5.1 downmixed to 2.1?

[jersy]

Hello All,

I need to downmix 5.1 audio to 2.1, but can find a piece of software that will allow me to do the .1 part. Unless I'm missing something Soundtrack Pro, Audition, Audacity, etc. wont allow it to happen as everything ends up L,R,C.


I have a properly encoded 5.1 ac3 as well as individual stems to work with.


The final format is going to be Blu-ray (both NTSC and PAL) so a solution that exports to ac3 would be great.


Any suggestions or ideas would be appreciated.


Thanks.

[Selur]

it should be possible to:

1. decode the 5.1 audio to a multi wave
2. pipe the wave output to sox
3. remix the audio any way you want with sox in example: "remix -m 1v0.3205,3v0.2265,4v0.2265,5v0.2265 2v0.3205,3v0.2265,4v-0.2265,6v-0.2265 4v1" (normal dpl 5.1->stereo downmix and keeping the lfe channel in an additional separate channel; not sure if there's an official downmix mapping for 5.1 to 2.1)
4. pipe the wav output of sox to aften
5. reencode the audio to ac3 with aften (channel config and lfe needs to be specified otherwise the output of sox would be interpreted as L,R,S)

also now that I think of it the whole think might even be possible with ffmpeg using it's 'new' audio filter options (http://ffmpeg.org/ffmpeg-filters.html#Audio-Filters)

after some thinking about it:

ffmpeg -y -v -10 -i "Path to input file" -ac 6 -f sox - | sox --multi-threaded --buffer 524288 -S -t sox - -t wav - remix -m 1v0.3205,3v0.2265,4v0.2265,5v0.2265 2v0.3205,3v0.2265,4v-0.2265,6v-0.2265 4v1 | aften -v 2 -pad 0 -dnorm 27 -readtoeof 1 -b 256 -chconfig 2/0+lfe - "path to output.ac3"

seems to do the job as requested.

Did a small test and analysed the output with mediainfo and got:

Audio
Format : AC-3
Format/Info : Audio Coding 3
Mode extension : CM (complete main)
Format settings, Endianness : Big
Duration : 33s 376ms
Bit rate mode : Constant
Bit rate : 256 Kbps
Channel count : 3 channels
Channel positions : Front: L R, LFE
Sampling rate : 48.0 KHz
Bit depth : 16 bits
Compression mode : Lossy
Stream size : 1.02 MiB (100%)

-> looks (and sounds) fine
(Like mentioned before, I'm not too sure regarding the down mixing, it's just the first thing that came to mind.)

Cu Selur