Downmixing 5.1 surround AC3 to stereo

[MOverride]

Hi. Before posting, I tried to :readfaq: and :readguid:, but didn't see anything helpful. If I missed it, please feel free to point out the error of my ways. :D

I have several movies that I'm converting to DivX that only have surround tracks, not stereo. For space reasons, and the fact that I don't have a surround system, I'm interested in downmixing these to stereo.

While I could do this fairly easily if I were going to reencode, as an AC3 or as some other format, I'd prefer to not do that if possible (simply because compressing lossily multiple times is generally not a good idea :p).

So, is there any way to downmix the AC3 into a stereo file (almost certainly another AC3; I know that I could do it into WAV files, but that wouldn't help the space issue any)? Or am I stuck reencoding the track if I want to downmix?

Thanks for any help you can provide! :)

[Skelsgard]

Unfortunately, you´re stuck with recompression.
If done properly, recompression can be "audibly impecable" (not really lossless).
It will be determined by the chosen transcoding source and output: AC3 to AAC, or AC3 to OGG, etc.
If you´re interested in downmixing, there wouldn't actually be multiple lossy compressions, just one: AC3 to WAV (lossless) and WAV to whatever (lossy).

Cheers

[HeadBangeR77]

Downmixing means reencoding as a rule - no other way to do this ;)

There are no free, open source & stable, mature AC3 encoders atm, and moreover I don't think AC3 5.1 => AC3 2.0 is a good idea. Believe me or not, AC3 is a lossy format, so transcoding into either high bitrate VBR MP3, Vorbis or AAC LC (which I prefer over HE-profile) is the only solution, if you go for stereo sound.

Btw. I remember I've written similar things in similar topics at least 3-4 times over the last few weeks ;)

[Skelsgard]

There are no free, open source & stable, mature AC3 encoders atm

Aften has proven itself as a very mature and high quality encoder so far, give or take tweaks between versions.
You can do your own test with Aften against Vegas AC3 and Soft Encode.

Cheers.

[HeadBangeR77]

Aften has proven itself as a very mature and high quality encoder so far, give or take tweaks between versions.
You can do your own test with Aften against Vegas AC3 and Soft Encode.

Cheers.
Downloaded recently, scanned its thread for some info, but I haven't given Aften a chance yet. Thanks for the tip :)

cheers

PS. Now I recall I used to have some problems with Aften and BeLight, and I finally gave up :o

[MOverride]

Believe me or not, AC3 is a lossy format,
Oh, I believe you. And I'm pretty sure that I said as much in my original post. ;)

Btw. I remember I've written similar things in similar topics at least 3-4 times over the last few weeks ;)
All of the posts I could find were going the other way: stereo upmixed to surround. Given that those were creating data, I didn't think they'd really be applicable.

Thanks, both of you, for your help. Time to break out BeSweet...

[Skelsgard]

PS. Now I recall I used to have some problems with Aften and BeLight, and I finally gave up
For all the respect I have for BeSweet, I think the best complement for Aften is kurtnoise's AftenGUI (others have joined him in this project). You should definitely start with that: Aften 1.04 + AftenGUI 1.2. Aften 1.05 had a few minor issues IIRC from what I´ve read lately (unimportant ones), I might be wrong or they might be fixed by now.

Cheers.

[HeadBangeR77]

@ MOverride
And? How is your transcoding? Sucessful?

Btw. I'm gonna give Aften a try, when I have finished some complicated filterchain for my current encode ;)

[MOverride]

@ MOverride
And? How is your transcoding? Sucessful?
Somewhat. :) I can get it into an MP3 easily enough; I'm now in the process of learning about AC3 encoding now that you pointed out Aften as a viable free encoder. I'm considering going with Dolby Surround 2.0 as opposed to a straight stereo file...

[HeadBangeR77]

Somewhat. :) I can get it into an MP3 easily enough; I'm now in the process of learning about AC3 encoding now that you pointed out Aften as a viable free encoder. I'm considering going with Dolby Surround 2.0 as opposed to a straight stereo file...
Actually, it was Skelsgard who pointed out Aften. ;) How is it going? What revision do you use? Btw. is Aften capable of encoding AC3 VBR or am I dreaming while still awake? :D If so, it would be a chance for me to lower AC3 bitrate a bit, hopefully without large impact on quality. Too busy to test it myself atm, since I'm into some deblocking filters, fighting against blocks and for details' preservation. ;)

Have you tried LAME 3.98 alpha, or do you stick to the recommended 3.97?

cheers,
HDBR77

[Pookie]

I recently did a test of 2ch 224kbs Aften AC3 and 3.98a Lame. Both are really, really good. Really...

