If you have a large board with common mode noise, i.e. potential differencies on signal ground between separate places on that board and you connect some piece of wire to the signal ground somewhere, then you create an antenna radiating electromagnetic waves. The motor of these electromagnetic waves is the common mode noise on the board. And a common mode filter can reject the radiation to some degree.

Way better is to arrange the cables leaving the board in such a way, that they sit at a place that is quiet in terms of common mode noise. So, put your circuit into a metall enclosure and let all cabels enter and leave the enclosure at only one place. Connect all cables shields to the metall enclosure at this place. Connect the signal ground of circuit and all cable filtering to the metall enclosure at this place. Follow the concept of a radio frequency plane here. By this you minimize common mode noise, the motor of radiation.

If you follow this design practice common mode filtering isn't needed in most cases. Of course, you need state-of-the-art differential filtering at input and output of dc-switcher, formed by appropriate pi-filters! Eventually a common mode ferrite choke like a clamp-on-ferrite can be useful on high speed digital communication cables. But the damping advantage is no more than 10dB usually. But make no mistake, heavy design mistakes cannot be cured by such a clamp-on-ferrite!

Use RS485 drivers for your midi communication that are slew rate limited. This is enormously helpful to enhance EMC!

Kai Klaas