So if I simply double a signal… I get the same sound but louder.
HOWEVER, if I EQ one of them a certain way, and do the exact opposite EQing on the other one, it sounds distinctly as the sum of the individual parts, and NOT the original signal.
Why?
Because filters have certain slopes that are not infinitely steep, can introduce phase shift, and depending on design have different filter curves for gain / attenuation with identical settings of the same bands.
All true.
But also be very careful with using this technique.
Reason being, if by any chance your music get played thru a monophonic sound system (AM radio etc) those two tracks might interfere with each other (not in a good way) or worst they might cancel out each other and disappear from the mix altogether.
If you use the ‘doubling thang’ as a technique, be sure to also check your mix in mono.
HTH
{’-’}