I am trying to understand how a SIM7600G 4G modem degrades the audio when configured for 8 kHz sampling rate. I created a reference audio using librosa.chirp to slowly sine sweep from 30 Hz to 3400 Hz. I saved the raw audio using 8 kHz sampling rate.

[![reference audio](https://i.stack.imgur.com/lRPAE.jpg)](https://i.stack.imgur.com/lRPAE.jpg)

Then I called the modem from a high quality digital land line and recorded the call. The modem answered the call and played the audio. This is the result:

[![degraded audio](https://i.stack.imgur.com/7aw6w.jpg)](https://i.stack.imgur.com/7aw6w.jpg)

The vertical lines intercept the original sine sweep at 1350Hz, 800Hz, 560Hz, and 440Hz.

**Question #1: Do you see an obvious pattern here?**

I find it confusing that the sine-sweep went trough, but something added all these artifacts that I guess are aliasing artifacts.

**Question #2: Do you know what is going on here?**

My only guess is that there is a somewhat-dumb noise canceling algorithm trying to remove specific frequencies from the audio, completely blind to the fact the I am playing a sweep. But I do not know yet what is going on.