Circuit design of output amplifiers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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In the models of color TVs of each generation, the circuitry changed quite significantly. Such changes also affected the output video amplifiers, which are described in the published material. The author provides interesting information about the parameters of the elements of the video path, which includes video amplifiers, explains why it is necessary to expand its bandwidth significantly more than the standard value of 6,25 MHz, and gives recommendations for improving the video amplifiers of old TVs.

The output video amplifier (VU), which connects the video processor (VP) with the kinescope, is a necessary and important part of every TV. The issues of its construction and calculation, unfortunately, are little considered in the domestic literature. The only book that contains a detailed presentation of all the problems can be considered [1]. This gap is partly filled by the information contained in the reference books of the "Repair" series, produced by the "Solon" company.

High demands are placed on the WU - they must provide a large transmission coefficient of the CP in a very wide frequency range with minimal signal distortion. There are no transient capacitors in the VP-VU-kinescope circuit, and it is a broadband DC amplifier with high-voltage outputs connected to the kinescope electrodes. Such amplifiers are characterized by a strongly pronounced dependence of their constituent elements on each other. For this reason, when considering possible schemes of the VU, it is necessary to take into account both the features of the construction of the VP and the parameters of the signals generated by them, and the characteristics of the kinescope. Let's start with the output link of this chain - the kinescope.

Any kinescope, as you know, has two types of inputs, to which a modulating signal can be applied: a cathode and a grid (modulator) for a black-and-white kinescope, cathodes and grids (modulators) for a color one.

In domestic black-and-white TVs, the video signal almost always goes to the cathode of the kinescope, and the modulator is either connected to a common wire, or beam-quenching pulses are connected to it during the reverse sweep. Feeding a video signal to the modulator was practiced only in the first TV models. The advantage of this method was the possibility of reducing the amplitude of the modulating voltage. However, this required a signal of positive polarity, which was not consistent with the subsequent use of negative polarity signals (sync pulses down) in the color path.

The VU of such TVs, as a rule, is single-stage and, before the advent of transistors, it was assembled on a 6P9, 6P15P lamp or the pentode part of a 6F4P lamp and their analogues. Such a WU is relatively simple. The parts used in it set the mode of operation of the lamps, made up the circuits for environmental protection and frequency response correction. The OOS circuit improved the linearity of the amplitude characteristic of the WU, which ensured an increase in the number of distinguishable gradations of brightness to the norm of eight steps of the gray scale of the test table. Frequency response correction circuits, which initially included a relatively large number of coils, kept the transmission coefficient constant in the video signal bandwidth, which created conditions for obtaining a good quality image. The bandwidth of such a VU usually reached 5 ... 5,5 MHz.

Two-stage VUs were rarely used and either to compensate for insufficient gain in the path (for example, in the Znamya TV), or to increase the stability of interlacing (Rubin-110). In modern black and white TVs, only transistor VUs are installed; they do not contain coils in the frequency response correction circuits.

A feature of color kinescopes with three electron-optical searchlights (EOP) can be considered the non-identity of the image intensifier tube, which manifests itself in the difference in their modulation and brightness characteristics.

The modulation characteristic of the image intensifier tube is the dependence of the beam current IL on the modulating voltage UM, determined by the power function: IL=f(UMg) where g is the coefficient of non-linearity of the modulation characteristic. The usual value of g for cathodes of color kinescopes of any company is 2,8 and a little more for modulators.

The parabolic nature of the modulation characteristic leads to the fact that the difference between the brightness steps of dimly lit image details worsens on the screen and the recognition of details whose brightness is near the white level in the video signal improves. According to [2], the most plot-important details, as a rule, are located in the region of the highest illumination, and the best image quality is observed at gGEN=1,2, where gGEN is the non-linearity of the through path (from the transmitting to the receiving tube). Since the indicated non-linearity of the modulation characteristic is a property of the kinescope, the color television standards provide for the use of measures on the transmitting side to reduce the value of gGEN to the level indicated above.

The modulation characteristics of the image intensifier tube of the same kinescope not only have a different coefficient g, but also begin at different voltages for closing (extinguishing) the beam. For these kinescopes, the scatter of the quenching voltages of the rays was allowed up to ±15 V. All this led to the fact that when the brightness of the image changed, the white fields acquired a color in one or another color tone.

