The demand for gamma correction stems from the invention of CRT TV monitors. The CRT uses an electron beam grating to illuminate the fluorescent coating behind the front panel of the display. The applied grid control voltage is controlled proportionally to the luminous intensity and complies with the rule of the underworld: luminous intensity = The gamma-time side of the control voltage. This control has an intrinsic nonlinear nature. The nominal gamma value of the CRT is about 2. 5. On the other hand, the human eye has a reverse response, which is relatively sensitive to the changes in the darker part of the gray level. Therefore, in order to make the final image show the real depth of gray level in human eyes, we must calibrate the red, green and blue signals before transferring the image. The school is being conducted in a video source system, such as a television broadcast camera.
This article describes how to ensure the brightness consistency and color matching of the automotive TFT-LCD panel by gamma Correction (also known as gamma calibration). We'll discuss how to use a 14-channel programmable gamma buffer to calibrate the gamma response of the LCD panel and introduce three examples of high, nominal, and low gamma settings.
The output voltage of the camera design is proportional to the luminous intensity, which equals 1/2 of the luminous intensity. 5 Times Square (0.) 4 Times Square. Because a slightly greater than 1 of the gamma value is favored, the broadcast standard usually uses 0. The gamma value of 5 makes the system gamma value of 1. 25。 Figure 1 shows the camera's built-in gamma response is the inverse of the CRT response, so no additional system gamma correction is required. However, the TFT-LCD signal must be adjusted to compensate for the camera's gamma tuning, which was initially set for the CRT monitor
Figure 1. Gamma value of traditional CRT system
Automotive LCD panel Gamma Calibration
The TFT-LCD panel is more and more applied in the automotive combination meter, information entertainment and navigation ontology unit and the Advanced Driving Auxiliary system (ADAS) intelligent rearview mirror. Instead of using electronic guns and fluorescent layers to produce luminous intensity, they control the transmittance of "backlighting" through the pixel by exerting a voltage on the LCD pixel. The use of cold cathode fluorescent lamp (CCFL) or light-emitting diode (LED) arrays to provide "backlight". The voltage applied to the LCD pixel can control the amount of backlight transmitted to the front to reproduce the transmitted image, which defines the transmittance curve for the TFT-LCD.
The gamma of TFT-LCD value differs from the CRT, but it has a slight gamma response. The gamma calibration of the traditional broadcasting system is carried out in the signal transmission process to compensate the CRT gamma response, which combines the reverse response of the human eye. Therefore, it must be applied to gamma correction before the signal is applied to the TFT-LCD panel. The TFT-LCD gamma correction is necessary to follow the gamma correction of CRT monitors. The input video signal is numeric and the gamma correction code is applied to a digital-mode converter (DAC) to generate voltages and apply to pixels. These gamma correction codes help the panel display manufacturer determine the appropriate correction code to satisfy the visual effects. The system can often store multiple gamma correction settings for different ambient light conditions. Figure 2 shows the normalized gamma correction for the TFT-LCD panel to implement the system gamma value 1.
Figure 2. Gamma value for LCD system
Digital video data, usually low-voltage differential signals (LVDS), must be converted using the DAC to produce analog voltages applied to pixels. The piecewise nonlinear DAC in the source/column drive of the panel is used for gamma correction (intentional nonlinearity). The source drive DAC decides how many different voltage steps can be exerted on pixels (such as a 8-bit DAC can produce 28 or 256 grayscale). Figure 3 shows the intensity change perception of gray levels caused by each voltage step, and is relative to the gamma response of the panel (voltage-transmittance or V-T curve) and the eye response.
Figure 3. Comparison of relative strength variations of different gamma responses
The nonlinear properties of gamma correction result in compressed image data with low brightness level, and the image data with high brightness level is compressed with little or no compression. Low level compression makes the image data in "dark" to "darker" areas are more eye-catching, and the human eye is more susceptible to detection. This improves the image depth. As an additional benefit, this compression and nonlinearity results in lower bits of the luminance encoding (for example, a nonlinear condition of 8 bits, and a linear case of 12 bits or 14 bits). In addition, compression can help reduce the likelihood of video signal noise.
