A digital PDM microphone works 1-bit stream as output with the direct output of the Sigma-delta modulator. The sample rate of the PDM is typically between a few hundred kHz to 3.072 MHz. This Sigma-delta modulator needs a decimation filter so that the PDM data can process further. The decimation filter is implanted in either the codec, or the DSP where the PDM microphone is connected. This output from the filter sends out data at a lower sample rate of 16 and 48 kHz. This is a weaker point than an analog microphone with an ADC system shown in Section 3.2. However, a PDM microphone does have some advantages over an I2S microphone. The PDM microphone typically requires only two signal lines (PDM clock and PDM data) for audio interface, while I2S requires three or four lines (bit clock, word clock, audio data, and occasionally master clock). This leads to a beneficial idea where the hardware engineers are easily able to minimize the physical interface line.

Figure 3-1 Digital PDM Microphone for 1-bit Output