged on one straight line with the cathode and the aperture of the control electrode. The projector is placed inside a long and rather narrow glass tube, sealed to a spherical bulb in which the mosaic is located. The inner surface of the tube, as well as part of the surface of the bulb, is coated with a metallic

* V. K. Zworykin, Journ. Radio Eng., Dec. 1929.

layer and serves as the second anode for the projector, and also as a collector of electrons emitted from the mosaic. The voltage on the first anode is usually equal to a fraction of the voltage of the second anode, whose value is approximately 1000 V.

Fig. 12. Device of the electron projector.

Focusing of the electron beam is carried out by means of an electrostatic field formed by applying a potential difference between the parts of the projector, and also between the projector itself and the metallized surface of the neck of the iconoscope.

Fig. 13. Action of an electric field on a moving electron.

If a moving electron enters an electric field and moves in the direction of the lines of force, the action of the field affects only the magnitude of the velocity, but not the direction of motion. If, however, the electron enters the field with a velocity directed at some angle \(\alpha\) to the direction of the lines of force, both the magnitude and the direction of the velocity undergo change, as can be seen from Fig. 13. In the case of an accelerating field, the angle between the direction of motion

between the electron and the axis of symmetry of the field will decrease, and in a decelerating field will increase. In this way it proves possible to make the electron beam convergent or divergent.

Fig. 14. Converging electric lens.

The described interaction between a moving electron and an electrostatic field is used to construct a kind of “lens.” Fig. 14 shows a converging electric lens, which, by a simple reversal of the poles, can be converted into a diverging one.

Fig. 15. Distribution of the electric field in an electron projector.

The same results can be obtained with the aid of a field created by a difference of potentials between two cylindrical electrodes or two diaphragms. In both these cases the field will force the electron beam to approach its axis, overcoming the natural tendency of the electrons to repel one another.

from one another. The action described is analogous to the focusing of a light beam by optical lenses. Electric lenses differ, however, from optical ones in that in them there is no discontinuity of the refractive index at the boundary of two media, and the refractive index changes continuously throughout the entire field. By properly arranging the electrodes and selecting the potentials, it is always possible to construct a complex electric lens equivalent to a system of converging or diverging lentils.

Fig. 16. General view of the iconoscope camera.

The distribution of the electric field in an electron projector is shown in Fig. 15. In this particular case the total action of the field on the beam is approximately the same as the action on a light ray of a system of two asymmetrical lenses shown in the same figure. The first lens directs the electrons through the aperture of the first anode and provides regulation of the beam intensity by means of the control electrode \(G\). The final focusing of the beam on the mosaic is carried out by the second lens, formed by the field between the end of the projector and the metallized neck of the iconoscope bulb. The size of the electron spot on the mosaic is determined by the dimensions of the emitting surface of the cathode and by the distances between the cathode, the lenses, and the mosaic.

The motions of the electron beam necessary for scanning the image are produced with the aid of alternating magnetic fields. The deflecting coils are placed on a yoke (Figs. 5, 6) fitted over the neck of the iconoscope. The scanning motions of the beam are rectilinear in both the vertical and the horizontal directions. They occur owing to sawtooth current pulses flowing through the coils and generated by special vacuum-tube generators.

From the curve of the spectral sensitivity distribution shown in Fig. 11, it is clear that the iconoscope can be used both for transmitting visible images and pictures invisible to the eye, illuminated by ultraviolet or infrared ...

light. The sensitivity of the iconoscope at the present time is approximately equal to the sensitivity of motion-picture film operating under the same conditions, i.e., at the same speed and with the same lens. The resolving power of the iconoscope proves to be greater than is needed for transmitting an image with 76,000 elements. Some of the instruments that have been built have proved suitable for transmission at 500 lines, and there is every reason to think that this is not yet the limit.

Since the iconoscope is not bulky, it proved possible to build a very compact camera containing it and two stages of amplification. This camera is connected with the remaining stages and with the scanning generators by means of a long cable. Thanks to the portability of such a device, the iconoscope can be installed at any place that is of interest for a television transmission. The general appearance of the camera is shown in Fig. 16.

