The XMOS USB Audio Reference Design, by default, contains an 18x8 mixer unit
(note that at sample rates above 96Khz only the first two outputs are
enabled).

This unit takes input takes 18 inputs:  USB OUT channels 1..10 and
DEVICE IN channels 1..6,9..10 and produces 8 outputs: Mixer Output
1..8

Before the mixer there is an unit that allows the selection of the 18 mixer inputs
from all the possible device inputs (DAW and physical audio).  This is 
an extension unit with id 50 in the descriptors

After the mixer unit there is are channel map units for each output terminal:
Each of these outputs can select a source from one of 28 channels sources: USB OUT
channels 1..10, DEVICE IN channels 1..10 and Mixer Output 1..8

The channel map units are extension unit with init ids 51 and 52. This unit
lets you implement arbitrary routings including loopbacks.

The mixer is control on MAC OS X via the command line utility
xmos_mixer. Running this application requires having the
libusb-1.0.0.dylib in the dynamic library load path. Sourcing the
setup.sh script will do this. Source code for the application is
provided as a guide on how to communicate with the device.

Here are the commands for the mixer application (note that the USB
audio reference design has only one unit so the mixer_id argument
should always be 0):

     --help

     --display-info

Show information about the device.

     --display-mixer-nodes   mixer_id

Display all the weights of all the mixer nodes (and their id) of a particular mixer.

     --display-min     mixer_id

Display the minimum allowable weights of a particular mixer.

     --display-max     mixer_id

Display the maximum allowable weights of a particular mixer.

     --display-res     mixer_id

Display the resolution of a particular mixer.

     --set-value       mixer_id  mixer_unit  value

Set the weight value in the mixer. The second argument should
correspond to the values shown by the --display-unit command. Values
can range from -127db to +128db with the special value -inf for mute.

     --get-value       mixer_id  mixer_unit

Get the weight value in the mixer. The second argument should
correspond to the values shown by the --display-unit command. Values
can range from -127db to +128db with the special value -inf for mute.

    --set-mixer-source  mixer_id, dst_channel_id, src_channel_id

Allows the selection of the mixer inputs.  Sets mixer input (dst) to src

    --display-current-mixer-sources  mixer_id

Displays the current inputs to a particular mixer

    --display-available-mixer-sources  mixer_id

Displays all the input channels available that can be fed into the inputs of a particular mixer

    --set-aud-channel-map dst src

Sets a channel map value for the device audio output

     --display-aud-channel-map          

Show audio output channel map i.e. for each audio output of the device what the source is.

     --display-aud-channel-map-sources  

Show the available audio output channel map sources.

    --set-daw-channel-map dst src

Sets a channel map value for the DAW output to the host

     --display-daw-channel-map          

Show audio output channel map i.e. for each DAW output to host, what the source is.

     --display-daw-channel-map-sources  

Show the DAW output channel map sources.

     --get-mixer-levels-input

     --get-mixer-levels-output

     --vendor-audio-request-get   bRequest, ControlSelector, ChannelNumber, UnitId

     --vendor-audio-request-set   bRequest, ControlSelector, ChannelNumber, UnitId, Data[0], Data[1],...
 
