According to [https://patents.google.com/patent/US4677890A/en Google patents for US4677890A], the patent for the SID chip is now expired.
*[https://youtu.be/CIo93AE8Fsw SID Phase Accumulator explained] by [[Jeri Ellsworth]]
*[https://youtu.be/yadWu7hs4MU Oscilloscope comparison of 4 different SID chips]
*[https://youtu.be/fQU-WZZ67yM C64 SID vs Amiga Paula sound chip]