Downmixing AAC 6 channel to stereo with dialogue audio gain?

[vonsworld]

Hi

In the old days when Xvid/avi was the most common format, I often used to demux the AC3 6 channel sound from videos and then downmix and convert to Mp3 using the Lame encoder. You could also boost the dialogue track only so that it wasn't too quiet when listening through stereo speakers.

With current Mp4 files that contain 6 channel AAC audio streams what tools do I need to accomplish the same task please?

1, Demux the AAC stream
2. Downmix to stereo AAC whilst also boosting the dialogue channel only
3. Remux with the original video stream

Thanks for your advice

[pandy]

ffmpeg with audio filter:

 -af pan=stereo|FL < FL+1.414FC+0.5BL+0.5SL+0.25LFE+0.125BR|FR < FR+1.414FC+0.5BR+0.5SR+0.25LFE+0.125BL"

[vonsworld]

Thanks for your reply, this seems to work very well, I am using the command line...

ffmpeg -i input.mp4 -c:v copy -ac 2 -af "pan=stereo|FL < FL+1.414FC+0.5BL+0.5SL+0.25LFE+0.125BR|FR < FR+1.414FC+0.5BR+0.5SR+0.25LFE+0.125BL" output.mp4

my input audio has a bitrate of 224 but the resulting output drops to 128, how can I specify a higher rate or maintain 224 please?

I'm guessing that FFMpeg is not too fussy and this will also work with MKV files?

[JVRaines]

Use the -b:a option to specify a bitrate, e.g. -b:a 224k. FFmpeg works great with MKV files.

[pandy]

ffmpeg -i input.mp4 -c:v copy -c:a aac -b:a 224k -ac 2 -af "pan=stereo|FL < FL+1.414FC+0.5BL+0.5SL+0.25LFE+0.125BR|FR < FR+1.414FC+0.5BR+0.5SR+0.25LFE+0.125BL" -f matroska output.mkv