I implemented the "Unison" mode in my musical synthesizer project with the IC YM2413, here I demonstrate how it turned out using the Violin type voice, I turn the unison on and off during the video. Unison is when multiple notes are playing the same pitch at the same time, in my case I am using 2 channels of YM2413 playing the same note, with the second note slightly out of tune in relation to the first. The result is a fat sound full of movement and life. The YM2413 has 9 sound channels (9-note polyphony), when I activate the unison it has a 4-note polyphony, which still allows you to play chords :)