AN3998

PDM audio software decoding library description

Doc ID 022391 Rev 1

7/10

5

PDM audio software decoding library description

The PDM library is composed of a structure and the implementation of four PDM filter

functions. The library uses two buffers, the PDM Input buffer and the PCM Output buffer; the

application must define these buffers in the main program.

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Input buffer (data) is a uint8 variable with a length equal to (Output frequency / 1000 *

decimation factor * Input Microphone Channels / 8) at least.

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Output buffer (dataOut) is a uint16 variable with a length equal to (Output frequency /

1000 * Output Microphone Channels) at least.

The structure is defined in the pdm_filter.h file and is used to configure the filter; it is

composed as follows:

typedef struct {

uint16_t Fs;

float LP_HZ;

float HP_HZ;

uint16_t Out_MicChannels;

char InternalFilter[34];

} PDMFilter_InitStruct;

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Fs: Defines the frequency output of the filter in Hz.

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LP_HZ: Defines the low pass filter cut-off frequency. If 0, the low pass filter is disabled.

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HP_HZ: Defines the high pass filter cut frequency. If 0, the high pass filter is disabled.

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In_MicChannels: Define the number of microphones in the input stream. This

parameter is used to specify the interlacing of microphones in the input buffer. The

PDM samples are grouped eight by eight in u8 format (8-bit).

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Out_MicChannels: Defines the number of microphones in the output stream; this

parameter is used to interlace different microphones in the output buffer. Each sample

is a 16-bit value.

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InternalFilter: Defines a 34 Byte memory used internally by the library during the PDM

decimation step.

The PDM audio software decoding library includes one header file pdm_filter.h and

binary/object codes for the following platforms:

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libPDMFilter_IAR.a

: for IAR compiler

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libPDMFilter_Keil.lib

: for ARM compiler

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libPDMFilter_GCC.a : for GNU compiler

This Library is provided in the "STM32F4-DISCOVERY board firmware package" (V1.10

and later) under the following path 'Utilities\STM32F4-Discovery'

5.1

PDM_Filter_Init

This function is used to initialize the filter of a single output channel. It is defined as follows:

Void PDM_Filter_Init (PDMFilter_InitStruct * Filter)