What MP3 stereo bitrate matches AC3 5.1 bitrate? 2010-05-18 22:26:12 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 KBpsSo 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 KBpsI 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?