The brightness characteristic of the image intensifier reflects the properties of the kinescope as a signal-to-light converter and is expressed by the ratio: L=lIL, where L is the brightness of the phosphor; l is the efficiency of the phosphor (luminescence intensity when exposed to the image intensifier beam). The stability of the parameter l for domestic kinescopes of old types is low, which over time caused the white fields of the image to be colored in color.

The non-identity and instability of the parameters g and l of the image intensifier tube of the kinescope require periodic adjustment of the white balance. To achieve white balance means to compensate for the change in the efficiency of the phosphors and the difference in the modulation characteristics of the image intensifier tube. White balance must be maintained over the entire brightness adjustment range if it is set at two points: at the level of minimum brightness (white balance at black level - BBL) and at optimal brightness (white balance at white level - BBB). The LIU is achieved by combining the start points of the modulation characteristics of all three image intensifier tubes, which leads to the simultaneous suppression of all beams. After that, the BBB is installed by giving the same slope to the modulation characteristics of all three image intensifier tubes (more precisely, by giving the same slope to the products of the amplitude characteristics of the VP and VU, the modulation characteristic of the image intensifier tube and the brightness characteristic of the phosphor). BBCh and BBB in TVs of different models are regulated differently, depending on the construction of the VP and VU.

The modulation of the color kinescope beams is provided in several ways, depending on where the color signals R, G and B are formed: in the kinescope, VU or VP.

The formation of signals R, G, B in the kinescope was used in the first domestic color TVs ("Record-102", "Rubin-401", "Rainbow-701" and then in all modifications of the ULPCT). As shown in the block diagram shown in Fig. 1, the cathodes of the kinescope connected together received a brightness signal Y, and the modulators received color difference signals RY, GY, BY Simultaneous exposure to brightness and color difference signals led to the formation of a beam as a color modulated one, for example: Y + (RY) \uXNUMXd R.

The use of this modulation method required the use of four VUs, which turned out to be complex both constructively and operationally. To obtain the required range of output signals while maintaining the required ratio of voltages on the cathodes and modulators of the kinescope, it was necessary to supply the VU with a voltage of 370 V. Adjusting the BBCh and BBB due to the presence of 12 tuning points interconnected by direct current in ULPCT TVs is a laborious procedure, executed cyclically several times. According to [3], the distortions in the brightness channel of ULPCT TVs, created by the video detector, the brightness path and the VU, reach 12%.

The non-linearity in the color path is even higher. It is created by demodulators (25% each), color difference signal amplifiers (10% each), and VU (15% each). In general, the total non-linearity of the luminance channel, color path and VU in ULPCT TVs can be equal to 50%. The main reasons for this are an unsuccessful way of forming the R, G, B signals, the imperfection of the chrominance demodulators, the VU and the green signal matrix, in which the constant component was also partially lost.

These values ​​may surprise the reader, who is accustomed to the fact that in audio technology the allowable non-linearity is measured in fractions of a percent. The matter lies in the different perception of non-linearity by human hearing and vision. Image distortions are manifested in a decrease in the number of reproducible gradations of brightness and color saturation, a reduction in the color palette, coloration of white fields, a decrease in horizontal and vertical clarity, and a deterioration in the sharpness of the edges of details. All these types of distortions are caused by a number of reasons, described in detail in [2], the main of which are the nonlinearity of the amplitude characteristic and frequency response of the VP and VU. In addition, they can be caused by the fact that the owner of the TV sets the brightness, contrast and saturation of the image incorrectly when the white balance is off.

Due to the very large non-linearity in the paths of the ULPCT TVs, the gamma correction mentioned above at the television centers could not significantly improve the image characteristics. The improvement occurred only with the advent of third-generation TVs, when the circuitry of all nodes changed significantly.

In TVs released later by ULPCT, the signals R, G, B were formed either in the VU, as shown in the block diagram of Fig. 2, or in the VP (according to the diagram in Fig. 3). In any of these cases, the received signals are fed to the cathodes of the kinescope, the modulators of which are connected to a common wire.

The formation of signals R, G, B in the VU is used quite rarely. An example of such a VU can be used in the TV SHIVAKI-STV202/208 [4].

