Why not always use AC3?

[agent2099]

There must be something I'm not understanding re: ac3. I have read comments that state something like "if I make the jump to 3 CDs, I'll use an AC3 track because of the extra space."


If an AC3 track is better, why not ALWAYS use it? When I chose AC3 for the audio (vs. mp3), the bitrate goes UP. The way I see the term ac3 used, it seems as if it lowers the bitrate (i.e. takes up more space).

1xac3 on my current movie is 82.6%, instead of 1x vbr mp3 which is 82.4%.


I must be missing something.

mp3 @ 73.7% (http://www.pbase.com/image/5680052)

ac3 @ 73.9% (http://www.pbase.com/image/5680057)

[Hiro2k]

whats the bitrate of the AC3? 192kbs???? If it is, then converting to a 160kbs mp3 make little difference since the filesizes will be almost the same and you now have more overhead from the mp3. Even a 128 kbs mp3 would not reduce the filesize by much and the overhead you have to calculate will ruin the benifit of using mp3 in your case. AC3 has very little overhead compared to MP3 and so that's why your movie has better percentage with AC3. Most AC3's are over 224kbs in which case it makes sense to compress it using mp3.

The reason that people only use AC3 on 2 or 3 cd's is becuse on 1 CD rips an mp3 at a low bitrate will give the main movie more bitrate.

[agent2099]

Originally posted by Hiro2k

The reason that people only use AC3 on 2 or 3 cd's is becuse on 1 CD rips an mp3 at a low bitrate will give the main movie more bitrate.

My results were quite different.

-------------------------
320kbps mp3 = 64.8% (http://www.pbase.com/image/5682147)

320kbps ac3 = 65% (http://www.pbase.com/image/5682127)
-------------------------
128kbps mp3 = 78.1% (http://www.pbase.com/image/5682086)

128kbps ac3 = 78.4% (http://www.pbase.com/image/5681981)
-------------------------
single cd 128kbps mp3 = 34.0% (http://www.pbase.com/image/5682113)

single cd 128kbps ac3 = 34.2% (http://www.pbase.com/image/5682052)
-------------------------

In each case, the AC3 resulted in a higher bitrate for the same filesize. Something isn't adding up. :confused:

[Hiro2k]

If they are at the same bitrate then the AC3 file will always win. The reason is because it has a lower fram overheard value than an mp3. So if they are at the same bitrate then the AC3 will always win.

[manono]

Hi agent2099-

There are a couple of things you can do to make the differences more obvious. One thing is that most people when converting AC3 to MP3 use VBR MP3. So, when I make a 160 VBR MP3 for a 1 CD rip, the average bit rate will typically be 110-115 KBPS. Isn't AC3 CBR?. The other thing, and something that will show a dramatic difference is that 6 channel AC3 5.1 soundtracks have bit rates of 384 or 448. If you convert that to stereo MP3, even very high quality VBR MP3, then the file size savings are huge, typically 250MB-300MB for a full length movie. Sure, you can transcode the AC3 to a lower bit rate, and the differences will be less, but still significant.

If you start with a 2 channel AC3 track at 192 KBPS, and compare that to a CBR MP3 also at 192 KBPS, then you have a case. It's just that no one does that. When people go to 2 CDs so they can use the AC3, they're talking about the 5.1 track (at least, that's the way I do it).

In your examples, you used the same file sizes, so you weren't comparing a 2 channel AC3 to a stereo VBR MP3 track, but a CBR MP3 track. But it's when you compare the 6 channel AC3 with the 2 channel VBR MP3 that you get the real file savings.

[agent2099]

manono, that's what I thought.


So my next question is, where is there an option for 6-channel AC3? I only see:

no audio
1x ac3
1x vbr mp3
2x ac3
2x vbr mp3
mp3 + ac3


How do I get the 6 channel action?

[agent2099]

Originally posted by Hiro2k
If they are at the same bitrate then the AC3 file will always win. The reason is because it has a lower fram overheard value than an mp3. So if they are at the same bitrate then the AC3 will always win.

So if the ac3 always wins at the same bitrate, why not choose it over mp3?


Also, what do you mean when you say "overhead." Are you referring to which (ac3 vs. mp3) uses more processing power? If so how do they compare?

