Color corrector. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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For the correct reproduction of a color image on a TV, it is necessary that the brightness and color difference signals arrive at the cathodes and control electrodes of the kinescope simultaneously. It is known that the time it takes for a signal to travel through a channel is inversely proportional to its bandwidth. The lower the channel bandwidth, the greater the signal delay. Since the bandwidth of the luminance channel is 6 MHz and the chrominance channel is 1,5 MHz, there is a delay in the chrominance signals of about 0,6...0,8 µs. Until recently, unregulated lines of the LZYAS-0,3/1000 type (300 ns, 1000 Ohm) or LZTsT-0,7-1500 (700 ns, 1500 Ohm) were used as delay lines, which require careful matching both at the input and and on exit.

If the width of vertical color transitions, as a rule, does not exceed 2 mm even in the SECAM system (due to coding features), then due to the mismatch of the midpoints of the leading and trailing edges of the color difference pulses with the midpoints of the brightness signal drops, the zone of color transitions horizontally when receiving a signal from the air reaches 4...6 mm, and when working with a VCR it can stretch up to 10 mm [1], which manifests itself as a "smearing" of the contours of brightly colored objects, the appearance of fringing from a distorted color, "not drawing" small details, etc. .

To eliminate this defect in TVs of the fifth generation, the Philips TDA4565 color corrector is used, the block diagram of which is shown in Fig. 1. Using it on the fourth generation TV "Horizon 51ТЦ412", you can significantly improve the quality of the "picture". The corrector consists of two functional units:

- adjustable gyratory delay line of the brightness signal Y;

- "sharpener" of color difference pulses RY, BY.

Rice. 1. Structural diagram of the color clarity corrector TDA4565

Gyrator delay line 4 consists of 10 cells with a delay of 90 ns each, and one cell with a delay of 45 ns. 6 cells of 90 ns are combined into a common block with a delay time of 6x90=540 ns. They are followed by three cells of 90 ns, which are connected in series with the input block by electronic switch 3 (three threshold comparators with switching thresholds of 4,5±1 V, 7,5±1 V and 10,5±1 V), controlled by the output 15. Thus, by increasing the voltage at pin 15, you can step by step increase the signal delay time up to 270 ms. An additional delay of 45 ns is enabled by shorting pin 13 to ground. A fixed delay 11 is included between pin 12 and pin 8 for another 180 ns. Thus, it is possible to adjust the delay of the brightness signal in steps of 45 ns from pin 11 from 540 to 855 ns (540 + 270 + 45 = 855), and from pin 12 - from 720 ns to 1035 ns. The output stages (pins 11 and 12) are made in the form of emitter followers with current generators in the emitter circuit.

The color-difference pulse sharpener includes pulse front detectors 1, 2; pulse shaper 5 and switches 6, 7. The delay in the color difference signal processing channel is approximately 150 ns (from 100 to 200 ns). At the moments of fronts and recessions of color-difference signals, by their differentiation and rectification at the outputs of the detectors, pulses of positive polarity are formed, the amplitude of which is proportional to the rate of change of the original signals. After the high-pass filter and limiter 5, these pulses control two identical color-difference signal sharpening channels, made in the form of electronic keys 6 and 7. They switch the paths RY and BY. Storage capacitors are connected to the outputs of the keys. "Total" key management provides simultaneous color changes in both channels with short color transitions. Capacitors on pins 1 and 3 of the IC are designed to suppress components outside the frequency band of color difference signals, including the residuals of color subcarriers, which improves the signal-to-noise ratio. The output stages (pins 7 and 8) are made in the form of emitter followers with cascode current generators in the emitter circuit.

The ratings of the external binding elements are indicated in accordance with the typical wiring diagram. You can read more about the work of the corrector in [2].

The scheme for switching on the corrector (Fig. 2) is borrowed from the Horizon TVs. It is distinguished by the presence of trimmers R5, R6 [3] at the outputs of the BY and RY signals, with which the color balance is adjusted. In modern TVs, there is no need for such regulators, because. in them the white balance is carried out automatically. The above switching circuit differs from the typical one by the ratings of RC circuits, the time constant of which determines the duration of the front, and by the presence of an active current source (AIT) on the transistor VT1. A brightness signal is fed to the AIT input through a differentiating capacitor with a capacity of 47 pF with a lead of 180 ns. AIT is connected in parallel with the internal current generator in the emitter circuit of the output stage. With such ratings of RC circuits, the duration of color transitions (about 150 ns) approximately corresponds to the duration of brightness changes in the brightness channel (about 6 MHz).