Schematic diagram of the VU is shown in fig. 4. Video processor DA1, having generated chrominance C and brightness Y signals, transmits the first of them to the SECAM detectors of the DA2 chip, and the second to the emitters of the VU transistors. As a result of processing the signal C in the DA2 chip, color difference signals RY, GY, BY are obtained, applied to the bases of the transistors of the corresponding VU. The addition of signals in transistors leads to the formation of color signals R, G and B on their collectors.

Each VU uses one modern high-voltage broadband transistor 2SC2271D, which provides a good frequency response with the simplest correction circuits: C2R5 in VU (RY) and their counterparts in others. WU is a cascade with a resistive load, assembled according to the scheme with OE. The features of the operation of such a cascade are described in [1], and the formulas for calculating the values ​​of the resistors and capacitors included in it are also given there. The LIU controls are the black level setting resistors found in all three VUs. The BBB is installed with resistors for changing the signal amplitude in VU (GY) and VU (BY). The signal span control in the VU (RY) is not provided.

The most widely used is the formation of R, G, B signals in video processors (VP). Such VIs can be divided into three groups in accordance with the method used in them to adjust the white balance: manual, automatic, microcontroller. The circuitry of the VU for the VP of each group is different.

Let us first consider the VU for the VP with manual white balance adjustment. Let's start with TV UPIMTST. Three M2-4-1 modules are installed on the BOS board of this device, each of which serves as a VU of one of the primary colors, assembled according to a circuit with a resistive load. Each VU contains five transistors. The scheme and operation of the module are described in [3]. Details related to white balance adjustment are located on the BOS board. Compared to ULPCT TVs, adjustment in UPIMCT has become easier: it has only six tuning points (this is also typical for other VUs in the group under consideration).

At the same time, the design of the VU of these TVs turned out to be very complex: they contain more than 100 parts, which is twice as many as in the ULPTTS, and much more than in any of the VUs considered below. The non-linearity of the demodulators in the color path remained at the level of the ULPCT, and in the color-difference signal amplifiers it increased to 14%. Distortions in the WU and the brightness path decreased to 8%. The total non-linearity decreased to 42%.

In [1], a somewhat more complex version of the VU for UPIMCT on seven transistors was proposed. Its main difference from the M2-4-1 module is the construction of the output stage according to the scheme with an active load. The cascade is assembled on two KT940A transistors, the first of which is an AB class amplifier, and the second is an emitter repetitive VU in [1] and in [5].

The advantages of a VU with an active load over a VU with a resistor load are to halve (from 4 to 2 W) power consumption and non-linear distortion, in the possibility of increasing the resistor values ​​in the collector circuits. Since the output signal is taken from the emitter follower, the construction of frequency response correction circuits is simplified.

On fig. 5 shows a schematic diagram of the VU used in the 3USCT TV with the MC-2 color module. It is an active load amplifier. Resistor R3 is used to transfer the OOS voltage to the signal preamplifier (in our case, the R channel), located in the DA1 VP. OOS provides a decrease in the nonlinearity of the amplifier up to 6%. The R8C1 circuit corrects the frequency response in the high frequency region. Zener diode VD2 serves as a source of exemplary voltage (ION) necessary to fix the operating point of the VU.

Adjusting the LIU with resistor R9 leads to setting the desired damping level in the output signal coming from the DA1 chip to the base of the transistor VT1. Adjusting the signal swing with resistor R7 provides the setting of the WU gain required to obtain the BBB. Resistor R10 in VU (G) and VU (V) has a nominal value of 1 kOhm.

Signal distortion in 3USCT TVs is much lower than in ULPCT and UPIMCT. In the luminance channel they are equal to 15%, in the color path - 8%, in general - 22%. The VU of the 3USTST TV with other color modules differ from those shown in Fig. 5 is basically the denominations of the parts. For the completeness of the description of such a variant of the WU, we point out that in [1] a circuit of a complementary WU, assembled on transistors BF469, BF470, for operation with the TDA2530 VP is considered. It is characterized by low (4%) non-linear distortion, low power consumption (0,5 W), but also narrow (4,8 MHz) bandwidth of output signals with a large span. The low-peak output bandwidth is up to 7 MHz.

According to a simpler concept, shown in Fig. 6, the VU of the ELECTRON-TK570 TV set was built [6].

