What could be the possible reason behind curved signal?

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Hi,
This question is not directly related to MATLAB, but if someone can help me, I would really appreciate.
I have a MEMS microphone connected to the microcontroller on a PCB. The microphone collects the data and sends the analog data to the microcontroller in real-time. On the microcontroller, this data is converted to digital form and then I extract this digital data from the microcontroller. But when I plot the signal, I get the signal as a curved one. It looks like the DC offset is not constant. All the recordings have this issue.
However, for ex., I record a song and when I play the file (after converting them to .wav files using MATLAB with specs such as sampling freq: 40KHz, Bits/sample:16), I don't see any problem in the audio files, I am able to hear the song properly. I have attached the plot as well as the audio file for reference.
Can anybody suggest what could be the issue here or what can I try to get rid of this? Thanks.
This is audio file: Audio file
  10 Comments
Walter Roberson
Walter Roberson on 4 May 2022
Calibration at different frequencies seems in order. If the chain seems to faithfully reproduce sine through the mic then there is probably no point in forcing the signal generator... but if through-the-mic is distorted then Yes, signal generator would be a classic debugging step.

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