Downmixing multi-channel tracks to stereo and normalising with FFmpeg's loudnorm

[GeoffreyA]

Back in the DVD days, I used Azid to apply DRC, downmix, and normalise audio. Downmixing those 5.1 tracks was always softer than the stereo tracks included on many DVDs. Coming back to encoding years later, the tools have changed but the problems haven't. Dialogue is still too soft.

FFmpeg's loudnorm filter does a good job at harmonising the volume and seems to be better than dynaudnorm. However, one has to adjust the target: loudness and true peak are easy enough (-23 and -1 or -2), but the range, or LRA, is up to the user's taste. Netflix recommends an LRA between 4 and 18; and I find that 18 gives good results, with audible dialogue.

My question is: what LRA values are others using that give good results with downmixed film material? Also, is it better to run loudnorm before downmixing or after?

Here is my batch file. The first, commented-out FFmpeg line is the measuring pass. After it runs, I set the variables to the measured values and run the second pass.

set out_i=-23
set out_tp=-2
set out_lra=12

set in_i=-16.8
set in_tp=6.7
set in_lra=23.2
set in_thresh=-29.4
set tg_offset=1.6

::ffmpeg -i %1 -map 0:a:0 -af aresample=ochl=stereo,loudnorm=i=%out_i%:tp=%out_tp%:lra=%out_lra%:print_format=summary -f null -

ffmpeg -i %1 -map 0:a:0 -af aresample=ochl=stereo:osr=192000:resampler=soxr:precision=33,
loudnorm=i=%out_i%:tp=%out_tp%:lra=%out_lra%:measured_i=%in_i%:measured_tp=%in_tp%:measured_lra=%in_lra%:measured_thresh=%in_thresh%:offset=%tg_offset%:linear=true:print_format=summary,
aresample=48000:resampler=soxr:precision=33 -c:a pcm_f32le -f wav - |
"%qaac%" --tvbr 91 --ignorelength --no-delay --verbose - -o "out\2.0-loudnorm.m4a"