Nexus 7 Multi Channel/Surround Sound with USB DAC

[mkrelli]

Hi.

I want to play audio from my Nexus 7 2013 (Razor) in a fixed install environment on a Mutli Channel USB Audio DAC. It is a 7.1 Channel device and has a CMedia CM6206 chip. I currently use the stock Android 5.1.1 build LMY47V.
From what I've read a multi channel source, e.g. a movie, should play fine, but the difficult part seems to be the spreading of stereo sound onto multiple channels. After reading through different articles related to ALSA I finally found an example configuration which does exactly what I need:

# for 5.1 speakers
pcm.ch51dup {
         slave.pcm surround51
         slave.channels 6
         type route
         ttable.0.0 1
         ttable.1.1 1
         ttable.0.2 1
         ttable.1.3 1
         ttable.0.4 0.5
         ttable.1.4 0.5
         ttable.0.5 0.5
         ttable.1.5 0.5
}

# for 4.1 speakers
pcm.ch41dup {
         type route
         slave.pcm surround41
         slave.channels 5
         ttable.0.0 1
         ttable.1.1 1
         ttable.0.2 1
         ttable.1.3 1
         ttable.0.4 0.5
         ttable.1.4 0.5
}

Sadly it is for Linux Desktop Editions and Android seems to be quite different (Driver Layer, Configuration ...). After figuring that out I came across a thread on XDA Developers where they want to use all built-in speakers for audio.
Thus, I finaly found the place the mixer configuration would probably go to (mixer_paths.xml). But now I'm stuck on translating the .asoundrc file into the right XML Syntax. This is where I need help... I already read through my configuration file, but the syntax is not fully clear to me.
Is there any document which describes the XML Elements valid in the mixer_paths.xml?

What I got so far:
- Device is rooted
- Alsa Mixer is installed
- Example .asoundrc
- The USB DAC enumerats (with alsa_aplay -L) as "default:CARD=Device" or as (with alsa_aplay -l) "card 1".

What I need to know:
- What is the relevant source elment?
- How do I route the audio to the actual device?
- What is the equivalent for "ttable" in .asoundrc?

Thanks and best regards,
Martin.