[tebasuna51]

I can't understand your problem, use -down16.

The uncompressed audio data are stored in 2 bytes for 16 bits and in 3 bytes for 24 bits, don't exist a especific 20 bits storage, when eac3to say is 20 bits means than the 4 less significants bits, from the 3 bytes/24 bits, are always 0.
Here down-convert from 24 or 20 bits is the same, forget the less significant byte.