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.
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:
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.