help anyone? - AAC to AC3(5.1)

[pootler]

Hello I am new to the forums.

I have videos with AAC 5.1 audio that I need to convert to AC3 5.1 to play in surround on my system.

Can Ffmpegx pass through ( i.e no encoding decoding) the video whilst only changing the audio stream from AAC to Ac3?

Do I have to replace the AAC track or can I add another track of AC3 encoding?
Does it maintain the correct channel mapping for AC3?

In order to passthrough video without any alteration do I de-select ''encode video'' or do I select passthrough in the dropdown and then select ''encode video''?

Thanks and many thanks for everybodys hard work on this project

Leo

[Case]

Can ffmpegX pass through the video whilst only changing the audio stream from AAC to AC3?

Only for AVI output and only to AC3 2.0 (stereo).

Do I have to replace the AAC track or can I add another track of AC3 encoding?

Either one can be done, but the latter is a bit more work.

Does it maintain the correct channel mapping for AC3?

No, as 5.1 is lost in the conversion to 2.0.

In order to passthrough video without any alteration do I de-select ''encode video'' or do I select passthrough in the dropdown and then select ''encode video''?

Select "Passthrough" in the dropdown and keep "Encode video" selected.

---------
It is possible to convert AAC 5.1 to AC3 5.1, but not with just ffmpegX 0.0.9y.
I have done it as a test a while ago, using QuickTime Pro and a guide on the Perian forum.
In short, the procedure was AAC 5.1 -> AIFF discrete channels -> AIFF 5.1 -> AC3 5.1 .

[pootler]

thanks for such a fast and comprehensive reply

I have found another program called avidemux2 - it seems to be able to do this (?)- I f anyone is interested I will let you know!

Thanks again

leo

[pootler]

thanks again for the reply CASE.

Here's an addendum.

Avidemux2 does indeed do what I require - but it messes up the AC3 5.1 channel mapping.

However - and I can only go via the information Plex provides - If I try with FFmepgx using ''encode video'' passthrough

and select AC3 (5.1) output IT DOES SEEM to produce an output with AC3 5.1 sound!

If I don't select ''encode video'' then it produces as you say AC3 in stereo.

SOOooo.. it seems to do what I require - I have now got to check the channel mapping.


By the way I find the conversion is slow - do you find this?

I can't see why the video has to be encoded if it is going to be passed through?

thanks

Leo

p.s Avidemux seems to be at least twice as fast

[Case]

It seems every tool that does AAC to AC3 has to do channel mapping, as it differs among file formats.


(from AviSynth Media Wiki)

Afaik, ffmpeg doesn't do channel mapping yet, so any app relying on ffmpeg will get it wrong.

The only free/low cost tool I could find that does 5.1 channel mapping is AviSynth (Windows).

[pootler]

Yes - seems windoze has some of the best programs - Mediacoder also does this and allows channel remapping.

Pains me but might have to resort to virtual windows for the conversions

Leo

[poisondeathray]

One option is to map the channels and do the conversion in audacity , free and crossplatform (pc/mac/linux) . It uses ffmpeg for the backend encoding if using ac3, aac

[pootler]

I guess if you want to there are ways to import and export the audio to achieve the same effect.

Problem is I am a lazy s*d and I would prefer a one stop - one button solution!

Leo

[poisondeathray]

Sorry not sure what you would use to do that more easily on a Mac. Windows does have more versatile programs in this regard

[johns0]

Thread moved to mac forum.

[emmgunn]

Hi Pootler,
If you don't mind being a guinea pig, you can try the latest beta of MKVtools. It'll do a one step 5.1 AAC to 5.1 AC3 conversion. Though mostly used for MKV files, the latest beta of MVKtools should (hopefully) have improved handling of other video formats like avi and mp4. I can test converting mp4 video with 5.1 AAC into avi video with 5.1 AC3, and that seems to work, but I have no way of testing mp4's with 5.1 AC3 audio, so I'd love to get some feedback. Also, since it sounded like you might be interested in having both audio tracks in your video, if you're feeling adventurous, I'd love to hear if you're able to successfully add multiple audio tracks to the video, and if its clear how you you can do that.

[pootler]

Thanks emmgun.

I'll give it a go!

In case you are interested, my current research has led me to a windows program called mediacoder which seems to give me the option to convert and channel map to other 5.1 audio streams.

Thankfully it opens with crossover in leopard so no need for virtual machines.

Still have to check channel mapping though.

Thanks again - will let you know my results with MKVtools

Leo

[AlanHK]

Avidemux2 does indeed do what I require - but it messes up the AC3 5.1 channel mapping.

There are several options under Filters/Mixer when converting AAC to AC3:
No change; 5, 5.1; Dolby...
Not sure which is appropriate for you; did you try these?

[emmgunn]

Hey Pootler,
I just found a major bug in MKVtools 2.4.b.1 where your original video file can be lost. DO NOT USE. I'll upload a fix soon.

[pootler]

thanks alan hk.

I have explored most of these options but they relate to the amount of channels.

What avidemux doesn't seem to help with is the mapping - i.e swapping the audio stream on channel one with ,say, channel two when converting to another audio format.
Because of the different channel mapping for different formats this is important if you are going to maintain the correct audio to the correct channel - hence you get ( in my case) center channel assigned to the left front!!

Leo

[emmgunn]

Hi Pootler,
MKVtools 2.4.b.2 is now uploaded which fixes the issue I mentioned above. Note, with MKVtools, you shouldn't have to worry at all about channel mapping, which is good because when I was playing around with it, it was a mess. It seemed almost random at times, and the documentation I found didn't help much. It basically came down to trial and error, but I think MKVtools should handle the following surround audio conversions:

• 5.1 AAC to 5.1 AC3
• 5.1 DTS to 5.1 AAC and 5.1 AC3
• 5.1 AC3 to 5.1 AAC

Let me know how it goes and feel free to e-mail me if you have any questions.

[AlanHK]

Because of the different channel mapping for different formats this is important if you are going to maintain the correct audio to the correct channel - hence you get ( in my case) center channel assigned to the left front!!

Yeah, obviously -- I would have expected "left front" mapped to "left front" in another format, if not I'd rate it as a bug.

But I do this in Avisynth and often find that I have to change the default mapping, perhaps due to encoding inconsistencies.

[kevinlane]

It seems every tool that does AAC to AC3 has to do channel mapping, as it differs among file formats.


(from AviSynth Media Wiki)

Afaik, ffmpeg doesn't do channel mapping yet, so any app relying on ffmpeg will get it wrong.

The only free/low cost tool I could find that does 5.1 channel mapping is AviSynth (Windows).

I have convrerted one aac audio from a mkv to ac3 using vlc player. my center channel audio comes from LS. It should come from FL as per this table. what's wrong? I was to remap it using eac3to but not sure of remapping order. What to do?