Fig. 17. Complete circuit diagram of the receiver and transmitter.

Fig. 17 depicts the complete circuit diagram of the receiver and transmitter, indicating the purpose of the individual parts and their mutual arrangement. Listing them in order, for transmission from the studio we have: the iconoscope camera, the amplifier of the picture signals and synchronizing signals, the control device, the modulator, and the radio transmitter. The elements of the television receiver are: the radio receiver, the kinescope, and the devices for deflecting the beam in the vertical and horizontal directions.

The name “kinescope” was assigned to the cathode oscillograph serving for the reproduction of the image, in order to distinguish it from the usual instruments of this kind. A special term is necessary, since the kinescope has certain substantial differences from an ordinary oscillograph, such as, for example, a special arrangement for regulating the intensity of the electron beam. In Fig. 18

one can see a photograph of the kinescope, with a screen diameter of 225 mm, thus making it possible to obtain images about \(160 \times 140\) mm in size. The arrangement of the electron projector of the kinescope is exactly the same as that of the projector of the iconoscope. The difference lies only in the operating mode—the kinescope projector operates at higher voltages on the second anode (4500 V).

As in the iconoscope, the electron projector of the kinescope is located in the long narrow neck of a large conical bulb, the inner surface of which is silvered or metallized in some other way so as to serve as the second anode. The purpose of this second anode is to accelerate the electrons leaving the projector and to form a field that gathers them into an extremely narrow beam.

Fig. 18. General view of the kinescope.

After leaving the first anode, the accelerated and narrowed beam falls on a fluorescent screen deposited on the flat bottom of the conical part of the kinescope bulb. This screen is a converter of the kinetic energy of the electrons into light energy. Under the action of the beam, a small bright spot is formed on the screen, with a cross-section approximately equal to the cross-section of the beam. Since the fluorescent layer is very thin, a considerable part of the light passes through it and is used in viewing the image.

To reproduce the brightness of the points of the transmitted object, it is necessary to vary the intensity of the light spot on the fluorescent screen. This is done by changing the intensity of the beam (the number of electrons) by means of the control electrode \(G\) (Fig. 12) of the electron projector. For correct reproduction it is necessary that the beam intensity be a linear function of the incoming signals (voltage pulses). In addition, it is highly essential that, when the beam intensity changes, the sharpness of its focusing should not decrease. Finally, the last condition is that the control of the intensity should not

was reflected in the velocity of the electrons in the beam. The latter is necessary in view of the fact that the deflection of the beam is inversely proportional to its velocity, and therefore changes in velocity will lead to distortion of the image, making the bright strokes shorter and the dark ones longer. Through especially careful development of the design, however, it was possible to achieve the result that, when the brightness of the spot is varied from zero to maximum, the change in the beam velocity is so negligible that no distortion of the image can be observed.

The characteristic of the kinescope is shown in Fig. 19. It is evident from it that a voltage of 10 V is sufficient for complete modulation of the electron beam, i.e., for changing the brightness of the spot from zero to maximum. The curve gives the relationship between the modulating voltage, the current at the second anode, and the corresponding brightness of the spot. (It should be pointed out here that the magnitudes of the current, voltage, and brightness indicated in Fig. 19 are rather illustrative in character than referring to any definite kinescope.)

Fig. 19. Characteristic of the kinescope.

From Fig. 20, which shows the dependence between the current at the second anode and the light energy emitted by the fluorescent screen, it is easy to see that

Fig. 20. Dependence between the current of the second anode and the brightness of the spot.

that there is a direct proportionality between them. On this basis we may conclude that the kinescope will correctly reproduce not only black-and-white images, but also images with halftones.

Fig. 21. Brightness distribution along the diameter of the spot.

