Here is a design article from EETimes about the common inter-IC audio protocols. The article covers the traditional audio signal chain, fully digital audio signal chains, I2S protocol, and PDM.
As audio integrated-circuit (IC) designs move to finer geometries, it becomes more difficult to design, and less cost-effective to integrate, high-performance analog circuits on the same piece of silicon with high-density digital circuits. Audio system architects are pushing analog portions of an audio signal chain further towards the input and output transducers and connecting everything in between digitally.
Interesting article, I didn’t know that PDM is now used in cell phone designs…
A curious coincidence is that PDM is precisely the way animal nervous systems represent sensory and other information.