Video sync pulse regenerators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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The problem of obtaining good quality video dubbing from video cassettes containing not the first copies is of concern to many video enthusiasts. To solve this problem, the authors of the published article propose to use video signal sync pulse regenerators.

Fans of videos quite often have to deal with their dubbing both on a VCR and on a computer. And there is great disappointment if the copy turned out to be of poor quality or did not work out at all. You can improve it or even record a protected one if you use the devices discussed below.

It should be recognized that with almost any analog method of dubbing video and audio signals, the copy will always be worse than the original. There are many reasons for this, but it would be inappropriate to dwell on all of them here. It should only be noted that as a result of rewriting the video signal, not only the clarity of the image deteriorates, but also its synchronization to a much greater extent. So already after the third copy, you can notice horizontal twitching of the image, especially in its bright areas. Obviously, if there are copy protection pulses in the original, then overwriting does not work at all. When recording a video signal on a computer, the requirements for the quality of sync pulses become even more stringent.

In professional practice, digital clock regenerators are used to correct synchronization signals, restoring all the time and amplitude parameters of the video signal pulses. In amateur practice, it is quite enough to restore the required amplitude of horizontal and vertical sync pulses, and only in more complex cases, also their duration and shape.

The principle of operation of the regenerators is simple: they remove the old sync pulses from the composite video signal, and new ones, generated by a special generator, are placed in their place. Depending on the requirements of the video amateur and the availability of components, two options for the regenerator are offered for selection - a simple one and a more complex one.

The basis of the first version was the device described in [1]. The schematic diagram of the regenerator is shown in fig. 1.

(click to enlarge)

The device consists of a video signal transmission channel and a generator. The video signal from the reproducing apparatus is fed to the input amplifier, assembled on transistors VT1, VT2. From its output, the signal passes through the R7C3C5 circuit to the generator and through the R8C4 circuit to the buffer stage on the VT3 transistor, matching the resistances of the input and output stages. The output stage is made on transistors VT4, VT5. It just ensures the replacement of old clock pulses with new ones, for which pulses from the generator act on this stage through the diode VD1. It should be noted that the video signal polarity is not changed by the transmission channel in the regenerator.

The clock generator in the regenerator is the LM1881 (DD1) microcircuit, which is a specialized multifunctional device [2]. In our case, the microcircuit uses a clock separation unit built according to the comparator circuit, the output of which additionally plays the role of a switch in the output stage of the video signal transmission channel. The sync pulses generated in the microcircuit and calibrated in amplitude from its output (pin 1) through the switching diode VD1 are fed to the base of the transistor VT5 of the output stage when the video signal sync pulses appear on it. As a result, the base of the transistor through the VD1 diode will be connected to a common wire, which achieves the removal of interference pulses and old synchronization pulses and their simultaneous replacement with new ones.

The device is assembled on a single-sided foil printed circuit board, the drawing of the conductors of which and the placement of parts on it are shown in Fig. 2. When mounting the DD1 chip, its output 7 is folded under it. Capacitor C7 is soldered to pins 4 and 8 of the DD1 chip from the side of the printed conductors.

To power the regenerator, you can use any suitable voltage source 9...12 V with a permissible load current of 100...300 mA. If we exclude the DA1 stabilizer chip, then it is possible to use power supplies with a voltage in the range of 4,7 ... 7 V, for example, from the "Electronics D2-1 OM" microcalculator.

When setting up the device, the voltages at the terminals of the transistors are checked for compliance with those indicated in the diagram. A deviation from them within ± 5 ... 10% is allowed. Then the output of the cathode of the diode VD1 is disconnected from the output 1 of the DD1 microcircuit and the device is included in the video signal path. If everything is assembled correctly, the same image should be observed on the control TV as without the device. Then, without turning it off, connect the broken circuit between the VD1 diode and the DD1 microcircuit. In this case, the image on the control TV should shift to the right by 1...5 mm, which is an indicator of the normal operation of the regenerator.

The second option - a more complex regenerator - has a video signal transmission channel similar to that described above. The changes affected only the generator, which in this case restores not only the amplitude of the clock pulses, but also corrects their duration. Its schematic diagram is shown in Fig. 3 (the numbering of the elements in the diagram continues the numbering of the parts of a simple regenerator). A part of the TRS-06 P/S cable TV translator was taken as the basis of the generator.

(click to enlarge)

Instead of the LM1881 microcircuit, the USR-1C module used in TV sets of the third or fourth generations and assembled on the K174XA11 microcircuit or its analogues was used as a node for extracting sync pulses from the video signal [3]. The frame trigger pulses received in the module from pin 8 of the XS1 connector pass through the matching transistor VT6 to the single vibrator DD2.2, which generates new frame sync pulses (FSI). Gating horizontal synchronization pulses from pin 2 of connector XS1 are fed to a single vibrator DD2.1 and a trigger DD3.1, which generates new horizontal sync pulses (FSI). Horizontal and vertical sync pulses are added after the diodes VD3, VD4 and affect the video signal transmission channel.

This version of the device requires a voltage source of 12 V with a load current of up to 300 mA. You can assemble it yourself according to any known scheme or use a finished product.

In a more complex version, the device is made on three boards. The video signal transmission channel is assembled on the first board. It is similar to the previous version, only parts related to its generator are not installed on it: R7, R9, C3, C5-C7, DD1, VD1. The second board is the USR module. On the third board (the author did not develop a printed one, but used a breadboard), the remaining elements of the generator are installed.

Before use, it is necessary to check the operability of the USR module. For this, power and a video signal are supplied to it. If all of its outputs have the required pulses (check with an oscilloscope), the module can be used. Unfortunately, there are a lot of defective products on sale.

In addition, before using the RRM module, small changes are made to it. First, you need to close with a jumper a resistor (56 kOhm) connected between pin 6 of the K174XA11 chip and pin 3 of the X4 connector (R20 in [3]); and secondly, remove the capacitor (150 pF) connected to the conductor going to pin 2 of the same connector (C16 in [3] or C4 on industrial TV circuits).

The establishment of the second version of the regenerator begins with checking the operation of the video signal transmission channel in the same way as described above. Then, the generator input is connected to it and the presence of pulses is checked with an oscilloscope at pin 12 of the DD2.2 single vibrator (KSI) and at pin 9 of the DD3.1 trigger (FSI). If necessary, set the duration of the pulses by selecting elements C14, R26 (4,4 ... 5,1 µs for FSI) and C15, R28 (192 µs for KSI). When recording video programs on a computer with unstable frame synchronization (slow vertical frame movement), you can try to increase the capacitance of the capacitor C15 to 0,068 uF. When connecting the anodes of the diodes VD3 and VD4 to the base of the transistor VT5, the image on the control TV connected to the output of the device should move, as indicated above.

In both versions, it is possible to use transistors of the KT315, KT361, KT3102, KT3107 series of the corresponding structure with any letter index. Resistors - MLT-0,25, capacitors - any suitable in size. Diode VD1 in a simple regenerator and diodes VD3, VD4 in a complex one must be germanium: D2 or D9 with any letter index.

In operation, both options are approximately the same. The author tested their performance when recording a noisy video signal on a computer. In both cases, the quality of the recorded image was much higher than with direct recording.

Literature

1. Domrin N. A. To the lover of video equipment. - M.: Energoatom Publishing House, 1992.
3. Elyashkevich S. A., Peskin A. E. Device and repair of color TVs. - M.: DOSAAF, 1987.

Authors: A. Vorontsov (RW6HRM), A. Korotkov (RA6FER), Stavropol

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