Driving High-Speed Data Against the Traffic – Part 1


  

Vision-based safety systems are becoming nearly ubiquitous in automobiles. Multiple high-definition displays are appearing in the center console, rear seatbacks, and the instrument cluster for both information and entertainment purposes. Car manufacturers are also increasingly deploying cameras to increase safety and for driver assist applications, such as improved visibility for backup and parking. The National Highway Traffic Safety Administration (NHTSA) has proposed new vehicle safety regulations calling for standard rear-mounted video cameras and displays in all vehicles by the year 2014. The regulation is aimed at reducing the hundreds of fatalities and thousands of injuries that occur each year as a result of back-over accidents. While unquestionably increasing safety and adding to the driving experience, the addition of all these cameras also raises new challenges for automotive system designers.

 Transmitting High-Speed Video Links
A dedicated high-speed video link connects each display or camera in a vehicle to a control (head-end) unit. In the simplest case, a single coaxial wire is used to display an NTSC (CVBS) signal from a back-up camera on a display in the center console. However, the trend is clearly to improve image clarity and quality with mega-pixel digital cameras displayed on high-resolution LCD panels.

High-speed serial digital links connect the video components, providing a seamless connection from the digital imagers used in cameras to a digital LCD display. The most common and reliable high-speed digital interface technology deployed for automotive video links is based on the ANSI/TIA/EIA-644-A Low Voltage Differential Signaling (LVDS) standard. LVDS provides a robust data transmission standard capable of long distances, low power, high noise rejection, and low EMI. Instead of a single-ended signal referenced to ground, LVDS uses a differential scheme to enable the desired attributes of the link.

Interconnect savings are also realized by deploying smaller connectors and cables to reduce system size and weight—both critical features in automobile applications. As shown in Figure 1, a serializer receives data from a video source, such as a camera’s image sensor, then converts the wide parallel bus of RGB color and control signals to an LVDS serialized stream transported over a single, twisted wire pair cable. A companion deserializer at the other end of the cable expands the video signals back into a parallel interface for connection to a display or head unit.

The FPD-Link III serializer/deserializer product family from TI offers a number of advanced features that address the challenges of high-speed system design. A single serial data stream transmitted over a single differential pair avoids data skew issues. The devices encode serial data to contain an embedded clock that they can recover without the need for a reference clock which allows for rapid initialization of the connection without special training sequences. Carefully randomized and scrambled video data minimizes electromagnetic interference (EMI), and is DC-balanced to allow signal transmission and recovery over long lengths (10m+) of twisted pair cables, or a single coaxial cable. These measures help reduce EMI which is particularly critical in automotive environments with strict standards for electromagnetic conformance (EMC). You can learn more about the Ser/Des chipsets here.

In Part 2, we will explore the different approaches to implementing control channels available to control data traveling against the direction of video data flow.