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Old 30th July 2011, 21:14   #2  |  Link
mariush
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Join Date: Dec 2008
Posts: 589
Hmmm....

The AAC compression is lossy... depending on the quality factor, it filters the signal and removes lows and high sides so that as much information can be retained in that bitrate.

For the main stereo signal, where there's not much bass and highs in the first place, the channels have a lot of redundancy so for that quality preset, the encoder may be able to retain a lot in a small bitrate... let's say 96-112kbps on average compared to 192 kbps for AC3 stereo.
The back speaker signal should be about the same, with some redundancy and less bass or highs, so .
But the bass and center speakers are paired in the last "stereo" pair and they have nothing in common, the encoder probably uses A LOT of bitrate to retain the bass (for the quality preset you use) and an average amount of bitrate for the center speaker, but as there's no similarities between the channels there's just a lot of bitrate... I'd guess instead of having 96-112 kbps, you probably average 160-224 kbps for that quality preset.
So what you gain on front and back channels you lose on the bass and center channel..
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