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Old 1st August 2011, 14:56   #13  |  Link
hello_hello
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Join Date: Mar 2011
Posts: 4,821
Quote:
Originally Posted by CarlEdman View Post
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.
Personally, (and I store video in much the same way as you) I stopped taking any quality hit at all and just keep the original 5.1 AC3. I only ever convert DTS to AAC.

Not that it answers your question, but I tried for myself just out of interest. I started with a DTS audio track from a BluRay.
I hour, 44 minutes long, 1.1GB.

Encoding to AC3 using AFTEN.
5.1 448Kb/s, 335MB
5.1 384Kb/s, 287MB
Stereo 192Kb/s, 144MB

Encoding to AAC using neroaacenc

q0.5, 5.1, 311MB
q0.5, Stereo, 125MB

q0.4, 5.1, 230MB
q0.4, Stereo, 91MB

q0.3, 5.1, 155MB
q0.3, Stereo, 62MB

I always use q.05 when encoding AAC so it's obvious why I just keep the AC3 audio. The AAC encode doesn't always end up larger than the 384k AC3 at q.05. Sometimes it's a little smaller. I guess it depends on the contents of the audio track. In which case the size of the q.04 and q.03 encodes would also be reduced.

All the above was converted using foobar2000 and it's own 5.1 to stereo plugin where appropriate.

A stereo 192k AC3 file is about half the size of a 5.1ch 384k AC3, but I assume it's using a fixed bitrate for each channel and I assume the front channels must get more bits?
For AAC, as it's VBR the difference between the size of the stereo encode and the 5.1 encode is probably proportional to the amount of activity in the other four channels.

Still in this case, compared with 5.1ch 384k AC3, 5.1ch q.04 offered a about a 20% reduction in file size while q.03 reduced the file size by about 46%.

Last edited by hello_hello; 2nd August 2011 at 09:13.
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