Automotive Electronics Quiet driving
By using highly integrated semiconductor technology it is possible, according to Kieran McDonald, to implement more effective automotive microphone biasing and fault detection
applications that require a microphone - these are voice detection and active noise cancellation (ANC). Voice detection is required for voice
Kieran McDonald T
here are growing consumer demands for more advanced automotive infotainment systems with higher levels of functionality and better quality audio. Buyers are also starting to expect lower noise levels within the cabin. As a result it is becoming increasingly commonplace for engineers to specify microphones into their system designs to give vehicle occupants a better user experience while travelling. Typically there are two in-vehicle
12 October 2014
streaming to cell phones and it is typically carried out via Bluetooth wireless synchronous transmission. ANC, meanwhile, is employed for road noise cancellation (RNC) and engine order cancellation (EOC), to reduce road and engine/exhaust noise respectively. It is done through the utilization of advanced digital signal processing (DSP) algorithms, along with the infotainment system's amplifiers and load speaker outputs to produce amplified anti-phase noise. The adoption of ANC techniques is increasing principally in internal combustion engine vehicles. While voice transmission tends to use a single microphone of an omni- or uni- directional type, three-dimensional ANC mainly relies on multiple error microphones positioned in different parts of the cabin. Critical to both of these is a very low noise biasing. Furthermore, due to their location outside the infotainment unit, effective fault detection is also required.
Key microphone supply requirements Normally the current drawn by an automotive microphone is relatively low, depending upon microphone impedance and the type of integrated amplification stage. It can be as low as 0.5 mA, for a
Components in Electronics
uni-directional microphone, or up to 20 mA for an omni-directional beam-forming microphone. The supply voltage tends to vary between 1.0 V and 15.0 V based upon the signal-to-noise ratio (SNR) required and the type of integrated amplifier, however most supply voltage levels will be within the 5.0 V to 8.0 V range. Biasing the microphone input lines
during assembly and maintenance. As a result the microphone power supply needs to be able to both detect and protect itself from faulty connections.
Principles of microphone power supply
An integrated microphone regulator device offers an attractive, beneficial alternative to
Figure 1: Example of a discrete microphone bias circuit (Without detection or fault diagnosis)
requires the power supply to be specified for low noise and high power supply rejection ratio (PSRR), particularly in the human audible range. Such requirements call for a low noise linear regulator. Further complications stem from the
microphone being located remotely from the infotainment system. The load being external introduces risk of misconnection
discrete circuitry or high side switches. A built-in current mirror provides it with the ability to detect the microphone and diagnose fault conditions in the load - something that is of particular importance in vehicle assembly and maintenance, where potentially there is a risk that the infotainment system, the microphone or the cabling between them may either be
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