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With the development of the automotive industry, the complexity and information density of automotive information systems are increasing. Display is no longer just a basic centralized instrument display, but to meet more and more detailed and diverse information needs for in-vehicle information display. The car display system has evolved from traditional pure audio, such as MP3 and CD, to an integrated display system integrating GPS navigation and audio-visual entertainment. The displayed content usually includes: GPS map information, DVD playback, digital broadcast TV, and reverse screen. For so much display information, it is usually necessary to equip multiple displays, or use a video switch to switch between different signals. These methods undoubtedly require increased cost, increased operational complexity, and only one display at a time. information.
The picture-in-picture (PIP) technology was produced in the 1980s and is mainly used in televisions where users can watch multiple programs at the same time. The position of the sub-picture in the picture-in-picture can be adjusted, can be enlarged, reduced or displayed statically; the content of the main sub-picture can also be exchanged. With these functions, people can monitor other channel programs at the corner of the screen when watching a TV program of a certain channel or Indoor and outdoor security; and can use the sub-screen function to appreciate some decomposition actions.
The use of picture-in-picture technology in this paper to achieve multi-picture simultaneous display in a special vehicle environment can solve these problems better.
Design
Figure 1 system block diagram
For the signal source of the car entertainment system, generally, the DVD player output signal is CVBS or S-VIDEO, the camera is CVBS or CCIR656, the TV is CVBS, and the GPS is Digital RGB. After the multi-channel composite signal is filtered, A/D converted and decoded, it enters the main picture and sub-picture processing. The video switch controls the switching of the main picture and the sub-picture signal in time, and the sub-picture is sampled in a certain proportion, stored in the memory, and then read out at a certain speed to realize zoom display. Just like opening a window somewhere on the main screen, embed the sub-picture in the window. The video switching pulse has a strict correspondence with the line sync signal of the main picture.
Figure 2 shows the principle of picture-in-picture display in which the main picture and the sub-picture are 8-order gray-scale pictures. During the field scanning process, a field window is established between lines A to B. During the line scanning process, a line window is established between the columns C and D, and a sub-picture signal is inserted in the interval. As can be seen from Fig. 2, the line field synchronization of the sub-picture is consistent with the line-field synchronization timing of the main picture, but the signal data is only sent between the C and D columns, and is a compressed full gray-scale waveform. The scaling of the sub-picture is controlled by the sampling frequency and the read/write speed. The lower the sampling frequency, the faster the reading speed is, the larger the compression ratio is. The position of the sub-picture is based on the synchronization of the line field of the main picture, and is strictly controlled by the timing. When the main screen starts scanning, the time after ΔTV is swept to the Ath line, and then ΔTH is swept to the Cth column.
Figure 2 Principle of picture-in-picture
At this time, the output signal source is switched to the sub-picture signal, and the sub-picture data is started to be read from the memory as the display data of the TFT screen. After THW, after reading the D column, the display data source switches back to the main screen. So, until the D column of the Bth line, the sub-picture display signal is completely scanned. The insertion window can be anywhere on the main screen, but it is usually placed on the four corners.
This program has the following characteristics: The TW8811 controller is used, and the decoding includes a 3D dressing filter to improve the dynamic display effect. Through the picture-in-picture technology, multi-signal source can be controlled to display at the same time, and the size and position of the main sub-picture can be flexibly adjusted, which satisfies people's demand for diverse display information in the car and increases entertainment. It can support resolutions up to 1280×1024; the interface of the screen supports TTL interface, TCONLESS interface, LVDS interface and analog screen interface, which is sufficient for the requirements of current car entertainment systems. The screen's Gamma voltage programmable control replaces the traditional resistor network string, which is more accurate and convenient.
