LCD TV DIAGRAMS
Today, the main technologies in the manufacture of displaysLCD, it'sTN+film, IPS and MVA. These technologies differ in the geometry of the surfaces, control plate and front electrode. The cheapest matrix is TN + film. It works this way: if no voltage is applied to the subpixels, the liquid crystals rotate relative to each other by 90° in the horizontal plane in the space between the two plates. Since the polarization direction of the filter on the second plate makes an angle of 90° with the polarization direction of the filter on the first plate, light passes through it. And if the red, green and blue subpixels are fully illuminated, a white dot appears on the screen. Ifred, green or blue subpixel is closed - a certain color is formed. Despite the worst viewing angles,matrix - TN + filmhas the shortest response time among all other modernLCDmatrices, which is why such TVs are bestsellers.
Description of the TV operationLCD
A brief description of the operation of the circuits of most LCD TVs: Turning onLCDTV to a 220 V network starts a switching power supply, which begins to supply stabilized voltages to the SLT analog-digital module, usually with the following values: 3.3 V, 5 V, 12 V and 33 V. In the SLT module, the processor conducts self-diagnosis to identify faults, and when the self-diagnosis test is passed, the TV starts working in STANDBY mode.So ohn is locatedmodeenergy saving, in which only the minimum required set of circuit elements remains powered. When a command is received from the remote control to the IR sensor, and then from the IR sensor, the detected command code is sent to the input of the video processor, or when a command is received from the keyboard located on the front panel of the TV to the input of the video processor, a power-on command is sent from the video processor via the I2C bus.
The SLT module is designed for analog-digital processing of video and audio signals, processing signals from the remote control, controlling the on and off of auxiliary voltages, and controlling the brightness of lampsLCDmatrix, sound control.Analog digitalThe module contains a video processor, a video signal switch, a sound processor, a clock switch, an RGB signal switch, a horizontal and vertical sync pulse generator, a tuner and SAW filters.LCDthe matrix has a digital input with an LVDS or TTL interface, depending on its model and the matrix backlight lamp, from which high-voltage wires go to the power converter.
When the processor turns on, it begins exchanging information with the matrix via the LVDS or TTL interface, depending on the typeLCDmatrices. If the TV is turned on in TV mode, the processor sends a code corresponding to the frequency of the desired channel to the Tuner block via the I2C bus. The tuner is tuned to the required frequency, and an intermediate frequency signal of the selected channel appears at its output. Then the signalintermediate frequencyfrom the tuner passes through surfactant filters to separateintermediate frequencyvideo andintermediate frequencysound that goes to the video processor, where the signal is convertedintermediate frequencyvideo to RGB color signals. In TV mode, RGB signals are sent through a switch to the processor input. The video processor extracts from the videointermediate frequencyhorizontal and vertical sync pulses, which are supplied to the HF and VF sync pulse generators - horizontal and vertical scan.
After the shapers, the clock pulses arrive at the switch. The processor converts RGB input signals into digital code and transmits them via an LVDS or TTL interface to the matrixLCD, which already displays the video. The IF audio signal arrives at the input of the sound processor, and from its outputs the audio signal of the right and left channels goes to the ULF inputs. The SLT analog-to-digital module has inputs for external audio and video signals. When the TV is turned on in video mode, the video signals are switched by the switch and supplied to the CVBS/Y input and input C of the video processor, and the audio signals of the right and left channels are supplied to the corresponding inputs of the sound processor.
When RGB mode is turned on, RGB signals go directly to the inputs of the video processor. When VGA mode is selected, the RGB signals from the VGA connector are switched by the switch to the RGB inputs of the processor. The horizontal and vertical sync pulses from the VGA connector are switched by the switch to the corresponding inputs of the processor and the VGA signal is decoded and transmitted to the matrix. When the video input is switched to DVI mode, digital signals from the DVI input go directly to the corresponding inputs of the processor. It decodes the given DVI signal and transmits it to the matrix.
Here is a collection of several dozen LCD TV circuit diagrams from all major manufacturers. Almost every archive contains several variants of circuits for different TV models. The diagrams are in the BOOKS section.
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The dream of “flat” TVs and monitors with a very small depth in depth arose more than a decade ago. But only in recent years has it become a reality: serial models on flat display panels have appeared.
Cathode ray tubes (picture tubes), which serve as the basis of any television, have existed for many decades and are constantly being improved. However, they also have disadvantages: the presence of high voltage, large volumetric dimensions (especially depth for large image sizes), etc. Therefore, developers have always strived for new ideas when creating display devices. One of them is the use of a liquid crystal substance as a valve for transmitting light fluxes. This idea was finally embodied in the form of LCD displays (panels) - LCD (Liquid Crystal Display). The rapid growth of their production abroad has led to the emergence of both a large number of models of “flat” TVs and computer monitors.
Let's consider the operating principle and design options for such displays. It is generally known that an LC substance (material) modulates the external light flux under the influence of an electric field or current. The specific operation of LCD displays is based on the use of the effect of rotation of the plane of polarization of the light flux by a layer of nematic LCD substance (the so-called twist effect).
The design of the LCD panel is shown in Fig. 1.
The panel contains two plane-parallel substrates made of transparent material (usually glass about 1 mm thick), located one relative to the other with a fixed gap into which the LCD material is introduced. Addressing electrodes are applied on the inner sides of the substrates in the form of a specific pattern. An indium oxide film is used as a transparent conductive layer of electrodes.
Layers of orienting coatings deposited on addressing electrodes are designed to set a specific orientation of LC molecules in the working material. The gap between the substrates is set by calibrated spherical or cylindrical spacer elements (spacers), the diameter of which can be in the range of 3...25 microns. After assembly (gluing), the panel is sealed around the entire perimeter, and the sealant layer also has spacers. Polaroids with a certain orientation of the plane of polarization are glued to the outer sides of the substrates.
The principle of operation of an LCD cell (pixel) of a panel using the twist effect is illustrated in Fig. 2.
The molecules of the LC material have a dipole moment. As a result of the interaction of the electric fields of the dipoles, a spiral structure of molecules of the liquid crystal substance is formed. Layers of orienting coatings on the upper and lower substrates, together with the dipole structure of the LC material, in the absence of an electric field, ensure a rotation of the polarization plane of the light flux by 90°. A layer of nematic LC substance oriented in this way has the property of polarizing the light flux passing through it. The polarization planes of the upper and lower polarizing filters are rotated relative to each other by 90°.
As can be seen in Fig. 2a, the light flux first passes through the upper polarizing filter. In this case, half of it, which does not have azimuthal polarization, is lost. The rest of the already polarized light, passing through the layers of LC material, rotates the plane of polarization by 90°. As a result, the orientation of the plane of polarization of the light flux will coincide with the plane of polarization of the lower filter and the flux will pass through it with virtually no loss.
If the LC substance is placed in an electric field, applying voltage to the addressing electrodes as shown in Fig. 2.6, the spiral molecular structure in it is destroyed. The light flux passing through the LCD material no longer changes the plane of polarization and is almost completely absorbed by the lower polarizing filter. Consequently, an LC substance has two optical states: transparent and opaque. The ratio of transmittances in both states determines the contrast of the image.
