Q value of filter
I am playing with the second order general filter in my adau1701.
I generate a noice input from PC, passing though the 1701 and back to the PC. Then I plot the freq response in the PC
I use the sigma studio to calculate two set of filter coeff
1) freq=15000, gain=-16, q=1.5 (as sigma_studio setting.jpg, correct me is I am wrong)
2) freq=1000, gain=-16, q=1.5
And I have two question about that:
1) By looking at the 1k filter freq response, the peak and the freq cutoff relation is not matching with the normal filter relation.
I use the calculator(http://www.sengpielaudio.com/calculator-cutoffFrequencies.htm) and get the high freq cut off should be at around 1.3kHz while I end up with the high freq cut off at 1.1kHz
2) By comparing the 1k filter and 15k filter with the same q, it seem the 15k filter is more narrow then the 1k filter. Is that a reason behind?
I am very new to those filter's, correct me if I doing somethings that didn't make sence
Audio Signal Processors
SigmaDSP Audio Processors
adau1701
I will answer question (1) tomorrow or Friday.
For question (2): Since the filter is constant-Q, that means it is variable bandwidth. The bandwidth is dependent on the center frequency. As the center frequency increases, the bandwidth will decrease. You can verify this using the calculator you referred to on your external link.
Looking forward for your answer, thanks
Hi Philip,
I gave this a try today. I used filters at 1 kHz, 2 kHz, 4 kHz, 8 kHz, and 16 kHz with a Q of 1.41.
I tested on an evaluation board with S/PDIF input and output.
You can see from my results that the bandwiths match up perfectly for 1 kHz through 8 kHz. The one at 16 kHz seems a little off, but I don't have time to look into that anymore at the moment. Let me know what you think.
Philip, actually I think I know why the 16 kHz filter is off. According to the calculation, the upper limit of the bandwidth should be above the Nyquist rate of 22 kHz, which is impossible. I guess for that reason, the BW gets shrunken a little bit near the high end of the spectrum.
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Thank you,
EZ Admin
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