AVAS: making acoustic vehicle alerts easy for engineers

AVAS (Acoustic vehicle alerting system) is a critical part of a vehicle’s safety features, but implementing audible cues is far from straightforward and can be a massive investment. Indeed, it’s not enough for the dashboard lights to turn on. AVAS is an essential part of the user experience, warning of potential collisions, inadvertent lane changes, object proximity, a rear trunk automatically closing, and many other events. Additionally, electric cars are now featuring virtual engine noises to alert pedestrians to their presence. However, one thing is clear: those audio cues can’t rely on the infotainment system that plays the driver’s music. Car makers need a completely different approach, and here are three principles that can help teams get started.

The infotainment system accounts for a substantial portion of the bill of materials. Consequently, many car makers try to reuse parts of it, or all of it, for their AVAS systems. The problem is that it can be pretty complex to program due to its digital signal processor, equalizer, and other features. It’s also not necessarily meant for critical systems. A warning bell must ring regardless of whether the radio is on or the speakers are turned down completely. That means creating overwriting guidelines, safety measures, and a ton of other subroutines to meet the urgent needs that may require an audio cue. And that’s not even accounting for the hardware challenges, such as using the CAN bus.

That’s why ST offers a much more straightforward approach to AVAS thanks to a reference design that uses one of our SPC58 automotive microcontrollers and  FDA903D Class D audio amplifiers. There are no more convoluted systems or cost-prohibitive devices. Engineers basically get an embedded system that they can plug into a power source and attach to speakers, and that’s it. The MCU supports the CAN bus. The FDA903D includes current sensing, so it can detect if speakers are disconnected without adding extra components. It’s even possible to use a dedicated pin to mute and unmute the system, making it vastly simpler than traditional systems.

AVAS is also a unique development challenge because, in essence, it requires little audio processing. A chime, a beep, or even a simulated engine noise pales in comparison to the computational power needed to play high-resolution audio from a phone, for example. In essence, the infotainment system was designed for powerful speakers tuned for all types of music and speeches that must fill the cabin. At the same time, AVAS often targets the driver or an outside pedestrian and doesn’t require nearly the same fidelity. Additionally, using the operating system of the central infotainment system, or creating a real-time application that runs alongside it, is hard to justify when the goal is simple yet critical sound reproduction.

It’s exactly for this reason that ST focused on how developers could write an application as efficiently as possible. Thanks to our SPC58 microcontroller, developers can simply use traditional I²C and I²S interfaces to talk to the flash or the amplifier. And since the system can play an uncompressed WAV file, engineers can simulate an engine accelerating or decelerating by simply modulating its pitch and playback speed. The ST platform can even reproduce notes to play complex melodies without storing large files or using an advanced audio pipeline. Put simply, using an MCU and a power amplifier like the FDA903D proves that it’s possible to do a lot with much less than previously required.

To help engineers benefit from this new approach to AVAS and take advantage of all ST solutions in a single reference platform, we created the AEK-AUD-C1D9031. The development board comes with an SPC58 microcontroller and two FDA903D Class D audio amplifiers to drive two loudspeakers. We also have ready-to-use demo code within our AutoDevKit software ecosystem with a pre-recorded engine noise to help developers get started. Additionally, the platform is customizable. For instance, engineers can add the AEK-CON-C1D9031 connector board, plug in a potentiometer, and play with the volume and playback speed to modulate what is produced. Designers can also reuse our PCB layout in their final product to reduce time-to-market.

Another advantage of a standalone platform like the AEK-AUD-C1D9031 is its high reusability. Module markers are increasingly exploring new markets, from trucks to scooters and forklifts. By nature, a system tied to a car’s infotainment can’t be used anywhere else. Conversely, porting our platform to a wide range of vehicles only requires a power source and speakers. As a result, design teams can apply the same proof of concept across multiple products and achieve a much better return on their investment. Ultimately, AVAS must not be about what the industry has to compromise because it focuses on other priorities like battery management systems. Instead, a good AVAS platform must deliver more features to end users while keeping costs down.