Yesterday my friend told me that she had set up an audio amplifier using TDA7297 and Raspberry Pi's 3.5 mm audio jack. And she was using omxd, a daemon for omxplayer, and playing mp4 video files. The audio output has been set to jack.
But the problem is that there's a strange noise in the background of audio. One is main audio+noise, the other is just noise with the main audio paused.) This problem occurs when the grounds of both systems (RPi and external audio amplifier) are connected. When disconnecting the grounds, this noise disappears.
But she need the common ground to use logic levels, control audio via MCU, use SPI, etc.
She tried simple RC low-pass, high-pass, band-pass, band-stop, notch, ... filters, but no help!
So, any suggestions ? What causes this noise ? And how can I hep her to eliminate it ?