What MP3 stereo bitrate matches AC3 5.1 bitrate?
Reply #1 – 2010-05-18 23:10:16
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.