Modulation effects are early staples of music production and electric instrument soundcraft. As producers, we almost all know how flange, chorus or phaser effects sound, and we know that the range of sonic possibilities are vast. Take a signal, make a copy and modulate the copy, applying some processes to its fast or slow movement. Modulation effects have been with us for a long time, because their early forms were quite easy to bring about.
It was a relatively simple step for an experimental engineer to duplicate a taped audio signal and to rhythmically slow one copy down and back up again. This would be exactly the same mechanism that first produced flanging, which is so called because the pressure
