Downmix drops low frequency 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
- § 8 Channel Layout
- § 17 Downmix Levels
- § 28 Resampler Options
- § 34 pan
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
Mixing Information for Dolby Pro Logic II