ProtoDAC TDA1387 X8 project

[eDad2003]

The volume increase from turning CDSP off and ALSA polarity inversion on was much greater than I expected. More  tests to come.

@DRONE7, @varunach, @Skip Pack

I notice it too, but really more at lower volumes IMO. Could it be as simple as CDSP uses a different volume curve from ALSA? For example, Spotify Connect offers Logarithmic, Cubic, and Linear as volume curve options.

I guess my concept is that the dynamic range should not change with volume -- if you adjust the volume (using software or hardware) from one player configuration so it matches the volume from another configuration (CDSP on versus CDSP off) they should sound substantially identical to our ears volumewize. there will be differences from the dithering and other effects. If there seems to be a 'contouring' effect that leads me to think of a loudness profile.

What I mean is that a volume setting of 32 "volumewise" is totally dependent on the curve that maps the digital volume value to dB volume. I think we can't compare the ALSA volume to the CDSP volume at 32 and say CDSP is reducing the volume. If one is using a linear volume curve, than 32 = 32% of max dB (and 99 = 99% of max), but if the volume curve is logarithmic, then 32 = 10% of max dB (and 99 is still about 99% of max). 

I think I have that right, I am still trying to wrap my head around exactly how the relationships work. I've found no evidence of loudness or contouring in CDSP unless you specifically introduce it in the pipeline.