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LED screen brightness of two control methods
Two brightness control methods of LED large screen are introduced.One is to change the current flowing through the LED. Generally, the LED tube allows a continuous working current of about 20 mah.Another method is to use the visual inertia of the human eye to realize the grayscale control with the method of pulse width modulation, that is, to periodically change the light pulse width (that is, the duty cycle), as long as the repeated lighting cycle is short enough (that is, the refresh frequency is high enough), the human eye will not feel the light pixel in the jitter.Because pulse width modulation (PWM) is more suitable for digital control, almost all LED screens use PWM to control the gray level in the era of microcomputer to provide LED display content.
LED control system is usually composed of master control box, scanning board and display control device.The master control box obtains the color brightness data of one screen pixel from the display card of the computer, and then redistributes it to several scanning boards. Each scanning board is responsible for controlling several rows (columns) on the LED screen, and the display control signal of the LED on each row (column) is transmitted in serial mode.There are two types of serial transmission display control signal way: one is a centralized control each pixel gray scanning board, scanning board will come from the control box of the brightness values of all pixels decomposition (pulse width modulation), then all LED to the opening of the signal pulse form (0) light is 1, dawn line by way of serial transmission to the corresponding LED on, whether to control its light.This method USES fewer devices, but the amount of data transmitted serially is large, because in a cycle of repeated lighting, each pixel needs 16 pulses at 16 levels of grayscale, and 256 pulses at 256 levels of grayscale. Due to the limitation of device working frequency, LED screen can only achieve 16 levels of grayscale in general.
Another approach is to scan the board serially to transmit not each LED's switching signal but an 8-bit binary brightness value.Each LED has its own pulse width modulator to control the lighting time.In this way, in a cycle of repeated lighting, only 4 pulses are needed for each pixel point at level 16 grayscale and 8 pulses at level 256 grayscale, greatly reducing the serial transmission frequency.With this method of decentralized control LED grayscale can be very convenient to achieve 256-level grayscale control.
The basic structure of LED display control system
LED display industry, as the dominant point in the future market, has attracted more and more attention, and LED display is increasingly widely used in every corner of daily life. LED display technology continues to improve, making the display effect more vivid and colorful, large clear display screen is becoming more and more difficult to describe with shock!
It is composed of signal source, control system and display screen. The signal source basically supports all input signals, including subtitler, video recorder, TV, camera, DVD player, PC and other signal input devices.
The control system completes the signal access, transformation, processing, transmission, imaging control and other operations; Display screen body is the control system through the signal source signal in the real display, text, pattern, video plus audio and other external equipment, to bring people a perfect display effect!
LED screen procurement of common errors
1. Life span of 100,000 hours
The technical data issued by the LED material manufacturer indicates that the life of LED illuminator is 10 thousand hours under ideal condition.The ideal state refers to the time of LED luminescence from luminescence to complete non-luminescence under the condition of constant pressure and constant current in the laboratory, which is 10 thousand hours (11 years).
The amount of water in a wooden bucket is determined by the lowest wooden board. LED display is currently used as a device of civilian grade with a service life of no more than 8 years.As the function of the display screen is to watch, when the display screen is bright only at night to see clearly, it is not qualified to have the use of value.
A car can run for 15 years and be scrapped if it sits idle for 3 years.The environment and method used have a great influence on the life of the product.
Ii. Abide by national standards
The general specification for LED displays was a ministerial standard in 1995.Up to now there are many companies claiming to meet the national standard, in the development of science and technology after eight years to look at the standard at that time, is not the standard.For example, out of control point, the national standard is three parts per million, with a 3.75 indoor dual base color display as an example.Generally do 640x480 standard resolution display for 7 square meters, per square meter for 43264 points, according to the national standard can have 90 points out of control.Who pays for such displays today?
The software is free
A common problem in the display industry is that Chinese companies produce rather than develop.At present, only a few enterprises have legitimate software.It is now illegal to use pirated copies.
Low price
Look at the performance-price ratio, not just the price.
Fifth, gray level
Gray scale is an important index of dual base color and full color display screen.The market is flooded with 1 -, 6 - and 64-level displays masquerading as 256 levels.The control cost is only 1/5 of that of the 256-level gray level control. The simplest way is to play a VCD on the surface of a competitive sports field and check whether it can be seen clearly on the LED display.
Buy the best
All purchasing power comes from the need to meet the needs and have a certain degree of advance.Blind pursuit will waste a lot of money to buy functions they do not need.
LED display to pay attention to the use of three things
I. precautions for display screen:
1 switch sequence:
When the screen is opened: turn on the machine first and then open the screen.
When turning off the screen: first turn off the screen, then shut down.
(turning off the computer screen first will result in high highlights on the screen body, burning the lamp tube and causing serious consequences.)
