A2B solution for car audio link slimming

With the increase in the number of nodes in the car audio link, the traditional audio link connection method will bring weight gain, difficult wiring, and electromagnetic compatibility to the car. ADI's A2B is a new type of automotive digital audio bus that can transmit 32 channels of audio data in two directions using only one unshielded twisted pair cable. It also has features such as phantom power supply and long-distance fault diagnosis, so it can solve many problems. The problem of traditional car audio bus such as weight gain, difficulty in wiring, and electromagnetic compatibility is not easy to reach the standard. This paper introduces the main characteristics of A2B by comparing the difference between the traditional audio link connection mode and the A2B audio link connection mode. It provides a car audio link system connection scheme using A2B, and introduces in detail in the car audio link system. How to use A2B to achieve remote fault diagnosis.

In-vehicle entertainment systems are increasingly demanding audio signal processing, such as echo cancellation and background noise cancellation for Bluetooth calls, active noise reduction in the cabin for engine and road noise, multi-channel high-fidelity music, etc., resulting in the number of audio nodes on the car. And the amount of audio data has increased dramatically. The use of traditional analog audio cables to connect multiple audio nodes directly increases the number of cables, increases the weight of the vehicle, and increases the complexity and cost of the in-car entertainment system.

1 Link of traditional analog audio transmission system

In a car audio system with active noise reduction, it usually includes multiple audio nodes such as a Bluetooth call microphone, an active noise canceling microphone, a car machine, and a car amplifier. Take a 5-seat passenger car as an example. Each of the front seats requires an error microphone. The rear seat requires two error microphones, plus a set of three microphone microphones, and seven microphones are distributed. 5 nodes. In addition to multi-channel audio transmission between the car, car amplifier and active woofer, this passenger car has a total of 8 audio nodes, more than a dozen audio data channels, as shown in Figure 1. Connecting with a traditional analog audio cable will require more than a dozen analog audio cables. This will greatly increase the complexity of the vehicle wiring and the weight of the vehicle.

2 Audio transmission system link using A2B technology

A2BTM (AutomoTIve Audio Bus) is a digital audio bus introduced by ADI for automotive applications. It can transmit 32 channels of audio data in two directions using only one unshielded twisted pair cable. It can also realize remote I2C control and faulty multiple nodes. Diagnostics can also provide phantom power supply of up to 50mA for each node. A2B supports up to 9 nodes (1 master node and 8 slave nodes), which are connected in a daisy-chain manner. The nodes support a maximum of 10 meters of cables, and the entire link supports cables up to 40 meters in distance. The audio data transfer delay between one node and the ninth node is only 50μs at the maximum.

Based on the characteristics of A2BTM, the audio link of the above five passenger cars can be reduced to the link of Figure 2, and only one bundle of unshielded twisted pairs can be connected to connect eight audio nodes to ensure audio data transmission between nodes. Under the premise of real-time, the complexity of the whole vehicle wiring and the weight of the vehicle body are greatly reduced, and the vehicle is more easily passed the EMC/EMI test.

At the same time, since A2BTM can also realize remote I2C control, the control interfaces such as MCU and CAN can be omitted on each audio node. In the case of phantom power, some nodes do not need to use a power chip. Thereby, the slimming process is performed for each audio node, which reduces the system cost.

Therefore, the introduction of A2BTM technology can help car manufacturers solve a series of problems caused by traditional analog audio transmission links in multiple audio nodes.

In automotive applications, fault diagnosis capabilities are critical. In the link of the traditional analog audio line, it is necessary to embed the processor in each node for fault diagnosis, and then transmit it to the host through a bus such as CAN. In the A2B audio link, since A2B can realize the transmission of remote fault diagnosis data, it can replace the CAN to complete the diagnostic function. A2B's fault diagnosis includes not only its own line diagnosis but also fault diagnosis of the devices on each node.

3.1 Line Fault Diagnosis

Line fault diagnosis includes short circuit to ground, short circuit to vehicle power, open circuit, reverse connection, etc. A2B will perform line fault diagnosis on the entire link during initialization, and also perform line fault diagnosis during normal operation after initialization.

When the initialization is completed, if the line fault occurs during the operation of the entire audio system, the interrupt information will be uploaded to the main power saving via the A2B bus. After receiving the interrupt information, the processor of the master node can obtain the corresponding fault code by reading the interrupt type register of A2B, thereby performing the corresponding fault operation.

3.2 Troubleshooting of Input and Output Devices on Nodes

There are microphones, ADCs, DACs, Codec and other input and output audio devices on the audio node. It is necessary to make corresponding fault diagnosis and working status adjustment for the microphone or various input and output devices. The two most common diagnostic modes are GPIO Query and PWM Pulse Width Modulation.

(1) GPIO query fault diagnosis

When the input/output device on the node fails, an interrupt signal is sent to the A2B transceiver, which is transmitted to the master node through the A2B bus. The A2B transceiver on the master node sends an interrupt signal to the processor, and the processor receives the interrupt. After the signal, query the A2B fault register to determine which node issued the interrupt request, and then issue an I2C command to the node to obtain the fault code, as shown in Figure 4.

(2) PWM pulse width modulation signal type fault diagnosis

The fault signal of some ClassAB/D power amplifier chips adopts real-time PWM form waveform transmission, and the processor judges various fault information according to different duty ratios of PWM signals. In the face of this type of fault transmission mode, since the A2B transceiver has two sets of I2S/TDM receiving ports, it is only necessary to connect the PWM fault diagnosis signal to a group of I2S/TDM interfaces of the A2B transceiver on the node, and at the same time The I2S/TDM interface corresponding to the A2B transceiver on the node can be connected to the PWM interface of the processor. After the main node processor receives the corresponding PWM waveform to determine the fault, it sends the corresponding I2C command to the power amplifier device on the node through the A2B bus to adjust the working state of the power amplifier device, such as the clamp off. It should be noted that in a multi-node audio link, the corresponding I2S/TDM interface on the entire audio link cannot be connected to other audio signals, and can only be connected as a PWM type diagnostic signal. This special use of A2B guarantees the real-time performance of node upload interrupt information, and eliminates the need to install a processor at the node for fault diagnosis and control.