Question about PDM microphones

Projects Discussion and Showcase Science and Measurement
1

Hello,

I have a question about how PDM microphones work when selecting the left or right channel. Selection between the 2 channels is done by connecting the appropriate pin to either VCC or GND.

Depending on how the connection is made data is collected on the rising or falling edge of the signal. This is the part that I do not understand. If I select the rising edge does this mean that it will send data to the microcontroller when the noise begins? If I select the falling edge does this mean that it will send data to the microcontroller when the noise ends?

If I have 2 independent systems that are identical except the channel selection and I play a sound will the system that is wired on the rising edge pick up the signal before the system wired on the falling edge?

This is purely a theoretical question to understand how the PDM microphone works. I hope it is not too silly but I do not know much about microphones.

2

The rising and falling edge is referring to the ws clock. The ws clocks in the left channel on one edge and the right channel on the other edge of the ws clock.

The right channel and left channel together form a "frame" they get clocked into i2s dma buffers serially.

You can think of the right channel and left channel being recorded at the same time. And played at the same time.

If your sample rate is 16KHz there are 16,000 frames a second.

As soon as you turn on the i2s peripheral it will start "recording"

3

See if this link helps: https://users.ece.utexas.edu/~bevans/courses/rtdsp/lectures/10_Data_Conversion/AP_Understanding_PDM_Digital_Audio.pdf

4

PDM is different from I2S
Here is a datasheet for a PDM mic
See the timing diagram figure 3 and the stereo set-up figure 5

MP34DT01-M-1.pdf (752.0 KB)

5

If the systems are truly independant then the terms rising/falling have no meaning.