From the “Inside Look” series we talked about everyday things, but, despite the abundance of material received in this direction over the past month, let’s still return to topics related to IT.
Especially for Defender of the Fatherland Day, LCD and E-Ink displays were placed on the preparation table, which, one way or another, I received in a somewhat battered state.
How Anton threw the phone against the wall, as well as the results of a meticulous analysis of the displays, read under the cut.
Preface
Once upon a time there lived Anton Gorodetsky.His wife left him, he was not sad like a child...
This is how the famous song of the group Umaturman begins. The story of display research begins in the same way. After the first publication on Habré, my friend, a graduate student at the Moscow State University, came to me and said: “I broke my mobile phone, do you want to cut it up?” I was surprised because this man always carried a Chinese phone with him, which I considered practically indestructible. Arriving home one day, Anton, out of habit, threw the phone into the closet, but, apparently, without calculating something, the display hit the edge of the shelf.
Realizing his ridiculous losses from the loss of his mobile and in view of the general bad mood that day, he acted like a true gentleman, throwing the lifeless body of the phone against the concrete wall again and again. When the remains reached me, half of the Chinese phone was simply missing, the display was covered with a small web of cracks.
I had to put it aside until better times (as I then thought, until someone did the same with the iPhone or other touchscreen smartphone) and started working on HDDs and CDs, then light bulbs, flash drives, etc.
After some time, my neighbor brings me a cracked E-Ink display. His friend broke the thin glass in the well-known e-reader with serial number 601 during an airsoft game, it seems, and gave the e-reader practically for nothing for repair and restoration.
This was already more interesting, the two technologies can be compared with each other, try to discern RGB subpixels and microcapsules in which charged particles float. But I was hoping to get a smartphone with a capacitive sensor in order to compare it with the resistive sensor of the Chinese phone.
And so Vasily (a scientific colleague in one of the department’s laboratories), having come to us at the ChemFak from Chernogolovka and seeing what I was actually doing with an electron microscope, said that he was ready to donate a phone from a well-known Korean manufacturer with a somewhat battered display for disassembly and cutting with a mark “for the sake of science, nothing is spared.”
Despite all the assurances that the sensor is capacitive, it turned out to be resistive, albeit of a more advanced design than the Chinese phone’s touch panel. An important part was extracted from this phone, which is waiting in the wings for cutting - the photo/video camera matrix...
Theoretical part
How does the LCD display work?
We have all been using flat-screen TVs, monitors, phones, smartphones for so long that we have already forgotten that once a good monitor weighed 10-15 kilograms (we still have one such mastodon and, most importantly, it works properly!).All this became possible thanks to centuries-old discoveries (liquid crystals were discovered in 1888) and the development of technology in the last 30-40 years (1968 - a device for displaying information using LCD, 1970s - the general availability of liquid crystals). You can learn a lot about liquid crystals and LCD monitors on Wiki.
So, almost any LCD monitor consists of the following main parts: an active matrix, which is a set of transistors with the help of which an image is formed, a layer of liquid crystals with filters that either transmit light or not, and a backlight system, which today They are trying to completely switch to LEDs. Although on my “old” Asus G2S the display of excellent quality is illuminated by fluorescent lamps.
How does it all work? Light coming from a source (LED or lamp) through a special transparent waveguide plate is scattered in such a way that the entire matrix has equal illumination over its entire area. Next, the photons pass through a polarizing filter, which allows only waves with a given polarization to pass through. Then, penetrating through the glass substrate on which the active matrix of thin-film transistors is located, the light hits the liquid crystal molecule.
This molecule receives a “command” from the underlying transistor at what angle to rotate the polarization of the light wave so that it, passing through another polarizing filter, sets the glow intensity of an individual subpixel. And a layer of light filters (red, green or blue) is responsible for the color of the subpixel. Mixing, the waves from three subpixels invisible to the human eye form an image pixel of a given color and intensity.
a) Schematic structure of an LCD display, b) structure of a liquid crystal film in detail.
It seems to me that this is demonstrated very clearly in the Sharp video:
In addition to the well-proven LCD + TFT technology (thin-film transistors), there is an actively promoted OLED + TFT organic light-emitting diode technology, that is, AMOLED - active matrix OLED. The main difference between the latter is that the role of a polarizer, an LCD layer and light filters is played by organic LEDs of three colors.
Essentially, these are molecules that are capable of emitting light when an electric current flows, and depending on the amount of current flowing, change the color intensity, similar to what happens in conventional LEDs. By removing the polarizers and LCD from the panel, we can potentially make it thinner, and most importantly, flexible!
What types of touch panels are there?
Since sensors are currently used more with LCD and OLED displays, I think it would be reasonable to talk about them right away.Very detailed description touch screens or touch panels are given (the source once lived, but for some reason disappeared), so I will not describe all types of touch panels, I will focus only on the two main ones: resistive and capacitive.
Let's start with the resistive sensor. It consists of 4 main components: a glass panel (1), as the carrier of the entire touch panel, two transparent polymer membranes with a resistive coating (2, 4), a layer of micro-insulators (3) separating these membranes, and 4, 5 or 8 wires, which are responsible for “reading” the touch.
Resistive sensor device diagram
When we press such a sensor with a certain force, the membranes come into contact, the electrical circuit is closed, as shown in the figure below, the resistance is measured, which is subsequently converted into coordinates:
The principle of calculating coordinates for a 4-wire resistive display ()
Everything is extremely simple.
It is important to remember two things: a) the resistive sensors on many Chinese phones are not of high quality, this may be due precisely to the uneven distance between the membranes or poor-quality micro-insulators, that is, the “brain” of the phone cannot adequately convert the measured resistances into coordinates; b) such a sensor requires pressing, pushing one membrane to another.
Capacitive sensors are somewhat different from resistive sensors. It’s worth mentioning right away that we will only talk about projective-capacitive sensors, which are now used in the iPhone and other portable devices.
The operating principle of such a touchscreen is quite simple. A grid of electrodes is applied to the inside of the screen, and the outside is coated, for example, with ITO, a complex indium tin oxide. When we touch the glass, our finger forms a small capacitor with such an electrode, and the processing electronics measures the capacitance of this capacitor (supplies a current pulse and measures the voltage).
Accordingly, the capacitive sensor reacts only to a firm touch and only with conductive objects, that is, such a screen will work every other time if touched by a nail, as well as by a hand soaked in acetone or dehydrated. Perhaps the main advantage of this touchscreen over a resistive one is the ability to make a fairly strong base - especially durable glass, such as Gorilla Glass.
Scheme of operation of the surface capacitive sensor()
How does an E-Ink display work?
Perhaps E-Ink is much simpler compared to LCD. Once again, we are dealing with an active matrix responsible for image formation, but there are no traces of LCD crystals or backlight lamps here; instead, there are cones with two types of particles: negatively charged black and positively charged white. The image is formed by applying a certain potential difference and redistribution of particles inside such microcones, this is clearly demonstrated in the figure below:
Above is a diagram of how an E-Ink display works, below are real microphotographs of such a working display ()
If this is not enough for someone, the principle of operation of electronic paper is demonstrated in this video:
In addition to E-Ink technology, there is SiPix technology, in which there is only one type of particles, and the “fill” itself is black:
Scheme of operation of SiPix display ()
For those who seriously want to get acquainted with “magnetic” electronic paper, please go here, there was once an excellent article in Perst.
Practical part
Chinaphone vs Korean smartphone (resistive sensor)
After a “careful” screwdriver disassembly of the remaining board and display from the Chinese phone, I was very surprised to find a mention of one well-known Korean manufacturer on the phone’s motherboard:
Samsung and Chinese phone are one!
I disassembled the screen carefully and carefully - so that all the polarizers remained intact, so I simply could not help but play with them and with the working big brother of the object being dissected and remember the optics workshop:
This is how 2 polarizing filters work: in one position the light flux practically does not pass through them, when rotated 90 degrees it passes completely
Please note that all the lighting is based on just four tiny LEDs (I think their total power is no more than 1 W).
