If we connect an amplifier to the analog output of the Ixora board (with i.mx6 module) we get noise every time we show a simple blending effect with ~100x100 pictures on the LVDS display.
The display does not need to be connected to create the noise.
During the blending, we can see this on the power line (the signal with the higher amplitude):
The Ixora board audio path is not optimized for high audio quality. High frequency noise (around 1MHz) is leaking from digital circuitry. The simplest way to fix it is to add a low pass filter between audio output and amplifier. Directly connected speakers or headphones act as a low pass filter by themself so the low pass filter is not required.
I forgot to mention that we have an audible noise, the EMC stuff is not relevant here. The frequency of the noise is pretty low.
We had it also really loud on the microphone input, but we could fix it here by using a linear regulator for AVCC, as described in the Apalis Carrier Board Design Guide.
For the audio output we currently have no solution.
We are using BSP 2.7.
We created an image without X11/Wayland.
We are running Qt5 software with eglfs output plugin and we use QML-based blending effects for a small slide show.
The CPU load and video processing does not affect the noise, it just seems to be related to 2D/3D effects.
We don’t need to play sound to hear the noise.