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.