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Since a lot of the distinctive sound of a SID comes from the analog components which still need to be emulated in a FPGA solution does an FPGA solution really g

• foft 5y ago
  FPGA does not really offer advantages of accuracy, though it does offer advantages in time precision. This is important when putting it inside a C64. Of course a much faster CPU can also be used to handle real cycle accuracy in sync with the original clock. Part of the sid sound comes from the non-linearity due to approximation in its design. For example the state variable filters are very non-linear on the 6581. It even differs significantly from each individual chip to chip[1]. The best software models I'm aware of are in residfp[2] and jsidplay[3]. After some work in octave I found the filter model can be recreated without the large lookup tables used in this software. Of course getting the digital part correct is also important. There is a lot of documentation on this resulting from the decap [4]. Some features like bitfade, multiple driven bits in the wave corruption etc do not translate well to FPGA. They can be approximated with timers and lookup tables. [1] Filter response chart: https://csdb.dk/forums/?roomid=14&topicid=105719 https://csdb.dk/forums/?roomid=14&topicid=105719 [2] residfp: https://bel.fi/alankila/c64-sw/index-cpp.html https://bel.fi/alankila/c64-sw/index-cpp.html [3] jsidplay2: https://haendel.ddns.net/~ken/ https://haendel.ddns.net/~ken/ [4] Sid internals: https://sourceforge.net/p/sidplay-residfp/wiki/SID%20internals/ https://sourceforge.net/p/sidplay-residfp/wiki/SID%20interna...