Asus Xonar D2X - Manual (Page 65)

59 ASUS Xonar D2
4. Go to Mixer Recording page and select “ALT” as the recording source. Click
“Reset” to make sure the recording volume is on the default highest level
(0dB). This is actually a high-quality onboard path from the Line-Out directly
to the Line-In. This allows you to get realistic output and input quality ratings
for the Xonar D2 audio card.
You may also choose to use an external 3.5mm line cable as follows (as short
as possible) to connect the Front-Out jack to Line-In jack for loop-back testing.
In that case, please select “Line In” as the recording source on the mixer page.
4
3.5mm Line Cable