Another Blu Ray hook up question

Cannot figure why i am only getting dolby digital 2 channel on standard dvd with my pioneer bdp 51fd blu ray player. I just hook it up to my older NAD T762 receiver's external outputs to use the internal dacs on the player to receive hd master & dolby true hd decoding but, now on standard dvd for some reason i am only getting dolby digital 2 channel. To receive the 5 channels of dolby digital, i had to re-connect my coax digital output and set the player back from analog to digital. Am i doing something wrong or is this what you have to do when going from a blu ray disc with hd master or dolby true hd to a standard dvd with dd or dts. My understanding was i thought the player would adjust accordly. Any help appreciated.

Thanks
Brian