PDM Microphone Encoding to Cut Switching Power in Always-On Audio
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Solution Overview
Problem
Digital MEMS microphones consume higher power due to switching of digital inputs and outputs, leading to increased system power consumption, which is undesirable in always-on applications with a wide dynamic range.
Innovation Solution
The use of encoding techniques such as singleton-suppression and doubleton-suppression encoding, combined with suppressed clock pulse duration modulation, to reduce the power consumption of PDM microphones by minimizing transitions in the bitstream.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If digital MEMS microphones are used to capture sound with wide dynamic range, then measurement precision is improved, but use of energy increases due to switching of digital inputs and outputs
Solution Approach 1:
The patent applies parameter changes by modifying the PDM bitstream through encoding techniques that alter the transition density parameter. Singleton-suppression encoding and doubleton-suppression encoding transform the original PDM signal to reduce the number of logic transitions, directly changing the energy consumption parameter while preserving the audio signal's dynamic range measurement capability
Solution Approach 2:
The patent introduces intermediary encoding circuits (singleton-suppression encoder and doubleton-suppression encoder) between the PDM microphone and the processing device. These intermediaries transform the high-transition PDM bitstream into a low-transition encoded bitstream, reducing power consumption without losing audio information, as the original signal can be recovered at the decoder
2Measurement precision
If digital MEMS microphones are used instead of analog microphones, then measurement precision is improved through noise shaping, but use of energy increases due to digital switching
Solution Approach 1:
The patent extracts the harmful high-frequency transitions from the PDM bitstream while preserving the audio information. By identifying and suppressing singleton and doubleton transitions (which carry minimal audio information but consume significant power), the encoding process removes the energy-wasting components while maintaining the noise-shaping benefits of digital conversion
Solution Approach 2:
The encoding process changes the transition density parameter of the digital signal. By transforming the PDM bitstream through XOR operations with delayed versions of itself, the patent reduces the switching activity parameter in the digital circuit, thereby lowering power consumption while preserving the audio signal's integrity and noise-shaping characteristics
Data Source
AI summary
An encoding technique for reducing the power of PDM microphones is disclosed. Digital MEMS microphones utilize a modulation technique called Pulse Density Modulation (PDM), where a single data line (PDMDAT) is used to convey the digital information from the microphone source to a receiver. A characteristic of PDM is that a low noise signal will produce the most transitions, a zero signal will produce an alternating bitstream of logic-1s and logic-0s, and low noise bitstreams will be rich in singleton and doubleton 1s/0s. Typically, CMOS drivers transmit the PDM bitstream signal. CMOS drivers consume power primarily when they transition, so a bitstream rich in singletons and doubletons will increase power consumption. Differential encoding with an XNOR function is used as a singleton-suppression encoder, and a differential encoding with an XOR function is used as a doubleton-suppression encoder. In some embodiments, such as a dual PDM microphone configuration, the microphones alternate sending data on the rising (transition to logic-1) and falling (transition to logic-0) edges of PDMCLK. In other embodiments, a Voice Activity Detection (VAD) function may be added. In some other embodiments, a suppressed clock pulse duration modulator may be added.