On the Methodology for Testing Television Camera Tubes

K. M. Shulzhenko

(Presented by the scientific seminar of the Radio Engineering Faculty)

The developed and published [1] methods for the basic testing of television camera tubes do not take into account their special features in the process of transmitting moving images. From the light characteristic of a camera tube it is possible, to a certain extent, to judge the process of forming the image signal of a moving object. However, the process of recording the light characteristic of the tube and the subsequent determination of the signal shape when transmitting a moving image are very complex and labor-intensive. In addition, such an indirect method cannot take into account all the features of a given tube when transmitting moving objects. It is therefore more expedient to include, in the set of tests for camera tubes, the recording of the transient characteristic of the tube when transmitting a moving boundary of a dark-and-light field. From this characteristic one can judge the sharpness (definition) of images when transmitting moving objects.

The image sharpness, estimated by the maximum steepness \(S_{\max}\) or by the rise time of the front \(x_{\text{э.р.}}\) of the transient characteristic, and the definition \(m\), determined by the number of dark and light lines reproduced on the receiver screen, are related to one another by the relation [2]:

\[ S_{\max}=\frac{\sqrt{\pi}}{\delta\sqrt{-\ln \Delta'_0}}, \qquad m \cong \frac{1}{x_{\text{э.р.}}}, \]

where \(\delta\) is the scanning pitch, and \(\Delta'_0\) is the visually sufficient amplitude level.

Measurement method

The sharpness of a television image of a moving object can be estimated by the maximum steepness or by the rise time of the front of the transient characteristic obtained when transmitting a moving boundary of a dark-and-light field. In order to observe the transient characteristic on the screen of an oscilloscope, it is necessary to create a displacement of this boundary along the lines, repeating at the frame-scanning frequency. To avoid uncertainty caused by the discrete displacement of the scanning element in the direction of the frame-

scanning, the transmission of a moving object along the lines is considered.

To increase the accuracy of reading the rise time of the edge of the transient response, and also to eliminate the error introduced by nonuniformity of the oscilloscope sweep, it is advisable to use time marks whose repetition frequency must be synchronous with the line-scan frequency. The time marks may be introduced, for example, by brightness modulation of the oscilloscope beam.

The proposed method was tested by the author in studying the effect of the motion of the transmitted object on the sharpness and definition of the television image of a moving object [2]. The measurements were carried out on a transmitting tube of the “image-transfer iconoscope” type with progressive image scanning. However, the proposed procedure is applicable for studying the transmission of the image of a moving object also by other types of transmitting tubes, with both progressive and interlaced image scanning.

To reveal the features of a particular transmitting tube, especially one with scanning by a beam of fast electrons, it is necessary to study the transmission of a moving boundary by different areas of the mosaic (target). For this purpose it is necessary to provide the possibility of selecting the image signal of any line of the frame and of any portion of that line. The position of the selected portion of the image signal may be monitored by feeding the signal from the line-selection circuit to a receiving device, where the transmitted image is observed.

Description of the experimental setup

The transient response of the transmitting tube at different speeds of motion of the object (a sharp boundary between dark and light fields) was obtained by the author on a laboratory television setup, the block diagram of which is shown in Fig. 1¹. The oscillograms were taken

Fig. 1. Block diagram of the experimental setup

with progressive scanning with a number of lines \(Z = 224\) at a frame repetition frequency \(N = 50\ \dfrac{\text{frames}}{\text{s}}\), synchronous with the frequency of the supply mains.

The transmitted object consisted of two disks placed together (Fig. 2a). One of them was cut from thick white paper. Its outer contour was formed by an Archimedean spiral. The other disk, made of matte dark paper, served as the background for the first disk. Both disks were fastened to the shaft of a synchronous three-phase motor, which provided a rotation rate \(n = 50\) rev/sec, synchronous with the line frequency. Uniform rotation of these disks, with one line scanning the image along \(OX\), makes it possible to obtain uniform translational displacement of the boundary of the dark-light field, with its rapid return to the initial position. The process of obtaining the moving boundary is shown in Fig. 2b.

