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M
mgr
Graduate
March 4, 2024
Question

STM32F411-disco low audio quality with PDM microphone

  • March 4, 2024
  • 1 reply
  • 2004 views

Hi,

I'm struggling with recording audio on STM32F411-disco (MB1115D) board.

I'm trying to run dedicated to this board example (Audio_playback_and_record), but out of the box it just doesn't work - recorded file is all silence.

I've spend quite amount of time on that, and after borrowing logic analyzer, I found that, microphone do not output any data - looks like clock is too low. Audio_playback_and_record example uses 16Khz audio sampling, when I increase it to 48Khz  by changing below line:

#define DEFAULT_AUDIO_IN_FREQ I2S_AUDIOFREQ_48K

It kinda start working, there is finally some sound being recorded but there is also a lot of glitch there.

Overall quality is very low.

Given, that do I miss something here? I was expecting that it will be super easy weekend project since I have my board, my PDM mic on it and examples but reality isn't that beautiful :face_with_rolling_eyes:

Attaching the recording, Thanks in advance.

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    1 reply

    A
    AScha.3
    Super User
    March 4, 2024

    Hi,

    I have no experience with pdm mics, but from ds MP45DT02 :

    AScha3_0-1709587707555.png

    ( with 2ch - stereo , i assume )

    at 16kHz *16b -> 256 khz (close to power down for the mics = zero output)

    With PDM (= 1 bit modulation) you need hi speed data , decimate, then get good resolution.

    The more data bits, the better the pcm signal.

    From example here:

    https://github.com/STMicroelectronics/x-cube-azrtos-f4/blob/befedcdd72388c9c9305eccf5208a19845c31b47/Drivers/BSP/STM32F411E-Discovery/stm32f411e_discovery_audio.c#L1048

    see : decimation is 64, so with 16bit frames need 4x frame rate for desired sampling rate,

    -> for 48kHz pcm audio need 192kHz input rate , giving 192k * 16 = 3,072 Mbit (close to max. frequ. for mic).

    So look at your program and what bit rates you give to the PDMDecoder , deciding what quality you can expect.

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