ffmpeg downmix 7.1 -> 5.1

[tebasuna51]

The simple way to do the downmix 7.1 -> 5.1 is:

ffmpeg -i INPUT71 -af "pan=5.1(side)|FL=FL|FR=FR|FC=FC|LFE=LFE|SL=0.5*BL+0.5*SL|SR=0.5*BR+0.5*SR" OUT51.wav

But I want preserve the volume balance between front and surround channels, like BeHappy/MeGUI can do in AviSynth, with a controlled gain by 2 without clip.

Then I make a separated chain for the surround channels with:

ffmpeg -i INPUT71 -filter_complex "asplit [f][s]; \
 [f] pan=4c|c0=c0|c1=c1|c2=c2|c3=c3 [r];  \
 [s] pan=2c|c0=0.5*c4+0.5*c6|c1=0.5*c5+0.5*c7, \
 acontrast=2, aformat=sample_fmts=s16:channel_layouts=stereo [d]; \
 [r][d] amerge=inputs=2,pan=5.1(side) [a]" -map "[a]" OUT51.wav

I tried other functions like 'compand' instead 'acontrast', other formats in 'aformat', 'join' instead 'amerge', ... and always obtain errors in formats, channels layouts, ...

Only the previous command work without errors, the output wav have the format, length, ... correct, but is empty. I only obtain silence.

Please, there are some ffmpeg guru than can help me.