49.6.3 Audio Filter
The PDMC embeds a predefined audio filter which is combined to the SINC filter in order to optimize an audio signal. This audio filter is made of IIR filters and includes a droop compensation filter made of a third-order SINC filter.
The audio filter has a decimation factor of 4. The overall OSR is defined by PDMC_MR.OSR and is 64, 128 or 256.
Note: For example, when the application requires a 32 Ksps sampling
frequency, the OSR can be configured to 64 and thus the GCLK frequency must be 32K x 64
x 16 = 32.768 MHz. The microphone is driven by 2.048 MHz. For the same GCLK frequency, an
OSR of 128 provides a 16 Ksps audio stream. Refer to the microphone reference manual for
the maximum frequency on PDMC_CLK.
When combined with the GCLK division factor of the PMC, the cut-off frequency can be optimized for any standard audio frequencies.
The figure below shows the spectral mask of the audio filter when the PDMC output frequency is configured to be 48 kHz. The mask scales with the output frequency.
