SigmaStudio's Bessel filter coefficient operation inquiry
Hi all
One of the customers is developing the product by applying ADAU1452 to the product.
There are inquiries about Bessel filters in the filter.
If measure another manufacturer's equipment in the Bessel filter and simulate it with MATLAB, the signal will decrease -3 dB in cutoff frequency as shown below.
However, when we simulate with the filter coefficients provided by SigmaStudio, the signal size is reduced by -5 dB at the cutoff frequency. If you actually apply a filter and measure the signal with the instrument, -5dB will fall.
In general, I know that the -3 dB drop in cutoff frequency in a filter.
Other filters in SigmaStudio fall -3dB at the cutoff frequency.
Is there a reason why only the Bessel filter drops -5dB?
Thank you
Audio Signal Processors
adau1452
SigmaDSP Audio Processors
Hello,
It looks like ADI set up their Bessel implementation to match the stopband response of the Butterworth filter as shown below -- both of these 1 KHz lowpass filters achieve 40 dB down at about 9 KHz. Also notice the phase shift traces, which cross at exactly 1 KHz as well. Since the Bessel filter has lower Q and a less-sharp knee, this forces the approx. 4.8 dB loss at 1 KHz.
If we multiply the Bessel filter's cutoff frequency by 1.275, its 3 dB point lines up with that of a Butterworth filter as shown below. However, now its 1 KHz phase shift as well as its stopband response no longer line up with the Butterworth filter.
So it depends on what's important to your application. As a workaround, you could just multiply your Bessel lowpass filter's cutoff frequency by 1.275 and the resulting filter will have its 3 dB drop at 1.0 x fc.
Best regards,
Bob
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