Advice on how to downgrade the audio bitrate of a MOV file with AC-3 audio.

[InsanelyOne]

I'm about at my wits end with this. I've got a MOV file (started with an MKV file which I've already deleted). The MOV file has h.264 video and AC-3 audio at 640 Kbps. My A/V setup can't seem to handle AC-3 audio > 448 Kbps. I've figured out how to convert it to H.264 video with Stereo audio but I want to preserve the 6-channels. I have access to both Window's and Mac's so the OS doesn't matter. Anyone know how I can strip the audio out of the MOV file all the while preserving the 6-channels... and then downgrade it to 448 Kbps. Please don't bother asking why I'm not using MKV files. I need either MOV or MP4 files for my A/V setup. In a nutshell... this is my need:

Input: MOV with H.264, AC-3 640 Kbps
Output: MOV or MP4 with H.264, AC-3 448 Kbps

[Frobozz]

Demuxing the audio stream from the video can be done with various applications such as MPEG Streamclip and probably QuickTime Pro. But there is nothing that will resample the AC-3 file at a lower bit rate and retain its existing structure. There may be a pro application that can do this but I doubt it.

I'm trying to figure how or why a 640 Kbps AC-3 file was created in the first place. I've never seen one greater than 256 Kbps except for some 320 Kbps ones I created in Toast. The maximum that Toast will create (2-channel only, though) is 448. What created that file?

[ArtemisG3]

I have seen audio @ 640kbps on two DVDs -- Appleseed and Appleseed Ex Machina (maybe Vexille also, I can't remember).

You should be able to downsample the audio with ffmpeg (and ffmpegX). i would try a command like this (edit it fit your needs):

ffmpeg -i your_source_file.mov -vcodec copy -acodec ac3 -ab 448 -ar 48000 -ac 6 your_new_file.mov

To use ffmpegX, you will want to go to the video tab and select Video Codec: Passthrough(ffmpeg), and uncheck encode video. Then go to the audio tab and select AC3, bitrate 448, sampling 48000, channels 5:1, and mode CBR.

Let me know if this works.

[InsanelyOne]

I don't know the source, but I don't think it's unusual for HD content to have higher bitrates. According to the Dolby Digital specification these are the AC-3 bitrates:

DVD -> 448 Kbps
HD DVD -> 504 Kbps
Blu-Ray -> 640 Kbps


Quicktime and MPEG Streamclip will result in Stereo audio. Not exactly what I'm looking for, but it appears to be my only option. (Same goes for DTS.... doesn't seem like there's any simple way to convert DTS to AC-3)

[InsanelyOne]

I have seen audio @ 640kbps on two DVDs -- Appleseed and Appleseed Ex Machina (maybe Vexille also, I can't remember).

You should be able to downsample the audio with ffmpeg (and ffmpegX). i would try a command like this (edit it fit your needs):

ffmpeg -i your_source_file.mov -vcodec copy -acodec ac3 -ab 448 -ar 48000 -ac 6 your_new_file.mov

To use ffmpegX, you will want to go to the video tab and select Video Codec: Passthrough(ffmpeg), and uncheck encode video. Then go to the audio tab and select AC3, bitrate 448, sampling 48000, channels 5:1, and mode CBR.

Let me know if this works.

I tried it with the same options you specified and got this error:
Error while opening codec for output stream #0.1 - maybe incorrect parameters such as bit_rate, rate, width or height

I removed the '-ab 448' option and got this error:
Unsupported codec (id=0) for input stream #0.1

Are you sure ffmpeg supports AC-3 as an input stream?

[ArtemisG3]

Are you giving ffmpeg the .mov file as the source or just an AC3 file? Can you paste the entire output from ffmpeg here so, I can get a better idea of what is happening?

[InsanelyOne]

Are you giving ffmpeg the .mov file as the source or just an AC3 file? Can you paste the entire output from ffmpeg here so, I can get a better idea of what is happening?

Well... before I lead you too far... I should let you know that I'm using the ffmpeg that comes with the latest version of Visual Hub. Not sure if that makes a difference. I presume he's using an up-to-date version. Here's the full output:

FFmpeg version SVN-r9226, Copyright (c) 2000-2007 Fabrice Bellard, et al.
libavutil: 49.4.0 libavcodec: 51.40.4 libavformat: 51.12.1
built: Feb 12 2008 19:58:15, gcc: 4.0.1 (Apple Computer, Inc. build 5367), i386
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/Users/xxx/Music/iTunes/iTunes Music/Movies/xxxxxx.mov':
Duration-8607
start-0.000000
bitrate-5214

0.0,,,(eng),,Video,h264,yuv420p,1280,536,23.98
0.1,,,(eng),,Audio,ac-3 / 0x332D6361,48000,5.1
0.2,,,(),,Audio,mpeg4aac,48000,2
Output #0, mov, to '/Users/xxx/Desktop/xxxxxx.mov':
0.0,,,,,Video,h264,yuv420p,1280,536, q=1-31,23.98
0.1,,,,,Audio,ac3,48000,5.1,0
Stream mapping:
Stream #0.0 -> #0.0
Stream #0.1 -> #0.1
Error while opening codec for output stream #0.1 - maybe incorrect parameters such as bit_rate, rate, width or height

[ArtemisG3]

You could try another build of ffmpeg like the one bundled with ffmpegX. If that fails too, then I'm not sure what else you can do. The audio may be non-standard AC3. I notice that ffmpeg is not reporting the bitrate of the AC3, but instead "0x332D6361".

[Herve]

I should let you know that I'm using the ffmpeg that comes with the latest version of Visual Hub

so... (I suppose VisualHub must have a recent version of ffmpeg):

ffmpeg -i your_source_file.mov -vcodec copy -acodec ac3 -ab 448k -ar 48000 -ac 6 ~/Desktop/your_new_file.mov

PS: I didn't try it (with .mov you usually must specify size and freamerate, ...) but there still was an error in the command line

bye

[Midzuki]

Just my $ 1.99:

decode with valdec.exe (part of the AC3Filter Tools package),
recompress with aften.exe.

[AntnyMD]

Any solution you're likely to find that works (and is free) is almost certain to cause your next problem: your new audio is not in sync with the video.

[ArtemisG3]

Any solution you're likely to find that works (and is free) is almost certain to cause your next problem: your new audio is not in sync with the video.

What makes you say that? Anytime I have audio sync issues with any ffmpeg output, it turns out the problem is with the original file and not the cause of ffmpeg.

[tomlee59]

There's a little semantics to be acknowledged here. Sync problems are very common, and while the origin may very well lie in the source, well-written software arguably should be able to handle it, if the source files are still compliant. The framerate-switching that seems to go on a lot in commercial movies is not handled at all well by many ffmpeg modules, leading to serious sync problems far too frequently.

[stanislawski]

demux w/mpeg streamclip then use Compressor from final cut studio

[InsanelyOne]

demux w/mpeg streamclip then use Compressor from final cut studio

MPEG Streamclip only seems to support Stereo Audio when exporting... at least as far as I can figure out.

[Midzuki]

InsanelyOne wrote:

I have access to both Window's and Mac's so the OS doesn't matter.

Therefore:

a) demux with YAMB ;

b) decompress into a 5.1 .WAV;

c) recompress into a 5.1 AC3;

HTH.