To ensure control of the optical state of the pixel cells (image elements) of the panel, it is necessary to generate such voltages on the addressing electrodes so that the state of each pixel changes without changing the state of the others. Based on this, the topology of the addressing electrodes of the LCD panel is a matrix formed by a system of row and column electrodes, structurally located on two parallel transparent substrates. Elements (pixels) of a television image in an LCD panel are formed at the intersection of row and column electrodes. To implement control of a large number of image elements (and in televisions this is almost always the case), signal multiplexing is used.
Several options for the topology of matrices used in LCD panels are presented in Fig. 3.
Option in Fig. 3a is the simplest and most popular. Option in Fig. 3.6 allows for a wider pin pitch for supplying columnar control signals. Options in Fig. 3,c - a variation of the Dual Scan (or Dauble Scan) architecture, which ensures a reduction in the number of multiplexed lines, which makes it possible to further increase the image contrast. In fact, in these cases, two separate screen fields are formed, the gap between which is invisible. Signal addressing for both fields occurs simultaneously.
There are two addressing methods in LCD panels: passive and active. Passive addressing uses temporary row multiplexing without using any key elements. The disadvantages of this method include a low multiplexing coefficient with low contrast, a strong manifestation of the cross-effect and a complex system for generating control signals.
With active addressing, a key element is created for each pixel at the intersection of a row and a column according to the scheme shown in Fig. 4.
Such elements allow the use of a lower multiplexing ratio. The contrast of the image is much higher. However, LCD panels with active addressing are much more expensive than panels with passive addressing, which also increases the cost of devices built on them. Active key elements are most often thin-film field-effect transistors TFT (Thin Film Transistor). In Fig. 5a shows a variant of the topology, and Fig. 5b - schematic diagram of the key element of active addressing on such a transistor.
Color filters are placed on the inside of the LCD panel substrate closest to the viewer. The materials used to make filters are thin films of various dyes. They are applied using various technologies: deposition from solutions or from gaseous media, printing, etc. Variants of the topology of color filters are illustrated in Fig. 6 (R - for red, G - green, B - blue).
The number of lines of LCD panels determines the multiplexing ratio. Most often, low-multiplexed panels are used with ratio values of 1:2, 1:3 and 1:4. Depending on this, several DC voltage levels are created in specific control devices, from which control voltages for rows and columns of the required shape are formed.
In Fig. Figure 7 shows diagrams of addressing voltages in LCD panels with a multiplexing ratio of 1:3. On it, BP0-BP2 indicate the signals of the line outputs; Sn-Sn+2 - column output signals; UDD - supply voltage of the panel control controller; Ulcd is the bias voltage that powers the output signal conditioners; Uobp equal to Udd - Ulcd. - reference voltage; Tk - personnel development period.
To create a luminous flux in LCD panels, a backlight device is used, which contains a radiation source, light distributors (light guides) and one or two reflectors. The source of radiation is incandescent lamps, LEDs, electroluminescent panels, most often fluorescent lamps.
In Fig. Figure 8 shows typical designs of backlight devices with front (Fig. 8,a) and end (Fig. 8,6) placement of the fluorescent lamp.
Let's consider the use of LCD panels using the example of one of the popular models LC-20C2E from SHARP. The company was one of the first to begin manufacturing “flat” TVs - back in 1996, 1997, having previously headed the list of developers and manufacturers of LCD panels. Now the list of models on these panels from SHARP exceeds a dozen, and the diagonal size of the screen has already exceeded 40 inches (about 92 cm).
The TFT LCD panel (LCD) of the described model has a screen size of 20 inches diagonally and is characterized by a significant viewing angle (160° both horizontally and vertically). The model has significantly lower power consumption compared to conventional TVs (no more than 45 W).
The TV is designed to receive signals in radio frequency standards B/G/L/D/K/l/M/N and PAL/SECAM/NTSC color systems. The channel selector (tuner) of the TV allows you to configure and store 197 television channels, including cable television (CATV) intervals. The 3H TV amplifier provides 2.5 W of power in two channels of audio playback.
The advanced matrix LCD panel has a resolution of 921x600 pixels. The brightness of the screen is no worse than 430 cd/m2. The service life of fluorescent lamps used for LCD backlighting is 60,000 hours.
The TV is powered from a DC voltage source of 13 V. When using a special network adapter included in the delivery package, the TV can also be powered from an AC voltage source of 110...240 V with a frequency of 50/60 Hz. TV dimensions (width, height, depth) - 476.6x556.4x229.4 mm. The weight of the device is 8 kg.
To ensure comfortable viewing, the plane of the TV screen can be tilted relative to the plane perpendicular to the stand by 5° forward or 10° backward, and also rotated 40° to the right or left relative to the middle position. The appearance of the TV is shown in Fig. 9.
The connection diagram of the boards and devices of the TV is shown in Fig. 10.
Each connector indicates the number of contacts and conventionally the method of connecting them with the contacts of the connector of another block: “1 in 1” or “crosswise”. Basically, the contacts are connected in the first way - contact 1 - with contact 1, 2 - with contact 2, etc. Only the MT and MA connectors between the tuner board and the main board are connected “crosswise”. For example, the contacts of MT connectors are wired like this: contact 1 to pin 20, contact 2 to pin 19, etc. The same applies to MA connectors, only they have 30 contacts. This must be remembered when studying circuit diagrams of blocks and repairs. The TV, in addition to the LCD panel, not shown in the figure, and two dynamic heads, contains seven boards: tuner (Tuner PWB), main (Main PWB) and video (Video PWB), audio output (S-Out PWB), switches (Switch PWB) and two inverters (Inverter A PWB and Inverter B PWB), as well as a backlight device (Back Light) of the LCD panel. Through the LS and LG connectors, the LCD panel receives initial control (Source) and strobe (or scanning) signals (Gate) from the main board.
The tuner board contains the tuner itself, as well as a control microcontroller with teletext and an OSD device (On Screen Display - display of service or additional information on the screen), ROM chips, programmable memory and microcontroller reset, switches for analog signals R, G, B ( both external and generated by the microcontroller), voltage stabilizers 5; 9 and 10.1 V, as well as connectors for external video and audio signals, including a SCART connector.
The main board houses most of the TV's devices, including a multimedia audio signal processor (it also houses the IF audio signal processing channel), a buffer amplifier, a 3H signal pre-amplifier, a sync selector, and a TV/AV mode selection switch. In addition, it contains a control microcontroller (different from the tuner installed on the board), EEPROM and microcontroller reset chips, a video processor with an ADC, an LCD panel controller with an external memory device (FIFO), an analog multiplexer, a backlight lamp error detector, reference voltage calibration devices and general control of the panel, DAC and switching power supply, which generates all the voltages necessary for the operation of the TV components: 3.3; 5; 8; - 8; 14; 28 and 31 V.
A small video card includes elements for matching the input jack J5001 (through which an external composite AV3 video signal is supplied) and a special jack SC5001 (designed to supply an external S-VHS signal, i.e., separately components of brightness Y and chrominance C) with subsequent TV circuits.
The audio output board contains an AF signal power amplifier, an amplifier supply voltage stabilizer, sound blocking stages, as well as error detectors for fluorescent backlight lamps.
On the switch board there are control keyboard buttons, an IR radiation receiver for the remote control system, a headphone connection socket and a standby voltage switching key.
Inverter boards A and B are required to convert a 13 V DC voltage supplied externally through connector J3702 of the tuner board into alternating voltages of 200...300 V with a frequency of 400 Hz, which are supplied through connectors P6751 and P6551 to the fluorescent lamps of the LCD panel backlight device.
