Here's a question I've heard from customers:
What is the difference between PDM and I2S digital microphones?
Analog Devices' digital microphones are available with two different output formats: PDM and I2S. The output of a PDM (pulse density modulation) microphone is a 1-bit high sample rate data stream that is the direct output of the Sigma-Delta modulator that's in the mic. The sample rate of this PDM stream is typically between 1 and 3.25 MHz. Before this PDM data can be further processed, it needs to be decimated to a lower sample rate that other devices, such as codecs and DSPs, can handle. This decimation filter is implemented in the codec or DSP to which the PDM microphone is connected. The output of this filter gives data at a lower sample rate, typically between 16 and 48 kHz.
A microphone with an I2S output, like the ADMP441, has all of the same pieces of the signal chain as the PDM chain, but the places where each piece is implemented is slightly different. An I2S output digital microphone includes the decimation filter, so its output is already at a standard audio sample rate that's easy to interface to and process. Since this filter is implemented in the microphone, the ADMP441 can connect directly to a DSP or microcontroller with an I2S input. This saves components in systems that don't otherwise need an ADC or codec, and saves the processor from the burden of implementing the decimation filter, either in hardware or in software.
A PDM microphone does have other advantages over an I2S mic. The PDM microphone typically has a smaller footprint, and only requires two signals (clock and data) for operation, while I2S requires three (data, frame clock, bit clock). The ADMP521 and ADMP421 are examples of Analog Devices' PDM MEMS microphones.
If you want to read more about different digital audio interfaces, check out an article that I wrote on that topic (pdf link), or post any other questions you might have below.