FFPMEG-DOWNMIX

An ffmpeg-docker wrapper for a script to appropriately downmix surround-sound audio in a media file to stereo, and append it to the media file.

The script requires a workdir folder to be mounted containing the input file. The output file is placed in the same directory. If the output file (arg#2) is not specified, the input file will be deleted at the end of the script and the output file will replace it.

Docker

Scripts

Build the container:

docker build --tag ffmpeg-downmix .

Run the script:

docker run -v /ABSOLUTE/PATH/TO/WORKDIR:/workdir --rm ffmpeg-downmix INPUT_FILE [OUTPUT_FILE]

Where /ABSOLUTE/PATH/TO/WORKDIR, INPUT_FILE and [OUTPUT_FILE] need to be modified per your usage. OUTPUT_FILE is optional. If specified, the input will remain untouched. If no OUTPUT_FILE is specified, the input_file will be overwritten with the output file at the end of the script. This second use is for automation cases where it is useful to replace the existing file.

Example 1:

Input and output files specified.

The following example mounts the existing workdir folder in the repo, and downmixes the primary audio source in input.mkv to a stereo source. It will append it to a new version of the file, output.mkv, as the new default source at a:0. The original sources and subtitles will all be preserved:

docker run -v C:\ffmpeg-downmix\workdir:/workdir --rm ffmpeg-downmix input.mkv output.mkv

The output output.mkv will be in the same workdir folder as the input file.

Example 2:

Only input file specified.

The following example mounts the existing workdir folder in the repo, and downmixes the primary audio source in input.mkv to a stereo source. It will append it to a new version of the file, input_new.mkv, as the new default source at a:0. The script will then overwrite input.mkv with input_new.mkv. The original sources and subtitles will all be preserved:

docker run -v C:\ffmpeg-downmix\workdir:/workdir --rm ffmpeg-downmix input.mkv

All temporary files will be deleted.

Mixing Ratios

The current mixing ratios in the script are as follows:

5.1 Surround

FL = "0.5*FC + 0.707*FL + 0.707*BL + 0.5*LFE"
FR = "0.5*FC + 0.707*FR + 0.707*BR + 0.5*LFE"

5.1(side) Surround

FL = "0.5*FC + 0.707*FL + 0.707*SL + 0.5*LFE"
FR = "0.5*FC + 0.707*FR + 0.707*SR + 0.5*LFE"

6.1 Surround

FL = "0.321953*FC + 0.455310*FL + 0.394310*SL + 0.227655*SR + 0.278819*BC + 0.321953*LFE"
FR = "0.321953*FC + 0.455310*FR + 0.394310*SR + 0.227655*SL + 0.278819*BC + 0.321953*LFE"

7.1 Surround

FL = "0.274804*FC + 0.388631*FL + 0.336565*SL + 0.194316*SR + 0.336565*BL + 0.194316*BR + 0.274804*LFE"
FR = "0.274804*FC + 0.388631*FR + 0.336565*SR + 0.194316*SL + 0.336565*BR + 0.194316*BL + 0.274804*LFE"

These can be adjusted in downmix.sh before building the container.

Sources

The script uses some of the mixing ratios suggested in the discussion: https://superuser.com/questions/852400/properly-downmix-5-1-to-stereo-using-ffmpeg