Pulse Width Modulation correction
Pulse Width Modulation (PWM) Correction
Pulse Width Modulation (PWM) is a widely used technique to control the brightness of LEDs. It can be used to process the video signal and also to perform uniformity correction. PWM is used instead of varying the current because changing the current of the LEDs would also change the colors as explained above. PWM works by flashing the LEDs either full on or full off at a very high rate. The flashes are so fast that the human eye cannot notice them and the duration of the individual flashes (pulse width) determines the perceived brightness.
PWM Uniformity Correction
PWM uniformity correction works by modifying the pulse widths to compensate for LEDs that are naturally brighter or dimmer or display a different color. In a non-corrected system, the video signal is turned into pulse widths by the video controller and then sent to the LED drivers to flash the LEDs. In a corrected system, the pulse widths are multiplied by correction coefficients before being sent to the LED drivers.
LED displays are display systems for media playing indoor and outdoor, with image quality as the key parameter
of their performance. Because of the limitation on the LED displays themselves and the manufacturing process,
LED displays suffer brightness/chroma ununiformity, which is also the most serious problem of this industry.
The causes of brightness/chroma ununiformity can be summarized as follow. 1. Difference of LED lights when
manufactured ,Optical characters of LED lights can not be precisely controlled at present because of the limitation
of manufacturing process. There is pretty big difference in brightness, chroma, visual angle and other
optical characters between different batches of LED lights. Even when of the same batch, working on
the same voltage and current, the brightness difference between different LED lights can be up to
40% - 50%, and the main wavelength range difference up to 15nm –20nm.
In order to improve the uniformity, when produced, spectrums of LED lights are measured
and then sorted into different grades according to their brightness and main wavelengths.
As sorting is time consuming and expensive, it can not be performed meticulously. Normally,
after sorted, the brightness difference of LED lights in the same grade is within 20% - 30%,
and the main wavelength range difference within 5nm. The remained difference of LED lights
when manufactured results in ununiformity. 2. Difference introduced from manufacturing process of LED displays
A LED display is an optical system with a 2-dimentional matrix of LED lights being one of the main components.
Many factors may affect the uniformity of this optical system, such as the difference between LED light driver chips,
welding of LED light modules, the flatness of module assembly and axis directions of plunged
LED lights. Especially, if the axis directions of plunged LED lights are not well controlled,
LED lights will differ greatly in the decay curves of brightness with angle, resulting in serious
lose of brightness uniformity. 3. Difference introduced from the usage of LED displays
When being used, lights of a LED display will decay in brightness and shift in the main wavelength.
This will worsen the uniformity of the LED display as the brightness decay and main
wavelength shift differ between different LED lights. For a LED display bad in cooling design,
temperature difference of different area will cause greater difference in brightness decay and wavelength shift.
So, in order to reduce brightness/chroma ununiformity, the whole manufacturing process of a LED display
need to be well managed and controlled. But at the same time, as we can see from the causes mentioned above,
brightness/chroma ununiformity of a LED display is not likely to be avoided completely.
Solution for brightness/chroma ununiformity calibration in pixel levelFacing this fact, pixel level calibration system does not
intervene the manufacturing processing ofa LED display to reduce its brightness/chroma ununiformity. Instead, it performs
brightness/chromaadjustment to the display after it has been completely produced. By adjusting the brightness/chormaof each LED
light according to the software analytical results from the measured brightness/chromavalues of the LED lights, the pixel level
calibration system can help the LED display acquiring perfectuniformity. As a new technology enabling great improvement of LED
displays brightness/chroma uniformity,pixel level brightness/chroma ununiformity calibration has been developed quickly at home
and abroad inthe recent couples of years, and has also become a core technology indispensable in improving image qualityfor
high-end LED displays.