TV decoder. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television, video equipment

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For several years in our city there has been paid coded broadcasting on the 29th channel. To implement a sufficiently reliable protection against unauthorized viewing of programs, a multivariant address coding system is used, developed in Russia and used by many commercial television studios. Visually, the encoded program lacks line and frame synchronization. When viewing the full television signal with an oscilloscope, it was found that there are no vertical sync pulses in the encoded signal, and instead of horizontal pulses, synchronization pulses are transmitted, shown in Fig. 1.

The number of lines during which the signals of Fig. 1(a) and Fig. 1(b) are transmitted periodically changes and this is one of the coding options. The duration of the high-level pulses (75% of the white level), shown in Fig. 1 by a dotted line, also changes. The subscriber's address and information about the encoding method are transmitted within 1 μs at the end of each line. However, the developers of the described television program encoding system made some oversights that make it easy to make a discrambler capable of converting

encoded program into a standard full color television signal (PCTS) when using on the transmitting side any of the coding methods embedded in the system. Such a discrambler can be made using the fact that the position of the transition point from low-level pulses (a level below black) to high-level pulses (Fig. 1) is constant in time and coincides with the beginning of the horizontal sync pulses. Frame sync pulses can be obtained by counting the number of transmitted lines. The electrical circuit of the discrambler that implements the described principle and provides automatic recognition of the encoded program is shown in Fig. 2.

(click to enlarge)

On the transistor VT3, a low-level pulse selector is assembled, which, after selection and inversion, charge the capacitance C6 and are fed to the input of the Schmitt trigger DD1.2. The time constant of the circuit R12, C6 is chosen to increase the duration of these pulses by 1..2 µs. After being inverted by the element DD1.3, these pulses come to one of the inputs of the element DD2.2. High-level pulses are emitted by the transistor VT2 and, after being inverted by the DD1.1 element, are fed to the second input of the DD2.2 element. Thus, in the presence of the encoded signal shown in Fig. 1 (a), at the output of the element DD2.2, horizontal synchronization pulses are formed. With the help of the elements VD4, R17, C9, their duration is brought to the standard (4,7 μs) and after being inverted by the DD1.4 element, they come to the base of the transistor VT8, which, opening, cuts them into the PCTS. Resistor R23 serves to adjust the level of these pulses.

To ensure the suppression of false sync pulses (see Fig. 1 (a)) elements VT4, VT5, DD2.1, DD1.5, VD5, R16 are used. After selection by the transistor VT3, all low-level pulses are fed to the emitter follower VT4, and then to one of the inputs of the DD2.1 element. The other input DD2.1 receives the signal generated by the element DD1.4 (inserted horizontal sync pulses). Chain VT5, R13, C7 serves to increase the duration of these pulses up to 70..110 µs. Therefore, at the output of the element DD2.1, in the case of receiving the signal shown in Fig. 1 (a), after passing the first encoded line, pulses appear. These pulses exactly match in duration and edge location to the false sync pulses present in the encoded signal.

Element DD1.5 inverts them and through a diode VD5 with a series-connected resistor R16, which serves to adjust the degree of suppression of false sync pulses, the signal is fed to the base of the emitter follower VT7. Frame synchronization is performed by counting the number of lines. To do this, it is convenient to use the filament voltage of the kinescope (CRT). (In almost all modern TVs, the filament voltage to the kinescope is supplied from a horizontal scanning transformer and contains the higher harmonic components that are necessary for the operation of the discrambler.) On the transistor VT1 and the oscillatory circuit L1, C2, the second harmonic of the horizontal frequency is selected.

After inversion on the DD3.1 element, the doubled horizontal scanning frequency comes to the counting input of the DD5 microcircuit. Elements DD3.2, DD3.3, DD3.4, DD4 are used to generate framing pulses that appear at the output of the element DD4.2, and reset the counter DD5. Button S1 is designed to adjust the phase of frame synchronization pulses. Thus, one of the inputs of the DD2.3 element receives frame rate pulses with a duration of 288 μs (4,5 lines). The other input of the DD2.3 element is connected to the capacitor C10, which is charged with horizontal synchronization pulses, in case of receiving a coded signal. When receiving conventional TV programs, the voltage at input 9 of the DD2.3 element corresponds to a logical zero, and the discrambler automatically stops working. So, when receiving encoded programs, after being inverted by the transistor VT6, frame synchronization pulses enter the input of the DD2.4 element, which, together with the elements VD8, R25, C11 and DD1.6, performs the function of their "cutting" (Fig. 3).

"Cutting" vertical sync pulses is necessary to ensure line synchronization during the passage of vertical sync pulses. After that, the vertical sync pulses are cut into the PCTS in the same way as the horizontal ones. The appearance of the decoded signal is shown in Fig. 4.

A voltage regulator is assembled on the VT9 transistor.

Construction and details. All resistors used in the discrambler are rated at 0.125W. The exception is R26, which should provide a power dissipation of the order of 0.5 watts. Deviations of the nominal values ​​of the elements: C2, C6, C11, R12, R25 - + 5%, the rest - + 20%. The inductance L1 is wound on a toroidal magnetic circuit made of M200NN ferrite with overall dimensions of 20x12x4 mm and contains 110 turns of the PEV 0.1 lead. There are no strict requirements for the quality factor of the L1 coil, so it can be wound on any other magnetic core. All transistors and diodes can have any letter index. Instead of DD1, you can use K533TL2; instead of DD2 - K133LA3, K155LA3, K533LA3, K1533LA3; instead of DD3 - K564LA7, K176LA7. DD4 - K564LE10, K176LE10. Capacitors C12, C13 must be located in close proximity to the microcircuits DD1, DD2.

TV connection. The described discrambler can be connected to almost any TV (except for tube ones), for this it is necessary to include it in the open circuit of a low-frequency video signal with a swing of 2..4.5 V. In 3USCT, 4USCT, 5USCT TVs, the discrambler is turned on at the output of the radio channel module. In Western-made TVs, as well as in 6USTST, the discrambler is turned on after the smiter follower connected between the video processor and ceramic band-pass and notch filters. An example of a connection diagram to a TV with a TDA8362A video processor is shown in Fig. 5

The dotted line in the figure shows the chain that needs to be broken.

Adjustment. Set the slider of the resistor R4 to the leftmost position according to the diagram. Turn on the TV to a coded program. Set using resistor R17 pulse duration at the output of the element DD2.4 equal to 4..4.7 µs. Connect the oscilloscope to the output of the discrembler and, by rotating the slider of the resistor R23, achieve equality in the amplitudes of the transmitted and embedded horizontal synchronization pulses. Then, using resistor R16, set the required amount of suppression of false sync pulses, while the signal present at the output of the discrembler must correspond to Fig.4. Lastly, by rotating the slider of the resistor R4, achieve the maximum quality of receiving the decoded program

The described discrambler was successfully installed in Philips, Samsung and Electron 51ТЦ4303 TVs. All televisions modified in this way received an encrypted channel with almost the same quality as non-encoded ones. After equipping a TV with such a discrambler, it becomes possible to record encoded programs on a VCR. To do this, just connect the low-frequency output of the TV to the low-frequency input of the VCR and turn on the latter for recording.

Literature

1. Brodsky M. A. Color television Mn. Wish. school., 1994-142s.
2. Khokhlov B. Video processor TDA8362A in modern TVs. - Radio, 1997, No. 6,7.
3. Digital Integrated Circuits: Directory / Ts75 P. P. Maltsev, N. S., Dolidze, M. I. Kritenko and others - M .: Radio and Communication, 1994.-240s .: ill.

Author: Vladimir Meshcheryakov

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