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Edit: from Wikipedia's Delta-sigma modulation it looks like since the microphone basically is an ADC, and that as such mics with PDM output may use a Σ‑Δ modulator internally. Also, from Selecting PDM Microphone Clock Frequencies and Decimation Ratios I read that "A Pulse Density Modulation (PDM) microphone uses a Sigma-Delta modulator to oversample an acoustic signal at a high sampling rate. This digital PDM signal is output..". Adding from Marcus' comment, maybe the "smart" is marketing only(?)

I see that there is something called "Smart MEMS microphones" which contain a Σ‑Δ modulator. Like T5828, T5837, T5838, T5848 from TDK InvenSense (SmartSound™ Overview).

Assuming that older MEMS mics with PDM output would not contain this, am I right in assuming that it is the addition of the Σ‑Δ modulator that makes it "smart"? Or would this be along another axis? Like level detection.

These mics still present a 1-bit datastream that must be decimated in SW.

What would a Σ‑Δ modulator in the end add?

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    $\begingroup$ ha! The only thing that makes any device "smart" is the marketing department's desire to call it "smart" :) But I think your question is interesting: $\Sigma-\Delta$ modulation is one way to generate PDM from an analog signal, but probably not the only one. I don't know which converter architectures other PDM microphones use! $\endgroup$
    – Marcus Müller
    Commented Feb 13, 2023 at 16:18

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The microphones feature 5th order sigma-delta converter which for a 1 bit output features the same performance as a data converter with a significantly higher number of bits. A standard ADC has an oversampling performance of 3 dB/octave (you gain half a bit after filtering for every doubling in frequency). In comparison a sigma-delta ADC adds 6 more dB for each order; a 5th order will offer 33 dB/octave of SNR improvement! So they were certainly smart to include a 5th order rather than try to get away with a first order solution! However I don’t think that is the “smart” they are referring to in their marketing.

The "smart" may actually be the "Acoustic Activity Detect w/ programmable thresholds and filters with power consumption as low at 20 uA" that is mentioned in the product flyers.

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    $\begingroup$ Thank you very much! This was clarifying! $\endgroup$
    – Øyvind Teig
    Commented Feb 14, 2023 at 8:13

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