The specific design of the LCD panel (TFT LCD) of the TV model under consideration is shown in Fig. eleven.
It is made in the form of a so-called “sandwich”. On the shielding board, two reflective plates are placed one after the other, which are part of the backlight device. The device also includes six fluorescent lamps (only two of them are shown in the figure). Light guides with a diffraction structure of a prismatic cross-section serve as a light distributor. The purpose of spacers has already been discussed in the first article of the series. Next are the diffusion and prismatic plates
The purpose of using all of the above devices is to make maximum use of the luminous flux and ensure its uniform distribution in the working area of the backlight.
The color filter plate, which was also discussed earlier, is located directly behind the panel. The LCD panel itself has contact connectors for supplying initial control signals (LSD Source) and strobe (scanning) signals (LSD Gate). The figure shows fragments of the ribbon cables through which these signals are sent.
The entire considered “sandwich” is tightened with eight screws (two of them are shown in the figure).
The block diagram of the tuner board is shown in Fig. 12.
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The diagram of the remaining components of the Sharp - LC-20C2E TV is shown in Fig. 13.
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The schematic diagram of the tuner board is shown in Fig. 14.
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The RF radio frequency signal goes directly to the antenna input of the tuner itself (see Fig. 12), located on the tuner board. The following signals are generated at its outputs: SSIF - audio IF signal, which passes through the SIF pin of the SC902/SC901 connector to the main board (see Fig. 13), namely to the multimedia audio signal processor IC901 (1X3371 CE); CCVS (see Fig. 12) is a full color television video signal, which, through the TV V pin of the same connector, comes to the video switch chip (see Fig. 13) of the main board IC402 (NJM2235M); AUDIO MONO (see Fig. 12) is a 3H monaural signal, which is also supplied to the IC901 chip of the main board via the MONOS pin of the same connector (see Fig. 13).
In addition, the CCVS signal (see Fig. 12) is supplied via transistors Q33, Q13, Q14 to the VIDEO OUTPUT pin of the connector for connecting external devices SC903 (SCART).
The tuner board also contains two sockets J902, J903, necessary for connecting left (L) and right (R) external speakers. SOUND L/R signals from the corresponding contacts (SC2 OUT L/R) of the SC902/SC901 connector, to which they arrive from the IC901 chip of the main board (see. Fig. 13).
Through the corresponding contacts (see Fig. 12) of the SC903 (SCART) connector, 34 AV SOUND L/R signals and AV PICTURE images are supplied to the TV. These signals come through the SC2 IN L/R and V2 IN pins of the SC902/SC901 connector to the main board (see Fig. 13), with audio signals going to the IC901 processor, and video signals to the IC801 video processor (VPC3230D).
From the main board, audio signals SC1 OUT L/R and video signals V2 OUT are supplied to the tuner board through the contacts of the SC901/SC902 connector. Moreover, the first ones are from the IC901 sound processor through the IC902 buffer amplifier (NJM4560M), and the second ones are from the IC801 video processor (VO output). Both signals ultimately go to the output pins of the SCART connector (AV SOUND OUT IVR and AV PICTURE OUT) for recording to the VCR (see Figure 12).
Generated by the IC901 audio signal processor (see Fig. 13), the 3H signals pass through to the preamplifier on the IC304 chip (BH3543F+), and from it, through the contacts of the P2003/P4004 connector, to the J4001 headphone socket located on the switch board. The schematic diagram of the switch board is shown in Fig. 15.
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The IC901 audio signal processor also generates the audio signals of the left and right DACM L/R channels (see Figure 13 in the previous part), which first pass the low-pass filter on the IC903 chip (NJM4560M), and then the channel switch IC303 (NJM2283F). The switch is controlled by the L/R command issued from the main board control microcontroller IC2001 (IX3565CE).
The 3H signals of the left and right channels through the contacts of the P3301/P3302 connector go to the sound output board, the circuit diagram of which is shown in Fig. 16. They come to the inputs of the 3H power amplifier on the IC3305 chip (L44635A+). Amplified signals through the contacts of connectors P304 and P305 are supplied to the dynamic heads of the left L and right R channels. The microcircuit is powered by a PA VCC source (see Fig. 13) with a voltage of 13 V. As already indicated, it first passes from the tuner board to the main board, and then to the sound output board through the pins of the P3301/P3302 connector.
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As already listed in the previous parts of the series, on the tuner board (see Fig. 12) there is a control microcontroller 19 (ST92R195), combined with OSD, teletext and extraction of necessary information from the signal devices. Directly connected to the microcontroller are EEPROM 13 (TMS27C2001 - 10), SRAM I6 (W24257 - AS - 35), Memory 12 (24C32) and RESET (TS831 - 4IDT) chips.
At the outputs of the microcontroller, signals of primary colors R, G, B (VPC - TEXT on the circuit diagram) are generated, corresponding to the selected mode of its operation: either teletext signals or OSD signals (program numbers, program settings, parameter adjustments, etc.) . These signals arrive at the inputs of a switch of analog signals R, G, B made on microcircuit 14 (TEA5114A). Its other inputs receive signals of primary colors R, G, B from another similar switch on the IZ chip. Signals R, G, B are supplied to it through the contacts of the external connector SC903 (SCART). The switches are controlled by the microcontroller via the FB.OSD (switch I4) and RGB CONT (switch I13) circuits. As a result, signals of primary colors appear at the outputs of switch I4, which pass through the contacts of the SC802/SC801 connector (see Fig. 13) to the video processor chip and ADC IC801 of the main board.
The schematic diagram of the main board consists of six parts. Three of them are shown in Fig. 17.1 - 17.3.
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The control microcontroller of the tuner board I9 (see Fig. 12 in the previous parts) also generates lowercase H and vertical V synchronizing pulses, which arrive through the contacts of the SC802/SC801 connector first (see Fig. 13 in the previous parts) to the IC801 video processor and the LCD control controller panel IC 1201 (IX3378CE), and from the latter - to the control microcontroller of the main board IC2001. Between the microcontrollers of the tuner board and the main board, information is exchanged through those shown in Fig. 12 and 13 synchronizing and control signals SUB CLK, SUB IN, SUB OUT, M/S IN, M/S OUT, H (HSY) and V (VSY).
The tuner board (see Figure 12) also contains a J3702 input jack for connecting a 13 V DC source and surrounding fuses. This voltage is supplied through the contacts of the P904/P901 connector to the main board, and through the contacts of the P702/P6555 and P703/P6755 connectors to the inverter boards B and A, respectively.
The video processor IC801 (see Fig. 13) receives the following analog video signals: AV1 - from the TV/AV video signal switch (from the IC402 chip upon command from the IC2001 control microcontroller); AV2 - from the SCART connector of the tuner board; AV3 - through the contact of connector P903/P5001, to which the external video signal V3 IN comes from one of the sockets of the J5001 connector of the video card, and the color signal V1 SC - through the contact of the same connector P903/P5001, to which the chrominance signal SC passes from the socket of connector SC5001 of the video card (S-VHS). The schematic diagram of the video card is shown in Fig. 18.
Through the contacts of the P903/P5001 connector (see Fig. 13), audio signals V3 IN L and V3 IN R (from the other two sockets of the J5001 connector on the video card) are also supplied, which are sent to the audio signal processor IC901. The brightness signal V1 SY (S-VHS) from the SC5001 connector socket on the video card goes to the TV/AV video switch (IC402 chip).
