I've considered making a phased array myself, but never got around to sending out the PCB. But here are two reasons by I2S is not the best option:
* I2S requires 3 instead of the 2 pins of PDM. However, in the datasheet that you provided, it shows how you can daisy-chain microphones which is really cool (even if not standard I2S.) So that argument goes away.
* PDM gives you access to way higher sample rates which in turns gives you more flexibility in choosing the delay for a delay-and-sum operation. For example, if the PDM clock is 2MHz, you could theoretically delay with a precision of 0.5us. In practice, you'll do that with lower precision, but with I2S, the clock will typically max out at 192kHz.
1) and 3) are valid, but 2) isn't really. In that sort of pipeline, you usually do IQ sampling which allows you to phase-shift by any arbitrary value with a complex multiplication.
* I2S requires 3 instead of the 2 pins of PDM. However, in the datasheet that you provided, it shows how you can daisy-chain microphones which is really cool (even if not standard I2S.) So that argument goes away.
* PDM gives you access to way higher sample rates which in turns gives you more flexibility in choosing the delay for a delay-and-sum operation. For example, if the PDM clock is 2MHz, you could theoretically delay with a precision of 0.5us. In practice, you'll do that with lower precision, but with I2S, the clock will typically max out at 192kHz.
* PDM microphones then do be cheaper.