When you record to SD card the Main Mix is recorded as a file called "Main Mix". When you are streaming to computer the main mix is on USB channel 19- 20.
The M20d will stream 20 channels of audio to your computer via USB, and receive a stereo return stream back from your computer. The use case for this would be when you’d like to record M20d unprocessed channels for editing in your DAW. Whenever a USB cable is connected to the M20d PC port, the following alert will be displayed in the Multitrack Recording window:
“Multitrack recording to SD/USB storage is not available while connected to a computer.” Input Streaming Configuration
The M20d streams 20 inputs to your computer and a stereo stream back to the M20d as follows:
• Channels 1-16: M20d Mic/Line Inputs 1-16, pre-processing / pre fader • Channels 17-18: M20d stereo Aux Input, pre-processing / pre-fader
• Channels 19-20: M20d stereo Main Outs, post-processing / post-fader
USB Stereo Return Stream:
The stereo return is streamed to the M20d’s stereo Aux Input (channels 17/18).
Note: In order to hear the stereo return, a Stage Icon Preset must be assigned to the Aux Input using the M20d touchscreen UI. The Laptop preset is ideally suited for this purpose, and can be found in the Stage Icon Gallery.
For USB audio streaming, you must download and install the Line 6 StageScape M20d Driver at http://line6.com/software.
On that page you will find 3 drop down menus. On the All Products drop down menu select StageScape M20d. On the All Software drop down menu select Drivers.
Select the OS of the computer you wish to use with your M20d. Once the download has completed, find and run the StageScape M20d Drivers Installer, following the onscreen instructions.
In your DAW you will need to change your audio interface to ASIO StageScape M20d. Then create new audio tracks in your DAW with inputs that correspond to the inputs on the M20d. Note: If you are using the StageScape DX/MME driver, make sure that StageScape is selected as the default audio device in Windows. Also, know that you will only get audio from inputs 1-2 if you are recording with DX/MME.
The Windows StageScape M20d USB 2.0 audio driver uses WDM and ASIO, supported on 32 and 64 bit OS configurations. The Mac OS X StageScape M20d USB 2.0 audio driver uses Core Audio, supported on 32 and 64 bit OS configurations.