They are also assembled according to the scheme with an active load, but unlike the VU, according to the scheme in Fig. 5, the OOS signal is not fed to the VP, but to the base of the transistor VT1 VU. Changes have also been made to the inclusion of peak-to-peak adjustment resistors and the supply of a fixed voltage to the emitters of the transistors. As an ION, a transistor assembly was used instead of a zener diode, which has a large differential resistance, causing a change in the stabilization voltage when the load current changes. A current flows through the divider R15R16, an order of magnitude greater than the base current of the VT7 transistor, so the voltages at its base and emitter practically do not change when the current fluctuates through the WU. The construction of the ION of various VUs is almost identical and differs only in the value of the output voltage and the values ​​​​of the divider resistors.

The output voltage is taken equal to the voltage in black mode (indicated in the reference books) at the outputs of the VP, from which the output signals R, G, B are taken. The corresponding values ​​for the TDA2530 and TDA8362 microcircuits are shown in fig. 5 and 6. In this case, a deviation of up to % 0,5 V is permissible, since the final setting of the operating point of each VU is provided by the black level trimmer during the adjustment of the LIU. It is provided for all rays. BBB beam R is missing.

Several resistors are included in the base circuit of the first transistor of each VU. The first of them, for example, R1 in the VU(R) is located near the VP and prevents its operation directly on the mounting tank and the cable connecting the VP to the VU. This has a beneficial effect on the bandwidth of the VU.

It should be noted that this and all subsequent figures show that the VU is no longer located in the color module, but on a separate board put on the kinescope base. The approach of the VU to the capacitive load - cathodes of the kinescope improved their frequency response and expanded the bandwidth.

On fig. 7 shows a schematic diagram of the VU TV TVT2594 [7]. The most important difference from the VU according to the schemes in Fig. 5 and 6 can be considered the use of an amplifier with a resistor load, assembled on a high-voltage broadband transistor BF871S. Its characteristics are the same as those of the already mentioned 2SC2271D transistor and the BF869, 2BC4714RL2, 2SC3063RL, 2SC3271N discussed below. In addition, if in the VU according to the scheme in Fig. 6, power from the ION is supplied to the emitter of the VU transistor, and the black level control circuit was connected to its base, then in the VU according to fig. 7 they changed places. Resistor R5 creates an OOS circuit. The C1R11 circuit provides RF frequency response correction, the VD1 diode protects the transistor from a voltage exceeding 12 V entering its base. The black level is adjusted in each VU, the signal amplitude is only in VU (G) and VU (B).

Let's move on to the VU for the VP with the automatic installation of the LIU (it is called the ABB system). They are widely used in televisions of the fourth and subsequent generations, although many companies (for example, SONY) continue to use VUs with manual white balance even today in the most modern mass-produced products, citing the high stability of the parameters of the kinescopes used.

The ABB system in each half-frame measures the dark currents of the image intensifier tube of the kinescope and corrects the damping levels of the R, G, B signals at the outputs of the VP in order to match the points of the modulation characteristics of the image intensifier tube corresponding to the beam current of 10 μA. Consequently, the LIU is set not for the moment of complete extinction of the rays, but at the point where the image intensifier tubes are still a little ajar. It is believed that this method of adjusting the LIU in mass equipment gives almost the same result as manual adjustment.

The functioning of the ABB system is described in detail in [1] and in [5]. We confine ourselves to pointing out that the sensors of this system are located in the VU, and the devices that control their operation are in the VP. It should also be noted that the ABB system is more complex than the manual adjustment system described earlier, but more efficient. The white balance is set in one cycle, while in the manual VU, it is necessary to repeat the adjustment of the LIU and WBB several times to achieve balance at all brightness levels. When using the ABB system, the LIU is set automatically and you only need to correct the LIU with the signal-to-peak resistors. In this type of VU, the number of adjustment points is reduced to two, since black level resistors are not needed. These VUs are implemented on transistors and microcircuits.

