I have noticed a problem with FFmpeg downmixing. If I run a command such as
ffmpeg -i infile.flac -ac 2 outfile.flac
It will have identical result as
ffmpeg -i infile.flac \
-map_channel 0.0.0 \
-map_channel 0.0.1 \
-map_channel 0.0.2 \
-map_channel 0.0.4 \
-map_channel 0.0.5 \
outfile.flac
That is to say, the fourth channel AKA 0.0.3 AKA LFE AKA low frequency is
gone. How can I downmix 6 to 2 without losing a channel?
The -ac 2 algorithm leaves out "LFE" as can be seen by the 4th column of this
output
$ ffmpeg -i infile.flac -ac 2 -v debug -f null -
0.414214 0.000000 0.292893 0.000000 0.292893 0.000000
0.000000 0.414214 0.292893 0.000000 0.000000 0.292893
To fix, set the LFE mix level
$ ffmpeg -i infile.flac -ac 2 -lfe_mix_level 1 -v debug -f null -
0.320377 0.000000 0.226541 0.226541 0.226541 0.000000
0.000000 0.320377 0.226541 0.226541 0.000000 0.226541
It should be noted that LFE typcially contains duplicate audio information
The Low-Frequency Effects (LFE) channel contains extra bass information necessary to make effects sound big enough. It should never contain elements that are not in other channels
and that including it in a downmix could cause problems
There are other concerns when adding an LFE signal to the mix. If the LFE is simply redistributed within the other channels of the mix, they will usually be subject to some low-frequency bandpass filtering. This filtering causes phase shifts of the LFE signal. When they are acoustically added within a room, these phase shifts are fairly subtle and often go unnoticed. However, when they are electronically added together with the five main channels in the encoder, they may produce less than desirable results at certain frequencies. For this reason, it is recommended that the LFE signal not be used in a Dolby Pro Logic II downmix
