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Old 30th July 2011, 21:00   #1  |  Link
CarlEdman
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Join Date: Jan 2008
Posts: 185
NeroAACEnc Multi-Channel Efficiency Puzzle

I have been using neroaacenc to transcode AC3 files (after first decoding with avisynth/NicAudio) to AAC format for years, so I have a bit of experience with the format and its typical efficiency, but the following I do not understand. I typically use neroaacenc quality settings in the 0.3 to 0.4 range.

1. AAC seems to be substantially more efficient than AC3. Stereo AC3 files typically compress to stereo AAC files about half the size.

2. For most material, I also downmix 5.1 AC3s to stereo AACs. That of course makes the size difference even larger. A typical stereo AAC is only about one third the size of the source 5.1 AC3.

3. For high-def video, where the video bit rate is so high anyway, I'm seriously considering stopping downmixing and taking the hit on larger resulting 5.1 AACs.

The problem is: 5.1 AACs are only slightly (~10%) smaller than the source 5.1 AC3s. This seems completely inconsistent the much higher efficiency observed when encoding stereo AC3s to stereo AACs (a reduction in size of about 50%, see above).

Before you respond along the lines of "You idiot! Of course 5.1 AACs must be larger than stereo AACs! They carry additional channels and therefore more information!" please let me reassure you that I understand that.

What puzzles me is the difference in efficiency AAC efficiency in 5.1 (where it hardly beats AC3) and in stereo (where it achieves 50% or so improvement). In fact, I would have thought that the additional surround channels would, for most scenes in most content, be very low or highly redundant to the stereo channels or (for the subwoofer channel) only cover very low frequencies (meaning little data), so adding them to a well-designed compressor which takes advantage of all this redundancy would mean a relatively small size-hit.

Instead, it is almost as if all the channels were encoded independently and encoding 5.1 channels takes just two or three times as much space as encoding 2 channels in AAC.

Can anybody explain this to me? Is it a general feature of AAC or just of neroaacenc? Or perhaps typical stereo AC3s are encoded at an excessively high bit rate, which AAC can squeeze out easily, but typical surround AC3s are not? Or perhaps the quality setting in neroaacenc just gives too many bits to multichannel audio--rather than achieving the same perceived sound quality as it would at stereo with the same -q setting? Or something else?
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