M-AUDIO Sonica Theater - Manual (Page 18)

Explanation of Surround Formats
You run into several surround formats when playing DVDs, some of which
include 2.0, 4.1, 5.1, 6.1 and 7.1. This section is meant to provide a quick
explanation of some of these formats.
2.0 - This is a stereo soundtrack.This soundtrack can be expanded to 7.1
channel playback with a Dolby Pro Logic® or Circle Surround II decoder.
It will play out of the left and right speakers without decoding. Headphones
are generally stereo or 2.0 devices.
2.1 - This is a stereo soundtrack with a separate subwoofer. Sometimes
this soundtrack can be expanded to surround playback with a Dolby Pro
Logic® or Circle Surround® decoder. It will play out of the left, right and
subwoofer speakers.
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