If the luminous spot on the screen of the kinescope is investigated, for example by means of an enlarged photographic image, it is easy to notice that its brightness is greatest at the center and gradually decreases toward the edges. The curve of brightness distribution along the diameter, taken with a microphotometer, is shown in Fig. 21. When the spot moves, its brightness decreases in proportion to the speed of motion. Its edges, being less bright, disappear more rapidly, as a result of which the moving spot appears smaller than the stationary one.

The material of the fluorescent screen is synthetic ortho-silicate zinc, almost identical with natural willemite. The use of ortho-silicate zinc is due to

Fig. 22. Energy distribution in the fluorescence spectrum of ortho-silicate zinc and the curve of eye sensitivity.

its comparatively high efficiency, low light inertia, its comparative stability and resistance to “burning out” under the action of the electron beam. The high efficiency of this material is explained by the fact that its radiation under the action of the electron beam is a narrow band

in the green part of the spectrum. The energy maximum falls at 5230 Å, which is very close to the maximum of the eye-sensitivity curve (5560 Å). This is readily seen from Fig. 22.

Fig. 23. Brightness of the luminous spot as a function of the voltage of the second anode

The luminous efficiency of a screen made of zinc orthosilicate, if expressed in lumens per watt (assuming that the maximum theoretically possible efficiency is 690 lm/W), amounts to from 1.8 to 2.7%.

Fig. 24. Light inertia of zinc orthosilicate.

In Fig. 23 a curve is presented expressing the brightness of the luminous spot on the fluorescent screen as a function of the voltage of the second anode. The dependence of the brightness of the spot on the beam intensity and on the voltage of the second anode may be expressed by the formula:

$$ J = AQ(V - V_0)^2, $$

where \(J\) is the brightness of the spot in candles, \(A\) is a constant depending on the material of the screen, \(Q\) is the beam intensity in amperes per \(1\ \mathrm{cm}^2\), \(V\) is the voltage of the second anode, and \(V_0\) is the minimum voltage necessary to excite the fluorescence of the given substance.

Fig. 24 illustrates the light inertia of zinc orthosilicate.

zinc. The curve shows that approximately \(0.06\) sec after the cessation of bombardment the glow may be considered practically nonexistent. For transmission at a frequency of 24 pictures per second, an ideal phosphor would have to have a decay curve falling to zero within \(1/24\) sec. If the decay time is too long, the moving parts of the picture will “creep,” just as in a photograph a rapidly moving ball is depicted in the form of something like a comet. If the decay is too rapid, flickering of the picture appears, arising from the fact that between two successive pictures there are intervals of complete darkness.

Scanning

The necessity of transmitting the picture by separate pulses gives rise to the need for its “scanning,” i.e., the “examination” of its surface element by element in some sequence. As much time may be allowed for the “examination” of each element as is compatible with the property of the human eye to retain visual impressions and to perceive the whole picture as a single whole.

One of the simplest methods of scanning consists in making a light spot run over the object along a series of parallel lines. The motion of the spot may take place either in one direction or in both. An example of scanning of the latter kind, applied to telecinema, was described in one of the preceding articles*. An example of the motion of a light spot in one direction is the scanning widely used in television with the aid of the Nipkow disk.

In the system described here, the motion of the scanning beam occurs only in one direction and is carried out by deflecting the electron beam by a magnetic field. In the kinescope the beam describes on the screen a sequence of equally spaced horizontal lines, sweeping the image, beginning at the top and ending at the bottom, exactly as its scanning was performed in the transmitter. After traversing the last line the beam jumps back to its initial position in order to begin a new picture.

To accomplish scanning and image sweep in this manner, two variable magnetic fields are created, acting on the beam as it leaves the electron projector. The field producing deflection in the vertical direction pulsates with a frequency equal to the number of pictures transmitted in one second. The number of pulsations of the horizontally deflecting field is greater by as many times as the picture contains lines.