Then I looked for a sensor for a long time, sincerely believing that it would be a rather thick socket. It turned out quite the opposite. In both Chinese and Korean phones, the sensor consists of several sheets of plastic, which are very well and tightly glued to the glass of the outer panel:
On the left is the Chinese phone sensor, on the right is the Korean phone sensor
Resistive sensor Chinese phone made according to the “the simpler the better” scheme, unlike its more expensive brother from South Korea. If I'm wrong, correct me in the comments, but on the left in the picture is a typical 4-pin sensor, and on the right is an 8-pin sensor.
Chinese phone LCD display
Since the display of the Chinese phone was still broken, and the Korean one was only slightly damaged, I will try to talk about the LCD using the example of the first one. But for now we won’t break it completely, but let’s look under an optical microscope:
Optical micrograph of the horizontal lines of the LCD display of a Chinese telephone. The upper left photograph has some deception of our vision due to the “wrong” colors: the white thin strip is the contact.
One wire powers two lines of pixels at once, and the decoupling between them is arranged using a completely unusual “electric bug” (lower right photo). Behind this entire electrical circuit there are filter tracks, painted in the appropriate colors: red (R), green (G) and blue (B).
At the opposite end of the matrix in relation to the place where the cable is attached, you can find a similar color breakdown, track numbers and the same switches (if someone could clarify in the comments how this works, it would be very cool!):
Rooms-rooms-rooms...
This is what a working LCD display looks like under a microscope:
That’s all, now we won’t see this beauty anymore, I crushed it in the literal sense of the word, and after suffering a little, I “split” one such crumb into two separate pieces of glass, which make up the main part of the display...
Now you can look at the individual filter tracks. I’ll talk about the dark “spots” on them a little later:
Optical micrograph of filters with mysterious spots...
And now a small methodological aspect regarding electron microscopy. The same color stripes, but under the beam of an electron microscope: the color has disappeared! As I said earlier (for example, in the very first article), it is completely “black and white” for an electron beam whether it interacts with a colored substance or not.
It seems to be the same stripes, but without color...
Let's take a look at the other side. Transistors are located on it:
In an optical microscope - in color...
And an electron microscope - black and white image!
This is seen a little worse in an optical microscope, but the SEM allows you to see the fringing of each subpixel - this is quite important for the following conclusion.
So, what are these strange dark areas?! I thought for a long time, racked my brains, read many sources (perhaps the most accessible was Wiki) and, by the way, for this reason I delayed the release of the article on Thursday, February 23. And this is the conclusion I came to (perhaps I’m wrong - correct me!).
VA or MVA technology is one of the simplest, and I don’t think the Chinese have come up with anything new: every subpixel must be black. That is, light does not pass through it (an example of a working and non-working display is given), taking into account the fact that in the “normal” state (without external influence) the liquid crystal is misoriented and does not give the “necessary” polarization, it is logical to assume that each a separate subpixel has its own LCD film.
Thus, the entire panel is assembled from single micro-LCD displays. The note about the edging of each individual subpixel fits in organically here. For me, this became a kind of unexpected discovery right as I was preparing the article!
I regretted breaking the display of the Korean phone: after all, we need to show something to the children and those who come to our faculty for an excursion. I don't think there was anything else interesting to see.
Further, for the sake of self-indulgence, I will give an example of the “organization” of pixels from two leading communicator manufacturers: HTC and Apple. The iPhone 3 was donated for a painless operation by a kind person, and the HTC Desire HD is actually mine:
Photomicrographs of the HTC Desire HD display
A small note about the HTC display: I didn’t look specifically, but could this stripe in the middle of the top two microphotos be part of that same capacitive sensor?!
Microphotographs of the iPhone 3 display
If my memory serves me correctly, then HTC has a superLCD display, while the iPhone 3 has a regular LCD. The so-called Retina Display, that is, an LCD in which both contacts for switching the liquid crystal lie in the same plane, In-Plane Switching - IPS, is already installed in the iPhone 4.
I hope that an article will be published soon on the topic of comparing different display technologies with the support of 3DNews. For now, I just want to note the fact that the HTC display is truly unusual: the contacts on individual subpixels are placed in a non-standard way - somehow on top, unlike the iPhone 3.
And finally, in this section, I’ll add that the dimensions of one subpixel for a Chinese phone are 50 by 200 micrometers, HTC is 25 by 100 micrometers, and the iPhone is 15-20 by 70 micrometers.
E-Ink from a famous Ukrainian manufacturer
Let's start, perhaps, with banal things - “pixels”, or rather the cells that are responsible for forming the image:
Optical micrograph of the active matrix of an E-Ink display
The size of such a cell is about 125 micrometers. Since we are looking at the matrix through the glass on which it is applied, I ask you to pay attention to the yellow layer in the “background” - this is gold plating, which we will subsequently have to get rid of.
Forward to the embrasure!
Comparison of horizontal (left) and vertical (right) “inputs”
Among other things, many interesting things were discovered on the glass substrate. For example, position marks and contacts, which, apparently, are intended for testing the display in production:
Optical micrographs of marks and test pads
Of course, this does not happen often and is usually an accident, but displays sometimes break. For example, this barely noticeable crack less than a human hair thick can forever deprive you of the joy of reading your favorite book about foggy Albion in the stuffy Moscow metro:
If the displays break, it means someone needs it... Me, for example!
By the way, here it is, the gold that I mentioned - a smooth platform "from the bottom" of the cell for high-quality contact with ink (about them below). We remove the gold mechanically and here is the result:
You"ve got a lot of guts. Let"s see what they look like! (With)
Under a thin golden film, the control components of the active matrix are hidden, if you can call it that.
But the most interesting thing, of course, is the “ink” itself:
SEM micrograph of ink on the surface of the active matrix.
Of course, it is difficult to find at least one destroyed microcapsule to look inside and see “white” and “black” pigment particles:
SEM micrograph of the surface of electronic “ink”
Optical micrograph of "ink"
Or is there still something inside?!
Either a destroyed sphere, or torn out of the supporting polymer
The size of individual balls, that is, some analogue of a subpixel in E-Ink, can be only 20-30 microns, which is significantly lower than the geometric dimensions of subpixels in LCD displays. Provided that such a capsule can operate at half its size, the image obtained on good, high-quality E-Ink displays is much more pleasant than on an LCD.
And for dessert - a video about how E-Ink displays work under a microscope.
LCDs (Liquid Crystal Displays) are made of a substance that is in a liquid state, but at the same time has some properties inherent in crystalline bodies. Liquid crystals were discovered a long time ago, but they were originally used for other purposes. Liquid crystal molecules under the influence of electricity can change their orientation and, as a result, change the properties of the light beam passing through them. Based on this discovery, and as a result of further research, it became possible to find a relationship between the increase in electrical voltage and the change in the orientation of crystal molecules to ensure imaging. Liquid crystals were first used in displays for calculators and in quartz watches, and then they began to be used in monitors for laptop computers. Today, as a result of progress in this area, LCD monitors for desktop computers.
The screen of an LCD monitor is an array of small segments (called pixels) that can be manipulated to display information. The LCD monitor has several layers, where the key role is played by two panels made of a sodium-free and very pure glass material called substrate or substrate, which actually contain a thin layer of liquid crystals among themselves. The panels have grooves that guide the crystals into specific orientations. The grooves are positioned so that they are parallel on each panel but perpendicular between two panels. Longitudinal grooves are obtained by placing thin films of transparent plastic on the glass surface, which is then specially processed. In contact with the grooves, the molecules in liquid crystals are oriented identically in all cells. Molecules of one of the varieties of liquid crystals (nematics), in the absence of voltage, rotate the vector of the electric (and magnetic) field in such a light wave by a certain angle in a plane perpendicular to the axis of beam propagation. The two panels are located very close to each other. The liquid crystal panel is illuminated by a light source (depending on where it is located, liquid crystal panels work by reflecting or transmitting light). The plane of polarization of the light beam rotates 90° when passing through one panel.