On fig. 8 shows a schematic diagram of the VU TV ELECTRON-TK550. With minor changes, such VUs are used in the ELECTRON-TC503, ORIZON-TC507, RUBIN-TC402/5143, HORIZONT-CTV501/525/601 devices. These RTs are considered in [6]. In terms of the construction of the collector circuits of transistors, the feedback circuits and the supply of exemplary voltage, they do not differ from the VU with manual white balance adjustment. The main difference is the availability of ABB system sensors. In VU(R), the sensor is the transistor VT3 and the measuring resistor R7. The values ​​​​of the measuring resistors in each VU are chosen so that the ratio of the currents of the three beams of the kinescope during the transmission of the measuring pulses provides the LIU. The method of their calculation is available in [1]. The R9C3VD3R8 circuit provides the transmission of measuring pulses to the VP. Resistors for adjusting the signal range are connected to the VP in the same way as it is done in 3USCT TVs (see Fig. 5).

An example of building a VU on microcircuits is shown in the diagram in fig. 9.

Such VUs are used in the HORIZONT-CTV-655 TV set [6]. They are assembled on TDA6101Q microcircuits - powerful high-voltage broadband op-amps. Their advantage can be called low power dissipation - they do not need heat sinks. In such WUs, resistors with a dissipation power of not more than 0,5 W are used, while in WUs on transistors, resistors with a dissipation power of 2 ... 5 W are required. The purpose of the pins of the microcircuit is shown in the figure and does not require explanation. BBB is regulated in VU(G) and VU(V). It is important to note that the microcircuit can also be used for manual adjustment of the LIU, if you do not install measuring resistors R6, R7, R11, R12, as done in [8], or, as recommended in [9], connect pins 5 of all three microcircuits together and connect through a 100 kΩ resistor to a common wire.

There are also three-channel integral VU. These are TEA5101A/W microcircuits with ABB and TDA6103Q with manual adjustment of the LIU. Schematic diagram of the inclusion of the first of them will be shown below, and the second - shown in Fig. 10, it is considered in [9].

The scheme is very simple and does not require additional explanations. For normal operation, the microcircuit needs a small heat sink: the power dissipation reaches 5 watts. The reference voltage is obtained from a voltage of 185 V across the divider R2R1.

The story of why in modern TVs the bandwidth of the video path reaches 10 MHz or more gives grounds for radio amateurs to appropriate improvements to domestic televisions of the third and fourth generation.

The most advanced can be called video amplifiers (VU) for video processors (VP) with microcontroller white balance adjustment, used in seventh-generation TVs, which use digital control of microcircuits. They can be divided into two groups. The first group includes VU for VU with automatic setting of the LCU (with the ABB system) and microcontroller adjustment of the BBB, the second - VU for VU with a microcontroller setting of both modes. Such VUs do not have tuning resistors.

The EC of the first group was used in TVT25152/28162 [7] and THOMSON-STV2160 [10]. In the first case, each VU (Fig. 11) is assembled on three transistors and is an amplifier with an active load (VT1, VT2) and a measuring transistor VT3. The DA1 chip is a video processor with an ABB system controlled via an I 2 C digital bus. The SDA20563A508 (DD1) digital chip is a microcontroller for the control system for the functions of all TV units, and SDA2586 (DD2) is a memory chip for digital values ​​​​of settings and adjustments. Cascade on the transistor VT10 - ION.

The construction of the WU has no significant differences from those described earlier. However, they function differently. As for the LIU, it is provided automatically. The signal ranges for obtaining the BBB are set during the manufacture or repair of the TV using the DD1 microcontroller when it is operating in the service mode. Using the menu on the screen of the kinescope and the remote control, the operator adjusts the parameters of each of the beams. Their required values ​​are stored in the DD2 chip, from which they are transferred to the VP during operation. The latter uses the incoming digital information to set the gain controls in the R, G, B channels. More detailed information on the functioning of the I2C digital control bus can be found in [1] and in [11].

On fig. 12 shows a schematic diagram of the VU of the mentioned THOMSON-STV2160 TV. DA1 chip - video processor with ABB system and digital control via I2C bus, DA2 - integrated three-channel video amplifier with ABB system circuits, DD1 - microcontroller, DD2 - memory device. ION is assembled on a transistor VT1. ABB system circuits contain elements R11, VD4, R14, VD5, R8, R4, C1. This VU functions in the same way as the previous one.

An example of a TV in which both the LIU and the BBU are installed by the microcontroller is the PANASONIC-TC-14L10R/21S2 [10]. The schematic diagram of its VU is shown in fig. 13. It uses the simplest of the considered amplifier with a resistive load on a single transistor. Chip DA1 - video processor, DD1 - microcontroller, DD2 - memory device. The functioning of this VU is the same as that of those assembled according to the schemes in Fig. 11 and 12, except that in the service mode, not only the BBB is tuned, but also the BCU.

