Why is the digital mic mono'ed on my 1761Eval?

Question

Hi there, 
 I'm using an EVAL-ADAU1X61EBZ Board with one(!) digital Mic ADPM521 on a Flex board. When plugging this mic onto the header J4 the signal is routed to both DSP Core inputs 0 and 1 although I can't find any mixing or input routing settings which I can blame for. It makes no difference if I choose L or R input (L/R Sel Hi/Lo). I also tried this ADAU1761_DigitalMicInToDAC.dspproj which I found somewhere here. 
 When disabling left or right ADC, I still hear the input on the opposite ear (e.g. mic on input L, left ADC disabled => still hearable on right ear). 
 Does it work as expected only when using two mics? 
 thanks, 
 Gary

ADIApproved · Accepted Answer

When only one PDM microphone is connected to a codec input, the same data will be present on both the right and left channels of the PDM data stream. So, the codec's input will read identical stereo data channels. If you're disabling one of the ADCs in the ADAU1761, first make sure that you're disabling the digital circuitry and not just the analog circuits, which don't have any effect on the PDM data inputs. After disabling one of the ADCs, if you still hear identical data in both the left and right channels, the cause is probably in the signal routing in the codec. 
 If you have further questions about getting this to work, it might be helpful if you can attach the SigmaStudio project with which you're working.)

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Why is the digital mic mono'ed on my 1761Eval?