Modulation on synths is everything. It is the most important part of synthesisers. There are so many different ways to modulate sound in your synthesiser and I am going to detail how a few of the methods work.
What is FM synthesis?
A cornerstone in all synthesis but especially bass music, FM synthesis is the kind of thing that you know of but you never really know what it means or what it actually does.
The FM in FM synthesis stands for frequency modulation. It involves 2 waves with one modulating the other creating a third, more complex waveform as a result.
If you modulate the pitch of a wave with an LFO, that is essentially what frequency modulation does just at a much much higher rate (several hundred or thousand oscillations per second). This leads to not the pitch changing but rather the timbre of the wave changing leading you to get that metallic sound known with FM. (Native Instruments)
The waveforms that are combined in a FM synth are called operators. Operators can play 2 roles, it can either modulate another operator (Modulator) or it can be connected to the synths output (Carrier). The routing of your modulators and carriers can change the sound drastically, these different routings are known as Algorithms. In some synths you can tweak the algorithms rather than in others you can use pre-set algorithms. (Native Instruments)
What is AM synthesis
AM synthesis is another type of waveform modulation.
When the modulation source is an oscillator and the destination is the Gain of a VCA in the audio signal path the result is called Amplitude Modulation, or AM for short. (Reid, G)
This leads to a very distinctive modulated sound.
What applications do they have?
FM and AM are incredibly useful for making bass sounds, pivotal even. Especially in bass music where the whole focus of the genre is a heavy bassline. FM is commonly used to make a foghorn style bass where filtering and distortion is applied and it genuinely can sound like a foghorn. I personally prefer using AM as I make more deep basses rather than mid range basses (silly statement but mid range basses are a real thing).
References
Native Instruments. (2022) What is FM synthesis?, Native Instruments Blog. Available at: https://blog.native-instruments.com/what-is-fm-synthesis/ (Accessed: 04 March 2024).
Bilbao, S. (2006) AM and FM Synthesis, AM and FM synthesis. Available at: https://ccrma.stanford.edu/~bilbao/booktop/node10.html (Accessed: 04 March 2024)
Reid, G. (2000a) Amplitude Modulation, Amplitude modulation. Available at: https://www.soundonsound.com/techniques/amplitude-modulation (Accessed: 17 April 2024).