Hello Cary,
When you adjust a filter's slider, SigmaStudio calculates the needed bi-quad coefficients and safeloads them to the DSP (as shown in the Capture Window). In a stand-alone system, your microcontroller must do at least some of this heavy lifting itself. Fortunately, SigmaStudio includes some tools to help you write the needed microcontroller code, also there's some good tutorials here in the forum.
If your adjustment need is for simple presets (Jazz, Rock, Classical etc.), you could try SigmaStudio's Lookup Filters, which allow you to set up responses that your micro could call up by setting a single parameter or index. Find these filters under Filters, Second-Order, Lookup. If you need more complete control (such as a graphic or parametric equalizer), your micro will need to access or calculate the coefficients at runtime.
Obtaining Filter Coefficients
SigmaDSP second-order IIR filters are implemented as Direct Form II (see Digital biquad filter - Wikipedia, the free encyclopedia)
The filter's response is determined by five coefficient numbers which typically vary from -2.0 to +2.0. The B coefficients feed forward to the filter's output, while the A coefficients feed back around the delays -- causing a resonant effect which would otherwise require a FIR filter many taps to accomplish. As a result, relatively small changes in these coefficients can drastically affect response.
SigmaStudio recalculates these coefficients whenever you adjust a filter. The results show up in the Capture Window, but there's other tools available to obtain the needed coefficients. The Fixed-Point Filter Table Generator (Tools menu) figures the coefficients for parametric and shelf filters. Notice how its results circled below match the what the Capture Window shows:
So, you can store sets of coefficients as a lookup table in your microcontroller code. At runtime, user-selected EQ settings address this lookup table, then your micro sends the needed coefficients to the DSP.
The more complete the adjustments you require, the larger the lookup table(s) become. You could alternatively have your uC calculate the needed coefficients on the fly. You could adapt the formulas in a spreadsheet like the one found at https://ez.analog.com/message/160192#160192 into the needed C code. Where I work we took a hybrid approach toward parametric EQ coefficients -- using two simple lookup tables, then combining their results with some runtime math.
Safeloading the Coefficients
If you just jam the new coefficients into the ADAU1401's parameter memory, very audible clicks and pops will result. SigmaDSPs implement a safeload procedure where you quarantine the new coefficients in dedicated safeload registers until ready, then the chip loads them all at once, in between audio samples. The safeload procedure is described in the ADAU1401 data sheet. The tutorial at https://ez.analog.com/message/61175#61175 shows how to safeload a ADAU1701/1401. Although each coefficient word has four bytes (28 bits plus a zero nibble padded at the MSB end), you actually safeload five -- there's a null byte in the middle.
Best regards,
Bob