Single PDM microphone at higher PCM sampling rate

[NewtoM]

Hello,

we need to feed mono audio data into the nRF52 and we are now evaluating the possibility to use PDM. The nRF52 seems to have a useful PDM interface, and even though an example does not seem to exist in the SDK [what a pity!], there is a PDM driver. There also seem to be many PDM microphones available, like from Knowles or InvenSense.

All of these mics are sensitive to 20+ KHz, which we would like to make use of. We planned to have 48 KHz sampling rate and I wondered how and where I could set that in the PDM driver.

After some search, on the PDM Interface site of the Nordic Infocenter I found that the PDM features "16 kHz output sample rate, 16-bit samples". So does it mean that the sample rate cannot be changed? Is this a driver issue or is this a limitation of the PDM hardware used? If the latter is able to handle stereo signal at 16 KHz, I would at least expect it to handle mono signal at 32 KHz without overhead.

Can you please comment on that or propose a hack to change the PCM sampling rate? Thank you!

NewtoM

[mkimhj]

I'm putting together a project right now with a single PDM mic and the nRF52832 and found your post super helpful.

I'm struggling a bit because there was no example provided by the SDK, would you be willing to share your source code?

[NewtoM]

Hi Ole,

I've seen a few more PDM frequencies added to the nRF5 SDK v16.0.0, which is good. Back in the day I used an oscilloscope to measure PDM clock frequencies and document the register setting for the given divisor. I still wonder, though, what is the closed equation for the relation between the register value and the PDM clock speed (or clock divider)? Can you provide me that?

Thanks,
Tamas

[mkimhj]

32000000 / (DIV * 64) = Sampling Rate

To get the PDM frequency:

32000000 / DIV

[Yonghun Song]

Hi, I'm Song

Now I'm using knowles microphone to analyze voice.

I don't know how to access PCM data.( easyDMA )

So, It would be great if you could share the code...

thank u.

- Song

[NewtoM]

The question is, how the register settings relate to the possible DIVs. E.g.

#define DIGITAL_AUDIO_SAMPLING_RATE_15625 (0x08000000UL)    // (32 MHz / 32)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_16129 (0x08400000UL)    // (32 MHz / 31)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_16667 (0x08800000UL)    // (32 MHz / 30)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_20000 (0x0A000000UL)    // (32 MHz / 25)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_31250 (0x10000000UL)    // (32 MHz / 16)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_33333 (0x11000000UL)    // (32 MHz / 15)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_45455 (0x16000000UL)    // (32 MHz / 11)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_50000 (0x18000000UL)    // (32 MHz / 10)  / 64
#define DIGITAL_AUDIO_SAMPLING_RATE_62500 (0x20000000UL)    // (32 MHz / 8)   / 64

So, say, if I wanted 4000 Hz sampling rate (i.e. 256000 Hz PDM frequency), what would be the register value? 0x... ? And why?

Thanks,
Tamas