3.4 Hands Free Car Mic Module Design

To avoid the danger of drivers are have to hold the phone during driving, a hands-free communication system for cars has been developed. Drivers can speak to the microphone module directly. It can be divided two major categories: car front-mounted and car after-mounted markets. The front-mounted microphone module is mostly installed in the roof map lights, the speaker is the existing speaker system in the car; the after-mounted type is clipped to the sun visor or fixed behind the steering wheel commonly. Most of the products are equipped with their speakers, both types are AEC function is required to avoid excessive echo during a call in the car, which will affect the quality of the call. The microphone module for the car is shown in Fig.4.1

Fig.4.1 Hands-free car mic module

The microphone is usually composed of ECM or MEMS microphone to the front-end receiving function. There are two types of microphone output interfaces, digital and analog. (Fig.4.2), and directivity can be divided omnidirectional and unidirectional (Fig.4.3).

Fig4.2 Digital vs Analog Interface
 
Fig.4.3 Mic Directional

The advantage of the digital type is it has better anti-noise, but the distance to the Audio Codec can't be too far. However, as long as the analog type is used with noise isolated wires or twisted wires, the transmission distance can usually be more than 2M. Regarding directivity, a single-pointing microphone can independently attenuate noise in a 180-degree direction, such as the engine noise, while 2 pieces array microphone can further only receive the voice in the driving area to make the sound quality better, but it has to be with beamforming calculations Law & DSP. (E.g. Fig.4.4)

Fig.4.4 Hands-free mic for car