The IC801 chip converts incoming analog video signals into digital ones: eight-bit luminance signals VPYO-VPY7 and chrominance UVO-UV7, as well as lowercase HSY, frame VSY and other (LLC1, LLC2, FIELD) synchronization and control signals. From the output of the IC801 chip, the analog full video signal VO, in addition to the SC901/SC902 connector, comes to the synchro selector on the IC401 chip (BA7046F). The CSYNC clock pulses allocated to it pass to the control microcontroller IC2001, and the HD pulses go to the analog switch made on the IC2007 chip (TC4W53U). The latter is also supplied with the HSYc clock pulses of the IC801 video processor. Depending on the state of this switch, controlled by the HSYNC SW signal coming from the microcontroller control 19 of the tuner board, a high or low level OSD HD signal is generated at its output. It goes to the same microcontroller 19 of the tuner board and controls the operation of OSD and teletext devices in it.
Control signals from the front panel keyboard SW4002-SW4004, SW4006-SW4008 and the IR radiation receiver RMC4002 pass to the control microcontroller of the main board IC2001 from the switch board through the contacts of the P4004/P2003 connector (see Fig. 15 in the previous parts).
The control microcontroller IC2001 (see Fig. 13) is connected to the EEPROM IC2004 (BR24C08F) and reset (RESET) IC2002 (PST529DM) chips.
Digital signals of brightness, color and synchronization generated by the video processor IC801 are sent to a large (160 pins) controller chip IC1201 (IX3378CE), which mainly generates digital control signals for the LCD panel: R0-R5 - red, GO-G5 - green, VO B5 - blue color and SK - synchronization. All of them pass to the panel through the contacts of the SC1201 (LCD Source) connector. The external memory chips (FIFO) IC1202 (PD485505) and the analog multiplexer 1C 1205 (TC4052BF) work together with the IC1201 controller. The multiplexed GCK signals arrive at the LCD panel through the contact of the SC1202 connector (LCD Gate).
The reference voltage REV from the IC1201 controller is supplied to the LCD panel reference voltage calibration device, made on the IC1102-IC1104 (NJM4565V), 1C 1106-IC1108 (NJM4580V) and IC1105, IC1110 (BU4053V) microcircuits. At the output of the device, five constant reference voltages (V0 V16 V32 V48 V64) are generated, arriving at the LCD panel through the contacts of the SC1201 connector and used to form the voltage levels of the rows and columns of the panel.
The DAC chip IC1101 (MB8346BV) creates ten constant levels A01-A08, A010, A012 that control the standard voltage calibration device, and the IC1101 chip itself, in turn, is controlled by the digital signals DAC1 SC, MPDA and MPCLK supplied to it from the microcontroller IC2001. The latter also generates the CONTROL signal, which controls the LCD panel controller IC1201.
The 1C 1109 (NJM353M) chip contains a device for general control of the rows and columns of the LCD panel. It creates control signals VCOM, CS COM and CS COM1, supplied through the contacts of the SC1201 and SC1202 connectors to the panel. The constant voltage A011 at one of the outputs of the DAC IC1101 provides the constant current mode (BIAS) of the general control device for the LCD panel.
To obtain variable supply voltages for the fluorescent lamps of the backlight device in the LCD panel, the TV has two identical inverter boards A and B. DC-to-AC converters are assembled on them according to the circuit shown in Fig. 19 for inverter A (the designations of the elements of inverter B differ only in the second digit) They are self-oscillating generators operating at frequencies of 30...65 kHz. Autogenerators include three (with parallel-connected primary windings) pulse transformers T6751-T6753 in inverter A and T6555-T6557 in inverter B (according to the number of lamps used) and two high-frequency transistors Q6751, Q6752 on board A and Q6551, Q6552 on board A board B.
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At the moment of supplying a supply voltage of 13 V, high-voltage (over 1 kV) pulses appear on the step-up (secondary) windings of all transformers, which ensures the initial ionization of the discharge gaps of the lamps and an avalanche breakdown in them. After the self-oscillators switch to operating mode, an alternating voltage with an amplitude of at least 300 V is created on the secondary windings of the transformers, which is supplied to the so-called “hot” (LIGHT HOT) terminals of all lamps through contacts LH1-LH3 of connectors P6751 and P6551. The “cold” (LIGHT COLD) terminals of the lamps (pins LC1-LC3) are connected to the sound card (see Fig. 16 in the previous issue). It has lamp error detectors made on assemblies of field-effect transistors Q3600-G3602. A simplified diagram of connecting three fluorescent lamps HL1-HL3 to inverter A and circuits on the sound output board is shown in Fig. 20. The error signal L ERR through the contact of connector P3302/P3301 (see Fig. 13) reaches the control microcontroller IC2001, which ensures a short-term transfer of the TV to STBY standby mode. After five cycles of turning the lamps on/off, if the error is not resolved, the TV turns off.
A constant (DC) supply voltage of 13 V passes through the contacts of the P904/P901 connector (see Fig. 12 and 13) from the tuner board to the main board, where the power source is located - a DC-to-DC converter (DC/DC converter), made on the key field-effect transistor Q702 (K2503), pulse transformer T701 and PWM controller chip IC702 (NJM2377M)
The power supply generates well-stabilized voltages of 3.3 V - stabilizer chip IC752 (BA033FP), 5 V - stabilizer chip IC751 (AN8005M) and transistors Q751, Q753, 31 V - transistor Q204 with op-amp chip IC201, 28 V - transistors Q201 , Q202 with a second op-amp chip IC201 and 8 V - dual transistors of different Q203 structures, and also stabilized only by feedback to the PWM controller IC702 voltage of 5 and -8 V. To turn off the power source in standby mode, a DC/DC converter comes command STBYc of microcontroller IC2001.
Control of most TV devices is provided by the IC2001 control microcontroller via the I2C digital bus (SDA data signals and SCL synchronization signals).
The remaining three parts of the main board circuit diagram are shown in Fig. 21.
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On the TV "Sharp - LC-20C2E" there are three possible ways to enter the adjustment mode of the main board microcontroller. To explain them in Fig. 22 and 23 show a view of the TV control panel located under the LCD display and a view of the remote control, respectively, and also indicate the purpose of the buttons and other elements.
In the first method, turn on the power of the TV and press the M button on the remote control.
The second method involves first simultaneously pressing the MENU and TV/VIDEO buttons on the TV control panel and turning on the power, and then simultaneously pressing the volume down (-) and channel number (CHv) buttons.
The third method involves connecting pin 81 or 82 of the control microcontroller IC2001 of the main board (control points TP2001 or TP2002, respectively) with a common wire and then turning on the power of the device. In this case, the memory will be initialized, i.e. this method is applicable when replacing IC2004 or IC2001 microcircuits during the repair process.
After entering the mode, by moving the cursor up or down using the Δ and Δ buttons on the remote control, select the required adjustment parameter:
- supply voltage +B5V (5.00+0.05 V);
- installation of the model (C2E);
- setting the screen size diagonally (20 inches);
- adjustment of the general mode (bias voltage COM BIAS) of the LCD panel (until the best contrast is obtained);
- setting the black level in the R and B signal channels (until optimal white balance is obtained).
In each case, by pressing the VOLUME+ and VOLUME- buttons on the remote control, set the required value.
To enter the tuner board microcontroller adjustment mode, first press the MENU button on the TV control panel. Then, by pressing the Δ button on the remote control, you achieve the image shown in Fig. 24, and press the M button on the remote control for 1 second. Next, by moving the cursor up or down using the D and V buttons of the remote control, select the required adjustment parameter.