Programmable Gamma Buffers
To turn the gamma response of the LCD panel into the desired V-T transfer function, the source (column) drive DAC of the panel can use a variety of reference voltages applied to multiple points of contact. These voltages force each DAC to have a specific ideal non-linear working condition. The reference voltage is often provided by the gamma buffer IC, which is usually a buffer amplifier that provides analog voltages to the DAC point. The gamma buffer IC can be static or programmable. The Intersil ISL76534 is a devices that can be used for automotive LCD panels, consisting of a 14-channel programmable gamma buffer based on I2C and a VCOM channel, both of which are 10 bits of resolution. The device can provide accurate and stable DC reference voltage to the automotive TFT-LCD source drive. Figure 4 shows a simplified diagram of the TFT-LCD panel system.
Figure 4. Simplified LCD panel diagram
By controlling the DAC points, the automotive LCD panel manufacturer can fine-tune the voltage to further adjust or calibrate the panel's nonlinear Gamma response (also known as gamma calibration). This helps the automotive LCD panel manufacturer to ensure that all LCD panels in one of its models have a consistent gamma response. This means that the difference between the potential visual effects caused by the LCD production and other factors is minimized, so the automotive LCD panel manufacturers can provide more consistent and visually attractive products. Consumers can be assured that the display of the car they purchased will have the same visual effect on the display that they see on the test drive.
Ultimately, the panel manufacturer decides how to calibrate gamma response, such as Gamma = 2. 2, γ = 2. 0, γ = 1. 8, or a combination of different gamma values based on expected brightness, so that the display has a specific visual effect. It is noteworthy that the gamma characteristics are easy to change with the angle of view and different ambient illumination conditions, so when purchasing a car should compare the display in the light and low light conditions under the intensity of the display, to determine whether the displayed image is satisfactory.
Image comparison of different gamma values
Fig. 5 compares the visual effects of the same image with different total gamma values, and it is easy to see the difference. The intermediate image (Figure 5B) is a nominal (primitive) gamma value (such as Γ = 2.) 2, the top image (Figure 5A) is greater than the nominal value, the bottom image (Figure 5C) Small Yu Biao value. The bottom image loses contrast with the dark color, and becomes somewhat reversed. The top image contrast is larger, but the overall darker area is more.
Figure 5A. Example of high gamma value image
Figure 5B. Example of nominal gamma value image
Figure 5C. Example of low gamma value image
Gamma value and automotive infotainment display settings
The automotive LCD panel has a black-Bai Duan point rank determined by the performance of the source drive. However, the signal can be adjusted digitally during video data processing. Alternatively, you can change the brightness by adjusting the backlight intensity of the LCD panel, which adds another system performance dimension. Display control is often adjusted for brightness and contrast, daytime, and night settings.
Programmable gamma buffers Simplify the task of evaluating and setting the display transfer function, i.e. transferring the task to the manufacturing cycle of the automotive display manufacturer. These gamma settings are programmed into the EEPROM and are transmitted to the DAC when the system is charged. Some buffers have multiple memory groups to hold multiple display settings files (profile) corresponding to different ambient light conditions, and custom settings that can be saved and stored by the user with the appropriate tools. The gamma settings file provided by the manufacturer is usually sufficient to satisfy most automotive applications. Programming is done during the initial setup, and then the settings file no longer changes unless you need fine-tuning to improve the visual effects of your display.
Automotive LCD panel makers strive to make the image visual effects of their display panels recognized by carmakers and consumers. This requires a stable gamma buffer reference value to ensure that the video color image displayed by its LCD panel has identical contrast, brightness, and color, not varies depending on the display. This goal can be accomplished using high-precision programmable gamma buffers, such as Intersil ISL76534. Learn more about ISL76534.