The required change in the speed of displacement of the optical image of the boundary over the mosaic can be accomplished by changing the distance from the disk to the camera lens, or by replacing disks having different lengths of the segment \(AB = X_T\) (Fig. 2). The speed of displacement of the boundary over the photocathode (or over the mosaic in the case of an “Iconoscope”-type tube)

\[ v_x = \frac{V_x f}{A}, \]

where \(V_x\) is the velocity of motion of the boundary \((V_x = X_T N)\), \(A\) is the optical transmission distance, and \(f\) is the focal length of the camera lens.

Fig. 2. Transmitted object creating a repeated uniform motion of the boundary of a dark-light field

To obtain different values of \(v_x\), changing the optical transmission distance \(A\) is inadvisable, despite its apparent simplicity, since it entails the need for repeated adjustment of some elements of the setup (optical focusing, maintaining constant illumination, setting the selected line). Therefore, to create different speeds of image displacement, it is more expedient to use a set of disks. The transmitting camera (Fig. 1) contained a lens, a transmitting tube with energy storage, a preliminary image-signal amplifier, and generators for line and frame scanning. A camera lens was used with focal length \(f = 20\) mm and relative aperture

\[ \frac{D}{f} = \frac{1}{2.8}. \]

Into the preliminary amplifier, signals for black-spot compensation were introduced into the image signal (when determining the influence of the tube’s black spot on the signal of the moving-boundary image, the compensation signals may be switched off), as well as blanking and synchronizing pulses. From the output of the preliminary amplifier, the image signal was fed

to the amplifier, and then to the input of the video amplifier of the television receiver and to the oscilloscope.

Synchronizing pulses from the sync generator were fed to the transmitting camera to start the sweeps of the camera tube. In addition, the vertical synchronizing pulses of the sync generator triggered the line-selection circuit, whose operating principle is analogous to that described in the work of M. I. Krivosheev [1].

The line-selection circuit produced rectangular pulses recurring at the frame frequency, the duration of which could be adjusted within the time of transmission of one line. The required line was selected by smoothly shifting the instant at which the line-selection pulse occurred relative to the vertical synchronizing pulse.

The rectangular pulses obtained from the line-selection circuit were applied to the control electrode of the oscilloscope tube for brightening. The oscilloscope sweep was triggered by the horizontal synchronizing pulses. With normal beam brightness on the oscilloscope screen, the signals of all lines of the frame were visible, but the signal of the selected line differed by its greater brightness. Therefore, by appropriately reducing the brightness, it was possible to extinguish on the oscilloscope screen the signals of all lines except the selected one. In the experimental setup an EO-4 oscilloscope was used, the passband of whose vertical amplifier had been specially widened to 4 Mc/s.

For a quantitative evaluation of the rise time of the leading edge of the transient characteristic, calibration pulses with a frequency of 2 Mc/s were fed from the time-mark generator to the control electrode of the oscilloscope tube. The time-mark generator was triggered by the horizontal synchronizing pulses.

Fig. 3. Oscillograms of the image signal of a moving boundary:
a) \(V_x = 0\), m.o.r. \(= 1\,\mu\text{s}\), \(E = 1000\) lx;
b) \(V_x = 0.8\,\text{m/s}\), h.r. \(= 1.5\,\mu\text{s}\), \(E = 1000\) lx;
c) \(V_x = 1.6\,\text{m/s}\), h.r. \(= 2.5\,\mu\text{s}\), \(E = 1000\) lx;
d) \(V_x = 1.6\,\text{m/s}\), h.r. \(= 2\,\mu\text{s}\), \(E = 2000\) lx.

