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Abstracts
A New System of Image Scanning in Television
N. S. Khlebnikov, Moscow
In generally known systems of television transmitters, the decomposition of an image into elements is carried out by one of the following methods:
A. Mechanical television
1) Scanning of the object by a light beam (operation in reflected light).
2) Decomposition of the optical image of an object by means of a moving mechanical device (direct vision).
B. Electronic television
3) Scanning of the electrical copy of the optical image by an electron beam (Zworykin system1).
4) Decomposition of the electronic image by moving it (Farnsworth system2).
Thus, systems of television scanning were divided into two groups—entirely mechanical or entirely electrical. Soviet engineer G. V. Braude proposed3 a new scanning system which, in accordance with the indicated division, may be called electromechanical, since electrical processes are used for scanning along the elements of a line, whereas mechanical displacement is necessary for frame scanning by lines. Of principal interest is the method of scanning along the elements of a line, based on the interaction between an electrostatic field and a conductor placed in it, provided the two have the corresponding geometry.
The phenomenon underlying the operation of the device had been known in a somewhat different form for a long time (and is not newly discovered, as the author erroneously indicates), and was observed as the blocking action of a voltage on a plate-like “grid” in a cathode tube with plane electrodes4 (Fig. 1).
The construction of the device is, in principle, extremely simple. Between the plates of a plane condenser (Fig. 2), perpendicular to their surface, there is placed a thin filament possessing photosensitivity. This filament cuts out, from the optical image projected onto the plane in which it is located, a line of the image. By moving the optical image in the plane of the filament in a direction perpendicular to it, it is evidently possible to obtain successively all the lines of the image.
Scanning along the elements of a line is based on the circumstance that, for a certain relation between the potentials of the plates \(\varphi_1, \varphi_2\) and of the filament \(\varphi_3\), namely
\[ \varphi_1 < \varphi_3 < \varphi_2. \]
this latter is divided into two zones, distinguished by the direction of the field gradient at the surface of the filament. Indeed, for the segment of the filament from plate $I$ to the point of its intersection with the equipotential plane $\varphi=\varphi_3$ (region $I$), the potential gradient (the intensity of the electrostatic field) at the surface of the filament is directed from the filament surface outward. At the point of intersection the gradient becomes zero and, in the region from this point to plate $II$ (region $II$), has the opposite direction. If the filament is regarded as infinitely thin (which proves not far from the truth in practically realizable cases), and the distribution of the initial velocities of the photoelectrons is also neglected (assuming, for example, that they are equal to zero), then the boundary of division between the two regions of the filament will be a geometrical point. As Braude indicates, in reality the boundary of division has a finite width equal to
\[ d=\frac{v_{\max}}{V}\,l, \]
where $v_{\max}$ is the maximum initial velocity of the photoelectrons in volts (from Einstein’s equation), $V$ is the potential difference between the capacitor plates, and $l$ is the distance between them. It follows from this expression that $d$ can be changed by changing $V$. Let us note that, according to Braude, the quantity $d$ determines the maximum size of the image element in the direction of the line.
Fig. 1. At a certain value of the negative potential applied to plate $G$, the electron current from filament $F$ to plate $A$ ceases.
For photoelectrons released from the filament, in region $I$ there is a retarding field, and in region $II$ an accelerating field. Therefore the points of intersection divide the filament into two parts: one emitting electrons and one not emitting them.
By applying an alternating potential to the filament, it is evidently possible to move the boundary of division between the regions and, consequently, to change the magnitude of the emitting region, thereby carrying out the decomposition of the image into line elements.
In this method, in order to obtain a constant scanning speed, as in electronic systems, a “sawtooth” voltage is used (Fig. 3), the amplitude of which, for moving the boundary along the entire filament, must be equal to the potential difference between the plates. Under these conditions the boundary will move uniformly along the filament and, upon reaching the end, will instantaneously return to the beginning, starting the decomposition of a new line.
The photoelectric current emitted by the filament at each given moment under the action of the sawtooth voltage, when the coordinate of the boundary $x$ is determined by the equality
\[ x=kt, \]
where $t$ is the time, $k$ is the velocity of motion of the boundary, and $x$ is the distance from the beginning of the line, will be expressed (when working with monochromatic light) as
\[ I=Sk\int_{0}^{kT}F(kt)\,dt, \]
where $F(kt)$ is the function of the distribution of illumination brightness along the line, and $S$ is the sensitivity of the cathode to light of the given wavelength.
Fig. 2. Diagram of Braude’s teletransmitter device; the arrows show the direction of the field gradient at the surface of the filament.
Thus, unlike all other scanning methods, in this case the instantaneous value of the current does not directly give the magnitude of the elementary pulse. Therefore the load of the device from which the voltage is taken, applied to the input of the amplifier, cannot be ohmic. Here it is necessary to use a certain “differentiating” load, i.e., one such that the voltage at its terminals would be proportional to the change of current with time, and not to the current itself. Such a load may be, for example, a self-induction coil. If the coefficient of self-induction of the coil is \(L\), then the elementary pulse will be expressed as
\[ V_L = L \frac{dI}{dt} = LSkF(kt). \]
The connection circuit when self-induction is used as the load is shown in Fig. 4. Another method is to superpose, on the filament together with the sawtooth voltage, a high-quality high-frequency voltage of small amplitude (compared with the amplitude of the sawtooth voltage) and to remove the detected high-frequency voltages from a resonant circuit.
Fig. 3. Sawtooth scanning voltage
Fig. 4. Circuit for connecting the device with inductive load \(L\)
As shown by calculations carried out by the author, the absolute sensitivity of the device described is of the same order of magnitude as the sensitivity of an ordinary mechanical system.
In instruments of such a device, secondary-electron multiplication can be used. For this purpose, in front of the plate that serves as the electron collector, a system of grids is placed, connected in the appropriate manner and serving as emitters.
A telecine transmitter was built according to this system. With a resolution of 240–360 lines, the image had high sharpness and contrast.
Literature
- V. K. Zvorykin, Uspekhi fizich. nauk, 14, 778, 1934.
- S. Yu. Lukyanov and A. A. Ravel, Uspekhi fizich. nauk, 15, 1935.
- G. V. Braude, Zhurnal tekhn. fiziki, 7, 1510, 1937.
- G. Barkhausen, “Cathode Tubes,” GIZ, 1926, pp. 47, 48.