- setting the horizontal size;
- setting the values of the video path parameters (brightness signal delay, contrast, saturation, color tone, AGC delay) in accordance with those indicated in the table.
The values are set using the same VOLUME+ and VOLUME- buttons on the remote control.
When repairing such TVs, you must be no less careful than when repairing conventional TVs. It is highly advisable to work wearing an antistatic wrist strap and on an electrically conductive mat, since all panels are “afraid” of electrostatic charges.
Before you begin the repair, you must make sure that the parameters are set correctly as described above. For guidance during repairs, see Fig. Figure 25 shows the placement of boards and other devices on the TV, as well as the location of the connectors. Wide black arrows on it show the directions for searching for connectors to facilitate the removal and installation of boards.
Let's look at possible TV malfunctions using specific examples.
1. No picture or sound.
First of all, check the integrity of fuses F2-F4 on the tuner board (see Fig. 14). If any of them (or several) has an open circuit, then check the load circuits for a short circuit. If it is detected, first of all, check the serviceability of the transformer T701 of the power supply and transistors Q702, Q751, Q753 and the key element Q752 of the main board (see Fig. 21, part 6).
If there is no short circuit, check the presence of constant voltages at the outputs of the rectifiers and power supply stabilizers. In the absence of all supply voltages, check the serviceability of the IC702 microcircuit, transistors Q702, Q703, as well as the absence of open fuses FB701, FB708, FB709 and the primary windings of transformer T701.
In the absence of any one supply voltage, check the serviceability of the corresponding rectifier in the secondary circuits of the T701 transformer and the voltage stabilizer.
2. No image.
Check the presence of digital video signals at the corresponding pins of microcircuits IC801 (see Fig. 17, part 3) and IC1201 (see Fig. 21, part 4) of the main board. If their absence is detected at the outputs of a particular microcircuit, then before replacing them (this is done as a last resort), check the direct current mode of the microcircuit. It should not differ from that indicated on the circuit diagram by more than ±10%. Only after this they decide to replace the microcircuit or any of the elements surrounding it.
If the necessary video signals are present at the outputs of the IC1201 chip and they arrive at the LCD panel, then first check the receipt of signals and voltages to the IC1205 chip, and then check the serviceability of it itself, as well as the receipt of multiplexed signals on the panel.
They also check the supply of the reference voltage REF from the IC1201 microcircuit (see Fig. 21, part 4) to the graduated voltage device (see Fig. 21, part 5), the serviceability of the IC1102-IC1108, IC1110 microcircuits included in it and the presence of graduated voltages on the contacts panel connectors (see Fig. 21, part 4).
At the end of the examination, they conclude that the panel itself is faulty.
3. There is no image when a signal is supplied to the antenna input.
First, check for the presence of voltages of 5, 9, 12 and 31 V at the corresponding contacts of the tuner connectors (see Fig. 14). It must be borne in mind that if voltages of 5.12 and 31 V come from the power source located on the main board, then the voltage of 9 V is stabilized by chip 15 of the tuner board, which may fail. Other stabilizers are also checked - NO, I1 microcircuits and transistors Q18 and Q28 located on the tuner board.
Then check the presence of the CCVS video signal at the tuner output. Its absence indicates a tuner malfunction. If there is a signal, it is necessary to monitor (TV V circuit) whether it is supplied to the input (pin 3) of the IC402 chip (see Fig. 17, parts 1 and 3) and to its output (pin 7). If there is no signal at the output of the microcircuit, then either the microcircuit is faulty, or its control inputs (pins 2 and 4) do not receive the corresponding command signals (TV/AV and AV/IR) from the control microcontroller IC2001 (see Fig. 17, part 2 and 3).
If there is a signal at the output of the IC402 chip, check the serviceability of the Q420 transistor on the main board (see Fig. 17, part 3) and the signal arrival at pin 73 of the IC801 chip. If there is a signal, then the microcircuit has failed.
4. There is no image when a signal is supplied to one of the video inputs.
With such a malfunction, three cases are possible.
If there is no image when the S-VHS signal is applied (first case) to the SC5001 socket of the video card (see Fig. 18), check the passage of the brightness signal V1 SY - V1 V through the video card, connector pins P5001/P903, microcircuit IC402 (pins 1 and 7 ) and transistor Q420 of the main board (see Fig. 17, parts 1 and 3) to pin 73 of the IC801 microcircuit with the corresponding commands from the control microcontroller IC2001 (see above). As in the previous fault, if there is a signal, the microcircuit is defective.
There may be no image when a video signal is supplied to pin 20 of the SCART connector (second case). Check the passage of the V2 V signal through the tuner board (see Fig. 14), the contacts of the SC902/SC901 connectors, the Q421 transistor of the main board (see Fig. 17, part 3) to pin 74 of the IC801 chip. If the signal arrives, the chip is faulty.
And finally, if there is no image when a video signal is supplied to the J5001 socket (third case) of the video card (see Fig. 18), check the passage of the V3 IN - SY OUT signal through the video card, connector contacts P5001/P903 (see Fig. 17, part 1 ), transistor Q820 of the main board (see Fig. 17, part 3) to pin 75 of the IC801 chip. If the signal is present, the chip is also faulty.
5. There is no sound from the dynamic heads.
Check the presence of signals 34 at the outputs (pins 12 and 8) of the IC3305 audio output board chip (see Fig. 16) and their arrival through the contacts of connectors P304 and P305 to the dynamic heads. If there are no signals, check the DC mode of the microcircuit and, first of all, the presence of a 13 V supply voltage at its pin 7. If the mode corresponds to that indicated in the diagram, check the receipt of 3H input signals to the microcircuit through pins 8 and 9 of the P3302/P3301 connectors from the main boards (see Fig. 21, part 6). It checks the serviceability of microcircuits IC303, IC903 (see Fig. 17, part 1) and the elements surrounding them, as well as the receipt of DACM R and DACM L signals from the IC901 processor (pins 27 and 28, respectively).
And finally, they check the serviceability of the IC901 processor itself, its surrounding elements and the receipt of MONOS (to pin 60) and SIF (to pin 67) audio signals from the tuner board at its inputs (see Fig. 14). The tuner itself may, of course, be faulty if both of these signals are missing.
Additionally, check the blocking voltage level at pin 53 of the IC2001 microcircuit (see Figure 17, part 2) which should be low. Otherwise the sound will be blocked
6. There is no sound in the headphones.
The search for the cause of the malfunction begins by checking the presence of sound signals on pins 24 and 25 of the IC901 processor on the main board (see Fig. 17, part 1). If they are not there, check the serviceability of the processor and its surrounding elements.
If the signals are present, first check the serviceability of the IC304 microcircuit and its surrounding elements, and then the passage of the HR and HL signals (see Fig. 17, parts 1 and 2) through the contacts of the P2003/P4004 connector to the J4001 headphone jack. It is located on the switch board (see Fig. 15).
7. There are no audio signals on the line output.
Check for the presence of 3H signals on pins 36 and 37 of the IC901 processor (see Fig. 17, part 1). If they are not there, examine the processor and its surrounding elements.
If there are signals, check the serviceability of the IC902 chip and, if it and its surrounding elements are serviceable, further passage of the V2R0, V2LO signals through the contacts of the SC901/SC902 connector to the SCART connector of the tuner board (see Fig. 14).
