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Topic: What MP3 stereo bitrate matches AC3 5.1 bitrate? (Read 16784 times) previous topic - next topic

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  • d1ll1nger
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What MP3 stereo bitrate matches AC3 5.1 bitrate?
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 don’t care if there are better programs out there for this purpose, this is the one I like.  AutoGK saves audio as stereo MP3.  That’s fine with me.

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

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.

Here’s 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 as…
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, that’s 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 channel’s 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 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.

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?

  • moozooh
  • [*][*][*][*]
What MP3 stereo bitrate matches AC3 5.1 bitrate?
Reply #1
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 — 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.

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.


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.
Infrasonic Quartet + Sennheiser HD650 + Microlab Solo 2 mk3. 

  • d1ll1nger
  • [*]
What MP3 stereo bitrate matches AC3 5.1 bitrate?
Reply #2
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.

  • moozooh
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What MP3 stereo bitrate matches AC3 5.1 bitrate?
Reply #3
Transparency is basically inability to see the difference (in this context, in sound between the original file and the resulting mp3). :)
Infrasonic Quartet + Sennheiser HD650 + Microlab Solo 2 mk3. 

  • ma-xell
  • [*]
What MP3 stereo bitrate matches AC3 5.1 bitrate?
Reply #4
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.


-V4 is a bitrate setting, but instead of specifying a bitrate, you specify a 'quality setting' such as -V0, which is the highest quality setting and would be equivalent to 320kbit quality. (This it when you're using Variable Bitrate VBR).

But, instead of converting to mp3 you could also just keep the ac3, most media player software can handle it fine. Like moozooh said you're losing quality because you're re-encoding, by applying mp3 compression on an already existing ac3 compression.

I do not however know the details about how ac3 works with bitrates among channels. 448kbit is very common so only if you dont want to keep the 5.1, should you re-encode it as stereo mp3, aac, whatever.. As for choosing a stereo bitrate I'd say safe 'transparent' zone for mp3 is 192+ and aac 160+.. More info on sound quality vs. bitrate.

  • 2Bdecided
  • [*][*][*][*][*]
  • Developer
What MP3 stereo bitrate matches AC3 5.1 bitrate?
Reply #5
If you google autogk keep ac3 you get a decade-old guide at doom9 that tells you how to do it. I don't know if it's still the same, but the facility seems to be there just by clicking a different option.

If you go down that route, you will want to check that your player(s) handle AC3 OK first.

Apart from compatibility or bitrate reduction, there's no point transcoding AC3 to mp3. You lose AC3's ability to play back the same track with different amounts of dynamic range depending on how you are listening (e.g. whether, with the dialogue locked at a reasonable loudness, you want the explosions to make you jump out of your chair, or have them made quieter so they don't wake up the rest of the house). You also decrease audio quality, though probably not by much.

"I don’t care if there are better programs out there for this purpose" 

If you're happy with 192kbps mp3, stick with it. The results aren't going to sound any worse if they're coming from a 448kbps AC3 - they'll potentially sound a little better, but not 2.3x better. It doesn't work like that. However, there's the possibility that 256kbps mp3 could sometimes sound a little better - that' usefully true from a better source, but can also be true from a poorer source.

These are all quite small changes though (apart from the loss of 3.1 channels). The best that you can do (unless it benefits from some intentional improvement) is just to keep the video and audio from the DVD as-is. That was impractical a decade ago, but very cheap (in terms of HDD space) today.

Cheers,
David.