It follows from the above that the construction of the VU in the transition from one generation of TVs to another changes towards simplification while improving technical and operational characteristics. Each time this is achieved through the use of more modern components and the complexity of the circuitry paths of color and brightness.

Let's see how the WU parameters changed.

The non-linear distortions in the first generation TVs (ULPTTS) were very high. For the WU of the brightness channel, they reached 12%, for the WU of color difference signals - up to 15%. This was explained by the twice as large range of these signals as compared to the brightness. In TV sets of the second generation (UPIMTST), the level of distortion in the VU was reduced to 8%, and in devices of subsequent generations - up to 5%.

The transmission coefficient of the WU in ULPCT TVs in the brightness channel reached 50, and the WU of color difference signals - 23 ... 47. The WU in the UPIMCT models had a transmission coefficient of 47. In 3USCT TVs, a WU with a transmission coefficient of 38 was used, and in the latest models it does not exceed 20. in color-difference VU. In televisions of the second or third generations, R, G, B signals were received from the TDA1,5, TDA3,2 EP with a span of 2530 V. For a more advanced TDA3505 EP, it is 2 V, and for the TDA4580, it is 3 V. The increased input signal span made it possible to reduce the gain of the VU , which provided a reduction in distortion and the possibility of expanding the bandwidth.

The bandwidths of the brightness, color difference and color signals in the UPIMCT and 3USST TVs (on TDA2530, TDA3501) are 5,5; 1,5...2; 5,5 MHz, respectively, in fourth-generation TVs - 5,2; 2; 10 MHz, and in modern devices (on TDA8362 and the like) - 8; 3,5; 9...10 MHz. This means that in televisions of the first or third generations, the brightness and color paths, as well as the VU, did not transmit the entire spectrum of the received video signal to the kinescope. Only in devices of the fourth and subsequent generations, the bandwidth of the EP expanded, surpassing the standard value of 6,25 MHz. EW with extended bandwidth required a corresponding increase in the bandwidth of the VU up to 9...10 MHz. And such WU appeared (see Fig. 4, 6-13). VU on TDA6101Q, TDA6103Q, TEA5101A/W provide linear frequency response up to frequencies of 7,5...8 MHz with minimal power consumption.

The question may arise: if the expansion of the bandwidth of the VP and VU to the 6,25 MHz transmitted by the telecenter is justified, why is a further increase necessary?

Recall that an impulse of any shape can be represented as a sum of sinusoidal components with corresponding frequencies, amplitudes and phases. The mathematical expression of such a representation is called the Fourier transform. It allows you to determine the values ​​of these parameters for the fundamental frequency of the pulse and its harmonics.

It is generally accepted that a line of a television image consists of 800 elements. At a horizontal frequency of 15,625 kHz, the duration of a rectangular pulse representing such an element is 80 ns. It corresponds to a set of sinusoids with frequencies of 6,25; 12,5; 18,75 MHz, etc. For approximate preservation of the pulse shape, it is necessary that at least part of the harmonics be transmitted without distortion of amplitudes and phases. With a bandwidth of 5,5 MHz, none of these harmonics will hit the kinescope and such an element will not be reproduced. With a video path bandwidth of up to 10 MHz, only sinusoidal oscillations of the fundamental frequency of 6,25 MHz will pass through it. As a result, an initially rectangular pulse will be transmitted to the cathode of the kinescope in the form of a positive half-wave of a sinusoid with a reduced amplitude and reproduced unsharply.

An impulse corresponding to an image detail with a duration of two elements of a line, with a bandwidth of 5,5 MHz of the VP and VU, will be transmitted at a fundamental frequency of 3,125 MHz, which corresponds to a horizontal definition of 340 lines of the test table scale. However, the image of this detail on the kinescope screen will be blurry and dim. With a bandwidth of 10 MHz, the fundamental frequency, second and third harmonics (3,125; 6,25; 9,375 MHz) will be transmitted. An even harmonic will increase the steepness of the pulse front, distorting its decay, and an odd harmonic will improve its squareness.

