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EQ Low- and High-Shelf filter coefficients comming back - Mysterious S factor.

Hi everybody

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.

For ex.:

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.