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VB-Audio makes free software that function as a virtual audio device and mixer. This allows you to send audio from one source to another.
Your DAW only recognizes one interface at a time. So if your mic or interface is connected, you can only record from that device. What virtual audio cable does is create a virtual device. You route all your sources (mic, interface, skype, youtube etc.) into the virtual device. Then you set that virtual device as your input to your DAW. This will allow you to record each source to a separate track.
You can also send a combination of sources to another application.
Example: You want your guest to hear your intro/outro/fx via zoom. You can combine your mic and whatever you use to play jingles and set it as your input to skype or zoom.
VB-Audio has a few programs I’ll list here. They all work on both PC and Mac.
Although if you are on Mac I highly recommend Loopback by Rogue Amoeba. It costs $100 but it’s better and much easier to use. It gives you unlimited inputs and outputs and you can even nest devices.
Virtual Audio Cable - A virtual audio device working as virtual audio cable. All audio coming in the CABLE input is simply forwarded to the CABLE output. (You'll need to install this for the other programs to work as well.)
Voicemeeter - Audio mixer application that uses virtual audio cable to mix and manage audio sources to and from devices and applications. This is their basic mixer that allows 3 inputs, and 3 outputs.
Voicemeeter Banana - This is an advanced version of voicemeeter. It gives you 5 inputs, and 5 outputs (3 physical, 2 virtual). It also gives you access to a parametric equalizer.
Voicemeeter Potato - This is their most advanced version. It gives you 8 inputs and 8 outputs (5 physical, 3 virtual).