The original Hammond used rotating tone wheels to generate harmonics which were added using drawbars; not true harmonics though. The principle is known as additive synthesis. The organ is usually played through a Leslie speaker which utilizes the Doppler principle to produce chorale and tremolo effects. Modern digital synthesizers such as the Nord Electro simulate both the drawbars and the Leslie speaker. In subtractive synthesis, used for example on the Korg MS20, tones with complex spectra are generated and unwanted components filtered out. The most common transient generator is the ADSR where the letters stand for Attack, Decay, Sustain and Release. The Yamaha DX7 uses Frequency Modulation (FM). Sidebands are generated in the frequency spectrum spaced at multiples of the modulation frequency from the carrier. MIDI allows you to control the sounds of one instrument from the keyboard of another.