In order that the beam of the receiver should follow the motions of the beam of the transmitter, the curves expressing the dependence of the intensity of both deflecting fields on time are given the form of saw teeth, dep—

* V. K. Zworykin, “Television with a Cathode Oscillograph as Receiver,” Radio Eng. IX, No. 12, 37—41, 1929.

shown in Fig. 25. Each cycle of voltage variation consists of two parts. The first is linear with respect to time and lasts practically throughout the entire cycle. The second, corresponding to the return of the beam to its initial position, constitutes only a negligible part of the period. Reproduction of the picture takes place during the first of these parts of the period, owing to the change in the potential of the projector’s control electrode in accordance with the brightness of the points of the image projected onto the mosaic, as was indicated above.

There exists a large number of methods by means of which sawtooth electrical impulses can be obtained. A simple method was described in one of the preceding articles. This method consists in charging a capacitor through a current-limiting device, for example a diode operating at saturation, and then discharging the capacitor through a thermionic or gas-filled tube. A practical obstacle to carrying out such a “sawtooth” generator is the circumstance that in reality there are no such things as the saturation current of a cathode tube. As a consequence, the increase in the potential difference between the plates of the capacitor will not occur strictly linearly, and therefore the line obtained on the fluorescent screen will not be absolutely straight.

Fig. 25. Dependence of the voltage of deflecting fields on time.

To straighten the lines and improve the quality of reproduction of the picture, one has to use a more complicated circuit, shown in Fig. 26, which includes a dynatron oscillator and two amplifier tubes. The capacitor \(C\) in the horizontal-deflection circuit is continuously charged through the resistance \(R\). Periodically, at preset intervals, the capacitor is discharged. The capacitance and the charging current are chosen in such a way that, during the charging time, saturation of the capacitance does not occur. The vacuum tube through which the discharge of the capacitor takes place is controlled by a dynatron generator having a distorted waveform of oscillation. The frequency of oscillation of the dynatron (which can be varied over wide limits) is set approximately equal to the scanning frequency of the transmitter, owing to which the received synchronizing signals easily bring the dynatron into step with the scanning motions of the transmitter beam. The discharge and charge of the capacitor \(C\) create “sawtooth” changes of potential, which are applied to the grid of the amplifier tube and produce in its anode circuit, with the deflecting coils connected into it, the same sawtooth oscillations of current.

The circuit that produces deflection of the beam in the vertical direc-

in appearance, is quite similar to the one just described, except, of course, for its parameters. Both deflecting systems act on the beam by means of magnetic fields arising in coils arranged around the neck of the kinescope.

Fig. 26. Circuit of the generator of sawtooth deflecting pulses.

The preference given to magnetic deflection over electric deflection is due mainly to considerations of economy. The manufacture of a kinescope with magnetic deflection is considerably cheaper. On the other hand, the devices for magnetic deflection are expensive to manufacture and consume considerably greater power in operation. The predominance of one factor or the other depends chiefly on the deflection frequency and the beam velocity.

Fig. 27. Signals entering the receiver.

Synchronization

When both deflecting circuits have been properly adjusted and synchronized with the transmitter, a series of parallel lines appears on the fluorescent screen; their sharpness and the perfection of the synchronization determine to a considerable degree the quality of the reproduced picture. These lines are converted into an image by superimposing the picture signal pulses on the control electrode of the kinescope projector, whereby the brightness of the spot on the fluorescent screen is changed in accordance with the brightness of the image points on the mosaic.

Fig. 28. Part of the receiving apparatus with a kinescope.

To send synchronizing signals to the receiver, the pulses of the iconoscope deflection generator are fed to an amplifier and then modulate the transmitter together with the picture signals, thus being transmitted over the same channel as the latter. They do not interfere with the transmission of the picture, since they are transmitted at moments when it is absent. Synchronization in the vertical direction is carried out in the same way, with the signals being transmitted after the end of each picture.

The use of a synchronization system in which the receiver beam is brought into step with the transmitter at the end of each horizontal line has considerable advantages in that any instantaneous disturbances of a static character do not exert a noticeable influence on the transmission.