The transient characteristics of the television system obtained in this way are shown in Fig. 3. The oscillogram in Fig. 3a represents the signal of a stationary boundary of a dark-and-light field, while the oscillograms in Figs. 3b, c, and d are signals of a moving boundary. The oscillograms in Figs. 3a, b, c were taken at an illumination of the transmitted object \(E = 1000\) lux. The blur zone of the image of the moving boundary, determined from these oscillograms, corresponds with sufficient approximation to the calculated rise time of the front of the transient characteristic [2]. Figure 4 shows the dependence of the relative loss of sharpness (definition) of the image on the velocity of motion of the transmitted object, calculated for the present case [2] from the formula:

\[ \frac{m}{m_0} = \frac{x_{\mathrm{oz.r.}}}{x_{\mathrm{z.r.}}} = \frac{1}{\sqrt{1+\left[0.32\,\frac{V_x T_{\mathrm{e}} f}{A h_{\phi}}\,Z(1-t_y)\right]^2}}, \tag{1} \]

where \(m_0\) is the definition of the stationary image, \(x_{\mathrm{oz.r.}}\) is the blur zone of the image of the stationary boundary of the dark-and-light field, \(T_{\mathrm{e}}\) is the exposure time, \(t_y\) is the relative duration of the return stroke of the frame scan, and \(h_{\phi}\) is the height of the image on the tube photocathode.

Fig. 4. Dependence of the relative loss of image sharpness on the velocity of motion of the transmitted object

In the experimental setup \(h_{\phi} = 8.8\) mm, \(t_y = 0.08\). The same figure shows the experimental points for the relative loss of image sharpness. At object illuminations not exceeding 1000 lux, the transmitting tube operated in a regime close to that of an ideal storage device. This is indicated by the symmetrical form of the image signal of a uniformly moving boundary of the dark-and-light field (Figs. 3b and c) and by the close agreement of the experimental dependence of the loss of image sharpness on the velocity of motion of the transmitted object with the theoretical dependence (1), corresponding to operation of the tube in the regime of an ideal storage device. At higher illuminations, a dependence was observed of the form of the image signal of the moving boundary on the object illumination. The oscillogram in Fig. 3d corresponds to the signal of the image of the moving boundary at an illumination of 2000 lux. In this case the signal form loses its symmetry, approaching an exponen-

initial. This indicates the dependence of the charge-accumulation time in the camera tube on the illumination of the object being transmitted. As a result of the reduction in the charge-accumulation time with increasing illumination, the blurring of the image of a moving boundary becomes smaller.

In Fig. 5 (curve 1) the dependence of the relative loss of image sharpness, calculated from formula (1), is shown. The experimental points (measurements of the relative loss of image sharpness were carried out from photographs taken from the screen of a television receiver of images of moving test charts [2]) for the relative loss of image sharpness at an illumination of 800 lux (points marked with crosses) lie close to the theoretical curve.

Fig. 5. Dependence of the relative loss of image sharpness on the speed of motion of the transmitted object

The experimental dependence (dashed curve 2, Fig. 5) of the loss of image sharpness on the speed of motion of the transmitted object at an illumination of 2000 lux lies above the theoretical curve. Thus, for a given speed of motion of the transmitted object, increasing the illumination reduces the loss of sharpness of images of moving objects.

In carrying out the experiments, a dependence was also observed of the form of the tube’s transient characteristic when transmitting moving objects on the operating mode of the tube. All this proves the advisability of supplementing the set of tests of camera tubes with charge accumulation by determining the transmission characteristics of moving objects: the transient characteristics of the tube when transmitting a moving boundary of a dark–light field, and the dependences of the relative loss of resolution (sharpness) of the image on the speed of motion of the transmitted object at several values of the objects’ illumination.

The proposed tests of camera tubes can supplement the existing information on camera tubes with charge accumulation, and may also prove useful both in establishing their operating conditions and in developing new television camera tubes.

References

1. M. I. Krivosheev, “Methods for the basic testing of television camera tubes,” Radiotekhnika, vol. 6, no. 1, 1951.

2. K. M. Shulzhenko, “On the influence of the motion of transmitted objects on the sharpness of a television image,” Candidate dissertation, LETI, Leningrad, 1953.

1. The experiments were carried out at the Department of Television of the Leningrad Electrotechnical Institute named after V. I. Lenin. ↩