I've noticed strange behavior with the biquad blocks in a device that we'll be selling soon. It has an ADAU1442 inside. I've notice some output signal after a biquad which has no input (only a bit noise, if this noise is removed by putting the level at -inf, the issue disappears). The filter I've used is a HPF (butterworth) at 20Hz but also with peak or other filters types in low freqencies, it happens. I've attached a scope image and what I see is like a very low frequency ocillation. I've also added some views of the debug software that I use and there I can see that level before the biquads is (lower than) -100dBFS and after the biquads it can go up to -30dBFS so it's noticable on the signal indication. The VU bars go up and down slow, like the wave of the scope. When I put the biquad off (b0 = 1, all others 0), the signal after the biquad also goes below -100dBFS and on the scope, I don't see the low frequency swings. I've read back the parameters from the DSP (for the HPF) and they seem correct to me (
b0 = 0x007FC366, b1 = 0x0F007934, b2 = 0x007FC366, -a1 = 0x00FF86B0 and -a2 = 0x0F807917). Is there anything I did wrong? Any hints how it could happen and maybe info how to solve this? Thanks in advance!