Dynamic / velocity

Hi
It's possible to have a function that can increase/ decrease the dynamic of single midi channel or the entire midi song

So you mean add or subtract a fixed velocity amount per track or entire song? Or do you mean compress or expand the dynamic range (loud notes are louder, quiet notes are quieter and vice versa)?

The second one compress or expand per tracks or the whole song the dynamic range

MIDI compression/expansion is a bit tricky, because you not only need to specify by how much, but also the 'average' or 'target zero' point for the compression/expansion. It is certainly something that works poorly on an entire mix.

TBH, I would suggest importing the SMF to a proper DAW/Sequencer and doing the compression/expansion on a Part by Part basis there, simply for the added convenience of FAR more control over how and where it happens, but also a visual confirmation that the track hasn't managed to bump the track into the 127 velocity limit and removed any dynamics at all. Plus you get the chance to do the compression/expansion thing on individual drums within a mix, which gives you far more control (often you need to squash just kick and snare while preserving the dynamics of cymbals and toms, for instance).

With velocity, volume AND expression all making an impact on how loud a track sounds, I usually find it takes a proper sequencer to control how they all interact...