Asus Xonar D2X User Manual page 68

4. Go to the 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 loop-back path from the
Line-out directly to the Line-In. This allows you to get realistic output and
input quality ratings for the Xonar D2X audio card.
4
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 the Line-In jack for loop-back
testing. In that case, please select "Line In" as the recording source on the
mixer page.
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3.5mm Line Cable
ASUS Xonar D2X