Hardware circuit design
1 DC/DC, Gamma buffer
Generally, the voltage in the automotive environment is in the range of 6 to 36V. The voltages required for this system are: +5V, +3.3V, +1.8V, +8.4V and LCD BIAS. First stabilize the input to +5V with a DC/DC BUCK circuit. TI's TFT dedicated power IC TPS65140 main output can be used to drive the LCD, and the charge pump is used to generate the VGH, VGL, VCOM, Gamma and other levels required for the TFT screen. It is worth mentioning that the power-on sequence of the TPS65140 is exactly the same as that required by the TFT, that is, the main power supply, then the VGL, and finally the VGH. The IC also features undervoltage protection, open circuit protection, and error detection. The main power circuit is shown in Figure 3.
Figure 3 main power circuit
To reduce the output voltage ripple, choose a 22μF, low ESR ceramic capacitor.
This design can use TI's BUF68120 as the gamma buffer of the screen. The device can set 14 Gamma and Vcom values ​​through internal registers and can be modified online in real time.
2 image processing circuit
Image processing is the core of the system. The sources supported by the TW8811 are CVBS, S-VIDEO, YCBCR, 24-bit Digital RGB, and Analog RGB.
After A/D conversion, the composite signal is separated into Y and C components by 3D comb filter, and C contains U and V components. The phase difference between the two is 90°, and then the chroma is demodulated, and finally the composite signal is decoded into 4: 2:2 YUV signal. For the digital RGB signal, it is directly converted into a YUV signal through the chromaticity space, and enters the PIP processing unit. When the PIP function is turned on, the sub-picture YUV signal stream is sampled at a certain frequency, stored in an external memory, and then read back into the data through the register control. The internal timing strictly controls the frequency and time point of sampling and reading. The final processed data and timing are output together to drive the TFT screen display. The internal block diagram of TW8811 is shown in Figure 4.
Figure 4 TW8811 internal block diagram
In order to ensure a better display effect, the signal input end needs to be filtered, as shown in Figure 5. Adding a 滤波-type filtering network at the input end, the attenuation frequency of 3dB is 10MHz, and the frequency bandwidth of the CVBS signal is 0~6MHz. The filtering network can effectively filter out high-frequency clutter.
Figure 5 CVBS input filter circuit
3 MCU and SDRAM control circuit
The MCU is the control center of the system, which is mainly used to initialize the TW8811, detect infrared interrupts or key scans, and perform corresponding operations. SDRAM is mainly used for buffering of PIP data and storage of OSD pictures.
software design
Software is also the core of the system, the software program flow is shown in Figure 6. The main program mainly completes the initial setting of the MCU, and initializes the register of the TW8811 through the I2C port to realize normal display. The main program is as follows:
Void main(void){
InitCPU(); //mcu initialization
System_init(); // tw8811 initializes while(1) {
Main_loop(); //pip button detection loop PowerOff();
WaitPowerOn();
}}
Figure 6 program flow chart
In main_loop(), after the MCU interrupt port detects the PIP function enable command, the signal channel of the input sub-picture is selected by changing the TW8811 register. And open the sub-picture window, that is, select the sub-picture data as the output display data source.
Taking the horizontal and vertical compression of the sub-picture as an example, the process of setting the sub-picture size is: interlaced the sub-picture YUV data of 4:2:2 at a speed of 1/2 of the original frequency, and buffered into the external SDRAM. . Under the control of the line sync timing, the original frequency is read from the SDRAM as display data.
The process of setting the position of the sub-picture is: setting the value of the start of the line, the end of the line, the start of the field, and the end of the field in the main screen by the register. The position of the sprite can be adjusted by modifying these four parameters.
Experimental result
According to the development trend of the car entertainment system, aiming at the characteristics of its multi-signal source, a scheme of multi-picture simultaneous display using PIP function is designed. Figure 7 and Figure 8 show the display effect of the solution. The resolution of the screen is 800 × 480, and the interface is 18-bit Digital RGB. The experimental results show that the user can conveniently control the picture-in-picture display of the two signals, and at the same time realize the display of POP (Picture on picture). The osd menu can be used to adjust the position and size of the sub-picture, and the main picture and sub-picture can be interchanged to achieve a good display effect, which is very suitable for the car entertainment system.
Figure 7 POP display effect
Figure 8 PIP display effect
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