now I tried the 16-bit thing with the R256-R DAC and it gives a big advantage. It is not as good as you would expect from 16-Bit because it is hard to get rid of all those digital noise in such a circuit. The problem with all those circuits is to keep away the digital noise from supply from audio output. As long as a digital output pin that is supplied from uC is used, you also hear all the uC noise. Maybe it gets better if you decouple the uC better or use something like a buffer. But simply said, using just those two additional resistors you get rid of those hiss noise. After playing a little bit around it seems that most noise comes from the USB port - maybe you get acceptable results by using battery power.
What I did is the following: I set the following mode in software:
SdPlay.init(SSDA_MODE_HALFRATE | SSDA_MODE_STEREO)
Then I prepared my file with the following modified batch-file:
FOR %%A IN (%*) DO sox %%A --norm=-1 -e unsigned-integer -b 16 -r 31250 -c 1 -t raw "converted\%%~nA.ahd"
Connections I made:
Pin 9 ----[220k]------+
Pin 10 -[1k trimmer]---+---||----[10k]--AudioOut--[1k]--GND
With trimmer the sound can adjusted for minimum hiss, I prepared also a special audio-file for easier trimming.