Digital Microphone Idle Tone Offset for Stereo Interference Noise
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Solution Overview
Problem
Digital microelectromechanical systems (MEMS) microphones experience noise interference due to the mixing of idle tone frequencies, particularly in stereo applications where manufacturing variations cause slight differences in idle tone frequencies, leading to unwanted noise in the output.
Innovation Solution
The solution involves offsetting the idle tone frequency of one microphone relative to the other by applying a DC offset to the digital signal before converting it to a pulse density modulated (PDM) signal, ensuring the intermodulation components fall outside the frequency range of interest, thereby reducing noise. This can be achieved by using microphones hardwired for different idle tone frequencies or selectively configuring them in stereo mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If two or more microphones are employed in stereo applications, then the microphone system can capture spatial audio information, but idle tone frequencies from different microphones mix to cause unwanted noise in the output
Solution Approach 1:
The patent applies asymmetry by intentionally creating a frequency offset between the idle tones of different microphones. Instead of all microphones operating at the same idle tone frequency, one or more microphones are configured with different idle tone frequencies, making the system asymmetric in frequency domain. This prevents the idle tones from aligning and mixing constructively, thereby reducing the unwanted noise while maintaining stereo functionality.
Solution Approach 2:
The patent changes the frequency parameter of the idle tone for different microphones. By adjusting the idle tone frequency of individual microphones to be different from each other, the system transforms the uniform frequency parameter into a varied set of parameters. This parameter differentiation ensures that idle tones do not overlap and mix to create noise, while the microphones continue to function in stereo configuration.
2Manufacturing precision
If idle tone frequencies are slightly different due to manufacturing variations, then each microphone operates independently, but the mixing of these frequencies creates intermodulation components in the operational frequency range causing noise
Solution Approach 1:
The patent applies preliminary anti-action by proactively designing different idle tone frequencies for different microphones during the manufacturing or configuration stage. Instead of allowing random manufacturing variations to create potentially harmful intermodulation products, the system pre-establishes a controlled frequency offset that prevents idle tone mixing. This preliminary measure counteracts the potential harmful effect before it can occur during operation.
Solution Approach 2:
The patent implements preliminary action by configuring microphones with different idle tone frequencies before they are deployed in stereo applications. This pre-configuration ensures that when multiple microphones operate simultaneously, their idle tones are already differentiated and will not mix to create intermodulation noise in the operational frequency range, thus preventing the problem before it arises.
Data Source
AI summary
A digital microphone device includes circuitry that can reduce the risk of noise caused due to an idle tone frequency component in a digital signal output by the digital microphone device. In stereo mode and other applications where interference occurs between two or more such microphones, each microphone device includes a digital output having a corresponding idle tone frequency, one of which is offset to shift noise components outside of a desired frequency range.