Scope
The intent of this article is to explain what To Source and From Source means in the Destinations page of the xNode and in some other locations throughout Livewire devices.
Description
A single Livewire Channel has send and return elements built into it. This is why you see To Source and From Source as options for stream mode/type in the VX's POH input, the VX's main input (Advanced Receive) section, the xNodes output (destination) section, etc.
These modes are used to specify which part of the Livewire stream is to be used at that interface
From Source:
This is the mode that will be used most of the time. It literally means it's going to use audio from the source, like an xNode source or a VX output channel.
To Source:
This is audio sent back to the source from an Axia console, also known as a backfeed.
With codecs and phone channels, the console engine automatically creates the backfeed for returning audio to the caller. In this case, the To Source or backfeed is always a mix-minus. This needs to be a mix-minus so that the caller doesn't hear echoes when they speak.
We can relate this to an analog recording setup too. The From Source would be the playback/monitor signal from the tape machine (the source) to the console. Inversely, the To Source stream is the tape-send from the console back to the tape machine (the source.) This also needs to be a mix-minus, so that there isn't a feedback loop when the tape machine is in "Input Monitor" mode and is being monitored at the console.
For Guest Microphone sources, the To Source is not a mix-minus because it is meant to go to the Guest's headphones.
To Source streams will also contain any talkback audio sent from the console operator back to the source.
Conclusion
Both of these audio streams can be represented by a single Livewire Channel number.
Just remember: From Source is the regular audio source, like from a node. To Source is the return from the console back to the source, the mix-minus or backfeed signal.
GPIO works in this same manner. There is Source GPIO and there is Console GPIO. More on that is explained in this example.