When electric field, the liquid crystal molecules are partially aligned along the field and the angle of rotation of the plane of polarization of light becomes different from 90°.
To display a color image, the monitor needs to be backlit so that the light is generated at the back of the LCD display. This is necessary so that the image can be viewed in good quality even if the surrounding environment is not bright. Color is produced by using three filters that separate three main components from the emission of a white light source. By combining the three primary colors for each point or pixel on the screen, it is possible to reproduce any color.
The first LCD displays were very small, around 8 inches, while today they have reached 15" sizes for use in laptops, and 19" or larger LCD monitors are being produced for desktop computers. An increase in size is followed by an increase in resolution, which results in the emergence of new problems that were solved with the help of existing ones. special technologies, all this is described below. One of the first challenges was the need for a standard to define display quality at high resolutions. The first step towards the goal was to increase the angle of rotation of the plane of polarization of light in crystals from 90° to 270°.
In the future, we should expect an increase in the penetration of LCD monitors into the market, due to the fact that with the development of technology, the final price of devices decreases, which makes it possible for more users to buy new products.
Let's talk briefly about permission LCD monitors. This resolution is one and it is also called native, it corresponds to the maximum physical resolution of CRT monitors. It is in the native resolution that the LCD monitor reproduces the image best. This resolution is determined by the pixel size, which is fixed on the LCD monitor. For example, if the LCD monitor has a native resolution of 1024x768, then this means that there are 1024 electrodes on each of the 768 lines, read pixels. At the same time, it is possible to use a lower resolution than native. There are two ways to do this. The first one is called "Centering"(centering), the essence of the method is that only the number of pixels that is necessary to form an image with a lower resolution is used to display an image. As a result, the image does not appear in the entire screen, but only in the middle. All unused pixels remain black, i.e. A wide black frame appears around the image. The second method is called "Expansion"(stretching). Its essence is that when reproducing an image with a resolution lower than native, all pixels are used, i.e. The image takes up the entire screen. However, due to the fact that the image is stretched across the entire screen, slight distortion occurs and sharpness deteriorates. Therefore, when choosing an LCD monitor, it is important to clearly know what resolution you need.
Special mention should be made of brightness LCD monitors, since there are no standards yet for determining whether an LCD monitor is bright enough. At the same time, in the center the brightness of an LCD monitor can be 25% higher than at the edges of the screen. The only way to determine if the brightness of a particular LCD monitor is right for you is to compare its brightness with other LCD monitors.
And the last parameter that needs to be mentioned is contrast. The contrast of an LCD monitor is determined by the brightness ratio between the brightest white and the darkest black. A good contrast ratio is considered to be 120:1, which ensures the reproduction of vibrant, rich colors. A contrast ratio of 300:1 or higher is used when accurate representation of black and white halftones is required. But, as with brightness, there are no standards yet, so the main determining factor is your eyes.
It is worth noting such a feature of some LCD monitors as the ability to rotate the screen itself by 90°, with simultaneous automatic image rotation. As a result, for example, if you are doing layout, now an A4 sheet can fit completely on the screen without having to use vertical scrolling to see all the text on the page. True, among CRT monitors there are also models with this capability, but they are extremely rare. In the case of LCD monitors, this feature is becoming almost standard.
The advantages of LCD monitors include the fact that they are truly flat in the literal sense of the word, and the image created on their screens is distinguished by clarity and color saturation. No distortion on the screen and a host of other problems inherent in traditional CRT monitors. Let us add that the power consumption and dissipation of LCD monitors is significantly lower than that of CRT monitors.
The main problem with the development of LCD technology for the desktop sector seems to be the size of the monitor, which affects its cost. As displays grow in size, production capabilities decrease. Currently, the maximum diagonal of an LCD monitor suitable for mass production reaches 20", and recently some developers have introduced 43" models and even 64" models of TFT-LCD monitors ready for commercial production.
But it seems that the outcome of the battle between CRT and LCD monitors for a place in the market is already a foregone conclusion. And not in favor of CRT monitors. The future, apparently, still belongs to LCD monitors with an active matrix. The outcome of the battle became clear after IBM announced the release of a monitor with a matrix having 200 pixels per inch, that is, with twice the density of CRT monitors. According to experts, the quality of the image differs in the same way as when printing on matrix and laser printers. Therefore, the question of transition to the widespread use of LCD monitors is only in their price.
Liquid crystal displays, also known as LCD monitors or LCD monitors (Liquid Crystal Display), contain the same components as a CRT monitor, but they use liquid crystals rather than electron beams to form image pixels. These substances are so named because they are usually in a liquid state, but at the same time have some properties inherent in crystalline solids. In fact, these are liquids that have anisotropy (heterogeneity in different directions) of properties (in particular optical) associated with order in the orientation of molecules. Under the influence of electricity, liquid crystal molecules, which have an oblong shape, can change their orientation and, as a result, change the properties of the light beam passing through them. Liquid crystals were first used in black-and-white (more precisely, black-and-gray) displays for calculators and watches, and then they began to be used in monitors for laptop computers. Currently, LCD monitors are becoming increasingly common in desktop computers.
The screen of an LCD monitor is an array of small segments (called pixels, just like in CRT monitors) that are used to form an image. An LCD monitor has several layers (Fig. 1.3.38), where the key role is played by two flat panels made of sodium-free and very pure glass material called a substrate or substrate, which contain a thin layer of liquid crystals between them. Therefore, LCD monitors, as well as plasma monitors, are often called flat panel monitors.
The panels have electrode grooves arranged so that they are parallel on each panel but perpendicular between the two panels. Liquid crystals located in cells formed by panels can change their orientation with the help of electrodes, therefore such cells are called twisted nematic (the word nema in Greek means needle). The liquid crystal panel is illuminated by a light source (depending on where it is located, liquid crystal panels work by reflecting or transmitting light).
Changing the intensity of the light flux passing through the LCD monitor from black to white is achieved by using the phenomenon of light polarization (see 1.3.5.3.1).
Rice. 1.3.38. LCD monitor
Since the light source produces unpolarized radiation, the first, internal, polarizer filter transmits light with only one direction of polarization. The polarization direction of the second, external polarizing filter is rotated 90° relative to the polarization direction of the first filter.
When voltage is applied to the electrodes of any pixel (Fig. 1.3.39a), the spiral of liquid crystals straightens and does not change the polarization direction of the light passing along it. In this case, the light will be blocked by the external polarizing filter, and the pixel will appear black. When the tension is removed (Fig. 1.3.39b), the spiral is twisted so that the crystals at its ends fit into the grooves. Light passing through the internal polarizing filter, following along the spiral, changes its polarization by 90° and is therefore transmitted by the external filter, i.e. a light (white) pixel is formed. By changing the voltage, you can get gray shades.
To display a color image, light must be generated from the back of the LCD monitor. This is necessary so that the image can be viewed in good quality even if the surrounding environment is not bright. Color is produced by using three filters (red, green and blue) that separate three main components from the emission of a white light source.
To display a color image, you can place several filters in the path of the rays, but this leads to attenuation of the transmitted radiation. The following property of a liquid crystal cell is more often used: when the electric field strength changes, the angle of rotation of the plane of polarization of the radiation changes differently for light components with different wavelengths. This feature can be used to reflect (or absorb) radiation of a given wavelength of light, i.e. specified color.
Rice. 1.3.39. Passage of light through an LCD monitor: a) with voltage applied to the electrodes; b) in the absence of voltage
The operating technology of LCD monitors cannot provide a quick change of data on the screen. The image is formed line by line by sequentially applying control voltage to individual cells, making them transparent. Due to the rather large electrical capacitance of the cells, the voltage on them cannot change quickly enough, so the picture is updated slowly. Additionally, the image does not appear smoothly and appears shaky on the screen. The low rate of change in crystal transparency does not allow moving images to be displayed correctly. Monitors with this imaging technology are called passive matrix monitors. Despite the use of technologies to improve image contrast by increasing the angle of rotation of the plane of polarization of light in crystals from 90° to 270° (in Super Twisted Nematic technology), these monitors are currently practically not produced.
