Posted by admin on February 21, 2013 under Tech Tips | 
If you have a source video file encoded with an AC3 Dolby Digital audio stream, you can extract the audio in it’s native format using FFMpeg.
The following example shows how to identify the available audio streams of the file video.avi. Just use ffmpeg without any output options, and you can see there are two streams (0.0 and 0.1), the second is AC3 audio.
ffmpeg -i video.avi
Input #0, avi, from 'video.avi':
Duration: 01:17:57.64, start: 0.000000, bitrate: 1587 kb/s
Stream #0.0: Video: mpeg4, yuv420p, 672x576 (snipped for brevity)
Stream #0.1: Audio: ac3, 48000 Hz, 5.1, s16, 448 kb/s
At least one output file must be specified
The following command will extract the AC3 audio stream to a file called audio.ac3.
ffmpeg -i video.avi -acodec copy audio.ac3
Input #0, avi, from 'video.avi':
Duration: 01:17:57.64, start: 0.000000, bitrate: 1587 kb/s
Stream #0.0: Video: mpeg4, yuv420p, 672x576 (snipped for brevity)
Stream #0.1: Audio: ac3, 48000 Hz, 5.1, s16, 448 kb/s
Output #0, ac3, to 'audio.ac3':
Stream #0.0: Audio: ac3, 48000 Hz, 5.1, s16, 448 kb/s
Stream mapping:
Stream #0.1 -> #0.0
Press [q] to stop encoding
size= 255799kB time=4677.51 bitrate= 448.0kbits/s
video:0kB audio:255799kB global headers:0kB muxing overhead 0.000000%
Verify the file was created. The output below shows that this stream is about 250Mb.
ls -lh audio.ac3
-rw-r--r-- 1 username gmendoza 250M 2010-02-21 09:47 audio.ac3
You can now use ffmpeg again to show that audio.ac3 only contains the ac3 audio stream.
ffmpeg -i audio.ac3
Input #0, ac3, from 'audio.ac3':
Duration: 01:17:57.46, bitrate: 448 kb/s
Stream #0.0: Audio: ac3, 48000 Hz, 5.1, s16, 448 kb/s
At least one output file must be specified
Now that you have extracted the audio stream, you can do anything you wish with it. Enjoy.
Posted by admin on September 26, 2009 under Tech Tips |
If you have Matroska Video (MKV) files encoded with AC3 Dolby Digital 5.1 or DTS audio tracks, you may want to simply extract the audio, convert it to a 2-channel stereo format like WAV, MP3 OGG, etc, and then add it back into the MKV as a separate audio track. This is useful when your media player (e.g. Western Digital Media Player WDAVN00) will not downscale the audio from a digital format like AC3 or DTS to stereo when you don’t have a receiver or TV with a built in Dolby Digital decoder. Now you’ll have the choice of either audio format depending on your technical requirements.
The great thing about the Matroska multimedia container is that you can easily manipulate these files without having to re-encode, saving lots of time. I’ll be using mkvextract to extract the AC3 audio, ffmpeg to convert ac3 to mp3, and finally mkvmerge to add and remux the new audio track to the MKV container. All of these are available to a number of platforms, but in my examples, I’m using Linux. Check out the MKVToolnix and FFMpeg websites for more info on the software.
If using Ubuntu Linux, install the relevant mkvtoolnix, mkvtoolnix-gui and ffmpeg packages.
sudo apt-get install mkvtoolnix mkvtoolnix-gui ffmpeg libavcodec-unstripped-52
To view the existing tracks of the MKV, use the mkvmerge -i option. In the following example, you see my “Cool.Video.mkv” file has an MPEG4 video in track 1, an AC3 Dolby Digital audio file in track 2, and subtitles in track 3.
mkvmerge -i Cool.Movie.mkv
File 'Cool.Movie.mkv': container: Matroska
Track ID 1: video (V_MPEG4/ISO/AVC)
Track ID 2: audio (A_AC3)
Track ID 3: subtitles (S_TEXT/UTF8)
Using mkvextract, extract the AC3 Dolby Digital audio from track 2, saving it to a file called audio.ac3.
mkvextract tracks Cool.Movie.mkv 2:audio.ac3
Extracting track 2 with the CodecID 'A_AC3' to the file 'audio.ac3'. Container format: Dolby Digital (AC3)
Progress: 100%
ls -lh audio.ac3
-rw-r--r-- 1 gmendoza gmendoza 432M 2009-09-26 11:58 audio.ac3
Convert the 6-channel ac3 file to a 2-channel stereo MP3 using ffmpeg. If you prefer a higher audio bitrate, adjust the -ab value as desired. e.g. 256, 384, etc, and adjust the audio rate to your liking as well.
ffmpeg -i audio.ac3 -acodec libmp3lame -ab 160k -ac 2 audio.mp3
[output omitted for brevity]
ls -lh audio.*
-rw-r--r-- 1 gmendoza gmendoza 432M 2009-09-26 11:58 audio.ac3
-rw-r--r-- 1 gmendoza gmendoza 87M 2009-09-26 12:08 audio.mp3
To simplify things, you could actually skip the digital format extraction process by running ffmpeg against the MKV file directly.
