Separating excitation from filtering for sound sythesis
Does the above make any sense or where did I go wrong?


Yes.


  Is this idea of separating excitation from filtering widely used in sound synthesis? (I believe there is some audio compression algorithm, which uses LPC)


Yes it does.


For decades it was the strongest approach for speech - and by extension singing voice - synthesis.
As for musical applications, in the framework of subtractive synthesis, filters or resonator banks (Moog 914) were used to model the coloration brought by the body of an instrument, or to create vocal sounds.
The vocoder uses this principle to apply the formants (filters) extracted from a vocal signal onto a synthetic excitation signal coming from a synth.
It is not uncommon to find a filter simulating the body of an instrument as the final step in a physical model (for example a simple bowed instrument model).
Modal synthesis is another sound synthesis technique based on the representation of sound as an excitation shaped through a filter.
One could also consider that the virtual amp/speaker modelling techniques used to build today's electric guitar sound are inspired by this approach - the signal coming straight from the guitar being considered as a raw excitation shaped by the footprint of an amp &amp; speaker.
Actually, any convolution processor (such as a reverberator) might be seen as an application of this technique - when you sing in a room, your voice is the excitation, the room the resonator.
This idea is also relevant to pitch-shifting. When a speaker speaks at a higher or lower pitch, the fundamental is modified, but the formants do not move (ideally...) - so a good pitch-shifting technique should preserve formants. To pitch-shift vocals or musical instruments sounds in a meaningful way, it might be a good idea to pitch-shift only the excitation part of the signal, while keeping the filter representation unchanged. In practice, though, we use techniques that do not explicitly make the actual separation (PSOLA in the time-domain, or spectral envelope manipulations in the frequency-domain when using a phase vocoder).



  Are there other methods which separate excitation from filtering?


Cepstral analysis. In some situations, adaptive filtering.