8. No white balance.
Depending on the color shade of the image, check the ranges of the RO-R5 signals on pins 18-23 of the SC1201 connector (see Fig. 21, part 4) of the LCD panel, the GO-G5 signals on pins 25-30 and the BO-B5 signals on pins 32- 37. If there are no R signals or their range is significantly reduced, check the serviceability of the resistors in the R1202, R1203 assemblies, if the G signals - in the R1204, R1205 assemblies, and if the B signals - in the R1206, R1207 assemblies.
In the case when all the resistors are working, but some of the above signals are missing or they are small, pay attention to the mode of the IC1201 controller and then make a decision about its malfunction.
9. The backlight lamps do not light up.
If all the lamps do not light up, then, most likely, the OFLO blocking command is sent to pins 2 of connectors R703/P6755 and R702/P6555 of the inverter boards (see Fig. 14 of the tuner board) via connectors SC902/SC901 from pin 34 of the IC1201 controller (see Fig. Fig. 17, part 1 and Fig. 21, part 4), stopping the operation of both converters. In normal operating mode, the indicated pin of the controller should have a high voltage level. In this case, the Q3603 key element located on the main board may also be faulty.
But the most likely malfunction is when three backlights do not light up. In this case, first check the integrity of fuses F1 and F5 on the tuner board (see Fig. 14), through which the 13 V supply voltage passes to the inverter boards. If the fuses are intact, check the functionality of the corresponding voltage converter (see Fig. 19), that is, the serviceability of its elements, primarily transistors and transformers.
If only one lamp does not light, then either it is faulty, or one of the windings of the corresponding transformer in the converters is broken.
Literature
- Samarin A.V. Liquid crystal displays. Engineer's Library. - M.: Solon-R, 2002.
- Krylov E. Backlighting of LCD displays. - Components and Technologies, 2001, No. 6, p. 18-20.
See other articles section.
All equipment can periodically fail, and the TV, which is found in almost every home, is no exception. To be able to repair it in a timely manner on your own, you need to understand the operation of the cascades, their purpose and interaction with each other, and also understand the basics of how a TV receiver works.
The basic principle (technology) of TV operation
One of the main devices of any TV that provides signal reception is a television antenna (TA), and the main parameter of its operation is the correct matching of the output R of the active vibrator with the resistance inherent in the reduction cable (CR). It is necessary in order to transmit the incoming pulse received by the TA and is a high frequency coaxial cable with sufficient efficiency (feeder).
Matching is necessary to achieve a higher TWR (traveling wave ratio) in the descent cable itself. The matching device is designed to convert R into a value close in value to the resistance possessed by the feeder.
Also, the TA must have certain values for the bandwidth; this is an important parameter, since its width directly determines the uniformity of its amplitude-frequency response (AFC).
The block diagram of a regular black and white TV can be imagined:
The signal arriving from the antenna enters the input selective device (ISD), which selects the television signal required at a certain moment. Taking into account the fact that its U is quite small, it is then amplified by a high-frequency amplifier (UHF).
After amplification, it goes to a frequency converter (IF), which is a mixer with a local oscillator, the accuracy of which is necessary to obtain a high-quality image (clarity, absence of any phase distortion and sound quality). Plus, correct and precise adjustment helps smooth out existing interference coming from other TV channels.
In terms of the number of oscillatory circuits, the local oscillator is completely similar to the VIU. After tuning the signal in the local oscillator, it goes to the mixer, where the parameter from the VIU also arrives.
According to the principle of operation of the mixer, which transfers the received frequency to an intermediate one, it multiplies the frequency of the existing image and the sound frequency by the frequency component of the local oscillator.
As a result, the output produces oscillations in the frequency of the image i, as well as the sound f (all of them are intermediate).
f PR = f G – f C
Thus, at the IF output there is an intermediate i of image and sound, while the first should be 6.5 MHz higher than the second.
Regardless of which channel is being tuned, these values are constant and have the following meanings:
- i image = 38 MHz.
- f sound = 31.5 MHz.
These oscillations, although high-frequency, contain smaller f received signals. If you need to fine-tune it, in such situations the parameters of the local oscillator can be adjusted by changing C (capacitance) in the oscillatory circuit circuit.
As a rule, modern models have an APCG unit that automatically adjusts the local oscillator.
Passing through the SC (TV channel selector), the intermediate frequencies enter the control unit, which converts the intermediate frequency of the resulting picture (UPCHIZ).
After it, the amplified pulse goes to the detector (VD).
VD has two main purposes:
- Video signal extraction.
- Obtaining a new, 2nd intermediate frequency of the audio component, which is the difference between the intermediate frequency components of the picture and the audio component and is equal to 6.5 MHz.
Thus, the VD is nothing more than an IF.
After the VD, the video signal goes to the amplifier (UVS), and then to the modulator of the kinescope itself (MK).
The resulting value (6.5 MHz) goes to the UPCHZ, after which it is transmitted to the detector (BH), which directly isolates the sound itself, after which it sends it to the UPChZ and subsequently to the loudspeaker (GR).
The synchronizing signal is isolated from the CCS by means of a synchronization unit (BS) and, without undergoing modifications, passes through all available blocks.
In the BS, it is divided into horizontal and also vertical pulses using units that carry out scanning (BKR, BSR), after which they go to the OS.
After the BS, all pulses received through the BKR and BSR go to the high-U rectifier (HV), which is necessary to power one of the anodes of the kinescope (K). Initially, the voltage to circuit U is supplied from the power supply unit (PSU).
As already mentioned, after the UVS, the horizontal and vertical pulses make up the complete finished video signal. Thanks to this, on screen K the electron beam moves synchronously and with the same phase as the beam that is transmitted from the television center tube.
The video signal contains pulses that dampen the beam in K, required for the reverse code of the specified scans (frame, line).
To select the synchronization pulses directly, there is a selector (SSI), which is always in a locked state and goes into the open state due to the synchronization pulses. Since the amplitude of the synchronization pulses is always higher than the amplitude of the image signal for the blackest elements, they are highlighted. Moreover, their meaning will correspond to the concept of “blacker than black.”
The SSI also has the function of dividing into horizontal and vertical sync pulses by measuring the difference in duration between horizontal and vertical pulses (the duration of the latter is higher).
Thus, through the differentiation procedure, horizontal sync pulses are obtained, and through integration, vertical sync pulses are obtained.
After the SSI, the vertical sync pulses go to the HRG (vertical scan generator), where a sawtooth voltage is obtained from the deflection coils at the output stage, which produces a sawtooth linear current I.
The OS deflection coils, which provide framing, are connected to the GKR using an output personnel transformer (VTK), which ensures complete coordination of the R stage (tube) with the R deflection coils. Alternatively, the connection can be made with GKR semiconductors, since their R is much smaller.
By means of the OS installed on the neck of the kinescope tube (K), the electron beam is controlled, and the effect on it is carried out using the magnetic field of the OS solenoids.
Horizontal sync pulses pass to a device that provides automatic frequency and phase adjustment of the horizontal scan itself (APChIF). There is also a comparison of the duration of the horizontal sync pulses and the reverse pulses of the horizontal scan itself, which come from the GSR.
If the duration of the horizontal sync pulses and the reverse pulses with the GSR coincides, at the output of the APCiF U will be equal to zero.
