Well I spent the weekend hacking my new idea for the SID filter, and I think it’s kinda sort of a success. Maybe. Here’s what it looks like:
The board in the background is my FPGA dev unit, and the board in the foreground is the analog filter/volume section. The FPGA is of course handling all the digital end while the board in front is doing the filter, volume control, and other analog functions.
It’s 10 chips, but to add a second SID will only take another 5. I got the controls down to 4 pots, but I can ditch those once I figure out the proper settings. I have totally redesigned the silly thing 8 times now, and I think I might possibly have a winner- time will tell. So far however I am very impressed with the output of it. It sounds like an R2 and/or an R3 SID. I have been using the SOASC MP3s for comparason purposes (and the SIDMan to a lesser extent). You can hit up the SOASC HERE.
I did a few tunes off of the “SID distortion” page (you can find that here) for your comparason pleasure. NOTE: He uses a Catweasel board and an R4 SID for his testing, which I believe to be a poor combination. The capacitors on the board are just as important as the SID when it comes to how the filter will sound! What value caps are on the Catweasel board? I cannot find that info anywhere. My friend has one and he can’t figure it out either.
Anyways, I believe a real stock C64 is the best test bed for how the SID “should” sound, since it will have the exact set of circuitry that is needed; this includes the specific output circuitry in the computer- an NPN transistor, 0.1uF cap and some resistors. These parts will add/change the sound somewhat (the .1uF cap in particular) and should really be taken into concideration.
The only problem I’ve found so far on my SID recreation is that the audio is too clean (there’s no bus noise) and there’s a slight resonance issue with my filter, but I am not 100% sure if this is due to my end or not. I am going to perform some more testing on this to find out.
Sooo, without further ado, here are the samples of my thinger in action:
Aaand, that’s all for now.