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Software-based active noise control in automotives

Posted: 11 Dec 2013     Print Version  Bookmark and Share

Keywords:active noise control  ANC  infotainment system  processors  DSPs 

Everyone wishes for a car that consumes less fuel. In fact, some studies indicate that car buyers now rank fuel economy as the top factor in choosing a new vehicle. To address this demand, as well as laws regulating fuel efficiency and carbon emissions, automakers have devised a variety of measures. These include variable cylinder management (i.e. deactivating cylinders under conditions of light engine load, such as steady-state highway driving), operating the engine at lower RPM, and cutting back on passive damping materials to decrease vehicle weight.

The good news is that these measures help improve fuel economy; the bad news is that they often result in objectionable in-vehicle noise. This noise, or "boom", exhibits one or more low-frequency tones under 150Hz and can cause driver fatigue during long-term exposure. To counteract this noise, automakers are employing active noise control (ANC) technology.

How does ANC work?
ANC relies on a fairly simple acoustic principle. To cancel unwanted engine noise where the driver or passengers are sitting, an ANC system emits synthesised "anti-noise" of equal amplitude but opposite phase through the car speakers. That said, achieving optimal performance is anything but simple. To begin with, every vehicle interior has unique acoustic characteristics that are affected by the location of seats, the materials used in the cabin, and the position, number, and type of speakers and microphones. Because all of these factors influence how an ANC system performs, the system must be calibrated and tuned separately for every vehicle model. The system must also adapt quickly to dynamic changes in cabin acoustics that result from acceleration and deceleration, windows opening and closing, changes in seat positions, and fluctuations in temperature. Moreover, the system must be robust—it cannot become unstable or degrade the audio quality inside the cabin should, for instance, a microphone stop working.

Figure 1: ANC systems use real-time engine data and sampled microphone data to construct "anti-noise" that is played over the vehicle speakers to reduce engine noise where occupants are sitting.

To manage these changes effectively, an ANC system needs real-time engine operating data, specifically RPM data; it also needs input from one or more microphones located near the heads of the driver and passengers. The real-time engine data helps determine the frequencies to be reduced, while the microphones monitor noise levels in the cabin, enabling the system to adapt to the constant changes in the vehicle's acoustic properties.

Considerations and trade-offs
Most commercial ANC systems use a dedicated hardware module. Automakers are beginning to realise, however, that it's more cost effective to run software-based ANC on existing hardware, such as the head unit or power amplifier of the vehicle's audio or infotainment system. Today's infotainment systems must accommodate an increasing number of audio tasks such as hands-free processing and voice recognition, so the DSPs and application processors in these systems often have the headroom to run ANC as well.

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