Thanks for your great support, can you help with some additional question regarding FastDSP on ADAU1787/88?
1) The FDSP can have two different channels for the headset L/R.
2) The configurations for these channels are independent configurations. They can have different filters/coefficients/gains etc.
3) They can have a different number of biquads. The configurations are independent.
4) For the FDSP only, this would have to be managed with ADC/PGA gain. If you are willing to route ADC to SDSP, and then interpolate back to FDSP, then a look ahead limiter could be employed in the SDSP.
'limiter' in SigmaStudio may helps for your forth questions.
If you go above or below full scale does the low latency ADC + deimator + biquads saturate or fold back? This is related to what happends if we cannot control level of in-signal.
hm....if the customer is using analog mic, usually the signal should be limited before input to ADAU1787's ADC. An external circuit may be needed but obviously it is not a workaround to the TWS-ANC(in-ear earphone) because of the size.
If the internal limiter or PGA are used, the workaround is like what Galvin says above.
Sure it is a big problem for ANC headphone design. but I think it is a system issue(maybe acoustic issue) more than a silicon problem.
I do want to interject here. Most digital MEMS microphones have 20 bits of resolution. The newest ones have 24 bits but I am not sure if the EOB is actually 24 bits but let's assume it is. So the input to the serial port will never clip.
So now to analog inputs. Now it is up to the designer to look at the max level that is likely to come out of the microphone and scale it so that it will be just before clipping of the ADC. This way the ADC can capture the full dynamic range of the microphone. Again, the analog mics do not have a huge dynamic range, 60 is common. So with the max setup to be just before clipping, then the noise floor will be -60 or a little less. So it is a matter of good analog design to ensure the ADC will not clip. Then in the DSP you can gain it up and have a peak limiter on it to keep it from clipping afterwards on the output or inside the DSP when filtering is done.
I have seen some ANC headphones that clip when a loud sound is outside and some that do not. It is not easy to design them this way but it can be done. However, even the good ones still have limitations. My son used them when playing his drums and they clipped. But this was extreme and not a typical use case.