[sdiy] Cyclone SID

[Rainer Buchty]

>Damn! I was hoping this would be an easy way to program my own SID. 

You're of course aware that the SID's oscillators are plain digital? You 
have 3 phase accumulators in there which bits get either output directly 
(saw), compared against a reference value (pulse), or inverted based on 
the MSB w/ the lower bits shifted left and output (tri).

Noise is based on the usual feed-back shift register.

How waveform "addition" (you can select more than one wave) is *really* 
done is still somewhat a mystery, as it is neither true addition, nor 
some easy logic (like AND, or OR) and furthermore depends on the SID 
revision. Ringmod is "faked" using XOR.

Rainer