Active matrix monitors use separate control elements (transistors) for each screen cell, compensating for the effect of cell capacitance and significantly reducing the time it takes to change their transparency. Since the transistors are located on the back side of the panel and must transmit light, they are implemented in plastic films using TFT (Thin Film Transistor) technology. Sometimes monitors using TFT technology are called TFT monitors.
In order to repair an LCD monitor with your own hands, you must first understand what main electronic components and blocks this device consists of and what each element is responsible for electronic circuit. Beginning radio mechanics at the beginning of their practice believe that success in repairing any device lies in the availability of a circuit diagram of a specific device. But in fact, this is a misconception and circuit diagram not always needed.
So, let’s open the cover of the first LCD monitor that comes to hand and in practice we will understand its structure.
LCD monitor. Main functional blocks.
The LCD monitor consists of several functional blocks, namely:
LCD panel
The liquid crystal panel is a complete device. As a rule, the assembly of an LCD panel is carried out by a specific manufacturer, who, in addition to the liquid crystal matrix itself, integrates into the LCD panel fluorescent backlight lamps, frosted glass, polarizing color filters and an electronic decoder board that generates voltages from digital RGB signals to control the gates of thin-film transistors (TFTs). ).
Consider the composition of the LCD panel of a computer monitor ACER AL1716. The LCD panel is a complete functional device and, as a rule, there is no need to disassemble it during repairs, with the exception of replacing failed backlight lamps.
LCD panel marking: CHUNGHWA CLAA170EA
On the back of the LCD panel there is a fairly large printed circuit board, to which a multi-pin cable is connected from the main control board. The circuit board itself is hidden under a metal bar.
The printed circuit board has a multi-pin NT7168F-00010 chip. This microcircuit is connected to the TFT matrix and participates in the formation of the image on the display. From the NT7168F-00010 microcircuit there are many pins, which are formed into ten loops under the designation S1-S10. These cables are quite thin and appear to be glued to the printed circuit board on which the NT7168F chip is located.
Control board
The control board is also called the main board ( Main board). The main board contains two microprocessors. One of them is a control 8-bit microcontroller SM5964 with an 8052 core and 64 kB of programmable Flash memory.
The SM5964 microprocessor performs a fairly small number of functions. A button panel and monitor operation indicator are connected to it. This processor controls turning the monitor on/off and starting the backlight inverter. To save user settings, a memory chip is connected to the microcontroller via the I 2 C bus. Typically, these are eight-pin non-volatile memory chips of the series 24LCxx.
The second microprocessor on the control board is the so-called monitor scaler (LCD controller) TSU16AK. This microcircuit has many tasks. It performs most of the functions related to converting and processing the analog video signal and preparing it for submission to the LCD panel.
With regard to an LCD monitor, you need to understand that it is inherently a digital device in which all control of the pixels of the LCD display occurs digitally. The signal coming from the computer's video card is analog and for its correct display on the LCD matrix it is necessary to carry out many transformations. This is what a graphics controller is designed for, or otherwise a monitor scaler or an LCD controller.
The tasks of the LCD controller include such as recalculation (scaling) of images for different resolutions, formation of the OSD menu, processing of analog RGB signals and sync pulses. In the controller, analog RGB signals are converted to digital via 3-channel 8-bit ADCs that operate at 80 MHz.
The TSU16AK monitor scaler interacts with the SM5964 microcontroller via a digital bus. To operate the LCD panel, the graphics controller generates synchronization signals, clock frequency and matrix initialization signals.
The TSU16AK microcontroller is connected via a cable to the NT7168F-00010 chip on the LCD panel board.
If the graphics controller of the monitor malfunctions, as a rule, defects appear related to the correct display of the image on the display (stripes may appear on the screen, etc.). In some cases, the defect can be eliminated by soldering the scaler leads. This is especially true for monitors that operate around the clock in harsh conditions.
During prolonged operation, heating occurs, which has a bad effect on the quality of soldering. This may cause malfunctions. Defects related to the quality of soldering are not uncommon and are also found in other devices, for example, DVD players. The cause of the malfunction is degradation or poor-quality soldering of multi-pin planar microcircuits.
Power supply and backlight inverter
The most interesting thing to study is the monitor's power supply, since the purpose of the elements and circuitry are easier to understand. In addition, according to statistics, malfunctions of power supplies, especially switching ones, occupy a leading position among all others. Therefore, practical knowledge of the device, element base and circuitry of power supplies will certainly be useful in the practice of repairing radio equipment.
The power supply for the LCD monitor consists of two. The first one is AC/DC adapter or in other words, a network switching power supply (pulse unit). Second - DC/AC inverter . Essentially these are two converters. The AC/DC adapter is used to convert 220 V alternating voltage into a small DC voltage. Typically, voltages from 3.3 to 12 volts are generated at the output of a switching power supply.
The DC/AC inverter, on the contrary, converts direct voltage (DC) into alternating voltage (AC) with a value of about 600 - 700 V and a frequency of about 50 kHz. Alternating voltage is supplied to the electrodes of fluorescent lamps built into the LCD panel.
First, let's look at the AC/DC adapter. Most switching power supplies are built on the basis of specialized controller microcircuits (with the exception of cheap mobile chargers, for example).
So in the power supply of an LCD monitor Acer AL1716 microcircuit applied TOP245Y. Documentation (datasheet) for this chip is easy to find from open sources.
In the documentation for the TOP245Y chip you can find typical examples of circuit diagrams of power supplies. This can be used when repairing power supplies for LCD monitors, since the circuits largely correspond to the standard ones indicated in the description of the microcircuit.
Here are some examples of circuit diagrams of power supplies based on TOP242-249 series microcircuits.
The following circuit uses dual Schottky barrier diodes (MBR20100). Similar diode assemblies (SRF5-04) are used in the Acer AL1716 monitor unit we are considering.
Note that the above circuit diagrams are examples. Actual circuits of pulse blocks may differ slightly.
The TOP245Y microcircuit is a complete functional device, the housing of which contains a PWM controller and a powerful field-effect transistor that switches with a huge frequency from tens to hundreds of kilohertz. Hence the name - switching power supply.
The operating diagram of a switching power supply is as follows:
AC rectification mains voltage 220V.
This operation is performed by a diode bridge and a filter capacitor. After rectification, the voltage on the capacitor is slightly higher than the mains voltage. The photo shows a diode bridge, and next to it is a filtering electrolytic capacitor (82 µF 450 V) - a blue barrel.
Voltage conversion and reduction using a transformer.
Switching with a frequency of several tens - hundreds of kilohertz of direct voltage (>220 V) through the winding of a high-frequency pulse transformer. This operation is performed by the TOP245Y chip. The pulse transformer performs the same role as the transformer in conventional network adapters, with one exception. It works for more high frequencies, many times greater than 50 hertz.
Therefore, the manufacture of its windings requires a smaller number of turns, and, consequently, less copper. But a core of ferrite is required, and not of transformer steel as in 50 hertz transformers. Those who do not know what a transformer is and why it is used, first read the article about the transformer.
The result is a very compact transformer. It is also worth noting that switching power supplies are very economical and have high efficiency.
Rectification of alternating voltage reduced by a transformer.
This function is performed by powerful rectifier diodes. In this case, diode assemblies labeled SRF5-04 are used.
To rectify high-frequency currents, Schottky diodes and conventional power diodes with p-n junctions are used. Conventional low-frequency diodes for rectifying high-frequency currents are less preferable, but are used for rectifying high voltages (20 - 50 volts). This must be taken into account when replacing defective diodes.
