Bitrate - Dolby Pro Logic II vs 6-channel AAC

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Kev_UK
Posts: 2
Joined: Sun Oct 14, 2007 11:44 am

Bitrate - Dolby Pro Logic II vs 6-channel AAC

Post by Kev_UK »

Hello, people.

Need to get my head around this. I currently output to stereo speakers, but anticipate that somewhere down the line, I will have a 5.1 setup. Basically, I'm thinking I'm best off keeping the 5.1 audio on DVDs intact for future proofing, in favour of the DPL2 audio.

However, I currently only listen in Stereo, and who knows when the 5.1 will happen! The question is about bitrate and quality.

6-channel discrete
Maximum Bitrate = 384kbps
Each channel gets = 64kbps

Dolby Pro Logic II
Maximum Bitrate = 160kbps
Each channel gets = 80kbps

Does this mean that the audio fidelity of a DPL2 track is greater than that of the 6-channel discrete? Is it really a choice between 6 channels or higher bitrate audio?

Cheers for the help!
PuzZLeR
Bright Spark User
Posts: 287
Joined: Tue Apr 24, 2007 4:01 am

Post by PuzZLeR »

Greetings.

I'm bumping this thread in the hopes that someone who has good knowledge on this can help us out.

AAC 5.1, my goal in converting the AC3 5.1, is a mysterious animal. I too have been backing up my AC3 5.1 tracks in the hopes that the future will provide a reliable converting app so I can later mux it into the video clip. I'm still looking into proper bitrate allocation and making sure a converter will also get the channels in the correct order. Even mixing the channels separately seems to be a science in itself.

If you want the original AC3 5.1 audio in your video you would need MKV as a container. But I personally would rather look into an AAC conversion since I find AC3, like MPEG-2, "old and fat".

Most experienced encoders do their video and audio encoding separately anyway so I'm not concerned about this workflow of separate content.

I've been looking into forums like hydrogenaudio.org to find my answer. I consider myself a n00b in this field so I'm doing alot of reading first before posting anything.

I'm sure we both can search our answer there. And if this forum doesn't provide one here, maybe one of us can revive this thread with an answer later on.
redraiderbum
Regular User
Posts: 132
Joined: Thu Jun 21, 2007 3:53 am

Post by redraiderbum »

I know little about this, but I was under the impression that you could demux the audio and video tracks. I have Apple's Logic Express for live band recording that I do on the side. With the separate audio track you could import the AC3 5.1 into logic express then do a mix down to an AAC 5.1 track. Could you not then encode the video separately then mux the two tracks back together after?

This of course requires that you have Logic Express or Logic Pro, perhaps Garageband will do it, but I haven't used it much. I have never used Final Cut HD, but it seems that should have the tools to mix down an AAC 5.1 track.

Although it feels like I'm misunderstanding the question? Honestly, I don't demux then remux the tracks (mainly because I don't know how to do it, I'm more familiar with audio only); or maybe there is some technical problem putting AAC 5.1 into an mp4 container, but I wouldn't think Apple would back mp4 if that was the case?
6-channel discrete
Maximum Bitrate = 384kbps
Each channel gets = 64kbps

Dolby Pro Logic II
Maximum Bitrate = 160kbps
Each channel gets = 80kbps

Does this mean that the audio fidelity of a DPL2 track is greater than that of the 6-channel discrete? Is it really a choice between 6 channels or higher bitrate audio?
I can, however, answer this question; the answer is, yes it is a choice between 6 channels or higher bitrate audio. However, I think you are equating audio fidelity and bitrate incorrectly. The reason is, typically when you are mixing down a track in 6-channel you are intentionally "assigning" each speaker less information by decreasing the range of sounds that each speaker must produce. That allows better overall sound quality/fidelity. So in stereo the reason the bitrate per channel is higher is because each channel is responsible for covering at least half the range (in practice it sounds funny if one channel in stereo produces half the range and the other produces the other half so they are both told to play the whole range).

There is one thing that does suck. If you play stereo sound over 6 channel speakers the playback device simply splits each channel in two and sends the information to the speakers, which causes the sound the be worse than it would be on 2 channel speakers.

Again maybe I missed your intent? If I did I apologize for the long post.

edit: @Puzzler, I'm not sure if logic express would faithfully import the AC3 audio channels, I suppose you could remix the audio yourself. It seems like it wouldn't be to difficult to demux the audio track faithfully with Logic or equivalent programs, I will try it when I get the chance.
Kev_UK
Posts: 2
Joined: Sun Oct 14, 2007 11:44 am

Post by Kev_UK »

No, you definitely understand me. It's clear that the best, or even only way to have a 6-channel AAC audio track with a higher bitrate is to demux and remux using other software. This is fair enough. It's a shame it requires more fiddling.
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