aften.exe -acmod 2 -v 0 -b 224 -dynrng 5 input.wav output.ac3
http://prdownloads.sourceforge.net/win32builds/aften-svn-r265-bin.zip?download

lame input.wav -b 224 --cbr -F -m s -h --noreplaygain output.mp3
http://www.rarewares.org/dancer/dancer.php?f=128

[Skelsgard]

Btw. is Aften capable of encoding AC3 VBR or am I dreaming while still awake?
Actually, IT IS!!!!
Awesome, isn´t it?
One thing I´ve seen is that when you encode a WAV (obtained from the decoding of AC3 file "A") to an AC3 file "B", A and B AC3 files are the exact same size. It would be interesting to see how a VBR AC3 would behave in this matter.
Pookie's test got me intrigued too. I think I´m gonna try to push it to 192 kbps, and even 160kbps, to see if Aften can provide same quality at these bitrates. AC3 is supposedly capable of good quality in the 192 kbps range.

Cheers

[Kurtnoise]

So, is there any way to downmix the AC3 into a stereo file (almost certainly another AC3; I know that I could do it into WAV files, but that wouldn't help the space issue any)? Or am I stuck reencoding the track if I want to downmix?

Why not downmix during the playback instead of reencoding ?

[tebasuna51]

@MOverride, some hints about topics in this thread:

- Of course you need re-encode after a downmix.

- The downmix method (stereo, Dolby ProLogic I, Dolby ProLogic II) is independent of re-encode format. You can have a dplII mix in mp3 format and instruct the decoder to expand to surround audio.

- If you need standalone player compatibility the audio must be AC3 or MP3.

- AC3 VBR is not compatible with avi container (or DVD format) then is unusable for movie audio track.

- MP3 VBR have also problems but some muxer (Nandub, AviMuxGUI) can obtain playable avi's. For a complete compatibility use MP3 CBR.

- At same bitrate a MP3 is better than an AC3. In others words an AC3 192 Kb/s is equivalent to a MP3 128/160 Kb/s (of course is not exact and source dependent).

- An AC3 format benefit is the Dynamic Range Compression info.
You can preserve the full dynamic range and add the DRC to be used in some conditions. The unique free encoder with DRC possibilities is Aften, but is in test state.

@KurtNoise, MOverride say at first post: "For space reasons, and the fact that I don't have a surround system, I'm interested in downmixing these to stereo."

[HeadBangeR77]

(...)
- MP3 VBR have also problems but some muxer (Nandub, AviMuxGUI) can obtain playable avi's. For a complete compatibility use MP3 CBR.
I must agree with all the statements you've made above, but with all due respect, the quoted one isn't fully correct IMHO.

"This is probably the piece of disinformation most widely spread about AVI. Seeking in an MP3-VBR audio stream in DirectShow works as if it was a video stream, not an audio stream: Each chunk contains data for a constant amount of time (usually 33 or 40 ms for video, or 24 ms for MP3 audio at 48 kHz). The chunk/frame to be loaded is just [time] / [time per frame], as if it was a video stream. The way MP3-VBR is sought in is valid for AVI files. Maybe it was not intended to be used for audio streams, maybe it was, fact is, the specification doesn't limit any particular seeking strategy to any particular stream type! There is a value in the stream headers, called dwSampleSize, which is 0 in order to trigger VBR stream seeking. This is officially documented in the MSDN and not a hack, bug or whatever. The way MP3-VBR and AAC are stored in AVI are specified and completely compliant with the AVI file specification."

Trying to undestand the above and using AVI-MUX for some time, I would never say MP3 VBR in an AVI container is "incompatible". I would rather say it's beyond specification, but not against it. Proper muxing + good spliter (even M$'s) work for me better than in mkv (?) - no problem while seeking.

@ Pookie
LAME 3.98 alpha11? For transcoding it behaves strangely with BeSweet/BeLight: defaults to -q0 instead of -q3 in case of -V2 and results in much higher bitrate (though I don't think these two are connected, are they?), what I definately don't need in soundtracks. :D
On the other hand I must try it in EAC for my audio CDs, yet I'm lacking hdd space atm.

cheers,
HDBR77

[MOverride]

How is it going? What revision do you use?
Kurtnoise13's 0.05 r278, with GUI 1.3, is what I used. And it's going quite well. :) I've got 2.0 downmixes of everything that'd been piling up on the hard drive. I can't guarantee for sure that these are actually surround encoded--should be, if I have BeSweet set up right--but I have no way to test at the moment. Maybe in a few years, though at that point I may just go back to 5.1. :p

Have you tried LAME 3.98 alpha, or do you stick to the recommended 3.97?
Actually, I'm still on 3.90. :scared: I didn't realize that the version that BeSweet 1.51r31 ships with was so out of date...

Thanks again, everyone! :D