It just means: To have 5.x sound as good as 2.0 at e.g. 224 kbps, you don't need 5/2 times the bitrate (224*5/2=560, next closest available block bitrate would be 576 kbps), but only about √5/√2 times as much (224*√5/√2~354 kbps, next closest available block bitrate would be 384 kbps). Well, channel coupling isn't really that good (the more channels, the more an accurate phase angle is important to avoid flanging effects). A 384 kbps discrete 5.1 AC3 sounds not certainly as good as a comparable 224 kbps ProLogic 2.0 AC3, better to have 448 kbps. And 640 kbps is even ... "generous", in comparison.
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