Schottky diodes have some features that you need to know. Firstly, these diodes have a low transition capacitance and are able to quickly switch - go from open to closed state. This property is used to operate at high frequencies. Schottky diodes have a low voltage drop of about 0.2-0.4 volts, versus 0.6 - 0.7 volts for conventional diodes. This property increases their efficiency.
Schottky barrier diodes also have undesirable properties that hinder their wider use in electronics. They are very sensitive to excess reverse voltage. If the reverse voltage is exceeded, the Schottky diode irreversibly fails.
A conventional diode goes into reversible breakdown mode and can recover after exceeding the permissible reverse voltage value. It is this circumstance that is the Achilles heel, which causes the burnout of Schottky diodes in the rectifier circuits of all kinds of switching power supplies. This should be taken into account when carrying out diagnostics and repairs.
To eliminate voltage surges that are dangerous for Schottky diodes and are formed in the transformer windings at the pulse fronts, so-called damping circuits are used. In the diagram it is designated as R15C14 (see Fig. 1).
When analyzing the circuitry of the Acer AL1716 LCD monitor power supply, damping circuits were also found on the printed circuit board, consisting of a 10 Ohm SMD resistor (R802, R806) and a capacitor (C802, C811). They protect Schottky diodes (D803, D805).
It is also worth noting that Schottky diodes are used in low-voltage circuits with a reverse voltage limited to a few tens of volts. Therefore, if a voltage of several tens of volts (20-50) is required, then diodes based on p-n junction. This can be seen if you look at the datasheet for the TOP245 chip, which shows several typical power supply circuits with different output voltages (3.3 V; 5 V; 12 V; 19 V; 48 V).
Schottky diodes are sensitive to overheating. In this regard, they are usually installed on an aluminum radiator to dissipate heat.
You can distinguish a diode based on a pn junction from a diode based on a Schottky barrier by the conventional graphic symbol in the diagram.
Symbol for a diode with a Schottky barrier.
After the rectifier diodes, electrolytic capacitors are installed to smooth out voltage ripples. Next, using the resulting voltages 12 V; 5 V; 3.3 V powers all LCD monitor units.
DC/AC inverter
In terms of its purpose, the inverter is similar to electronic ballasts, which are widely used in lighting technology to power household fluorescent lamps. But, there are significant differences between the electronic ballast and the LCD monitor inverter.
An LCD monitor inverter is usually built on a specialized chip, which expands the range of functions and increases reliability. For example, the backlight inverter of the Acer AL1716 LCD monitor is built on the basis of a PWM controller OZ9910G. The controller chip is mounted on a printed circuit board using planar mounting.
The inverter converts direct voltage, the value of which is 12 volts (depending on the circuit design), into alternating voltage of 600-700 volts and a frequency of 50 kHz.
The inverter controller is capable of changing the brightness of fluorescent lamps. Signals for changing the brightness of the lamps come from the LCD controller. Field-effect transistors or their assemblies are connected to the controller microcircuit. In this case, two assemblies of complementary field-effect transistors are connected to the OZ9910G controller AP4501SD(Only 4501S is indicated on the chip body).
There are also two high-frequency transformers installed on the power supply board, which serve to increase the alternating voltage and supply it to the electrodes of the fluorescent lamps. In addition to the main elements, the board contains all kinds of radio elements that serve to protect against short circuits and lamp malfunctions.
Information on repairing LCD monitors can be found in specialized repair magazines. For example, in the magazine “Repair and Service of Electronic Equipment” No. 1, 2005 (pp. 35 – 40), the device and circuit diagram of the LCD monitor “Rover Scan Optima 153” are discussed in detail.
Among monitor malfunctions, there are quite often those that can be easily fixed with your own hands in a few minutes. For example, the already mentioned Acer AL1716 LCD monitor came to the repair table due to a broken contact of the socket outlet for connecting the power cord. As a result, the monitor turned off spontaneously.
After disassembling the LCD monitor, it was discovered that a powerful spark was formed at the site of the poor contact, traces of which were easy to detect on the printed circuit board of the power supply. A powerful spark was also formed because at the moment of contact the electrolytic capacitor in the rectifier filter is charged. The cause of the malfunction is solder degradation.
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| Solder degradation causing monitor failure |
It is also worth noting that sometimes the cause of a malfunction can be a breakdown of the diodes of the rectifier diode bridge.
Creating an LCD Display
The first working liquid crystal display was created by Fergason in 1970. Previously, LCD devices consumed too much power, had a limited service life, and had poor image contrast. The new LCD display was introduced to the public in 1971 and then it received warm approval. Liquid crystals are organic substances that can change the amount of light transmitted under voltage. A liquid crystal monitor consists of two glass or plastic plates with a suspension between them. The crystals in this suspension are arranged parallel to each other, thereby allowing light to penetrate the panel. When an electric current is applied, the arrangement of the crystals changes, and they begin to interfere with the passage of light. LCD technology has become widespread in computers and projection equipment. The first liquid crystals were distinguished by their instability and were of little use for mass production. The real development of LCD technology began with the invention by English scientists of a stable liquid crystal - biphenyl (Biphenyl). First generation liquid crystal displays can be seen in calculators, electronic games and watches. Modern LCD monitors are also called flat panels, dual scan active matrix, thin film transistors. The idea of LCD monitors has been in the air for more than 30 years, but the research has not led to an acceptable result, so LCD monitors have not gained a reputation for good image quality. Now they are becoming popular - everyone likes their elegant appearance, thin body, compactness, economy (15-30 watts), in addition, it is believed that only wealthy and serious people can afford such a luxury.
Characteristics of LCD monitors
Types of LCD monitors
Monitor Composite Layers
There are two types of LCD monitors: DSTN (dual-scan twisted nematic) and TFT (thin film transistor), also called passive and active matrices, respectively. Such monitors consist of the following layers: a polarizing filter, a glass layer, an electrode, a control layer, liquid crystals, another control layer, an electrode, a glass layer and a polarizing filter. The first computers used eight-inch (diagonally) passive black-and-white matrices. With the transition to active matrix technology, the screen size has increased. Almost all modern LCD monitors use thin-film transistor panels, which provide bright, clear images of a much larger size.
Monitor resolution
The size of the monitor determines the workspace it occupies and, importantly, its price. Despite the established classification of LCD monitors depending on the diagonal size of the screen (15-, 17-, 19-inch), a more correct classification is based on operating resolution. The fact is that, unlike CRT-based monitors, the resolution of which can be changed quite flexibly, LCD displays have a fixed set of physical pixels. That is why they are designed to work with only one resolution, called working. Indirectly, this resolution also determines the diagonal size of the matrix, however, monitors with the same operating resolution may have different matrix sizes. For example, 15- to 16-inch monitors generally have a working resolution of 1024 x 768, which means that a given monitor actually physically contains 1024 horizontal pixels and 768 vertical pixels. The operating resolution of the monitor determines the size of the icons and fonts that will be displayed on the screen. For example, a 15-inch monitor can have a working resolution of both 1024 x 768 and 1400 x 1050 pixels. In the latter case, the physical dimensions of the pixels themselves will be smaller, and since the same number of pixels is used when forming a standard icon in both cases, then at a resolution of 1400×1050 pixels the icon will be smaller in its physical dimensions. For some users, too small icon sizes with a high monitor resolution may be unacceptable, so when purchasing a monitor you should immediately pay attention to the working resolution. Of course, the monitor is capable of displaying images in a different resolution than the working one. This mode of monitor operation is called interpolation. In the case of interpolation, the image quality leaves much to be desired. The interpolation mode significantly affects the quality of display of screen fonts.
