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Strange output on biquad

Question asked by guytaar on Mar 1, 2013
Latest reply on Mar 1, 2013 by mmmike

Hi all,

 

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!

 

Best regards,

 

Guy

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