AnsweredAssumed Answered

Algorithm Information in help file

Question asked by Tony on Oct 20, 2009
Latest reply on Dec 14, 2011 by twest820



As I'm trying to build an interface to control the Biquad filter, I'd just like to get confirmation on the following statement made in the Help File of Sigma Studio - > Algorithm information - > EQ.

"In the implementation on the DSP, when the coefficients are stored  in parameter RAM, a1 and a2 need to be inverted. This is done in software before  the parameters are written to memory."


When I implement the Biquad equations in my custom uC code and pluggin the coeffs into an IIR block, a1/a2 don't have to be inverted to create the correct filter. On the other hand, the results I would get from the "Fixed point filter table generation tool" would indeed have a1 and a2 inverted. Just confused by the "this is done in software" line and where I actually have to invert the coeff...


Also just wondering as It is stated that these equations come from the work of Robert Bristow-Johnson, if the implementation is different. For example, comparing with his other publication, I've noticed that b0 and b2 would have to be inverted (-) in order to work correctly for both High Pass filter (Butterworth and Bessel).

b0  = -(1 + cos(ω0)) * gainLinear / 2

b2  = -(1 + cos(ω0)) * gainLinear / 2

Should it be the following instead?

b0  = (1 + cos(ω0)) * gainLinear / 2

b2  = (1 + cos(ω0)) * gainLinear / 2

Maybe if you could confirm the equations from the help file that would be great!