This article refers to the address: http:// The fastest change in the new automotive electronic signal format is video. A few years ago, the video display in the in-vehicle device was limited to the small-sized display of the navigation system. To be precise, it was just a navigation electronic device, and some luxury cars used the same The display plays a TV signal. The video signal needs to be transmitted a long distance from the TV receiver to the display output. The image format is an analog signal called a composite video baseband signal (CVBS). In recent years, with the development of automotive electronics technology, the development of video sources, display devices and video transmission lines has made great progress. This paper discusses related development trends. Figure 1. First generation LVDS transceiver with 8 outputs The first generation of LVDS devices has the drawback of requiring four pairs of twisted pairs (eight outputs) to achieve the required data transfer rate, and eight data lines that make the mechanical structure complex and difficult to install, compared to a pair of transmission lines. It is much higher. This was improved with second-generation LVDS devices, such as the MAX9247/MAX9248 (Figure 2), which used a pair of twisted pairs to simultaneously transmit data and clocks. Figure 2. The second generation LVDS transceiver has 2 outputs. An important function of the second generation chipset is the ability to select the capacitive output coupling mode, which is typically not available in LVDS devices. With this coupling, the ground potential deviation between the transmitter and the receiver can be avoided. With DC coupling, this potential potential difference can cause data to be untransmitted and even cause excessive current damage to the device. Figure 3. Spread spectrum technology reduces EMI The second generation of LVDS data transmission devices are primarily designed for large screen applications. The connection of various cameras inside the car does not require high transmission speeds. For this application, Maxim's third-generation LVDS devices use a low clock rate and reduce the width of the parallel data bus. AHD Camera,AHD Camera Kit,HD Security Camera Dedi Technology (HK) Limited , http://www.chlcddisplay.com
Separating the navigation display from the electronic system allows the display to be mounted in a position that is easy for the driver to observe. This separation requires the addition of a video transmission line. Now, as more and more display devices are installed in the car, including electronic dashboards for displaying speed, speed, and car status, and rear-seat multimedia players (passengers can watch TV or DVD, etc.), each display needs Video transmission line.
A new generation of cars may also be equipped with a variety of cameras for assisted driving, such as rearview mirror cameras, night vision goggles, and road sign recognition cameras, each of which requires a video transmission line to connect to the display device.
The rapidly increasing transmission lines inside the vehicle body, especially the longer and longer these transmission lines, make transmission of analog CVBS signals very difficult. These signal formats cannot withstand the electromagnetic interference of automobiles, and large-screen displays and increasingly higher resolutions further exacerbate video interference (such as multipath interference).
One solution to reduce video interference is to replace the analog signal with a digital signal that does not itself interfere. Low voltage differential signaling (LVDS) has proven to provide the most reasonable connection for digital video transmission. The small signal amplitude (0.35V) and differential structure allow the LVDS transmission line to have minimal electromagnetic radiation.
The first generation of LVDS transmission devices (such as the MAX9213/MAX9214) have been installed in the car to provide one clock output and three channels of data, using an LVDS transmitter/receiver to connect to the navigation display (Figure 1). The three-way parallel output needs to reach the rate required for image transmission, and the clock is used for synchronous transmission.
When using capacitor decoupling, ensure that the transmitted data does not charge the capacitor in one direction for a long time, for example, when sending a long string of "1"s. Second-generation devices such as the MAX9247 or MAX9248 solve these problems with "DC equalization" technology. When it is detected that there is a long string of 1 or 0 in the transmission data, part of the data is inverted and transmitted. When the data arrives at the receiver, the data is inverted again and restored to the original format. In this way, overcharging of the capacitor can be avoided, and the transmitter notifies the receiver whether each batch of data is sent in the normal format or the inverted format.
The second generation devices are capable of speeds up to 42MHz with data rates up to 1.15Gb. An increase in the clock frequency results in stronger electromagnetic radiation, and spread spectrum transmission techniques can be used to reduce EMI. Spread spectrum technology adds jitter to the clock frequency, spreading the original EMI peak energy over a wider frequency band. Since the energy is constant, the maximum peak of EMI is reduced (Figure 3).
Third-generation devices are used to control the transmission of data (mainly for camera connections and also for displays) to set display brightness and contrast, or camera sensitivity. Currently, CAN, LIN or UART transmission buses are used in the system. These solutions require more devices and cables, occupy more space and cost more. The third generation of devices will use the LVDS interface to transmit control data, avoiding the use of other interfaces.
February 03, 2019