I have some questions with sample code of STM32F407G discovery board ，How do i know to set these value?

[cTomm]

I am learning I2S to record voice but I met some questions.

Q1: I try to change the Filter.Fs form 16000 to 24000 ,but I can't get clear voice.

I don't Know why. How can i do?

Q2: I read the AN3998 "PDM audio software decoding library description "

​ and I calculate Input buffer length and Output buffer length are 64 and 8

respectively, why PCM_OUT_SIZE  of this code is 16 not 8 ?

Output frequency = 8000​

decimation factor = 64

Input Microphone Channels =1

Output Microphone Channels =1

Input buffer length = 8000 / 1000 * 64 *1/8 =64

Output buffer length= 8000 / 1000 *1 =8 <=

*following description is "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.

�? Input buffer (data) is a uint8 variable with a length equal to (Output frequency / 1000 * decimation factor * Input Microphone Channels / 8) at least.

�? 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;

*following code is sample code.

• following AudioFreq is 32000

/* Audio recording frequency in Hz */

#define REC_FREQ             8000  

/* PDM buffer input size */

#define INTERNAL_BUFF_SIZE   64

/* PCM buffer output size */

#define PCM_OUT_SIZE      16

uint32_t WaveRecorderInit(uint32_t AudioFreq, uint32_t BitRes, uint32_t ChnlNbr)

{ 

 /* Check if the interface is already initialized */

 if (AudioRecInited)

 {

  /* No need for initialization */

  return 0;

 }

 else

 {

  /* Enable CRC module */

  RCC->AHB1ENR |= RCC_AHB1ENR_CRCEN;

  /* Filter LP & HP Init */

  Filter.LP_HZ = 8000;

  Filter.HP_HZ = 100;

  Filter.Fs = 16000;

  Filter.Out_MicChannels = 1;

  Filter.In_MicChannels = 1;

  PDM_Filter_Init((PDMFilter_InitStruct *)&Filter);

  /* Configure the GPIOs */

  WaveRecorder_GPIO_Init();

  /* Configure the interrupts (for timer) */

  WaveRecorder_NVIC_Init();

  /* Configure the SPI */

  WaveRecorder_SPI_Init(AudioFreq);

  /* Set the local parameters */

  AudioRecBitRes = BitRes;

  AudioRecChnlNbr = ChnlNbr;

  /* Set state of the audio recorder to initialized */

  AudioRecInited = 1;

  /* Return 0 if all operations are OK */

  return 0;

 }  

}

void AUDIO_REC_SPI_IRQHANDLER(void)

{  

  u16 volume;

  u16 app;

 /* Check if data are available in SPI Data register */

 if (SPI_GetITStatus(SPI2, SPI_I2S_IT_RXNE) != RESET)

 {

  app = SPI_I2S_ReceiveData(SPI2);

  InternalBuffer[InternalBufferSize++] = HTONS(app);

  /* Check to prevent overflow condition */

  if (InternalBufferSize >= INTERNAL_BUFF_SIZE)

  {

   InternalBufferSize = 0;

   volume = 50;

   PDM_Filter_64_LSB((uint8_t *)InternalBuffer, (uint16_t *)pAudioRecBuf, volume , (PDMFilter_InitStruct *)&Filter);

   Data_Status = 1;    

  }

 }

}