This application note describes the audio recording application based on SAM G53 Xplained Pro, MEMS MIC Xplained Pro, and I/O1 Xplained Pro kits. The recording process is based on MEMS MIC Xplained Pro using WM7220 MEMS microphone hardware with a PDM hardware interface controller on SAM G53.
The Pulse Density Modulation Interface Controller (PDMIC) from SAM G53 is a PDM interface controller and decoder that support both mono/stereo PDM format. It integrates a clock generator driving the PDM microphones and embeds filters which decimate the incoming bit stream to obtain most common audio rates.
The PDMIC includes a DSP section containing a decimation filter, a droop compensation filter, a sixth-order low pass filter, a first-order high pass filter and an offset and gain compensation stage.
Pulse-density modulation, or PDM, is a form of modulation used to represent an analog signal with digital data. The audio signal coming from MEMS microphone is in the form of PDM signal. PDM uses only one bit to convey audio.
The PDM clock (PDMCLK) is used to sample the PDM bit stream. The SAM G53 device provides the Master clock to Microphone to transmit the PDM data.
The one-bit data is asserted on the data line on either the rising or falling edge of the master clock. For PDM stereo operation, in which one microphone asserts the data line on the rising edge of the master clock which is routed to PDMIC0, while a second microphone asserts the data line on the falling edge which is routed to the PDMIC1.
The data coming from the microphone is sent to the decimation process where the 1-bit PDM data is converted to 16-bit PCM data and it is decimated by a factor of 64 or 128