D.
[quote user="D. Hartl"]Are you 100% sure that the digital source material is not band limited?[/quote]
Yes. I actually have two different boards. One uses a 12MHz crystal with the settings similar to those shown above. When I use two of these boards (audio in->ADC->I2S->DAC->audio out) the passband is flat from 15Hz to >20KHz. However, when I use the 12.288MHz crystal with your setting on the receive side (it's actually a radio link) the passband is limited at ~7.5KHz as shown in the screen shot.
[quote user="D. Hartl"] If you do not have any biquads enabled [/quote]
Nope, no biquads are enabled.
[quote user="D. Hartl"]Do you measure this signal directly at the AIC3204 or is this after an analog filter on the output?[/quote] This is measured directly at the output of the AIC3204. It is an identical circuit to the one that uses a 12MHz crystal, just with different initialization settings.
[quote user="D. Hartl"]The roll-off at lower frequencies also doesn't look right, assuming you measure the signal directly at the output?[/quote]
The low end roll off is due to undersized output capacitors on this board. This has been fixed on the newer design. As mentioned above, the new design is flat down to about 15Hz.
Yes, this seems very strange. But I have measured it several times with the same result. Is there anything else it could be?