GEP causing some wave channels to change duty cycles by TheUltimateKoopa at 10:03 AM EDT on September 10, 2017
I think that's the correct word for it. In some games such as Donkey Kong Land, on the wave channel, the square wave will suddenly change from 50% to 25%. For example, in the "Mountain" theme at around 34.4 seconds. https://drive.google.com/open?id=0B_G73t0I71FdYVRWdW9zZkV1OUE
I don't have a Game Boy to test it on. All I know is that every single version of that song (Construction Site/Building/whatever), the wave channel is always a square wave.
Maybe someone has uploaded a "directly recorded from a Game Boy" soundtrack on Final Fantasy Shrine, I could compare it with.