Hello Bob/Brett,

Hope you don't mind if i ask another question directly from you guys.

The application that I'm working on requires extremely HIGH Quality Factor Q of at least 29, based on customer's specification of .05 - 0.5 bandwidth (octave). The woofer selects frequency range, bandwidth, and EQ level. Based on their relative position defines the filter settings. My problem is the high Q. Based on what I see on the Sigma Studio library, 16/15 is the actual maxima on all filters I've examined and tried. Is there a library/block for filters with HIGH Qs? or is there a way to improve the Q higher than 16? I'm using SigmaDSp ADAU1701 and Sigma Studio 3.9 build 2, rev1246.

Thanks a lot in advance!!!

Regards and more power,

Dan

Hello Dan,

The Q is limited not by the algorithms in the DSP itself, but by the calculations within SigmaStudio. Thus, you can get a higher Q when you bypass these calculations. I know of two ways to accomplish this:

The State Variable (Q input) FilterFind this filter under

Miscellaneousfilters. It calculates its own coefficients within the DSP in response to its variable F and Q inputs. Use DC sources (as shown below) or aux ADCs to set the frequency and Q as needed. Because the "Q" input actually responds to (1/Q), a small input here produces a high-Q filter:Calculate Your Own IIR CoefficientsThe second-order, double-precision General IIR Filter has a IIR Coefficient choice, where you type in your own IIR coefficients. Since this also bypasses SigmaStudio's calculations, you can enter coefficients that make a higher Q filter than otherwise possible. Calculate the coefficients with a spreadsheet, as described at:

http://ez.analog.com/docs/DOC-1819

If you need (for example) a peaking filter instead of a parametric filter, you can use the formulas found at:

http://wiki.analog.com/resources/tools-software/sigmastudio/toolbox/filters/general2ndorder

to have a spreadsheet calculate those coefficients. You could also use Matlab (that's out of reach for most of us, however). In this example I used Brett's spreadsheet (available at the above link) to calculate the coefficients for a parametric EQ filter with boost of 20dB, f = 63Hz, and Q = 33. This filter's response is shown via Stimulus/Probe below:

Note that the peak is only 14dB, not 20dB as expected. This is from an imperfection in Stimulus/Probe -- for more info, see http://ez.analog.com/message/132533#132533

Also, you cannot use Stimulus/Probe with the State-Variable filter, because this tool cannot evaluate the filter's coefficients that are figured at runtime in the DSP.

Best regards,

Bob

Message was edited by: Robert Blowsky (fixed error in schematic)