The SN7 noise channel can "steal" the pitch of the third square
channel for increased range, and this percussion set takes
advantage of that. The kick and snare drums use CH3 pitch for
two frames- CH3 is muted for the first frame and outputs a
square wave on the second. The hi-hats use CH3 pitch for only
one frame, during which CH3 is silent.

In order for this to work, an instrument must be placed on both
CH3 and CH4, whenever a kick or snare is triggered. (Because
the hi-hats only need one frame of CH3 silence to sound
correct, and CH4 pitch macros take precedence over CH3 macros
in Furnace, you don't need to place an instrument in CH3 when
a hi-hat is playing by itself.) This is the "KickSnare CH3"
instrument in the folder. The "CH4" instruments are for CH4,
obviously.

The "ExampleInst" instruments show a setup where a regular
melodic instrument is combined with the CH3 percussion
instruments, so that CH3 can play something useful with minimal
interruption from CH4. Of course, now you have to keep track of
three instruments for one timbre, and you can't have a note 
sustain through a percussion sound, but I think it's worth it.


Note also that the CH4 instruments need to be played on either
C or D to sound correct.