Monitor interface
LCD monitors by their nature are digital devices, so the “native” interface for them is considered to be the DVI digital interface, which can have two types of convectors: DVI-I, which combines digital and analog signals, and DVI-D, which transmits only a digital signal. It is believed that the DVI interface is more preferable for connecting an LCD monitor to a computer, although it is also possible to connect via a standard D-Sub connector. The DVI interface is also supported by the fact that in the case of an analog interface, a double conversion of the video signal occurs: first, the digital signal is converted to analog in the video card (DAC conversion), which is then transformed into a digital electronic unit of the LCD monitor itself (ADC conversion), As a result, the risk of various signal distortions increases. Many modern LCD monitors have both D-Sub and DVI connectors, which allows you to connect two system units to the monitor at the same time. You can also find models that have two digital connectors. In inexpensive office models, there is basically only a standard D-Sub connector.
LCD matrix type
The basic component of the LCD matrix is liquid crystals. There are three main types of liquid crystals: smectic, nematic and cholesteric. According to their electrical properties, all liquid crystals are divided into two main groups: the first includes liquid crystals with positive dielectric anisotropy, the second - with negative dielectric anisotropy. The difference lies in how these molecules react to an external electric field. Molecules with positive dielectric anisotropy are oriented along the field lines, and molecules with negative dielectric anisotropy are oriented perpendicular to the field lines. Nematic liquid crystals have positive dielectric anisotropy, while smectic liquid crystals, on the contrary, have negative dielectric anisotropy. Another remarkable property of LC molecules is their optical anisotropy. In particular, if the orientation of the molecules coincides with the direction of propagation of plane-polarized light, then the molecules do not have any effect on the plane of polarization of the light. If the orientation of the molecules is perpendicular to the direction of propagation of light, then the plane of polarization is rotated so as to be parallel to the direction of orientation of the molecules. The dielectric and optical anisotropy of LC molecules makes it possible to use them as a kind of light modulators, allowing the formation of the desired image on the screen. The principle of operation of such a modulator is quite simple and is based on changing the plane of polarization of light passing through the LCD cell. The LCD cell is located between two polarizers, the polarization axes of which are mutually perpendicular. The first polarizer cuts out plane-polarized radiation from the light passing from the backlight lamp. If there were no LC cell, then such plane-polarized light would be completely absorbed by the second polarizer. An LCD cell placed in the path of transmitted plane-polarized light can rotate the plane of polarization of the transmitted light. In this case, part of the light passes through the second polarizer, that is, the cell becomes transparent (fully or partially). Depending on how the rotation of the plane of polarization in the LC cell is controlled, several types of LC matrices are distinguished. So, an LCD cell placed between two crossed polarizers allows the transmitted radiation to be modulated, creating gradations of black and white color. To obtain a color image, it is necessary to use three color filters: red (R), green (G) and blue (B), which, when installed in the path of white light, will allow you to obtain three basic colors in the required proportions. So, each pixel of an LCD monitor consists of three separate sub-pixels: red, green and blue, which are controlled LCD cells and differ only in the filters used, installed between the top glass plate and the output polarizing filter
Classification of TFT-LCD displays
The main technologies in the manufacture of LCD displays: TN+film, IPS (SFT) and MVA. These technologies differ in the geometry of surfaces, polymer, control plate and front electrode. The purity and type of polymer with liquid crystal properties used in specific developments are of great importance.
TN matrix
TN cell structure
A TN-type (Twisted Nematic) liquid crystal matrix is a multilayer structure consisting of two polarizing filters, two transparent electrodes and two glass plates, between which the actual nematic liquid crystal substance with positive dielectric anisotropy is located. Special grooves are applied to the surface of the glass plates, which makes it possible to create an initially identical orientation of all liquid crystal molecules along the plate. The grooves on both plates are mutually perpendicular, so the layer of liquid crystal molecules between the plates changes its orientation by 90°. It turns out that LC molecules form a spirally twisted structure (Fig. 3), which is why such matrices are called Twisted Nematic. Glass plates with grooves are located between two polarizing filters, and the polarization axis in each filter coincides with the direction of the grooves on the plate. In its normal state, an LCD cell is open because the liquid crystals rotate the plane of polarization of light passing through them. Therefore, plane-polarized radiation generated after passing through the first polarizer will also pass through the second polarizer, since its polarization axis will be parallel to the polarization direction of the incident radiation. Under the influence of the electric field created by transparent electrodes, the molecules of the liquid crystal layer change their spatial orientation, lining up along the direction of the field lines. In this case, the liquid crystal layer loses the ability to rotate the plane of polarization of the incident light, and the system becomes optically opaque, since all the light is absorbed by the output polarizing filter. Depending on the applied voltage between the control electrodes, it is possible to change the orientation of the molecules along the field not completely, but only partially, that is, to regulate the degree of twist of the LC molecules. This, in turn, allows you to change the intensity of the light passing through the LCD cell. Thus, by installing a backlight lamp behind the LCD matrix and changing the voltage between the electrodes, you can vary the degree of transparency of one LCD cell. TN matrices are the most common and cheapest. They have certain disadvantages: not very large viewing angles, low contrast and the inability to obtain perfect black color. The fact is that even when the maximum voltage is applied to the cell, it is impossible to completely spin the LC molecules and orient them along the field lines. Therefore, such matrices remain slightly transparent even when the pixel is completely turned off. The second drawback is related to small viewing angles. To partially eliminate it, a special scattering film is applied to the surface of the monitor, which allows you to increase the viewing angle. This technology is called TN+Film, which indicates the presence of this film. Finding out exactly what type of matrix is used in the monitor is not so easy. However, if there is a “broken” pixel on the monitor resulting from the failure of the transistor that controls the LCD cell, then in TN matrices it will always light up brightly (red, green or blue), since for a TN matrix an open pixel corresponds to a lack of voltage on the cell. You can recognize a TN matrix by looking at the black color at maximum brightness - if it is more gray than black, then it is probably a TN matrix.
IPS matrices
IPS cell structure
Monitors with an IPS matrix are also called Super TFT monitors. A distinctive feature of IPS matrices is that the control electrodes are located in the same plane on the bottom side of the LCD cell. In the absence of voltage between the electrodes, the LC molecules are located parallel to each other, the electrodes, and the polarization direction of the lower polarizing filter. In this state, they do not affect the polarization angle of the transmitted light, and the light is completely absorbed by the output polarizing filter, since the polarization directions of the filters are perpendicular to each other. When voltage is applied to the control electrodes, the generated electric field rotates the LC molecules by 90° so that they are oriented along the field lines. If light is passed through such a cell, then due to the rotation of the polarization plane, the upper polarizing filter will transmit light without interference, that is, the cell will be in the open state (Fig. 4). By varying the voltage between the electrodes, it is possible to force the LC molecules to rotate at any angle, thereby changing the transparency of the cell. In all other respects, IPS cells are similar to TN matrices: a color image is also formed through the use of three color filters. IPS matrices have both advantages and disadvantages compared to TN matrices. The advantage is the fact that in this case the color is perfectly black, and not gray, as in TN matrices. Another undeniable advantage of this technology is the large viewing angles. The disadvantages of IPS matrices include a longer pixel response time than for TN matrices. However, we will return to the issue of pixel reaction time later. In conclusion, we note that there are various modifications of IPS matrices (Super IPS, Dual Domain IPS) that can improve their characteristics.
