To enhance road safety, a growing number of governments have passed legislation prohibiting drivers from using mobile phones, unless they use a hands-free kit. Frequently, such kits are judged by their voice quality.

Thus, as the market for hands-free kits continues to grow, manufacturers are dedicating their efforts to improving voice transmission quality. The key considerations in the design of a high-quality hands-free solution are: to reduce background noise, to reduce and cancel acoustic echo, and to reduce clipping.

Background Noise
The key challenge that a hands-free car kit must overcome is noise generated by the vehicle and its operating environment. The most commonly encountered noise sources in a vehicle are as follows:

1) Engine Noise
This noise is present at all times when the vehicle is operating and it can degrade the voice quality performance of a hands-free communication system. Engine noise peaks when a vehicle accelerates, becoming even more noticeable during rapid acceleration - e.g., in a sports car.

2) Road Noise
This noise is generated by friction between the tires and the road. Its intensity depends on the composition of the tire tread and the road surface.

3) Wind Noise
Air flowing over a vehicle encounters uneven surfaces, for example mounted racks, mirrors, open windows, etc., thus generating an oftentimes significant level of wind noise. The noise intensity will vary from car to car, as no two are completely identical.

4) Acoustic Echo
There are two sources of acoustic echo in an enclosed vehicle:
• direct acoustic coupling between the speaker and microphone
• reverberation when voice (acoustic) signals are reflected and "broken up" by the cabin interior surfaces.

5) ) Clipping
A hands-free car kit designed to barely meet the requirements when driving with the windows closed will not function under more challenging ambient conditions. Opening a window, for example, increases wind noise and thus also the microphone signal level. In turn, the signal gets clipped and the analog-to-digital converter is overloaded.

To achieve good background noise reduction, a hands-free car system must be capable of dynamically modifying the microphone gain in a manner that allows the user to be clearly understood by the other party, but without causing the microphone signal to be clipped.
A good hands-free car kit relies on an acoustic echo filter (AEC) and a non-linear processor (NLP). The two must interact in such manner that the AEC continuously tracks changes in the acoustic path and reacts to them, while the NLP helps overcome the AEC's imperfections by
reducing echo further and processing non-linear effects.

To prevent signal clipping under severe noise conditions, like those found in a car traveling at over 100km/h with the windows open, a well-designed clipping compensation circuit is
required. This compensation circuit prevents the signal from being clipped, and furthermore
permits the AEC to converge to echo and the noise reduction circuit to identify and filter out background noise.

ZARLINK Semiconductor's Galileo family of chips offers solutions that meet the demanding
requirements of today's hands-free car kits. The ZL38004 is a dedicated voice processor that integrates a dual-channel CODEC with multiple inputs and offering echo cancellation, noise
reduction and clipping compensation. The chip's patented software algorithms continuously track changes in the echo path, including during full-duplex operation. In addition, they reduce bothersome background noise while simultaneously maintaining high voice quality. During full-duplex operation, the ZL38004 supports natural conversation at low voice levels.

ZARLINK customers have been impressed by the quality and performance of the ZL38004, as one of the leading manufacturers of quality hands-free car kits has attested: "Last year we looked at all solutions for hands-free car kits. We decided to go with ZARLINK because this chip offers the best quality at a price that a discrete solution (DSP and CODECs) can't
match!"

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