Originally Posted by freqmax
That means there's more than plain D/A and standard analog filter. So could anyone describe what's special in technical terms?
"More than" a DAC and filter, no. But again, even a system with just a DAC and filter can have its own distinct character, because those components color the sound in their own ways. No electronic component, especially no analogue
electronic component, is actually a mathematically ideal implementation of its nominal function. As has been noted, the Amiga's DACs aren't even close to linear - that's going to distort the output. Also, samples are fed in at varying rates and played back without interpolation, which means that any aliasing noise is going to come out at a different frequency, likely well within the range of human hearing, which is also going to color the sound.
And finally, there is not a single analog filter in existence that has a mathematically ideal, linear frequency roll-off. (Which virtually every emulator in existence gets wrong; even reSID was wrongly doing linear roll-off until very recently.) Even the cleaner filters have a curved roll-off, and a lot of them will have different ranges roll off at different curves.
Additionally, real-world filters don't just attenuate frequency components, they change their phase relationships. All of this varies wildly
from filter to filter, with the result that just about every design is at least subtly different. (A good place to go for further reading on this subject would be installments 4
of Gordon Reid's excellent Synth Secrets
series for Sound on Sound.
The man knows his stuff.)
So, really, even with a very simple DAC + filter system, there's any number of factors that can give a distinct character to the sound. It's probably not impossible to emulate them properly, but I don't know of an emulator that does.
Many get things basically right, but I'm not surprised that folks like magnetic feel that only the real deal provides the same sound. It's complex.