Digital Microphone Idle Tone Offset for Stereo Noise Attenuation
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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 preserving 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, solving the contradiction between stereo capability and noise reduction.
2Manufacturing precision
If idle tone frequencies are slightly different due to manufacturing variations, then each microphone operates independently, but the mixing of these different frequencies creates intermodulation components in the operational frequency range
Solution Approach 1:
The patent applies preliminary anti-action by proactively designing different idle tone frequencies for microphones during the manufacturing or configuration stage, before the intermodulation problem can occur. Instead of allowing random manufacturing variations to create problematic frequency alignments, the system pre-configures intentional frequency differences that prevent intermodulation components from falling within the operational frequency range. This anticipatory measure eliminates the harmful effect before it can manifest.
3Ease of manufacture
If the same clock frequency is used for digital circuits in multiple microphones, then the circuit design is simplified and manufacturing is easier, but the idle tone frequencies align causing noise interference
Solution Approach 1:
The patent applies local quality by allowing different microphones to have different idle tone frequency configurations while maintaining the same clock frequency. Instead of standardizing all frequency parameters across microphones, the system locally differentiates the idle tone frequency for each microphone. This localized differentiation prevents idle tone alignment and interference while preserving the benefit of using a common clock frequency for simplified circuit design and manufacturing.
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.