Quality Control of A2B Microphone Modules

Quality Control of A2B Microphone Modules

A2B (short for Automotive Audio Bus) is a digital network technology from AD (Analog Devices). It is designed to connect all audio devices and components within a car. The practical feature of A2B is that the connections are realized using a 2-wire, Unshielded Twisted Pair (UTP) cable. This provides a big reduction in weight and complexity of cable harnesses. The 2-wire A2B interface provides power supply, network control, as well as bidirectional high-quality digital audio on multiple channels.

Due to the rapidly-growing use of automotive voice recognition applications, A2B modules always contain MEMS microphone arrays. To ensure flawless operation, tests must be carried out against the absolute specification values, and, even more important, the values of all microphones in the array relative to each other. This page explains how to interface A2B microphone modules to an acoustic test system and how to measure the relevant key parameters in a reliable QC procedure.

特長

  • For measurements of A2B microphone modules
  • Seamless integration into NTi Audio microphone test solution
  • Characterization of single microphone components as well of complete module

How to interface to an A2B microphone module

The A2B interface definition is proprietary. To get access to the audio data, in order to test the quality of the microphone array, for example, a device is required that decodes the A2B network data to either analog or AES/EBU digital audio format. The decoded audio channel signals (e.g. of the digital MEMS microphones) can then be routed to the audio analyzer for the testing. There are a few such decoding devices commercially available, such as the Mentor A2B Analyzer. This device is programmable and can be fully integrated into the NTi Audio microphone test system.

Once the audio signal is accessible to the audio analyzer, the measurement tests for digital MEMS microphone arrays are performed.

The typical parameters of interest for a QC test are the same as for the testing of most other microphones; Sensitivity, Frequency response, Distortion, and sometimes Signal to Noise ratio (SNR). For a complete microphone characterization typically performed in a lab environment, parameters such as EIN (Equivalent Input Noise), and Dynamic Range are also measured or calculated.

For all absolute measurements (those that are not expressed in % or dB) the units for digital MEMS microphones are different. While the sensitivity of analog microphones is expressed in mV/Pa or dBV/Pa, the unit for digital microphone is dBFs. This stands for “decibels below Fullscale” and describes the headroom of a digital microphone from 94dBSPL (1Pa) to the maximum digital output of that microphone. This point of maximum digital output is also referred to as the AOP (Acoustic Overload Point).

Acoustic vs. Digital Observation

For characterizing the performance of an A2B module containing several MEMS microphones, it is of interest how the assembled MEMS microphones behave relative to each other. A typical parameter is the “Sensitivity Span”; the difference between the highest and lowest sensitivity measured on the assembled MEMS microphones.


Digital MEMS microphone peculiarities

Digital MEMS microphones deliver data in the ½ cycle PDM format. The microphone requires a CLK input, and delivers its data on a DATA output. Furthermore, two microphones share one data line. Therefore, each microphone is configured to be either a “left” or “right” microphone. This is done by hardwiring the L/R input pin to either Vdd or ground. MEMS microphones are supplied mostly by 1.8V or 3.3V.

In normal operation, the “left” microphone writes a data bit on each rising edge of the clock signal, while the “right” microphone writes a data bit on each falling edge. While one microphone is writing data, the other one puts its DATA output into a high-impedance mode. On the DSP that is receiving the data, the left and right signal data are then separated and put together into two signal streams.

Normal operation of two digital MEMS microphones

But what happens when one of the two microphones is not assembled correctly or is missing?

Operation with one inoperative or missing MEMS microphone

In this example, the right microphone is missing, therefore only the left microphone is writing to the data line. At the falling edges, the left microphone puts its DATA line to high-impedance state. Therefore, the DATA line keeps its state as it was previously written by the left microphone. As a result, from the receiving DSP perspective, the right microphone seems to deliver the exact same data as the left microphone. The two data streams are identical! This problem must be addressed by the test system, as detecting a missing microphone is a fundamental feature when testing a A2B microphone module.

The recommended NTi Audio solution

The basic measurement system for testing A2B microphone modules consists of an audio analyzer, a Mentor A2B Analyzer with A2B Service API, a reference loudspeaker and a reference microphone. The system is controlled by a PC software.

A2B module test system overview


The FX100 Audio Analyzer

The FX100 generates the test signals for the reference loudspeaker, and analyzes the signals coming from the MEMS microphones as well as from a reference microphone. Depending on the number of MEMS microphones and the time constraints, additional parallel channels or input switchers can be used.


Mentor A2B Analyzer

MEMS Mic Test Box

A2B Analyzer from Mentor Automotive

Provides an interface to connect the the DUT on the A2B bus. Configures the A2B bus and the connected bus nodes. The A2B Analyzer is fully controlled by the NTi Audio microphone test software.


The reference loudspeaker

This has to provide sufficient bandwidth and sound pressure to cover the required test conditions. It is recommended to use a coaxial design (point-source) loudspeaker to avoid non-uniform sound distribution.


The reference microphone

This is to measure the true signal coming from the loudspeaker during each measurement. With this information, any deviation or drift from the reference loudspeaker can be compensated.


The PC Software

For EOL (End Of Line) testing of A2B microphone modules, RT-Mic software is the ideal choice. It offers an easy-to-handle configuration, guided workflows for calibration, reference data collection, and limit calculation. Each microphone is measured and judged against PASS/FAIL criteria. The results of the individual MEMS microphone tests are summarized in an overall DUT (Device Under Test) result.

RT-Mic EOL QC software


Options & Accessories

An environment sensor can measure and log temperature, relative humidity, and barometric pressure along with the measurement data.

A barcode scanner can be used to read the serial number of the measured DUT. A turntable is used to determine the directional characteristic of a single MEMS microphone.

Benefits

  • Seamless interface to your A2B microphone module
  • Fast and accurate measurement of all relevant acoustic parameters.
  • Single MEMS microphone and complete PCB evaluation.
  • Integrated detection of inoperable or missing MEMS microphones.
  • Turnkey solution for EOL testing applications.

オーダー情報

Flexus FX100 Audio Analyzer

A2B Microphone Module Test System

セット内容

  • Flexus FX100 オーディオアナライザ
  • Mentor A2B Analyzer with A2B Service API
  • 測定用マイクロホンM2010
  • RT-Microphone Software for Flexus

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