understanding PDM gain

[mjaszczykowski]

Dear Nordic,

In our application, we use nRF52840 (nRF5 SDK) and a PDM microphone to listen to the very quiet signals.

We would like to understand how exactly gain works. I could not find more information than this:
https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fpdm.html

Is the gain just purely digital multiplication/division and can be applied after processing producing the same result?
Or can increased gain improve the SNR for a quiet signal? 

Best regards,
Marcin

[Kazi Afroza Sultana]

Hello Marcin,

You can not increase SNR ratio with gain. I will explain rest of the queries later on Monday.

Thanks.

BR

Kazi

[mjaszczykowski]

Hello Kazi Afroza Sultana,

Thank you very much for your reply.

I am looking forward to seeing more details.

[mjaszczykowski]

Hello Kazi Afroza Sultana,

I hope you are well.

Do we have any update on this one?

[Kazi Afroza Sultana]

Hello,

The PDM module of the nRF52832 converts the PDM signal to PCM.

The gain set in the register will alter the numerical value for each sampling. The sampling rated will be reduced by the oversampling factor. Decimation process is done to accomplish this through a digital filtering operation. The input of the filter is the two-channel PDM serial stream (with left channel on clock high, right channel on clock low). Depending on the RATIO selected, its output is 2 × 16-bit PCM samples at a sample rate either 64 times or 80 times (depending on the RATIO register) lower than the PDM clock rate.

The gain is controlled by GAINL and GAINR registers. 

[mjaszczykowski]

Kazi Afroza Sultana said:

The gain set in the register will alter the numerical value for each sampling

Thank you very much for your reply.
What do you mean by alter in mathematical terms? Is it multiplication for a gain greater than zero?