The USB-MIDI interface makes ground loops with the audio link from the same laptop to the same keyboard (Line). The ground loop goes through the Midi-In (the USB Midi interface), there is probably no optocoupler. About the Interface Midi Out (Midi In the Keyboard) on the other hand is not, perhaps the keyboard has an optocoupler, was also significantly more expensive. However, the ripple level is compared to the reinforced laptop headphone output signal is sufficiently low, a USB sound card had less, since it was already clear. For professional claims probably recommends a more expensive USB-MIDI cable, which is decoupled.
