I've set up an audio amplifier using TDA7297 and Raspberry Pi's 3.5 mm audio jack. And I'm 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. (The recorded audio is http://vocaroo.com/i/s0c0RC3uJE2o and http://vocaroo.com/i/s0Ek3z3ltDSO. 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 I need the common ground to use logic levels, control audio via MCU, use SPI, etc.
I 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 eliminate it !?