How PDM_Filter_XX_XX works?

[Volodymyr Kukhar]

Posted on December 29, 2016 at 15:42

I'm trying to connect SPH0641LU4H-1 microphone to STM32F411RE Nucleo board.

According to the Wikipedia 

https://en.wikipedia.org/wiki/Pulse-density_modulation

 format should represent lowest and highest amplitude as 0b0000...(all 0) and 0b1111...(all 1) accordingly.

When I connect microphone pin to the ground ->  0b0000... PDM data is transferred to PDM_Filter_XX_XX and output sound is silence. But when I connect it to 3.3v source to emulate PDM 0b1111... ->  the output I get is 1KHz noise. (

mailto:16bit@16KHz

 PCM). PDM_Filter_128_LSB function outputs big(depends on micGain) int16_t values with + or - sign.

But I expect that 0b0000... and 0b1111... represents maximum and minimum diffuser deflection. Am I wrong?

In general, I receive strange binary from PDM_Filter which doest'n looks like a wave and sounds like very noisy 1KHz square wave oscillator.

Some ideas where I went wrong? I can provide details for every step I did.

Thanks!

(configuration:

   I2S:   MasterClock: ON

            PLLI2SN: 192

            PLLI2SM: 16

            PLLI2SR: 2

            AudioFreq: 64000

      PDM_Buffer size: 128 * uint16_t

      PCM_Buffer size: 16 * uint16_t

)

#nucleo #microphone #i2s #stm32f4-i2s

[Imen.D]

Posted on December 29, 2016 at 16:08

Dear

Kukhar.Volodymyr

‌,

This application Note

http://www.st.com/content/ccc/resource/technical/document/application_note/ca/18/be/bb/f8/53/47/a5/DM000408pdf/files/DM000408pdf/jcr:content/translations/en.DM000408pdf

,

in

'5.2 PDM_Filter_xx_xx' section, may help you with more details and clarification aboutthis topic. Also, t

here is anAudioexample under STM32CubeF4 firmware package, so you can get inspiredto develop your application and

update it according to your needs:

STM32Cube_FW_F4_V1.0\Projects\STM32F4-Discovery\Applications\Audio\Audio_playback_and_record

You shouldfollow the instructions on the

readme.txt file describing how to usethe example.

Hope this helps you.

Best Regards

Imen

[Volodymyr Kukhar]

Posted on December 30, 2016 at 13:56

Yes, I have read

/external-link.jspa?url=http%3A%2F%2Fwww.st.com%2Fcontent%2Fccc%2Fresource%2Ftechnical%2Fdocument%2Fapplication_note%2Fca%2F18%2Fbe%2Fbb%2Ff8%2F53%2F47%2Fa5%2FDM000408pdf%2Ffiles%2FDM000408pdf%2Fjcr%3Acontent%2Ftranslations%2Fen.DM000408pdf

m

any times, and my application done similar

to

https://github.com/fboris/STM32Cube_FW_F4/tree/master/Projects/STM32F4-Discovery/Applications/Audio/Audio_playback_and_record

and

http://www.st.com/en/embedded-software/x-cube-memsmic1.html

examples with some differences:

1. External microphone connected to i2s2,

2. Icollect 0.5sec of sound and transmit it via UART to pc.

My question is more narrow about PDM_Filter_xx_xx function, and the way it works.