[manono]

Hi-

So if the ac3 always wins at the same bitrate, why not choose it over mp3?

Because the AC3 bit rate (for AC3 5.1, and not stereo AC3) is a much higher bit rate. Look at it this way. Say you have a 128 CBR MP3-then that's 64 per channel (there are differences between joint stereo and true stereo, but I think the principle's the same). And if it's 5.1 AC3, and there are 64/channel, then the total bit rate is 384 (6x64-sometimes used on DVDs). So, by converting the DD5.1 to stereo, you've already saved yourself 2/3 of the file size (or thereabouts).

where is there an option for 6-channel AC3?

Since there isn't one, I guess the overhead for 2 channel and 6 channel AC3 must be nearly identical.

Also, what do you mean when you say "overhead."

Well, all this time, when you've been comparing the 1x vbr mp3 with the 1x ac3, that's all you've been doing-comparing the overhead (they're in the Interleaving & AVI-Overhead Box of GKnot). I guess it's the amount of data required to get the interleaving correct. But I'm far from an audio expert. So if DSPGuru sees this and wants to correct me (again), then he's more than welcome.

[bb]

Doesn't VBR MP3 sound better than AC3 two-channel at the same bitrate?
(Why not use OGG Vorbis, BTW?)

bb

[agent2099]

I'm used to following the guide when using GKnot.

So to get a 6 channel audio in my Divx file. Do I follow these steps.



1. Under avi overhead: select 1x ac3

2. Under audio A: select a bitrate equal to the bitrate in the .ac3 created from my DVD2AVI project.

3. At the save an encode screen, slect "just mux" instead of the standard "MP3, Custom Parameters."

The GKot guide only gives you the directons for creating mp3s when making a divx. If I follow these steps will I get a 6 channel AC3 divx that will use 5.1 speakers. Will powerdivx be able to decode this 6-channel ac3 file, much like a software dvd player would decode a 5.1 dolby digital or dts dvd?

Do you still select "demux" audio when you are creating your DVD2AVI project?

[DSPguru]

mp3 is a better 2ch audio compression method than ac3. (i'm not talking about codecs/tools. talking about method/domain/signal representation).
anyway, transcoding a 2ch AC3 into a 2ch MP3 wouldn't enhance the sound quality, on the contrary! every transcoding step like this will decrease sound quality. that's also true for ac3->ogg or ac3->aac.
further more, the best you could do is take a 2ch ac3 @ 192kbps and transcode it into a VBR mp3 @ 128kbps. not much of bits savings.
also, like manono mentioned, VBR mp3 muxing also consumes more bits overheads in order to implement the AVI spec hack.

all in all, the conclusion would be -
never transcode a 2ch ac3 into mp3. keep the original ac3 !


as for 5.1 ac3 source @ 448kbps,
well, in that case, you better transcode in order to lower bitrate.
if you'd transcode to vorbis instead into mp3, the benefit would be :
- better quality at same (smaller?) bitrate.
- low muxing overheard due to ogg nature, unlike avi's.

btw, more and more ppl uses AC3Machine in order to re-encode a 448kbs ac3 into a 224kbps ac3 and find it cost/effective :
http://forum.doom9.org/showthread.php?s=&threadid=33128&perpage=18&pagenumber=2

imho, the best 5.1 codec is aac ;)

[manono]

Hi-

1. Yes.
2. I think I'd just fill in the Audio Filesize in the BitRate Tab and point the Select Button to it in the Audio 1 Tab of the Encoder.
3. Yes, and get your interleave settings from the AC3 Q&A (http://forum.doom9.org/showthread.php?s=&threadid=18318) in the Audio Forum.
...will I get a 6 channel AC3 divx that will use 5.1 speakers.
Yes, if DD5.1 came on the DVD.
Will powerdivx be able to decode this 6-channel ac3 file,
I don't know, but I don't think so. Maybe your sound card can. I send mine to a decoder box on the way to the speakers when I watch the .avi on TV.
Do you still select "demux" audio when you are creating your DVD2AVI project?
I demux when using SmartRipper or DVDDecrypter, but DVD2AVI can do it also.

[agent2099]

Thanks. Trying my first ac3 divx now.