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What MP3 stereo bitrate matches AC3 5.1 bitrate?
d1ll1nger
post May 18 2010, 22:26
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I have spend hours in this forum, doom9, and in general Google searches trying to find an answer to this, so if this question has already been addressed somewhere here already and I failed to dig deep enough in the right place then I apologize in advance. This is about bit rates and file size, not necessarily quality or down mixing.

I like to use AutoGK for backing up my legally owned DVDs. I donít care if there are better programs out there for this purpose, this is the one I like. AutoGK saves audio as stereo MP3. Thatís fine with me.

So when a DVD movie has a stereo AC3 file, DGIndex lists it asÖ
AC3 2/0 192 KBps

So in this case, all things being equal, I save it as a CBR MP3 192 KBps. Iíve compared the differences in file size and the MP3 audio file size is exactly identical to the AC3 audio file size. So I feel fairly certain that I am not loosing any quality here.

Hereís where things start getting complicated. More and more movies today are not offering a stereo option. Instead they only have 5.1 audio. So DGIndex lists that asÖ
AC3 3/2 384 KBps -or-
AC3 3/2 448 KBps

I have read that in 5.1 audio, you have 5 full channels and 1 channel at 1/3 the bit rate of the full channels. So, for example, in a 448 KBps file, thatís 5 channels at 84 KBps and the LFE channel at 28 KBps. So since no one channel is more than 84 KBps, and stereo has 2 channels then that would be a bit rate of 168 KBps, which is rounded down to 160 KBps. Can anyone validate this about 5.1 audio? Is that the real way to determine each channelís bit rate?

I guess that makes sense in theory, but it seems strange that a 448 KBps 5.1 audio track would have a lower bit rate per channel than a 192 KBps stereo audio track. I donít know all the technical details about how all the various channels in a 5.1 audio track are combined to form a stereo audio track (and I donít care) to really know how to calculate what the stereo trackís bit rate needs to be to accommodate for the conversion.

So thatís the question: When converting a 448 or 384 KBps AC3 5.1 audio track to a CBR MP3 stereo audio track, what should the MP3ís bit rate be set at to match as closely as possible the original bit rate of the AC3 file so as not to loose any quality or create any extra over sampling bloat?
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moozooh
post May 18 2010, 23:10
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QUOTE (d1ll1nger @ May 19 2010, 01:26) *
So in this case, all things being equal, I save it as a CBR MP3 192 KBps. I've compared the differences in file size and the MP3 audio file size is exactly identical to the AC3 audio file size. So I feel fairly certain that I am not loosing any quality here.


Filesize is a function of bitrate ó when you set bitrate to a certain value, you explicitly say how much data should each second of the stream take. But the problem is, this says very little about the resulting quality because the relation is indirect.


Besides, AC3 (unlike, say, DTS or LPCM) is a lossy format susceptible to compression artifacts, so you are undergoing a double loss when transcoding from it to MP3. You're losing quality either way.

QUOTE (d1ll1nger @ May 19 2010, 01:26) *
I guess that makes sense in theory, but it seems strange that a 448 KBps 5.1 audio track would have a lower bit rate per channel than a 192 KBps stereo audio track. I don't know all the technical details about how all the various channels in a 5.1 audio track are combined to form a stereo audio track (and I don't care) to really know how to calculate what the stereo track's bit rate needs to be to accommodate for the conversion.


Generally, in a multichannel track, much like in a stereo track with a channel coupling method (like joint-stereo) being used, the bitrate allocated to each separate channel isn't just (overall bitrate)/(number of channels) because every channel is basically silent or very quiet most of the time, except when the primary sound sources are mapped to it. I can't vouch for AC3, though.


QUOTE (d1ll1nger @ May 19 2010, 01:26) *
So that's the question: When converting a 448 or 384 KBps AC3 5.1 audio track to a CBR MP3 stereo audio track, what should the MP3's bit rate be set at to match as closely as possible the original bit rate of the AC3 file so as not to loose any quality or create any extra over sampling bloat?


Assuming you downmix 5.1 to 2.0, use whatever LAME setting that generally brings you transparency with audio. -V4 should do well.


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d1ll1nger
post May 21 2010, 01:59
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QUOTE (moozooh @ May 19 2010, 01:26) *
Assuming you downmix 5.1 to 2.0, use whatever LAME setting that generally brings you transparency with audio. -V4 should do well.


Thanks for the response, and the info. AutoGK does use LAME 3.97, but it does it all automatically. So whether it's using a -V4 switch or not is out of my control. I know it downmixes from 48KHz 5.1 to 48KHz joint stereo qval=2. But all that I can do is choose MP3 CBR or VBR, and a corresponding bit rate. I'm not sure what audio transparency means, or what it does. Is there a minimum bit rate size that allows this feature to be enabled that I should be using? Thanks again.
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moozooh
post May 21 2010, 02:18
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Transparency is basically inability to see the difference (in this context, in sound between the original file and the resulting mp3). :)


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