2. Switch screen time interval shall be more than 5 minutes.
3. The screen can only be switched on after the computer enters the engineering control software.
4. Avoid opening the screen under the condition of full white screen, because the impulse current of the system is the maximum at this time.
5. Avoid opening the screen when it is out of control, because the impulse current of the system is the maximum at this time.
A the computer does not enter the control software and other programs;
B. The computer is not powered on;
C the power supply of the control part is not turned on.
6.When the environment temperature is too high or the heat dissipation condition is not good, should pay attention not to open the screen for a long time.
7.When a line of very bright light appears in part of the display screen, please turn off the screen in time. Under this condition, it is not recommended to open the screen for a long time.
8.If the power switch on the display screen often trips, please check the screen or replace the power switch in time.
9.Check the fastness of the joint regularly.If there is a loose phenomenon, pay attention to timely adjustment, re-reinforcement or update the lifting parts.
10.According to the environment of the display screen and the control part, avoid insect bites, and place rodenticide when necessary.
Ii. Matters needing attention for the change of the control part
1.Computer, control part of the power line zero, fire can not be reverse connection, should be in strict accordance with the original position plug.If there are peripherals, after the connection is completed, the chassis should be tested for electrification.
2.When moving the computer and other control equipment, the connection line and control board should be checked for looseness before power on.
3.The position and length of communication line and flat connection line cannot be changed at will.
4.After moving, if short circuit, trip, burning, smoke and other abnormal appearance is found, should not be repeated power test, should promptly find the problem.
Iii. Precautions for software operation and use
Software backup: WIN2003, WINXP, applications, software installer, database, etc.
It is recommended to use "one-button restore" software, which is easy to operate.
2. Proficient in installation method, original data recovery and backup.
Master the setting of control parameters and the modification of basic data presetting
4 proficient in using programs, operations and editing.
Check viruses regularly and delete irrelevant data
6.Non - staff, do not operate the software system.
Introduction and comparison of several driving solutions for LED displays
LED driver chip manufacturers' product structure, the current mainstream chip is mainly divided into three grades.The first level is the chip with gray level mechanism, which has PWM function in the chip, and can generate gray level according to the input data, which is easier to form deep gray level and display high-quality picture.The second level is the chip with output open circuit detection (LOD), temperature overheat protection (TSD) and brightness adjustment. These chips are more suitable for special occasions due to additional functions, such as variable intelligence board, which requires the chip to have the function of detecting LED errors.The third stage is constant-current source chip without any additional functions. Such chip only provides constant-current source for LED to ensure good screen quality.The following are some references:
TLC5941 driver chip
TLC5941 chip is the latest launch of TI (Texas instruments), with point correction, high gray level (PWM control) and other characteristics.TLC5941 all internal data registers, grayscale registers, point correction registers and error status information are accessed through the serial interface, the maximum serial clock frequency is 30MHz, the interchip current error is generally within 6%, the interbit current error is generally within 4%, the maximum output current of each channel is 80mA
TLC5941 each channel can be PWM mode according to the value of the internal grayscale register for 4096 level grayscale control, the register is 12 bits, each channel LED drive circuit by 6 points correction register value for 64 level control, and the maximum driving current can be set through the off-chip resistance.Current control provides a level 64 LED point of gray level correction ability, level 4096 grayscale adjustment is to ensure the even under the low grey scale, each point in the lattice has as many as 256 level gray scale, said to all red, green and blue color can be 16 m color color expression ability, both for high quality color large screen display is especially important compared with traditional color large screen display system, focus on the PWM gray-scale control, programmable logic chip (or high-speed CPU only need to handle the cache management, gray and point calibration data output, reduce the design complexity,Moreover, because PWM gray level control has nothing to do with data serial removal, higher frame frequency can be easily obtained and good dynamic display effect can be achieved.
In order to ensure the reliable operation of color large screen, TLC5941 provides each LED open circuit (LOD) and over temperature detection (TSD) capability, built-in collector open circuit output circuit, for error alarm.Whichever channel has 16 channel error occurs, the built-in open collector output circuit output pin can be pulled to low level, by querying the chip's internal state information, you will know that which way fails, all the TLC5941 built-in open collector output circuit in the system output pin can be got together, through the pull-up resistor receives high level, by monitoring the signal, make self diagnosis system can be in the running process.TLC5941 is suitable for the harsh working environment and high requirements for display effect and safety performance, such as highway LED information signage, large outdoor LED TV, etc..