We have seen that the radio transmitter is modulated by the picture signals, and also by the signals of vertical and horizontal synchronization. Therefore the signals entering the receiver are complex and have the form shown in Fig. 27, where the upper curve represents the picture signals, which have an irregular form, are very often asymmetrical with respect to the time axis, and usually have large ordinates on the positive side of the voltage axis. Both systems of synchronizing signals, on the contrary, give pulses toward the negative half-axis. The difference in the form of the horizontal- and vertical-synchronization pulses is due to the difference in their duration and is used to separate them. All three indicated kinds of signals differ from one another in frequency and amplitude. The maximum amplitude of the picture signals is chosen so as to be smaller than that of both synchronizing pulses, whose amplitudes are approximately equal to one another.

Fig. 29. General view of the receiving device.

The separation of the three kinds of signals is carried out in the receiver by a very simple method, described in detail in another article. Here, therefore, only the main points will be indicated. If, using Fig. 17, we trace the path of the signals in the receiver, we shall see that, after passing through the antenna and amplifier, they enter three independent circuits: the vertical-deflection circuit, the horizontal-deflection circuit, and the kinescope circuit. The synchronizing pulses do not act on the kinescope, since they are transmitted at moments when the beam is making its return motion. The picture signals do not act on the deflection circuits, because their amplitude is chosen sufficiently small that they cannot bring into operation the input tubes of the deflection circuits. The separation of the deflection signals from one another is based on the difference in the form of these and the other pulses; for this purpose, in the input

TELEVISION

filters are placed in the circuits of both contours, quite satisfactorily accomplishing the selection of the required signals. In addition, the anode circuits of both dynatrons contain contours tuned

Fig. 30. Photograph of the kinescope screen during transmission of the picture in Fig. 2.

approximately to resonance with the operating periods of the corresponding deflecting circuits, which further increases selectivity.

Fig. 31. Photograph of the kinescope screen during transmission of the picture in Fig. 3.

When the beam returns to the position from which it is directed to trace a new line, and from the bottom of the picture in order to begin the next one, on the fluorescent screen there appears

bright line, the so-called “return line.” To avoid its appearance, synchronizing pulses which, as was indicated, have a direction opposite to the direction of the picture pulses, are applied to the control electrode of the kinescope. They charge it negatively and thereby extinguish the beam for the duration of its return.

For reproducing the picture, the intensity of luminescence of the fluorescent screen is varied by the picture signals applied to the control electrode of the kinescope projector. If the bias voltage on this electrode is chosen, according to the characteristic of the kinescope (Fig. 19), so as to obtain the greatest amplitude of variation in the intensity of the screen luminescence, the image will possess maximum contrast. The brightness of the picture background, or, what is the same thing, its average illumination, can be regulated by selecting the bias voltage on the control electrode of the kinescope.

Reproducing Apparatus

The design of the television receiver is shown in Figs. 28 and 29. The first of these is a photograph of the part of the apparatus comprising the kinescope and the deflecting devices.

Fig. 32. Image transmitted by means of a kinescope from the studio.

Fig. 29 depicts the entire receiving installation, consisting of power sources, the kinescope, two radio receivers—the sound receiver and the television receiver—and a loudspeaker. The reproduced picture is viewed in a mirror mounted on the inner side of the receiver lid. In this way, on the one hand, a more suitable viewing angle is obtained, and on the other, the lid blocks extraneous light and the image can be viewed without completely darkening the room. The absence of moving mechanical parts makes the receiver quiet in operation.

The following figures illustrate the results obtained with the television system described. Figs. 30 and 31 are photographs of the fluorescent screen when transmitting the same pictures that are shown in Figs. 2 and 3. Figs. 32 and 33 are images transmitted by means of the iconoscope from the studio and in the open air.

Fig. 33. Image transmitted by the iconoscope under natural illumination

The further development of devices with such capabilities as those embodied in the kinescope and the iconoscope opens new horizons for high-quality television. Apart from television in the usual sense, a great future awaits the iconoscope in the role of an artificial eye for observing phenomena that until now have been entirely hidden from us, as, for example, in the case of ultramicroscopy.