The reproduction of an image detail with a length of three elements of a line will be noticeably improved, which corresponds to a horizontal clarity of 230 lines. With a bandwidth of 5,5 MHz, two harmonics (2,083 and 4,167 MHz) will be transmitted, and with a bandwidth of 10 MHz, four (another 6,25 and 8,333 MHz).

Therefore, a TV with a video path bandwidth of 5,5 MHz provides sharp reproduction of no more than 230 picture details per line. Details with dimensions corresponding to 230...340 lines will be rendered unsharply, with blurred borders. Smaller ones will either merge into a common light gray stripe, or will not be reproduced at all.

If the bandwidth of the video path is extended to 10 MHz, then the boundary of the sharply reproduced strokes of the test table will be a level of 340 lines, and the strokes in the interval of 340 or more lines will be slightly blurred.

It is known that the video signal at the output of VHS video recorders has a horizontal definition of 230 ... 270 lines, and of the S-VHS format - 400 ... 430 lines. Broadcast programs are transmitted with a clarity of 320...360 lines. This means that a receiver with a bandwidth of 5,5 MHz will reproduce well all but the smallest details of the VHS format, slightly degrade the sharpness of on-air programs and significantly degrade the playback of S-VHS signals, reducing their clarity by almost half (from 400...430 lines up to 230...340).

At the same time, TVs with a bandwidth of 10 MHz video path will reproduce VHS signals in high definition, as well as broadcast programs, and only the finest details of the S-VHS image will be reduced sharpness.

So, for satisfactory playback of VHS format programs, it is enough to have a video path bandwidth of 5,5 MHz, and when using an S-VHS VCR, a 10 MHz bandwidth is needed.

The question remains, why do we need a wider band (than 6,25 MHz) when receiving on-air programs?

The fact is that in televisions of the fourth and subsequent generations, measures are being taken to improve the shape of the received video signals. Due to a number of reasons (they are detailed in [1, 2] and in [12]), the pulses that make up the video signal transmitted by the TV center do not have a rectangular shape. The duration of the rises and falls of the pulses in the brightness signals can be (depending on the amplitude) up to 150 ns. The same is the duration of the drops in the color difference signals of the PAL and NTSC systems. In the SECAM standard, they have a duration of up to 1800 ns, which is caused by the use of a different method of modulating subcarriers with chrominance signals. In PAL and NTSC systems, varieties of amplitude modulation are used, and in the SECAM standard, frequency modulation is used. As a result, the duration of the differences in the color difference signals depends on the value of the subcarrier frequency shift when moving from an image detail with one color to a detail with another color.

To increase the slope of the SECAM color difference signals, color transition correctors are introduced into TV sets. The basis of such a corrector is the TDA4565 microcircuit (analogs - K174XA27, KR1087XA1). The principle of operation of the corrector is described in detail in section 8.5 in [5]. The corrector reduces the duration of the drops from 800 to 150 ns, equalizing their steepness in the brightness and color difference signals and combining them in time. However, it cannot handle signals that have very shallow edges. In [1], it was proposed to use an additional corrector together with the microcircuit, which reduces the duration of the color transition from 1800 to 800 ns and then allows the TDA4565 microcircuit to reduce this duration to 150 ns. The scheme of such a corrector on one transistor is considered in [1].

In the most modern TVs, correctors for signal drops are also used in the brightness path, for example, image enhancement processors TDA9170, TDA9171 [9]. By statistical analysis of the repetition rate in the frame of five brightness levels, it corrects the overall nonlinearity of the video path gtot to the standard value of 1,2. As a result, all 10 gradations of brightness are displayed on the scale of the test table, the range of saturation of blue and especially blue colors, which are poorly reproduced within the R, G, B colorimetric system used, is expanded. The TDA8362 chip has built-in circuits to improve image clarity.

Increasing the steepness of the drop is a change in its shape by introducing into the signal composition higher-frequency harmonics that were absent in the received signal. The use of such a procedure in TVs with a bandwidth of VP and VU equal to 5,5 MHz is ineffective, since most of the harmonics introduced by the corrector are located outside this band and playback will not improve. At the same time, broadening the bandwidth improves the transmission of harmonics. We note in passing that the color transition corrector does not correct aperture distortions in the kinescope. To reduce them, only precise focusing of the kinescope beams is needed, which reduces their diameter.