MVA matrices
Domain structure of an MVA cell
MVA is an evolution of VA technology, i.e. vertical molecular alignment technology. Unlike TN and IPS matrices, in this case, liquid crystals with negative dielectric anisotropy are used, which are oriented perpendicular to the direction of electric field lines. In the absence of voltage between the plates of the LC cell, all liquid crystal molecules are oriented vertically and have no effect on the plane of polarization of the transmitted light. Since the light passes through two crossed polarizers, it is completely absorbed by the second polarizer and the cell is in a closed state, while, unlike the TN matrix, it is possible to obtain a perfectly black color. When a voltage is applied to the electrodes located above and below, the molecules rotate 90°, orienting themselves perpendicular to the electric field lines. When plane-polarized light passes through such a structure, the plane of polarization rotates by 90° and the light passes freely through the output polarizer, that is, the LC cell is in the open state. The advantages of systems with vertical ordering of molecules are the ability to obtain ideal black color (which, in turn, affects the ability to obtain high-contrast images) and short pixel response time. In order to increase viewing angles, systems with vertical ordering of molecules use a multidomain structure, which leads to the creation of MVA-type matrices. The idea behind this technology is that each subpixel is divided into several zones (domains) using special protrusions, which slightly change the orientation of the molecules, forcing them to align with the surface of the protrusion. This leads to the fact that each such domain shines in its own direction (within a certain solid angle), and the totality of all directions expands the viewing angle of the monitor. The advantages of MVA matrices include high contrast (due to the ability to obtain perfectly black color) and large viewing angles (up to 170°). Currently, there are several varieties of MVA technology, for example PVA (Patterned Vertical Alignment) from Samsung, MVA-Premium, etc., which further enhance the characteristics of MVA matrices.
Brightness
Today, in LCD monitors, the maximum brightness stated in the technical documentation ranges from 250 to 500 cd/m2. And if the brightness of the monitor is high enough, then this is necessarily indicated in advertising brochures and presented as one of the main advantages of the monitor. However, this is precisely where one of the pitfalls lies. The paradox is that it is impossible to rely on the numbers indicated in the technical documentation. This applies not only to brightness, but also to contrast, viewing angles and pixel response time. Not only may they not correspond to actual observed values at all, but sometimes it is even difficult to understand what these numbers mean. First of all, there are different measurement techniques described in different standards; Accordingly, measurements carried out using different methods give different results, and you are unlikely to be able to find out exactly what method and how the measurements were carried out. Here's one simple example. The measured brightness depends on the color temperature, but when they say that the monitor brightness is 300 cd/m2, the question arises: at what color temperature is this maximum brightness achieved? Moreover, manufacturers indicate brightness not for the monitor, but for the LCD matrix, which is not at all the same thing. To measure brightness, special reference generator signals with a precisely specified color temperature are used, so the characteristics of the monitor itself as a final product may differ significantly from those stated in the technical documentation. But for the user, the characteristics of the monitor itself, and not the matrix, are of paramount importance. Brightness is a really important characteristic for an LCD monitor. For example, if the brightness is insufficient, you are unlikely to be able to play various games or watch DVD movies. In addition, it will be uncomfortable to work at the monitor in daylight conditions (external illumination). However, it would be premature to conclude on this basis that a monitor with a declared brightness of 450 cd/m2 is somehow better than a monitor with a brightness of 350 cd/m2. Firstly, as already noted, declared and real brightness are not the same thing, and secondly, it is quite enough for the LCD monitor to have a brightness of 200-250 cd/m2 (not declared, but actually observed). In addition, the way in which the brightness of the monitor is adjusted is also important. From a physics point of view, brightness adjustment can be done by changing the brightness of the backlight. This is achieved either by adjusting the discharge current in the lamp (in monitors, Cold Cathode Fluorescent Lamps, CCFLs are used as backlights), or by so-called pulse-width modulation of the lamp power supply. With pulse-width modulation, voltage is supplied to the backlight lamp in pulses of a certain duration. As a result, the backlight does not glow constantly, but only at periodically repeating time intervals, but due to the inertia of vision, it seems that the lamp is constantly on (the pulse repetition rate is more than 200 Hz). Obviously, by changing the width of the supplied voltage pulses, you can adjust the average brightness of the backlight. In addition to adjusting the brightness of the monitor using the backlight, sometimes this adjustment is carried out by the matrix itself. In fact, a DC component is added to the control voltage at the electrodes of the LCD cell. This allows the LCD cell to open completely, but does not allow it to close completely. In this case, as the brightness increases, the black color ceases to be black (the matrix becomes partially transparent even when the LCD cell is closed).
Contrast
An equally important characteristic of an LCD monitor is its contrast, which is defined as the ratio of the brightness of the white background to the brightness of the black background. Theoretically, the contrast of the monitor should not depend on the brightness level set on the monitor, that is, at any brightness level, the measured contrast should have the same value. Indeed, the brightness of the white background is proportional to the brightness of the backlight. Ideally, the ratio of the light transmittance of an LCD cell in the open and closed state is a characteristic of the LCD cell itself, but in practice this ratio may depend on both the set color temperature and the set brightness level of the monitor. Recently, the image contrast on digital monitors has increased significantly, and now this figure often reaches 500:1. But here everything is not so simple. The fact is that contrast can be specified not for the monitor, but for the matrix. However, as experience shows, if the passport indicates a contrast of more than 350:1, then this is quite enough for normal operation.
Viewing angle
The maximum viewing angle (both vertical and horizontal) is defined as the angle from which the image contrast in the center is at least 10:1. Some matrix manufacturers, when determining viewing angles, use a contrast ratio of 5:1 rather than 10:1, which also introduces some confusion specifications. The formal definition of viewing angles is quite vague and, most importantly, has no direct bearing on the correct color rendering when viewing an image at an angle. In fact, for users, a much more important circumstance is the fact that when viewing an image at an angle to the surface of the monitor, it is not a drop in contrast that occurs, but color distortions. For example, red turns into yellow, and green turns into blue. Moreover, such distortions different models manifest themselves in different ways: for some they become noticeable even at a small angle, much smaller than the viewing angle. Therefore, it is fundamentally wrong to compare monitors based on viewing angles. It is possible to compare, but such a comparison has no practical significance.
Pixel response time
Typical pixel turn-on timing diagram for a TN+Film matrix
Typical pixel turn-off timing diagram for a TN+Film matrix
Reaction time, or pixel response time, is usually indicated in the technical documentation for the monitor and is considered one of the most important characteristics of the monitor (which is not entirely true). In LCD monitors, the pixel response time, which depends on the type of matrix, is measured in tens of milliseconds (in new TN+Film matrices, the pixel response time is 12 ms), and this leads to blurring of the changing picture and can be noticeable to the eye. A distinction is made between pixel on and off time. The pixel on time refers to the period of time required to open the LCD cell, and the off time refers to the period of time required to close it. When we talk about the reaction time of a pixel, we mean the total time the pixel turns on and off. The time a pixel turns on and the time it turns off can vary significantly. When they talk about the pixel response time indicated in the technical documentation for the monitor, they mean the response time of the matrix, not the monitor. In addition, the pixel response time indicated in the technical documentation by various manufacturers matrices are interpreted differently. For example, one of the options for interpreting the time to turn a pixel on (off) is that this is the time the pixel brightness changes from 10 to 90% (from 90 to 10%). Until now, when talking about measuring pixel response time, it is assumed that we are talking about switching between black and white colors. If there are no issues with black (the pixel is simply closed), then the choice of white is not obvious. How will a pixel's response time change if measured as it switches between different halftones? This question is of great practical importance. The fact is that switching from a black background to a white one, or vice versa, is relatively rare in real applications. In most applications, transitions between halftones are usually implemented. And if the switching time between black and white colors turns out to be less than the switching time between grayscale, then the pixel response time will not have any practical significance and you cannot rely on this characteristic of the monitor. What conclusion can be drawn from the above? Everything is very simple: the pixel response time declared by the manufacturer does not allow us to clearly judge the dynamic characteristics of the monitor. It is more correct in this sense to speak not about the time a pixel switches between white and black colors, but about the average time a pixel switches between halftones.