Rice. 2. Scheme for switching on the corrector

Resistor R3 must be selected with an accuracy of ± 1%, because the delay time is proportional to its resistance. In the case of using TDA4565A microcircuits, the resistance R3 is 1,15 kOhm (1,21 kOhm is connected in parallel with the 27 kOhm resistor).

When the R4 resistor slider is in its highest position, one additional 90 ns delay line is turned on, in the middle position - two 90 ns each (total delay - 180 ns), in the lowest position - three (270 ns).

Construction and details. A drawing of the printed circuit board of the additional submodule with dimensions of 60x45 mm is shown in fig. 3, and the assembly drawing - in fig. 4. Electrolytic capacitor C12 - type K53-4 for a voltage of 20 V, the rest - K50-35. Non-polar capacitors - type K22-5. Trimmer resistors - SPZ-38a.

Rice. 3. PCB

Rice. 4. Assembly drawing

Before mounting the submodule, it is necessary to solder wire leads about 4 mm long at points 5, 7, 8 and 10 and unsolder elements R38, R39, R42, R55, C31, C32, C3, VT2, VT3, as well as the VT2 delay line ( LZYAS-0,3/1000) and elements of its coordination R63 and R88. Resistor R60 with a resistance of 150 ohms must be replaced with a resistor with a resistance of 10 ohms, capacitor C48 with a capacity of 22 microfarads - by 0,047 microfarads. Near the connection point of R60 and C28, as well as on the edge of the board in the "common wire" on the same line with the upper terminals (right according to the diagram) of the capacitors C31 and C3, holes are drilled for connecting the power circuits (pins 4 and 7) of the submodule.

Before mounting the submodule, the KN4 output must be shortened by 2...3 mm, and the capacitor C28 must be bent to the board. Conclusions 4 and 5 of the submodule are soldered directly to the main board instead of the upper terminals of the dismantled capacitors C31 and C3, and points 1 and 2 are connected with flexible wires (55 and 70 mm long, respectively) to the lower terminals of these capacitors. Output 3 of the submodule is connected with a shielded wire (150 mm) instead of the right output of the dismantled resistor R88, and output 6 is connected with a flexible wire (55 mm) to the place of output 2 of the dismantled delay line.

Adjustment. When setting up, a "colored stripe" signal is sent from the output of the corresponding generator to pin 1 XN2 (A1), after removing the jumper. Applying a signal to the RF input (due to possible interference), as well as using a TV center signal (due to image repetitions) is not recommended. The delay line is regulated using a resistor R4 and a jumper between terminal 13 DA1 and a common wire along the minimum horizontal color transition zone. The color balance is set by resistors R5 and R6. The optimal swing of the RY signal is about 1,05 V (maximum - 1,5 V), the BY signal is about 1,33 V (1,9 V maximum).

In some TVs, it is possible that the SD-41 decoder submodule has been replaced with a more modern one, for example, "Electronics 016" or similar with output signal level controls. In such blocks, these controls must be set to the position corresponding to the maximum output signal, and the optimal levels should be set from the outputs of the color clarity corrector.

With careful adjustment of the corrector, a pronounced effect of increasing the clarity and purity of the image is observed, it acquires relief and purity. If the brightness is insufficient, it may be necessary to adjust the mode of the input stage of the gyratory delay line using a 220 ... 510 kΩ resistor connected between pins 17 and 16 of DA1. This is determined by the best reproduction of the leftmost white band [3]. The indicated resistor is not shown in the diagram, but the board provides a place for its installation.

Literature

1. K. Filatov. Color transition corrector. - Radio, 1990, N9 P.41.
2. L. Keves, A. Peskin. Color module MTs-501. - Radio, 1992, N5 P.28.
3. A. Pakhomov. Color corrector. - Radio, 1999, N2 P.10.

Author: A. Petrov, D. Petrov, Mogilev; Publication: cxem.net

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