In televisions with a frame rate of 100 Hz, the bandwidth of luminance and R, G, B signals is increased to 15 ... 22 MHz, and for color difference signals it is 13 MHz. In such devices, a VU is used on a TDA6111Q chip with a cutoff frequency of 16 MHz.

All considered VUs were used in industrial TVs produced in large series and proved to be efficient. Therefore, they can be tried to be used to upgrade older TVs. Let's consider this possibility.

As for ULPCT TVs, replacing four tube VUs with transistor ones would significantly improve image quality, get rid of several lamps operating in forced mode, and reduce power consumption and heat dissipation. But this is hindered by the fact that the VUs of such TVs are powered by a voltage of 370 V, and the maximum voltage for promising transistors (BF871S and similar) reaches only 250 V. It is impossible to reduce the supply voltage while maintaining the kinescope modulation method. Consequently, the replacement of the VU in ULPCT TVs is possible only with a significant alteration of the color block with a change in the modulation method of the kinescope. Bearing in mind the construction of modern televisions, it should include the introduction of a VI into it for generating R, G, B signals, which will make it possible to change the kinescope modulation method and assemble the VU according to any scheme shown in Fig. 4-7, 9, 10.

In TVs of the UPIMCT series, it is possible (and even desirable) to replace the KT940A transistor in each M2-4-1 module with any of the following similar foreign transistors. The result will be more stable operation of the VU, improved color reproduction. The option described in [1] seems very rational: instead of a cascade on a KT940A transistor with a resistive load, use a cascade on two KT969A transistors with an active load. This will improve the quality of work with a halving of the power consumed in the +200 V supply circuit. It is also advisable to change the design of the VU more significantly: replacing the M2-4-1 modules with any of the ones considered according to the diagrams in Fig. 4-7, 9, 10, mounted on a small board attached to the kinescope board. This will expand the bandwidth of the VU while drastically reducing the number of parts used and power consumption.

In 3USCT with VU built according to the schemes in Fig. 5 and 8, the KT940A transistors (VT1 and VT2) can be replaced by BF869 and BF422, respectively (see Fig. 11) without any changes. It is also advisable to transfer the VU from the color module to the kinescope board.

Transistors BC557N, BC558, BC558B can be replaced by KT3107I. Instead of BF422, BF423 transistor KT3157A can be used. Transistors 2SC2271D, 2SC3271, 2SC3063RL2, 2BC4714RL2, BF869, BF871S are interchangeable. According to reference books, the domestic transistor KT969A has similar parameters, but this replacement is not equivalent. Diode 1N4148 can be replaced by KD522B.

Literature

1. Khokhlov B. Decoding devices for color TVs. - M.: Radio and communication, 1992.
2. Jaconia V., Gogol A., Druzin Y. et al. Television: a textbook for universities. - M.: Radio and communication, 1997.
3. Elyashkevich S., Chisinau S. Blocks and modules of color unified TV sets. - M.: Radio and communication, 1982.
4. TV SHIVAKI-STV202MKII, SHIVAKI-STV208MKII. - Radio constructor, 1998, No. 02, p. 19-30.
5. Elyashkevich S., Peskin A. Televisions 3USCT, 4USCT, 5USCT. Device, adjustment, repair. - M.: Symbol-R, 1993.
6. Lukin I., Koryakin-Chernyak S., Yankovsky S. Practical circuitry of modern televisions. Series "Repair", no. 8. - M.: Solon & Science and technology, 1996.
7. Peskin A., Konnov A. TVT TV repair. Series "Repair", no. 16. - M.: Solon, 1997.
8. Brylov V. Chip TDA8362 in 3USCT and other TVs. - Radio, 1998, No. 9-12.
9. Ponomarenko A., Anikeenko V. Television chips PHILIPS, book. 1. Series "Foreign electronics", no. 3. - Kyiv: Science and technology, 1998.
10. Peskin A., Konnov A. TV sets of foreign companies. Series "Repair", no. 17. - M.: Solon, 1997.
11. Peskin A., Konnov A. Digital control bus I2C. - Radio, 1996, No. 10, p. 14, 15.
12. Peskin A., Konnov A. Digital control bus I2C. - Radio, 1996, No. 10, p. 14, 15.
13. Dombrugov R. Television: a textbook for universities. - Kyiv: Vishcha school, 1979.

Author: V.Brylov, Moscow

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