ffmpeg -i Cool.Movie.mkv -acodec libmp3lame -ab 160k -ac 2 audio.mp3
If you prefer encoding with more advanced options, you could extract the audio as a 2-channel WAV file instead, and then process it with LAME, Oggenc, or some other encoder of your choosing. The following shows the extraction to WAV, and then conversion to various formats for fun, e.g. MP3, OGG, and FLAC.
ffmpeg -i Cool.Movie.mkv -acodec pcm_s16le -ac 2 audio.wav
lame -V0 -q0 --vbr-new audio.wav audio.mp3
oggenc -q6 audio.wav
flac audio.wav
Use mkvmerge to combine the original MKV with the MP3 audio track to create a new file called Cool.Movie.New.mkv. Make sure you have enough disk space for both the original and new MKV file.
mkvmerge -o Cool.Movie.New.mkv Cool.Movie.mkv audio.mp3
mkvmerge v2.4.1 ('Use Me') built on Dec 13 2008 21:03:46
'Cool.Movie.mkv': Using the Matroska demultiplexer.
'audio.mp3': Using the MP2/MP3 demultiplexer.
Warning: 'audio.mp3': Skipping 32 bytes at the beginning (no valid MP3 header found).
'Cool.Movie.mkv' track 1: Using the MPEG-4 part 10 (AVC) video output module.
'Cool.Movie.mkv' track 2: Using the AC3 output module.
'Cool.Movie.mkv' track 3: Using the text subtitle output module.
'audio.mp3' track 0: Using the MPEG audio output module.
The file 'Cool.Movie.New.mkv' has been opened for writing.
Progress: 100%
The cue entries (the index) are being written...
Muxing took 270 seconds.
Verify that the audio track has been added. You can see Track ID 4 has been successfully added.
mkvmerge -i New.Cool.Movie.mkv
File 'New.Cool.Movie.mkv': container: Matroska
Track ID 1: video (V_MPEG4/ISO/AVC)
Track ID 2: audio (A_AC3)
Track ID 3: subtitles (S_TEXT/UTF8)
Track ID 4: audio (A_MPEG/L3)
That’s really all there is to it. There are quite a few options available when editing MKV container files. For example, I wanted nice descriptions for my tracks since various media players will read and display them for you during menu navigation. I recommend using the mkvmerge gui application as shown in this screenshot.
It’s really just a front-end application to mkvmerge, and the following text shows the commands that were used to specify the language for each tag, re-order the audio tracks, disable subtitles by default, and give useful descriptions to each Track ID.
mkvmerge -o "Cool.Movie.New.mkv" \
--language 1:eng \
--track-name "1:Cool Movie (MPEG4)" \
--default-track 1:yes \
--display-dimensions 1:40x17 \
--language 2:eng \
--track-name "2:Dolby Digital 5.1 (AC3)" \
--default-track 2:yes \
--language 3:eng \
--track-name "3:English Subtitles" \
--default-track 3:no \
-a 2 -d 1 -s 3 Cool.Movie.mkv \
--language 0:eng \
--track-name "0:2-Channel Stereo (MP3)" \
--default-track 0:no \
-a 0 -D -S audio.mp3 \
--track-order 0:1,0:2,1:0,0:3
mkvmerge -i Cool.Movie.New.mkv
File 'Cool.Movie.New.mkv': container: Matroska
Track ID 1: video (V_MPEG4/ISO/AVC)
Track ID 2: audio (A_AC3)
Track ID 3: audio (A_MPEG/L3)
Track ID 4: subtitles (S_TEXT/UTF8)
Tags: AC3, Audio, Dolby Digital, FFMpeg, LAME, Linux, Matroska, MKV, mplayer, Multimedia, Video
Posted by admin on April 9, 2009 under Tech Tips |
I recently wanted to convert some of my 720p and 1080p Matroska Video (MKV) files to the Xvid format so that I can play them on my Xbox 360 (check out ushare). I really wanted to make sure that the video quality and Dolby Digital 5.1 audio would remain intact, and was pleased to get the job done with mencoder.
In the following example, I decided to use a single pass, fixed quantizer value of 4. The audio will simply be copied.
mencoder movie.mkv -channels 6 -ovc xvid -xvidencopts fixed_quant=4 \
-vf harddup -oac copy -o movie.avi
The Dolby Digital 5.1 (AC3) output was a major pain to figure out because by default, mencoder (and mplayer) only will select 2 audio channels. So increasing the value to 6 ensures you receive them all. Otherwise, you end up getting standard stereo out all channels.
There’s a ton of options that you can use, so just be sure to read the man pages for mencoder.
NOTE 1: This is not an exhaustive or definitive post on quality retention. This is just an easy way to re-encode a source video file to Xvid.
NOTE 2: MKV is only a container file format, meaning that you store audio and video tracks within an MKV file, as well as a number of other data types. e.g. Subtitles, Pictures, Fonts, etc. Many times, these video and audio tracks may already have been encoded with a codec supported by your media player. You could potentially extract the appropriate audio and video tracks with mkvextract (a component of the mkvtoolnix package), and recombine them into a container format supported by your platform. This is a great option because you would not have to re-encode, saving time and quality loss. I’ll update with more examples later.
