dxvxd wrote:Going back to the little experiment I proposed in the initial post to this thread. You sequence a series of repeated notes and then speed up the tempo until the repeated notes become audible as an oscillator. My question is, why is this frequency modulation and not amplitude modulation? I'm not changing the pitch, i'm just playing the same note over and over again so it seems to me the amplitude is modulating from zero to something not zero repeatedly.
It's frequency modulation because you're varying the tempo, which determines the time between each successive note played and thus the frequency at which they are sounding. Think of it as 'notes-per-second', if that helps.
If you have 440 of them per second then your 'virtual oscillator' is generating an A4 tone of some kind. The manner in which this interacts with the actual frequencies of each individual note being played is complex, and I'm not going to pretend I understand the details, but as with many things FM-related it's worth experimenting!
dxvxd wrote:If you held a note with an LFO acting on the pitch, and you increased the LFO speed eventually you would get frequency modulation.
Well, technically you've got frequency modulation at all points there, because as soon as you start modulating the pitch (which is the frequency) you're, well, modulating the frequency.
Where it becomes really interesting is when the frequency at which the pitch is being modulated reaches the audio range itself. In this case the note being modulated is the carrier, and the LFO is the modulator. Depending on whether the frequencies of the carrier and the modulator are harmonically related (ie: integer multiples or divisions of one another) or not, you'll get a harmonic or inharmonic tone as a result.
dxvxd wrote:If you did the same thing but with the LFO acting on the amplitude of the note, eventually you would get amplitude modulation.
By definition, yes. As with frequency modulation you actually have amplitude modulation as soon as you start modulating the amplitude. Initially it's just an obvious variation of the loudness of the tone but once your modulation is operating in the audible range you'll hear new resulting tones emerge based on that modulation frequency.