If deviations in one direction or another are observed in duration, the output is U, proportional to the magnitude of this deviation. In this case, the polarity of the voltage will depend on the time of arrival of pulses from the SSI and GSR.
Due to the existing inertia of the APCiF, pulsed noise, which also comes along with the incoming signal, does not have any effect on its operation.
The output voltage from the APCiF goes to the GSR, which in turn changes the frequency component of the sweep voltage.
Simplified electrical circuit diagram (block diagram) of a TV
According to the structural diagram presented in the previous subsection, the location and interaction of individual blocks with each other becomes clear.
Taking into account the development of technology, the principles of circuit design and operation have changed significantly, since over time, televisions with a black and white screen were replaced first by color, and then by LCD and plasma.
In this regard, new elements were added to the classic block diagram in connection with the transition to color broadcasting, such as:
- BC – color block.
- BDU is a unit that provides remote control.
- BKVU is a unit that provides switching of all external devices.
As for modern LCD and plasma panels, the number of different blocks in them is much greater.
Design, principles of operation of black and white models (analog)
All black and white TVs, both lamp and semiconductor models, have a similar structural layout.
As can be seen from the presented figure, the following devices have been added:
- Meter channel selector (SCM).
- Decimeter channel selector (SCD).
- Intermediate image intensifier (IFIA).
Sound and picture signals, amplified and converted in the block that switches TV channels (PTK), enter the UPCHI.
Taking into account the fact that the oscillation frequency of the local oscillator differs in value from f of the incoming pulse (above), as already indicated, the difference between the intermediate i of the picture and sound is 6.5 MHz.
To obtain the highest quality images, it is necessary to precisely tune the local oscillator at the input to the desired frequency, which ensures the clarity of the video image and the purity of the audio signal, as well as the absence of phase distortion.
All such TVs have both manual and automatic adjustment functions
Manual tuning helps ensure correct tuning when receiving a test pattern.
Automatic tuning is extremely necessary for various switching situations, such as turning on and warming up the device itself (the frequency component of the local oscillator changes), a power surge, external interference, or switching the required channels.
APCG (automatic frequency adjustment of local oscillator)
APCG is executed with the OS and contains a distinguisher and a control element.
The discriminator is nothing more than a phase discriminator, where the input is U intermediate frequency. Thus, if the TV is tuned precisely, the U output will be zero.
If there is a deviation in the local oscillator frequency (from 38 MHz, nominal), a control detuning U appears at the output.
U detuning goes to a device called a varicap, which is connected to the local oscillator circuit in the PTC. Thus, this U changes the f of the local oscillator to the side that is opposite to the detuning.
But the APCG is not able to completely eliminate the existing disorder, because its residual values are always available. In this case, the higher the auto-tuning coefficient, the lower the value of the residual detuning will be.
Often, the standard solution in devices of this type is the use of APCG via intermediate f and UPT (constant I amplifier). With this scheme, the residual detuning is about 50 kHz (initially present at 1.2 MHz).
Also, many first-generation models are equipped with the following units:
- Automatic gain control (AGC), ensuring constant maintenance of any values.
- Automatic construction by f and phase (APChiF).
In these models, due to the APCiF in the GSR, frequency and phase auto-synchronization is provided with similar parameters of clock pulses from the television center. It also ensures reliable synchronization of the horizontal scanning of the signal at the input if it is weakened or impulse noise is present, which is important for models with a large screen diagonal.
Further, at the output of the PD (phase detector), which is necessarily present in such models, there will be a constant U, while its polarity and value will be directly proportional to the phase angle of the pulses.
If this angle is zero, the voltage at the PD output will also have a zero value. For other values, this U goes to the control grid of the MRG (master relaxation oscillator) through a low-pass filter (LPF).
If the voltage begins to change, changes also occur in the natural oscillation frequency of the ZRG. Thus, these oscillations will decay only when their discrepancy with the phase shift angle and f of the synchronizing pulses is also reduced to zero.
Depending on the construction scheme, AChiF is not always able to compensate for all possible deviations f of the ZRG. To avoid such a problem, manual adjustment is installed in such TVs with a simple AChF circuit.
As for the first-class models, due to the correct choice of the APC&F circuit with a wide band range covering the f SRG, there is no need to install the possibility of manual adjustment. This is achieved by a controller, a phase discriminator, which remembers the last value of the peak U difference f.
Design, principles of operation of color televisions (analog)
These models are analog and are made on semiconductors.
Unlike the previous image, the following new components have been added to the semiconductor color TV:
- Remote control board (RC).
- Video processor equipped with a color decoder.
- Decoder providing teletext.
- DVD player.Player-USB.
Circuit, device, operating principles of LCD and plasma panels
In these models, the circuit is significantly changed, since, unlike analog, the signal is processed digitally.
The main blocks inherent in such devices are as follows:
- Inverter. Thanks to it, the voltage necessary to power LEDs or backlight lamps is provided.
- The memory in which settings data is stored is ROM.
- Random access memory, which is directly involved in their processing - RAM.
Thus, the principle of operation of the TV in all models remains the same, however, due to the development of modern technologies, the constituent elements have undergone significant changes.
A television receiver is a device for receiving television signals and converting them into visual and audio images.
A television consists of a visual information display device (kinescope, liquid crystal or plasma panel); chassis - a board that contains the main electronic units of the TV (television tuner, decoder with an amplifier of audio and video signals, etc.), a housing with connectors, control buttons and speakers located on it.
Television radio signals received by the antenna are fed to the radio frequency (antenna) input of the TV. Next, they enter the radio frequency module, also called a tuner, where the signal of the exact channel to which the TV is currently tuned is isolated and amplified. The tuner also converts the radio frequency signal into low-frequency video and audio signals.
The video signal, after amplification, is fed to a color module (color TVs only), which contains a color decoder, and then to a visual information display device. The color decoder is designed to decode color signals of a particular system (PAL, SEC AM, NTSC).
The audio component is fed into the audio channel, where the audio signal is isolated and amplified as necessary. After amplification, the audio signal is sent to a loudspeaker (speaker), which converts the electrical signal into audible sound. If the TV is designed to reproduce stereo or multichannel sound, its audio channel contains a corresponding multichannel audio decoder that divides the audio component into channels.
CRTs come in black and white and color, and they differ in design.
The inside of a black-and-white kinescope screen is covered with a continuous layer of phosphor, which has the property of glowing white under the influence of a flow of electrons. A thin electron beam is formed by an electronic spotlight placed in the neck of the kinescope. The electron beam is controlled electromagnetically, as a result of which it sequentially scans the screen line by line during scanning, causing the phosphor to glow. The intensity (brightness) of the phosphor glow during scanning changes in accordance with the electrical signal (video signal) carrying information about the image.
The inside of a color picture kinescope screen is covered with a discrete layer of phosphors (in the form of circles or lines), glowing red, green and blue under the influence of three electron beams generated by three electronic spotlights. All color picture tubes in front of the screen have a color separation shadow mask. It serves to ensure that each of the three electron beams, simultaneously passing through numerous holes in the mask during scanning, precisely hits “its” phosphor (the first - on the phosphor grains that glow in red, the second - on the phosphor grains that glow in green, the third - on the phosphor grains, glowing blue).
Each electron beam is modulated by its own video signal, which corresponds to three components of a color image. Entering the kinescope, video signals control the intensity of the electron beams and, consequently, the brightness of the phosphors (red, green and blue). As a result, 3 single-color images are simultaneously reproduced on the screen of a color kinescope, which together create a color image.
