We can differentiate between high and low bandwidth technologies. Typically, sensors require a low bandwidth. The most common accelerometers used in cars have an output data rate (ODR) of a few kHz. When it comes to infotainment, audio and video data require a data rate in the range of several Mbps.
However, what is really raising the bar is the adoption of HD multicamera systems for parking assistance, 360° vision systems (also known as bird’s-eye view or surround view monitor systems), radar (RF microwave), and lidar (optical) to enhance advanced driver-assistance systems (ADASs). The coexistence of all these systems is a key factor in the development of autonomous vehicles, but it represents a big challenge for any communication bus.
Traditional buses used in automotive are:
- Local interconnect network (LIN): with a speed of up to 20 kbps, it is mainly used within subsystems where low cost is essential and the speed/bandwidth ratio is not important
- Controlled area network (CAN): with a transmission rate of up to 1 Mbps, it is mainly used for communication between the electronic control unit (ECU) and sensors in start/stop systems, parking assist systems, and electric park brakes
- FlexRay: faster than CAN (up to 10 Mbps), it is more expensive. It was initially adopted for x-by-wire (drive-by-wire, steer-by-wire) systems and conceived to accommodate multiple network topologies.
- Media oriented systems transport (MOST): with a maximum speed of 150 Mbps, it’s designed to transport audio, video, voice, and data signals. It defines all seven layers of the ISO/OSI model, from the physical level up to the application layer. It is a proprietary solution.
With such evolution in network technology, another aspect has become important. Many different buses used for different subsystems include very complex (and expensive) cabling. Size and weight are new challenges in automotive applications because meeting new environmental regulations means developing new systems that can, for example, reduce CO2 emissions. In such a landscape there’s no easy answer to the need for a high bandwidth, low latency, deterministic, robust, and cheap communication bus.
Automotive Audio Bus (A2B)
One of the major contributions to the total cabling weight comes from car audio systems because analog wiring requires expensive shielded cables for each audio source/sink (loudspeaker). Moreover, active noise cancellation (ANC) and road noise cancellation (RNC) systems require several microphones inside the car, adding many other inputs to the audio network.
The actual cabling inside a car for a traditional audio system is well represented by Figure 1.
Automotive Audio Bus (A2B) is an innovative technology from Analog Devices that allows the implementation of in-line topology with a single master connecting to up to 10 slaves daisy chained. With a speed of 50 Mbps, A2B is optimized for audio applications. Connectivity is dramatically simplified by using an unshielded twisted pair (UTP) cable, reducing the total weight of the harness by up to 75%. The distance between nodes can be as long as 15 m, while the maximum network length is 40 m. The same UTP supplies power (phantom powered configuration), up to 300 mA—ideal for digital microphones.