MBI5028 driver chip
MBI5028 is an LED driver chip with programmable current gain.Built-in series-parallel shift register and output latch, and PrecisionDr technology is used to get better electrical characteristics.The maximum serial clock frequency of MBl5028 is 25MHz, the interchip current error is generally within 6%, the interbit current error is generally within 3%, and the maximum output current is 90mA
MBI5028 built-in current gain control logic unit, programmable current gain function using share-io technology, no need to add additional pin, just in the corresponding pin input a specific sequence signal, can enter the MBI5028 special function mode -- current adjustment mode.In this mode, the data of different current gains can be written to the logic unit of the current gain control through the system microcontroller, and these data can be latched, and the output of current can be effectively controlled through the built in converter Shared by digital and analog.Due to the change of working environment and aging of LED screen, the brightness of LED screen will be reduced. If a fixed forward current is used, the brightness deviation of LED screen will be small.By using the programmable current gain function and PrecisionDr technology, the current deviation can be adjusted to compensate the brightness of the LED screen and obtain high quality images.Using PrecisionDr technology and built in the converter of digital and analog sharing, the relative output current can be obtained by changing the digital code in the same precision, thus improving the imaging quality of LED screen.
Current technology can provide 256 current levels for LED displays, making it 1200% of the total dynamic range, and 256 output current levels.In terms of electrical characteristics and chip packaging, MBI5026 has good compatibility. Users can obtain the current gain technology with share-io technology without changing the PCB board previously designed for the same type of chip, which can greatly reduce the upgrade cost.MBI5026 is suitable for working environment conditions is not harsh, but requires high quality imaging LED screen drive solution, such as indoor large LED display screen and other low-end screen.The MBI5028 is also suitable for upgrading older driver chips.
ST2221C driver chip
ST2221C is an LED screen driver chip launched by SITI technology (Taiwan, China).With built-in series-parallel shift register unit, output latch unit and current output control unit, the electrical characteristics are excellent.ST2221C has a maximum serial clock frequency of 25MHz, and the inter-chip current error is generally within 10%, and the inter-bit current error is generally within 6%. The maximum output current is 120mA. ST2221C contains 16-channel constant current drive unit, which can drive 16-channel LED at the same time.
There is a problem
Power consumption and heating problems
Because of the large output current, the power consumption and heat of LED display chip has been the first factor that impedes the development of driver chip.This problem will become particularly acute in the future when it comes to the way handheld LED displays are driven.With the improvement of LED device manufacturing technology and the decrease of driving current, the problem will be solved step by step.
Application cost problem
A mainstream 16-bit steady current LED display driver chip can only drive 16 LED devices.With a resolution of 1024 768LED display, multiple driver chips must be used to achieve the desired effect, which makes the material cost higher.If the driver chip itself adopts the scanning method, then a mainstream driver chip can drive multiple LED devices at a time, which will reduce the application cost a lot
LED display use notes and daily maintenance
I. precautions for display screen:
1 switch sequence:
When the screen is opened: turn on the machine first and then open the screen.
When turning off the screen: first turn off the screen, then shut down.
(turning off the computer screen first will result in high highlights on the screen body, burning the lamp tube and causing serious consequences.)
2. Switch screen time interval shall be more than 5 minutes.
3. The screen can only be switched on after the computer enters the engineering control software.
4. Avoid opening the screen under the condition of full white screen, because the impulse current of the system is the maximum at this time.
5. Avoid opening the screen when it is out of control, because the impulse current of the system is the maximum at this time.
A the computer does not enter the control software and other programs;
B. The computer is not powered on;
C the power supply of the control part is not turned on.
6.When the environment temperature is too high or the heat dissipation condition is not good, should pay attention not to open the screen for a long time.
7.When a line of very bright light appears in part of the display screen, please turn off the screen in time. Under this condition, it is not recommended to open the screen for a long time.
8.If the power switch on the display screen often trips, please check the screen or replace the power switch in time.
9.Check the fastness of the joint regularly.If there is a loose phenomenon, pay attention to timely adjustment, re-reinforcement or update the lifting parts.
10.According to the environment of the display screen and the control part, avoid insect bites, and place rodenticide when necessary.
Ii. Matters needing attention for the change of the control part
1.Computer, control part of the power line zero, fire can not be reverse connection, should be in strict accordance with the original position plug.If there are peripherals, after the connection is completed, the chassis should be tested for electrification.
2.When moving the computer and other control equipment, the connection line and control board should be checked for looseness before power on.
3.The position and length of communication line and flat connection line cannot be changed at will.
4.After moving, if short circuit, trip, burning, smoke and other abnormal appearance is found, should not be repeated power test, should promptly find the problem.
Iii. Precautions for software operation and use
Software backup: WIN2003, WINXP, applications, software installer, database, etc.
It is recommended to use "one-button restore" software, which is easy to operate.
2. Proficient in installation method, original data recovery and backup.
Master the setting of control parameters and the modification of basic data presetting
4 proficient in using programs, operations and editing.
Check viruses regularly and delete irrelevant data
6.Non - staff, do not operate the software system.