Chirp in i2s Recording

[umer]

Hi there,

So in my previous posts, I have been asking questions about setting i2s on two nRF devices in different modes. I have been successful in making it work. 

However, as I have now observed the data, there is an unusual chirp waveform in the start of the recording as soon as I trigger start. See the following figures for a few examples:

The recorded audio signal initially is riding on the chirp waveform and as a result, is very noisy. But after 2.5 sec, the audio signal settles down and stays there (cant confirm for longer periods of time as my application records for only few seconds). 

Here is my sequence of API function calls for I2S in Zephyr:

i2s_configure(mic, I2S_DIR_RX, &config) // config: master mode, i2s standard frame, 16 kHz sample frequency
i2s_trigger(mic, I2S_DIR_RX, I2S_TRIGGER_START)
i2s_read(mic, &mem_block, &data_size)   // read calls made till the time threshold for the application arrives
i2s_trigger(mic, I2S_DIR_RX, I2S_TRIGGER_STOP)

Please advise what could I be doing wrong here.

Cheers.

Update: 

I have modified the figures to reflect time axis as well. 

[mathiaso]

Hi, Umer.

Would you be able to include the time axis as well in your plots?

Best regards,
Mathias

[umer]

Thanks for the reply. I have modified the original post to reflect the time axes.

[mathiaso]

Thank you, Umer.

From your code snippet, it looks like the microphone provides the I2S data directly. As I2S is a purely digital signal, I'd think the offset/chirp is already there when the data is sent from the microphone to the nRF5340. Does your microphone's datasheet mention any filters that can be adjusted?