Number of colors displayed
All monitors by their nature are RGB devices, that is, the color in them is obtained by mixing in various proportions the three basic colors: red, green and blue. Thus, each LCD pixel consists of three color subpixels. In addition to the completely closed or completely open state of the LCD cell, intermediate states are also possible when the LCD cell is partially open. This allows you to form a color shade and mix the color shades of the base colors in the desired proportions. In this case, the number of colors reproduced by the monitor theoretically depends on how many color shades can be formed in each color channel. Partial opening of the LCD cell is achieved by applying the required voltage level to the control electrodes. Therefore, the number of reproducible color shades in each color channel depends on how many different voltage levels can be applied to the LCD cell. To generate an arbitrary voltage level, you will need to use DAC circuits with a large bit capacity, which is extremely expensive. Therefore, modern LCD monitors most often use 18-bit DACs and less often - 24-bit ones. When using an 18-bit DAC, there are 6 bits per color channel. This allows you to generate 64 (26=64) different voltage levels and, accordingly, obtain 64 color shades in one color channel. In total, by mixing color shades of different channels, it is possible to create 262,144 color shades. When using a 24-bit matrix (24-bit DAC circuit), each channel has 8 bits, which makes it possible to generate 256 (28=256) color shades in each channel, and in total such a matrix reproduces 16,777,216 color shades. At the same time, for many 18-bit matrices the data sheet indicates that they reproduce 16.2 million color shades. What is the matter here and is this possible? It turns out that in 18-bit matrices, through all sorts of tricks, you can bring the number of color shades closer to what is reproduced by real 24-bit matrices. To extrapolate color tones in 18-bit matrices, two technologies (and combinations thereof) are used: dithering and FRC (Frame Rate Control). The essence of dithering technology is that the missing color shades are obtained by mixing the closest color shades of neighboring pixels. Let's consider a simple example. Let's assume that a pixel can only be in two states: open and closed, with the closed state of the pixel producing black, and the open state producing red. If instead of one pixel we consider a group of two pixels, then, in addition to black and red, we can also obtain an intermediate color, thereby extrapolating from a two-color mode to a three-color one. As a result, if initially such a monitor could generate six colors (two for each channel), then after such dithering it will already reproduce 27 colors. The dithering scheme has one significant drawback: an increase in color shades is achieved by reducing the resolution. In fact, this increases the pixel size, which can have a negative impact when drawing image details. The essence of FRC technology is to manipulate the brightness of individual subpixels by additionally turning them on/off. As in the previous example, a pixel is considered to be either black (off) or red (on). Each subpixel is commanded to turn on at a frame rate, that is, at a frame rate of 60 Hz, each subpixel is commanded to turn on 60 times per second. This allows the color red to be generated. If you force the pixel to turn on not 60 times per second, but only 50 (at every 12th clock cycle, turn the pixel off rather than turn it on), then the resulting brightness of the pixel will be 83% of the maximum, which will allow the formation of an intermediate color shade of red. Both color extrapolation methods discussed have their drawbacks. In the first case, there is a possible flickering of the screen and a slight increase in reaction time, and in the second, there is the possibility of loss of image details. It is quite difficult to distinguish an 18-bit matrix with color extrapolation from a true 24-bit matrix by eye. At the same time, the cost of a 24-bit matrix is much higher.
Operating principle of TFT-LCD displays
The general principle of image formation on the screen is well illustrated in Fig. 1. But how to control the brightness of individual subpixels? It is usually explained to beginners this way: behind each subpixel there is a liquid crystal shutter. Depending on the voltage applied to it, it transmits more or less light from the backlight. And everyone immediately imagines some kind of dampers on small hinges that rotate to the desired angle... something like this:
In reality, of course, everything is much more complicated. There are no material flaps on the hinges. In a real liquid crystal matrix, the luminous flux is controlled something like this:
The light from the backlight (we follow the picture from bottom to top) first passes through the lower polarizing filter (white shaded plate). Now this is no longer an ordinary stream of light, but a polarized one. Then the light passes through translucent control electrodes (yellow plates) and encounters a layer of liquid crystals on its way. By changing the control voltage, the polarization of the light flux can be changed by up to 90 degrees (in the picture on the left), or left unchanged (right there). Attention, the fun is about to begin! After the layer of liquid crystals, light filters are located and here each subpixel is colored in the desired color - red, green or blue. If we look at the screen with the top polarizing filter removed, we will see millions of glowing subpixels - and each one glows with maximum brightness, because our eyes cannot distinguish the polarization of light. In other words, without the top polarizer we will simply see a uniform white glow over the entire surface of the screen. But as soon as you put the upper polarizing filter in place, it will “reveal” all the changes that liquid crystals have made to the polarization of light. Some subpixels will remain brightly glowing, like the left one in the figure, whose polarization was changed by 90 degrees, and some will go out, because the upper polarizer is in antiphase to the lower one and does not transmit light with the default polarization. There are also subpixels with intermediate brightness - the polarization of the light flow passing through them was rotated not by 90, but by a smaller number of degrees, for example, by 30 or 55 degrees.
Advantages and disadvantages
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Symbols: (+) advantage, (~) acceptable, (-) disadvantage |
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| LCD monitors
| CRT monitors
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| Brightness | (+) from 170 to 250 cd/m2 | (~) from 80 to 120 cd/m2 |
| Contrast | (~) 200:1 to 400:1 | (+) from 350:1 to 700:1 |
| Viewing angle (by contrast) | (~) 110 to 170 degrees | (+) over 150 degrees |
| Viewing angle (by color) | (-) from 50 to 125 degrees | (~) over 120 degrees |
| Permission | (-) Single resolution with fixed pixel size. Optimally can only be used in this resolution; Depending on the supported expansion or compression functions, higher or lower resolutions can be used, but they are not optimal. | (+) Various resolutions are supported. With all supported resolutions, the monitor can be used optimally. The limitation is imposed only by the acceptability of the regeneration frequency. |
| Vertical frequency | (+) Optimal frequency 60 Hz, which is enough to avoid flickering | (~) Only at frequencies above 75 Hz there is no clearly noticeable flicker |
| Color registration errors | (+) no | (~) 0.0079 to 0.0118 inches (0.20 - 0.30 mm) |
| Focusing | (+) very good | (~) from satisfactory to very good> |
| Geometric/linear distortion | (+) no | (~) possible |
| Broken pixels | (-) up to 8 | (+) no |
| Input signal | (+) analog or digital | (~) analog only |
| Scaling at different resolutions | (-) is absent or interpolation methods that do not require large overheads are used | (+) very good |
| Color Accuracy | (~) True Color is supported and the required color temperature is simulated | (+) True Color is supported and there are a lot of color calibration devices on the market, which is a definite plus |
| Gamma correction (color adjustment to the characteristics of human vision) | (~) satisfactory | (+) photorealistic |
| Uniformity | (~) often the image is brighter at the edges | (~) often the image is brighter in the center |
| Color purity/color quality | (~) good | (+) high |
| flicker | (+) no | (~) not noticeable above 85 Hz |
| Inertia time | (-) from 20 to 30 ms. | (+) negligible |
| Image formation | (+) The image is formed by pixels, the number of which depends only on the specific resolution of the LCD panel. The pixel pitch depends only on the size of the pixels themselves, but not on the distance between them. Each pixel is individually shaped for superior focus, clarity and definition. The image is more complete and smooth | (~) Pixels are formed by a group of dots (triads) or stripes. The pitch of a point or line depends on the distance between points or lines of the same color. As a result, the sharpness and clarity of the image is highly dependent on the size of the dot pitch or line pitch and on the quality of the CRT |
| Energy consumption and emissions | (+) There are practically no dangerous electromagnetic radiations. Power consumption is approximately 70% lower than standard CRT monitors (25 to 40 W). | (-) Electromagnetic radiation is always present, but the level depends on whether the CRT meets any safety standard. Energy consumption in operating condition is 60 - 150 W. |
| Dimensions/weight | (+) flat design, light weight | (-) heavy design, takes up a lot of space |
| Monitor interface | (+) Digital interface, however, most LCD monitors have a built-in analog interface for connecting to the most common analog outputs of video adapters | (-) Analog interface |
Literature
- A.V.Petrochenkov “Hardware-computer and peripherals”, -106 page ill.
- V.E. Figurnov “IBM PC for the user”, -67 pages.
- “HARD "n" SOFT" (computer magazine for a wide range of users) No. 6 2003.
- N.I. Gurin “Working on a personal computer,” - 128 pages.
