STM32H7A3ZI-Q: 4× INMP441 via SAI A/B in I²S/PCM mode – one stereo pair is noisy & distorted
Question:
I’m using an STM32H7A3ZI-Q with four INMP441 MEMS microphones. Each INMP441 outputs PCM (not PDM), and all four share the same BCLK (bit clock) and LRCLK/WS (word select). I’ve split the data lines into two stereo pairs:
SAI_A (master) → Microphones 1 & 2
SAI_B (synchronous slave) → Microphones 3 & 4
Both SAI blocks run in I²S/PCM mode, each with its own DMA stream.
Only one stereo pair connected → perfectly clean audio
All four microphones connected → one stereo pair becomes very noisy and distorted
hsai_BlockA1.Instance = SAI1_Block_A;
hsai_BlockA1.Init.AudioMode = SAI_MODEMASTER_RX;
hsai_BlockA1.Init.Synchro = SAI_ASYNCHRONOUS;
hsai_BlockA1.Init.OutputDrive = SAI_OUTPUTDRIVE_ENABLE;
hsai_BlockA1.Init.NoDivider = SAI_MASTERDIVIDER_ENABLE;
hsai_BlockA1.Init.FIFOThreshold = SAI_FIFOTHRESHOLD_3QF;
hsai_BlockA1.Init.AudioFrequency = SAI_AUDIO_FREQUENCY_16K;
hsai_BlockA1.Init.SynchroExt = SAI_SYNCEXT_DISABLE;
hsai_BlockA1.Init.MonoStereoMode = SAI_STEREOMODE;
hsai_BlockA1.Init.CompandingMode = SAI_NOCOMPANDING;
hsai_BlockB1.Init.TriState = SAI_OUTPUT_NOTRELEASED;
if (HAL_SAI_InitProtocol(&hsai_BlockA1, SAI_I2S_STANDARD, SAI_PROTOCOL_DATASIZE_24BIT, 2) != HAL_OK)
{
Error_Handler();
}
hsai_BlockB1.Instance = SAI1_Block_B;
hsai_BlockB1.Init.AudioMode = SAI_MODESLAVE_RX;
hsai_BlockB1.Init.Synchro = SAI_SYNCHRONOUS;
hsai_BlockB1.Init.OutputDrive = SAI_OUTPUTDRIVE_DISABLE;
hsai_BlockB1.Init.FIFOThreshold = SAI_FIFOTHRESHOLD_3QF;
hsai_BlockB1.Init.SynchroExt = SAI_SYNCEXT_DISABLE;
hsai_BlockB1.Init.MonoStereoMode = SAI_STEREOMODE;
hsai_BlockB1.Init.CompandingMode = SAI_NOCOMPANDING;
hsai_BlockB1.Init.TriState = SAI_OUTPUT_RELEASED;
if (HAL_SAI_InitProtocol(&hsai_BlockB1, SAI_I2S_STANDARD, SAI_PROTOCOL_DATASIZE_24BIT, 2) != HAL_OK)
{
Error_Handler();
}This is my SAI Configuration.
void Interleave4ch(const int32_t *bufA, const int32_t *bufB, uint32_t count)
{
for (uint32_t i = 0; i < count; i++) {
uint8_t *dst = &wavBuf[i * 4 * 3]; // 4 Channel × 3 Bytes
int32_t a0 = bufA[2*i + 0];
int32_t a1 = bufA[2*i + 1];
int32_t b0 = bufB[2*i + 0];
int32_t b1 = bufB[2*i + 1];
// Channel 0
dst[0] = a0 & 0xFF;
dst[1] = (a0 >> 8) & 0xFF;
dst[2] = (a0 >> 16) & 0xFF;
// Channel 1
dst[3] = a1 & 0xFF;
dst[4] = (a1 >> 8) & 0xFF;
dst[5] = (a1 >> 16) & 0xFF;
// Channel 2
dst[6] = b0 & 0xFF;
dst[7] = (b0 >> 8) & 0xFF;
dst[8] = (b0 >> 16) & 0xFF;
// Channel 3
dst[9] = b1 & 0xFF;
dst[10] = (b1 >> 8) & 0xFF;
dst[11] = (b1 >> 16) & 0xFF;
}
}This is my interleave function
What I’m looking for:
Which additional SAI or DMA settings become critical when handling four INMP441s in I²S/PCM mode?
Could an internal resource limit or clock jitter cause this?
Would it make sense to use a single SAI block in TDM-4-slot mode instead of two separate stereo instances, ist it even possible since i receive PCM?
Any best practices for reliably interfacing four digital MEMS mics with an STM32H7 over I²S/PCM?
- I need the Data from all Microphones at the same time.
Thanks in advance for any suggestions or insights!