Hands Free Car Kit & Telematics
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THE NEED FOR HANDS-FREE COMMUNICATION Demand for higher quality in-vehicle hands-free voice systems is growing. Many governments have already passed or are considering legislation that prohibits the use of handheld mobile phones while driving. In addition, car manufacturers are responding to consumer demand for higher performance in-car communication and audio systems. However, moving vehicles pose a hostile environment for mobile voice communications technology. In particular, acoustic echo coupled with high levels of road and car noise decreases the sound quality of a conversation. Previous hands-free car kit solutions have met limited market success, primarily because of their inability to support high-quality voice communication. A poorly designed car kit will deliver voice service far worse than any cellular voice call. The key elements of designing a high performance car kit are noise reduction, acoustic echo cancellation, double-talk performance and clipping compensation.
CHALLENGES FOR DESIGNERS Acoustic Echo There are two sources of acoustic echo - direct acoustic coupling between the speaker and the microphone, and voice signal reverberation inside the passenger cabin. The majority of acoustic echo results from direct acoustic coupling, which occurs when the microphone picks up the voice signal directly from the speakers. This is often exaggerated in a car environment, as the volume of the hands-free car kit is set loud to overcome engine and road noise. |
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Noise Sources Noise, whether created by the vehicle or the operating environment, is the main challenge that hands-free car kits have to overcome. Of particular concern are variable levels of noise. The most significant sources of noise inside a car are engine noise, road noise, and wind noise. Noise reduction algorithms can be used to alleviate the effects of wind noise, road noise and engine noise on the sound quality of the communication system. To achieve good noise reduction, the car kit must be capable of dynamically modifying the microphone gain using a smart automatic gain control circuit on the input of the system to eliminate signal clipping and preserve the settings of the end user. Double Talk Performance Good double-talk performance relies on the AEC and the NLP designed to interact in such a way that the AEC tracks changes in the echo path and continuously converges during double-talk and the NLP conceals the imperfections of the AEC by reducing echo further and handling non-linear effects. For best-in-class echo cancellation performance, the algorithm must converge continuously during double-talk conditions. Signal Clipping To compensate for signal clipping under severe noise conditions, for example a car driving at over 100 km/h with the windows open, a good clipping compensation circuit is required. Under these conditions, the microphone signal is clipped and the ADC is overloaded. The clipping compensation circuit prevents the signal from being clipped and allows the AEC to converge to echo and the noise reduction circuit to identify and reduce background noise.
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ZARLINK'S HANDS FREE VOICE PROCESSING SOLUTION The company's newest hands-free solution, now commercially available, is a dedicated voice processor that combines integrated dual-channel wideband codecs with multiple interfaces. The device supports advanced echo cancellation, psychoacoustic noise reduction, full-duplex operation and is flexible and field-upgradeable. The chip's patented software algorithms continuously track changes in the echo path, even during double-talk conditions, and reduce background noise while preserving high voice quality. The solution is based on a flexible platform that targets introductory systems with basic features and high-end systems with advanced features. It's a field upgradeable solution that allows the designer to build on platform designs with future system upgrades without re-designing the hardware.
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