Harmonics. Harmonics are what make sound, colour and the universe three dimensional.
Anyone who has ever dealt with the subject of light knows that the instant there is light, there is reflection.
Sound is not so different.
What happens when we digitize a sound, is that we take what should be a smoothly rising and falling sine wave, and divide it into steps - the difference between a rolling hill and steps - literally.
We have made it better and better by putting in more steps(bits), but the are still steps.
Imagine a car travelling up a hill - smooth climb, smooth hill - put in steps, and immediately the ride is dependent on the number of steps.
Now here's the part it may be hard to get your head around.
The reflections from a stepped or digitized sound or wave, must produce very different reflections and harmonics from a smooth sine wave.
The complex sound is then producing thousands of different square waves, altering the whole characteristic of the sound.
Real musical instruments produce sine waves, no matter what the instrument.
Further to that:-
Valves can be likened to listening to sound via a glass tube, and solid state like listening through sand, so why do transistors sound different to valves?
Harmonics.
The last concept is extreme, but quite a good analogy.
Let us make everything as simple as possible, but no simpler - Einstien
