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USB Audio Design Guide 23/61

Set the mapping for physical outputs which can either come directly from the

inputs or via the mixer.

There is enough ﬂexibility within this conﬁguration there will often be multiple

ways of creating the required solution.

Using the XMOS Host control example application, consider setting the mixer to

perform a loopback from analogue inputs 1 and 2 to analogue outputs 1 and 2.

This must be run with the MultiChannel Audio device connected to the host you

run the mixer app from.

First consider the inputs to the mixer:

./ xmos_mix e r -- display - aud - channel - map 0

shows which channels are mapped to which mixer inputs:

./ xmos_mix e r -- display - aud - channel - map - so u rces 0

shows which channels could possibly be mapped to mixer inputs. Notice that

analogue inputs 1 and 2 are on mixer inputs 10 and 11.

Now examine the audio output mapping:

./ xmos_mix e r -- display - aud - channel - map 0

This shows which channels are mapped to which outputs. By default all of these

bypass the mixer. We can also see what all the possible mappings are:

./ xmos_mix e r -- display - aud - channel - map - so u rces 0

So now map the ﬁrst two mixer outputs to physical outputs 1 and 2:

./ xmos_mix e r --set - aud - channel - map 0 26

./ xmos_mixer --set - aud - channel - map 1 27

You can conﬁrm the eﬀect of this by re-checking the map:

./ xmos_mixer --display - aud - channel - map 0

This now makes analogue outputs 1 and 2 come from the mixer, rather than

directly from USB. However the mixer is still mapped to pass the USB channels

through to the outputs, so there will still be no functional change yet.

The mixer nodes need to be individually set. They can be displayed with:

./ xmos_mixer --display - mixer - nodes 0

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