I have to calculate EQ coefficients before start my ADAU1701 every time. The first filter in EQ is LowShelf filter and last one - High-Shelf. Rest are peaking filters. There are some threads on this forum about calculation of EQ filter coefficients, but only about EQ peaking filter. I found new problem: mysterious S factor in EQ shelving filters.
I thought the S factor means "Slope" parameter of Low- or High-Shelf filter. Slope can vary from 0 to 2 in graphics editor. But there is not Slope in FilterTableGen. There is Q parameter, which can vary from 0,01 to 20.
I detected the problem when I started to change the parameter S in excel -> argument of sqrt function sometimes is going under the 0.
Boost = 5dB; S=1,7
Boost = -5dB; S=0,7
Does somebody knows what t. h. is S? And how to mold the curve of the shelving filter using coefficients?
There is well known excel file: Parameteric_EQ_IIR_Coefficient_Calculator.xls with calculations, but only for peaking filter type of EQ filter. Maybe someone could expand on the functions of shelving filters.