Modern means of displaying visual information include liquid crystal screens, projection systems, and plasma panels.
In LCD (Liquid Crystal Display) televisions, the image is formed by a system of liquid crystals and polarizing filters. From the rear side, the liquid crystal panel is evenly illuminated by a light source. The cells (pixels) of liquid crystals are controlled by a matrix of electrodes to which a control voltage is applied. Under the influence of voltage, liquid crystals unfold, forming an active polarizer. When the degree of polarization of the light flux changes, its brightness changes. If the polarization planes of the liquid crystal pixel and the passive polarizing filter differ by 90°, then no light passes through such a system.
A color image is obtained by using a matrix of color filters that separate three primary colors from the radiation of a white source, the combination of which makes it possible to reproduce any color. LCD TVs are compact, lack geometric distortion, have no harmful electromagnetic radiation, are lightweight and have low power consumption, but at the same time have a small viewing angle.
In projection TVs, the image is obtained as a result of the optical projection of a bright light image created by the projector onto a translucent or reflective TV screen. Projectors used in projection televisions can be built on cathode-ray picture tubes, liquid crystal matrix semiconductor elements, and laser projection tubes.
The main disadvantages of projection TVs are their bulkiness, high power consumption, low clarity of the enlarged image and a narrow area for placing viewers in front of the TV screen.
The operation of a plasma TV is based on the principle of controlling the discharge of an inert gas in an ionized state between two plane-parallel glasses of a cellular structure located at a short distance from each other. The working element (pixel), which forms a separate point in the image, is a group of three pixels, respectively responsible for the three primary colors. Each pixel is a separate microchamber, on the walls of which there is a fluorescent substance of one of the primary colors. The pixels are located at the intersection points of transparent control electrodes, forming a rectangular grid. During a discharge in the thickness of an inert gas, ultraviolet radiation is excited, which, acting on phosphors of primary colors, causes them to glow. The image is displayed sequentially, point by point, in lines and frames, on the screen.
The brightness of each image element on the panel is determined by the time it glows. If on the screen of a conventional kinescope the glow of each phosphor spot continuously pulsates at a frequency of 25 times per second, then on plasma panels the brightest elements glow constantly with an even light, without flickering. Plasma panels are available in 16:9 image format. The thickness of a panel with a screen size of 1 m does not exceed 10-15 cm, which allows them to be used as a wall-mounted option. The reliability of plasma panels exceeds the reliability of traditional picture tubes.
Today, when it comes to choosing a new TV, many buyers do not know which TV to buy: plasma or LCD. This question is especially important for those who want to buy a large-diagonal TV and use it in the future as a home theater. But to figure out which TV is better, you should know what each technology is and compare their characteristics.
LCD TV uses liquid crystals, which are located between transparent panels with electrodes. When exposed to electricity that passes through the electrodes, the molecules of these crystals can freely change their position, transmitting light through them. Thanks to this technology, it is possible to create a light switch that will operate on electricity if the crystals are located behind the backlight.
Depending on how the light passes and the plane of its polarization, dark and light pixels will be visible on the screen, of which there are quite a lot in the TV matrix. After the light passes through the crystals, it hits the light filter, which consists of green, red and blue subpixels. In this case, three subpixels are used for one pixel. Their colors are primary and form other shades that create a color picture on the screen.
Plasma screen technology
Plasma TV also consists of transparent panels with electrodes, between which microlamps are located, filled with ionizing gas. Each of these cone lamps is filled with a gas that begins to emit ultraviolet light when electricity passes through it. Each cone is coated with a phosphor of a certain color. When ultraviolet radiation passes through the phosphor, we see a certain light. Moreover, each of the pixels consists of 3 microlamps of the main color, which, when combined, allow you to create additional shades and colors. The brightness of the emitted glow will depend on the voltage level.
Comparison of characteristics
|
Characteristic |
Plasma |
LCD |
| Screen size | Can choose a model with a diagonal of up to 100 inches or more. | Today, the production of large LCD monitors has improved, and there is no difference with plasma. |
| Contrast | Plasma panels convey contrast better because they can emit light on their own. | In LCD TVs, contrast depends on the intensity of the light and crystals, and this does not allow the same level of contrast to be achieved. |
| Brightness | The brightness in such TVs is high, but has limitations. | In conventional LCD displays, the brightness is quite low. But LCD models with LED backlighting are superior to plasma. |
| Black depth | They have better black depth, since the level of pixels can glow at different brightnesses. | On LCD TVs, if the picture is quite dark, some parts of it will disappear. |
| Viewing angle | For such screens it is at least 170 degrees in all directions. | Older models had a viewing angle of 45 degrees, and today they reach the same viewing angles as plasma. However, at a certain angle, the contrast decreases. |
| Permission | Plasma TVs with high resolution do not yet exist. | It has better resolution, since it is easier to reduce a pixel than a cell with gas. |
| Fast response | Electricity passes through gas at maximum speed, which allows for increased response speed. | Liquid crystals do not transmit electricity as quickly, but thanks to the use of transistors, it was possible to achieve the same speed of response. |
| Illumination Uniformity | Each cell is a separate light source. In this regard, the screen is illuminated evenly. | In LCD models, the uniformity of illumination depends on the quality of the backlight and other characteristics. |
| Functionality | They have a large selection of different functions. | The number of interfaces, functions and connectors is no different from plasma models. |
| Energy efficiency | Consumes much more electricity as it requires constant operation of fans for cooling. | Consumes a small amount of electricity. |
| Durability | Plasma TVs operate for no more than 30,000 hours. However, it may last less due to overheating. | Service life – up to 100,000 hours. When the backlight lamp burns out, it can be replaced, but after this there is a possibility of “broken” pixels appearing. |
| Price | Large screen TVs are not that expensive. | A large LCD screen is quite complex to manufacture, so a TV with the same diagonal as a plasma will cost significantly more. |
Advantages and disadvantages of plasma
To put it all more briefly, plasma has the following advantages:
- Has uniform screen illumination;
- Better contrast and depth of black;
- Better brightness, saturation and color rendition;
- Price, especially for a large diagonal TV.
- Good viewing angle.
However, plasma also has certain disadvantages:
- Consumes a lot of electricity;
- It weighs quite a lot, which makes it difficult to hang it on the wall;
- Over time, each pixel begins to glow weaker;
- Cooling fans begin to emit noise over time.
Pros and cons of LCD TVs
Now you should understand what advantages LCD displays have:
- High screen resolution;
- Lightweight and compact TV;
- Large selection of screen sizes;
- Better energy efficiency.
We will also determine what disadvantages LCD TVs have:
- Black color is not deep enough;
- Color rendering level and contrast;
- Decrease in brightness over time;
- Cost of large diagonal TVs.
So which is better: LCD or plasma?
In fact, sitting near the TV, you will not notice any particular difference in these two technologies. But if you look at the characteristics and listen to the reviews, it becomes clear that plasma is still better in many respects. However, despite this, plasma TVs get quite hot. This may cause noise from fans to disturb you. In addition, plasma weighs quite a lot, and this also creates certain problems, especially when installing the TV on the wall. Therefore, plasma is suitable only for those people who have a fairly large room with ventilation, and you have money not only for a TV, but also a set of HD signal sources. But buying an LCD TV is more practical. It supports any video signals and digital TV signal variants. In addition, it is suitable for small rooms and will last you much longer.
