Hello and welcome aboard,
Not all the toolbox filters can be controlled via GPIO (auxiliary ADC) inputs, but hopefully you can satisfy your requirements with those filters that are GPIO-controllable.
- General Filters
As you have noticed, the General Second-Order filters have no control input pins. These filters are controlled by a set of five coefficients which SigmaStudio calculates for you whenever you move a slider or type in an adjustment. Thus when I set the filter shown for 6 dB boost at 50 Hz, SigmaStudio calculated the five coefficients and sent them to the DSP.
In other words, the magic is inside SigmaStudio and not the SigmaDSP chip, which simply works as a multiply-and add machine. Since the chip doesn't figure the coefficients at runtime, these filters are not GPIO-controllable. Typically, then, a multi-band parametric EQ design includes a microcontroller which accepts user input, calculates the coefficients, and then safeloads these to the DSP via I2C or SPI. SigmaStudio includes some tools (such as the Fixed Point Filter Table Generator) to help with the calculations, and searching the SigmaStudio forum turns up an abundance of info.
- Index Filters
You can have SigmaStudio build a set of preset filters to choose via GPIO. Several flavors of index-controlled filters are available. Each gives you different options, but all depend on an integer index which you can derive from a an Aux ADC input. The example below uses two varieties of index-controlled filter:
The Aux ADC output goes from 0 to 1.0 in 5.23 format -- this is a 1 before the binary point with 23 binary zeros after. The filters expect an integer index input, in 28.0 format. This is a common sticking point, so you may wish to read the tutorial at What are the number formats for SigmaDSP?. The DC sources, adder, and multiplier shown convert the Aux ADC output to a suitable integer index. In turn this index selects between all the filter settings.
Another flavor of index filter is shown below, its index control works the same way.
- State-Variable Filter
Of all the filters in the toolbox, there's exactly one for which the SigmaDSP can calculate all needed coefficients at runtime. It's the State-Variable Filter, whose low-pass, high-pass, and band-pass outputs are simultaneously available (no parametric EQ, however), You could easily have two pots control frequency and Q, but since the Eval-ADAU1701MINIZ board has only one pot I show a fixed DC source for Q. Note that the Q input is actually 1/Q, forcing you to take your own reciprocal to get a linear Q adjustment -- and that divide block is quite a resource hog. The circuit shown below sounds rather like a Wah-pedal as you turn the pot.
I will attach these example projects below.
Best regards,
Bob