Additive synthesis
The additive synthesis is the exact oppposite of the subctractive one :)
Additive synthesis was born in the same period as the subctractive one, but it never encountered a large success (until recent times) since it's very complex and it costs too much to build a synthtizer of this kind. Now thanks to the DSP technology and software it's possibile to create this kind of synth.
The base of this synth if the Fourier theory that each signal is made by different sinusoids with different amplitude and frequency, the sum af all this base harmonics give us the complete signal analized.
An additive synth is made by 2 main parts: the oscillators and the envelope generators. Tha dditive synth has an envelope generator for each oscillator, this make possibile to chenge the amplitude of each harmonic during the lenght of the sound produced. The different harmonics are then summed together to get the final sound.
Thanks to the very fine control on each harmonic, with this kind of synth is possibile to get sound that move along the timeline in a very interesting way, so in my opinion is very useful for pads, noises and effects in general, but for leads or basses a subtractive synth gives often much more fuller and rich sounds.
A curiosity, the first additive synth of the history is the Hammond organ that has 9 different oscillator whch amplitude is modifiable using the 9 drawbar controls.