Audio mixer and mixer control

[Harry Schmalzbauer]

Hello,

today I wanted to utilize my optical S/PDIF out with an external D/A 
converter to empower my garden radio.
Unfortunately, it seems mixer(8) isn't really doing what I understand a 
mixer's job is.

As far as I understood, mixer(8) is just controlling/pushing settings to 
the dsp's specific hardware mixer (if that's true, mixctl(8) was more 
clear e.g.).

So if I have dsp0 with line-in and line-out, and dsp3 with a S/PDIF out, 
there's no way to get the dsp0-"mix" over to dsp3?
What I'm looking for is a mixer which processes various input sources 
and sends them to arbitrary output devices.
Does anybody know if there's such kind of mixer available?

Or is it possible to interconnect different dsp channels? (ugh, I don't 
really know anything about contemporary audio hardware :-( )

I also have problems understanding the mixer(8) channels.  Hard to find 
the corresponding dsp channel... The relation of "speaker", "mix", the 
invible "monitor" and "rec" is completely unclear to me, likewise the 
difference of "vol" and "pcm".

Is it common that S/PDIF out is a separate dsp?  I never had to 
investigate on other OS, where I get the same signal on analog and 
digital outputs simultaniously.

Thanks for any hints,

-harry