I'm new to studying audio domain and using the starter kit, and there are some questions about my setup.

It has hum from audio input if connected from my PC speaker output jack, but it works fine from the smartphone jack.
Furthermore, the noise will differ if I change the block size(audio frames) in IDE settings.
After quickly searching the forum, I found no explicit reference to the same issue as mine, but could it be because the board is using USB power from my PC? (I'm using the IDE via USB connection)

Are there any resources to help for understanding or something I may have missed?

Attachment is the audio input from Scope
alt text

When connecting audio from the same computer that's powering the board, there is a ground loop between the two which causes this disturbance. This should be done with an isolating transformer.