Drivers in the new vehicle market are actively seeking quieter vehicles that provide a superior communication experience between passengers and the outside world. With increased electrification, cars have become quieter; however, in these quiet cars, especially at high speeds, passengers typically hear more outside noise than in noisy combustion-engine cars.
To further reduce cabin noise, advanced technology using microphones, amplifiers, speakers and advanced digital signal processing can be used to reduce background noise, clear voice communication between passengers, and emergency and high-fidelity hands-free voice calls.
Introduction to Active Noise Cancellation
Traditional noise reduction technologies such as passive sound insulation and specialized tires can make vehicles heavier and reduce fuel efficiency. As shown in Figure 1, the Active Noise Cancellation (ANC) approach can achieve the same benefits compared to passive insulation approaches while reducing overall weight and without compromising fuel efficiency.
The ANC system works by strategically placing two to six microphones in the vehicle interior cabin. These microphones measure internal noise and transmit audio data to the audio subsystem, which in turn sends an anti-audio signal to the built-in speakers. Since these speakers are the same speakers used for audio playback applications, the additional cost of adding an ANC system is relatively low.
Figure 1: An ANC system uses a microphone and built-in speakers to reduce interior noise.
ANC systems in entry-level cars use two to four microphones, while high-end cars use as many as six. Figure 2 shows a typical ANC implementation.
Figure 2: Block diagram showing how microphone signals are combined to generate a noise-resistant playback signal.
Several strategically placed microphones in the cabin capture noise from sources such as the engine, road and tire contact, wind, structural vibration, and heating, ventilation and air conditioning systems.
An audio analog-to-digital converter (ADC) digitizes the audio signal from the microphone array and sends the digitized output to the application processor over a time division multiplexed (TDM) bus.
The application processor implements the feedback ANC algorithm and produces an anti-noise signal equal to and opposite in magnitude to the measured audio noise. This anti-noise signal is played on an existing speaker system, thereby eliminating active ambient noise.
Similar to ANC systems, in-car communication (ICC) and hands-free voice beamforming systems enhance the communication experience between passengers and with the outside world, requiring multiple microphones and multiple speaker systems. All these systems coexist and communicate effectively with each other to create an enhanced experience in the cabin. Depending on system requirements, vehicle size, and luxury level (low-end, mid-range, and high-end), OEMs use different numbers of microphones for each application.
Key Design Considerations
ANC systems use two to six microphones. These microphones need to be biased to the recommended bias voltage provided by the microphone manufacturer. They also need to be diagnosed for different failure situations, such as:
Microphone signal shorted to ground
Microphone signal shorted to MICBIAS
microphone signal on
Microphone differential signals are shorted to each other
Microphone signal shorted to battery voltage (V BAT)
For example, the programmable microphone bias and comprehensive troubleshooting capabilities available in Texas Instruments’ audio ADC PCM6260-Q1 allow users to accommodate both in a very cost- and size-optimized manner.
The device supports fully programmable microphone input DC fault diagnostics for direct coupled or DC coupled input while recording microphones. Programmable thresholds are used for these faults. Individual faults and individual channels can be enabled or disabled individually. In the event of a diagnosed fault, an interrupt will be sent to the interrupt pin. Reading back the status register can provide additional details such as the type of fault and which channel has failed.
In addition, the device integrates a built-in low-noise, programmable, high-voltage microphone bias pin for biasing analog microphones from 5 V to 9 V. The integrated bias amplifier supports load currents up to 80 mA for multiple microphones and is designed to provide high power supply rejection ratio, low noise and programmable bias voltage for fine-tuning bias for specific microphone combinations.
For the two to six microphones required for an ANC system, the PCM6260-Q1 is one of a family of audio ADCs that offers two (PCM6020-Q1), four (PCM6240-Q1, PCM6340-Q1), six ( PCM6260-Q1, PCM6360-Q1) and eight (PCM6480-Q1) channels. Figure 3 is an application block diagram of a six-microphone ANC system using the PCM6260-Q1.
Figure 3: The PCM6260-Q1 is used to implement a six-microphone ANC system.
What are the future trends in implementing audio in ANC systems?
Looking ahead to future implementations, some OEMs are also considering audio hubs, as shown in Figure 4.
Figure 4: Audio Hub Module
The audio hub will receive audio signals from ANC, ICC, eCall and hands-free microphone systems. It will then digitize and accumulate the audio signal and send the digitized signal to the appropriate audio subsystem for further processing.
While audio hubs are designed to support an increasing number of microphone inputs, an audio ADC such as the PCM6260-Q1 can serve this architecture well as it allows up to 24 microphones to share the same control I 2 C bus and TDM bus .
As the trend towards ANC, ICC and hands-free beamforming continues, audio hubs will reduce microphone routing and implementation complexity, and reduce costs associated with microphone cables.
Reviewing Editor: Tang Zihong