Exploring Automotive Audio Bus technology

Modern cars with hands-free technology are typically equipped with one or two built-in microphones, which are hooked up to the head unit via an analogue connection. In the future, additional microphones will be needed for new applications and features, such as hands-free calling from the rear seat, in-car communication (ICC) and active noise cancellation. For various reasons, hooking these microphones up with a classic analogue connection is not ideal or sometimes not even possible.

In many head units there is no room for all the necessary plug connections, and there is also a huge amount of wiring involved if each microphone needs to be directly connected to the head unit. Furthermore analogue-to-digital converter (ADC) channels need to be set up in the head unit for subsequent processing of the microphone signals, which would only be fully utilised in some vehicles or would require a wide variety of head units.

With all that in mind, Analog Devices has developed an audio bus, which enables several microphones to be connected via a simple two-wire line. The "Automotive Audio Bus" (A²B) is a bidirectional bus with a data word width of up to 32 bits and supports sample rates of 44.1kHz and 48kHz. As a result, it can be used not only to connect microphones but for almost all in-car audio connections. This means that various audio components, such as head units and amplifiers, can be connected with A²B in a very cost-effective manner.

One of A²B's major advantages is its simplicity. The principle is based on an I²S/TDM (audio multi-channel) connection, which is normally only used for connections between components on the same circuit board. A²B makes this multi-channel connection via a cable, measuring one or more meters long, whereby only two A²B wires are required for the four TDM signals (SYNC, BCLK, DTx, DRx) through which clock and data are transferred in both directions. The A²B chip has both an I2S/TDM interface and the transceiver for the A²B connection. The A²B connection looks like a normal TDM line both for the master and the slave. For the microphone connection, the slave chip has the option to accept a PDM (pulse-density modulation) signal as an input.

A variety of digital microphones support PDM as digital output. The PDM output emits a "sigma-delta" transformer signal without filtering; the digital filtering takes place in the A²B chip. This allows up to four digital microphone capsules to be directly connected to the A²B bus without any additional circuitry.

There is no need for a software stack, which processes incoming and outgoing data in one form or another, to be put in place on the master or the slave. Implementing the bus simply requires a host controller to configure the master and slaves via I²C. The slaves are configured via the bus and require no further intelligence. In addition to the audio data, the bus provides the transfer option for an I²C channel, whereby the host ECU communicates with its I²C master via the I²C slave interface of the A²B master.

This communication is then relayed via the bus and the A²B slave with its I²C master connection to the corresponding component. Both for the host and an external component, this works as if both were directly connected to each other (figure 4). As such, even external components (e.g. ADCs) can be configured in the slave node via the bus.